Fired Equipment and Design radiant section – and in a bundle in the upper portion – the convection section. Convective heaters are a special application in which there is only a convection section.
Fired equipment transfers heat produced by combustion of the fuel t o the process process strea m. In gas processi processing ng equipment, the fuel is usually na tura l gas; h owever, wever, etha ne, propane, propane, or light oils are sometimes used. The process stream varies widely, e.g., natural gas, heavier hydrocarbons, water, glycol, am ine solutio solutions, ns, heat tra nsfer oils, oils, and molten salts.
2. Firetube heat heat ers where the combustion combustion gases ar e concontained in a firetube that is surrounded by a liquid that fills the hea ter sh ell. This liquid ma y be eith eith er the process ess stream or a heat transfer medium medium tha t surrounds the coil coil bundle conta conta ining th e process process strea m.
Fired equipment can be classified as: 1. Direct Direct fired heaters w here the combustio combustion n gases occup occupyy most of the hea ter volume a nd hea t th e proce process ss strea m conconta ined in in pipes pipes a rra nged in front front of refractory walls – the
Fig. 8-2 l 8-2 l ists th e commo common n a pplic pplicat at ions ions a nd general cha racteristics of these heaters.
FIG. 8-1 Nomenclature a = con st st a nt nt , E q 88-21 2 A = a r ea , m AO = D r y co com b us us t io ion a i r m ol ol s pe per d r y f u el el m ol ol s, s, for st ochiomet ochiomet ric combust ion, m ols/mols B = pa r a m et et er er d ef ef in in ed ed b y E q 88 -21 c = n u m be be r of of ca ca r b on on a t om s i n fu fu e l m ol ol ec ecu le le C = con st s t a n t, t , E q 88-4 a n d E q 88-8 C d = burner burner spud discharge discharge coeff coeffic icie ient nt C p = spec specif ific ic heat, kJ /(kg • K ) CO = carbon mono monoxide xide mols mols in mols mols of dry flue flue gases, mol s/m ols C O 2 = ca rbon diox dioxide ide mols mols in mols mols of of dry flue gases, mol s/m ols C O 2O = ca rbon diox dioxide ide mols mols in mols mols of of dry flue gases for for st ochiomet ochiomet ric combust ion, m ols/mols d = d ia ia m e t er er of of p ip ip e or cy cy l in in d er er or or f in in , m m D = d ia ia m e t er er of of pi pi pe pe or cy cy l in in d er er , m EA = volume volume perce percenta nta ge of exce excess ss com combusti bustion on air, air, % f f = F a n n i ng ng fr fr i ct ct i on on f a ct ct or or F = v ie iew f a ct ct or or, di d i me me n s io ion le l e ss ss F l o = mols mols of dry flue gases gases per per mols of dry fuel for for st ochiomet ochiomet ric combust ion, m ols/mols g = accel acceleratio eration n due to gravity gravity = 9.806 9.8067 7 m/s 2 G = R a t i o of a i r t o f u eell , k g/ g/k g; g; or = ma ss velo veloccity, ity, kg/(s • m 2) G H I = g r os os s he he a t in in pu pu t , k W GTE GTE = gross gross thermal effic efficie ienc ncyy, Eq 8-1 8-17a G r = G r a s h of of n um um be b e r, di d i me me ns n s io ion le le s s 2 h = h ea ea t t r a n s f er er c oe oef fi fi ci ci en en t , W/ W/(m • ° C ) H = h ea ea t c on on t e n t or en en t h a l py py, k J /k g ; or = height height of of stack, stack, m; height height of fin, mm H a v = a v a il il a b le le d ra r a f t , P a (g (g a ug ug e) e) 3 H H V = h ig ig h er er or or g r os os s he hea t i n g va va l u e, e, kJ kJ /S m h y = n u m be be r of of h y d r og og en en a t om s i n fu fu el el m ol ol ec ecu le le I = a v er er a g e t u be be r a d ia ia n t h ea ea t f lu lu x i nt n t en en s it it y (circumf er ent ia l), W/ W/m 2
k = t h e rm rm a l c on on d u ct ct i vi vi t y, y, W/ W/(m • ° C) L = l e n g th t h of of h ea ea t t r a n s fe f e r s ur u r fa f a ce ce , m ; or or = m ea ea n b ea ea m l en en gt gt h , m; m; or or = chara cteristic teristic dimensio dimension, n, m 3 L H V = l ow ow e r or n et et h ea ea t i n g v a l ue ue , k J /S /S m m = con s ta ta n t , E q 88-4 a n d E q 88-8 or = d im im en en si si on on , m M = m a ss ss fl fl ow r a t e, e, kg kg /h r n = n u m be be r of of f in in s pe pe r m et et e r, r, n u mb mb er er o f t u b e r ow ow s n i = n u m be be r of of n i t r og og en en a t o m s in in f u el el m ol ole cu cu le le N u = N u ss ss el el t n u m be be r, r, d im im en en s io ion le le ss ss N HI H I = n e t he h ea t in in p u t, t, kW kW N P S = n om om in in a l p i pe pe s iz iz e, e, m m NTE NTE = net thermal effic efficie ienc ncyy, Eq 88-17b 17b o = n u m be be r of of ox ox yg yg en en a t o m s in in f u el el m ol ol ec ecu l e mols in mols of dry flue gases, mols/ mols/mols mols O 2 = oxygen mols O 2O = oxygen xygen mols in in mols mols of dry flue flue gases for for st ochiomet ochiomet ric combust ion, m ols/mols P = p a r ti t i a l p r es es s ur ur e of C O2 + H 2O, atm P B = b a r om om et et r i c p r es es su su r e, e, kP kP a ∆P = pr es es su su re re di d i f fe fe r en en ce ce, kP kP a P g = b ur ur n er er fu f u el el ga ga s pr p r es es su su r e, e, kP kP a ( a bs bs ) P r = P ra r a nd n d t l n u m b e r = C p µ /k, dimensionless q gs = g a s f lo low r a t e a t s t a n d a r d co con d it it i on on s , m 3/d a y q l = l i qu q u id id f lo low r a t e , m 3/hr Q = h ea ea t t r a n s f e r or h ea ea t i n p ut ut or or h ea ea t c on on t e nt nt (r (r a t e s ), ), W r = ratio of of flue flue gases gases to heat release, release, kg/(MW • h r ) r f = f ou ou li li n g h e a t fl flow r es es is is t a n ce ce , (m 2 • ° C )/W R = f r ac a ct io ion of of t ot ot a l heat l heat libera libera tion absorbed in ra diant section section (Fig. 8-20) 8-20 ) R D = r e l a t iv iv e de d e n si si t y R e = R ey ey no n ol d s n um u m be be r , D Vρ/µ or LVρ/µ, dimensionless s = n u m be be r of of s u lf lf u r a t om om s in in f u el el m ol ol ec ecu l e S = t ub u b e s p a ci cin g, g, m m S m 3 = standa rd cubi cubicc meters meters at 101. 101.32 325 5 kPa and 15° 15° C
8-1
FIG. 8-1 (Cont’d) Nomenclature T = = t = U = UHT = V = w = wt = ∆x = Y = Greek β =
∆T
ε1, ε2,
=
µ ρ σ
= = =
π
=
Subscripts a = a t m o sp sp h er er i c a i r a t op op er er a t i n g con d it it i on on s a s = a ir ir a t s t a nd n d a r d co con di d i t io ion s B = b a rro om et ri ri c b = bu l k c = con vveect iv iv e cs = cr os os s s ec ect i on on , pr pr oj oject i on on f = f in in ; fo fou li li ng ng ; fr fr i ct ct io i on ; F a n n in in g g = ga s g s = g a s a t st s t a n da da r d con d it it io i on s i = in si si d e, e, in i n t er er n a l L M = log me m ea n ba ba se se e m = m i d d l e su s u r fa fa ce ce o = ou t si s i de de, ex ex t er er n a l, l, ov ov er er a l l p = pi p e r = r a d ia n t s = s t a ck w = w a ll 1 = b u r ne ne r op op e ra ra t i n g con di d i ti t i on on s 2 2 = b u r ne ne r op op e ra ra t i n g con di d i ti t i on on s 2
t em em p er a tu t u r e, e, K t em e m pe per a t u r e d if if fe fer en e n ce ce , ° C f i n t hi hi ck ne ne s s , m m overall heat tra nsfer nsfer co coeffic efficie ient, nt, W/ W/(m 2 • K ) u se s ef u l h ea ea t t r a n s fe fe r or h ea ea t d u t y, y, W v el el oci t y, y, m /s w ei ei g h t of a ir i r, k g w a ll ll t hi h ic k n es es s , m m d is is t a n ce ce in in d i re re ct ct i on on of of h ea ea t t r a n s f er er , m or or m m ex pa pa n s io ion f a ct ct or or, di di me me ns ns io ion l es es s volumetric volumetric coeff coeffic icie ient nt of thermal expansion, expansion, 1/(° C or K) em i ss ss iv iv it it i es es of of co com b us us t io ion g a s es es a n d w a l l , respectively v is is co cos it it y o f fl fl ui ui d, d, m P a • s density density of of fluid, fluid, kg/m 3 S t e ffaa n -B -B o lt lt z m a n n con s t a n t , 5.67 (10 –8 ) W/(m 2 • K 4) 3. 14 16
FIG. 8-2
Q
= 1000 • k • A •
∆T
Heater Applications and Characteristics Direct Fired
For heat transfer in cylindrical geometry where the heat tra nsfer is norma norma l to the axis, as in heat flow flow t hrough a cylincylindrical vessel or pipe pipe wa ll:
Firetube Applications
Hot oil heater Regeneration Regeneration ga s heaters Amine and stabilizer reboilers
Indirect Indirect fired wa ter bath heaters (line heaters)
Q
Propane and heavier hydrocarbon vaporizers
=
Q
Glycol and amine reboilers
=
Low pressure steam steam generators
Higher therma l efficie efficiency ncy
Ea sily skid mounted
Requires less plot space
Forced or na tura l dra ft combustion
Forced Forced or or na tura l draft combustion
=
2•
π • L • k • ∆T ln (d o /d i)
Eq 8-2a
2 • π • k • ∆T 1 ( / D i ) + (1 / D o )
=
π • k • ∆T (500 /d i ) + (500 /d o )
Eq 8-2b
Fig. 8-3 gives the thermal conductivities and densities of commercial commercial refr actories a nd insulation. insulation. Similar Similar da ta for for metals a re given given in Fig. 8-8 a 8-8 a n d Fig. 9-8. 9-8 .
Characteristics Heat duty usua lly less less tha n 2930 kW
2 • π • L • k • ∆T ln (D o /D i )
For radia l heat flow flow t hrough a spherical spherical vessel:
Hot oil and salt ba th heaters
More ancillary equipment and controls controls
Eq 8-1
wt
Example 8-1 — Estima te the loss per per linear meter t hrough a 25 mm la yer of block block insulat ion covering covering a 200 200 mm NP S S ch 40 40 steam header. Assume:
Less likely to have hot spots or tube rupture
Ti = 120°C To = 1 0 ° C k = 0.0721 0.07 21 W/(m • ° C )
HEAT TRANSFER
Solution Steps
Conduction
d o = 269.9 269.9 mm
Fourier’s Fourier’s law of conduction conduction gives gives th e rat e of of heat tra nsfer thr ough substances resulting from vibrations and interactions between adjacent m olecules olecules a s opposed opposed to overa ll motion or mixing of t he molecules. cules. Conduction Conduction a lwa ys a pplies pplies to solids solids an d ra rely to fluids.
d i = 219.1 219.1 mm L = 1m From E q 8-2a 8-2a
Funda menta l equations for for stea dy heat conductio conduction n thr ough ough some common common solid solid sha pes, ignoring border conditions, conditions, a re:
Q
For unidimensional perpendicular heat flow through flat wa lls, lls, as in hea t flow through a squa re or or very large cylindric cylindrical al tank wall:
= =
8-2
2•
π • L
• k (Ti
− To ) = ln (d o /d i )
239 239 W per linea r m
2 (3.1416 )(1)(0.0721 )(120 −10 ) ln (269.9 / 219.1)
FIG. 8-3 Properties of Commercial Refractories and Insulations Thermal Conductivity, W/(m • °C) at Mean Temperature, °C 540 815 1100
1370
3. 52 0 1. 87 0 1. 40 0 1. 20 0
2 . 94 0 1 . 84 6 1 . 42 8 1 . 26 9
2 . 8 27 1 . 8 89 1 . 4 71 1 . 3 27
2. 8 27 1. 9 61 1. 5 43 1. 3 70
2. 84 1 2. 07 7 1. 60 1 1. 42 8
29 95 25 15 22 91 2114
10 7 58 8 4 8 2 63 7 3 7 74 1 4 4 79
1 65 0 1 37 0 1 26 0 1 09 0
0. 37 5 0. 15 9 0. 14 4 0. 1 3 0
0 . 44 7 0 . 18 7 0 . 17 3 0 . 15 9
0 . 5 19 0 . 2 16 0 . 2 16 0 . 2 02
0. 5 77 0. 2 45 0. 2 45
0. 64 9
97 7 60 9 49 7 46 5
8 10 1 1 51 7 1 00 0 8 62
HEAVY CASTABLE AP G r ee n G r ee n ca s t 9 4 AP G r e e n M i z z ou
1 87 0 1 65 0
2. 09 0 1. 12 0
1 . 81 7 1. 111
1 . 6 30 1 . 0 82
1. 5 43 1. 0 6 7
1. 55 8 1. 06 7
26 11 22 11
3 9 9 90 2 0 6 84
2, 3 , 4 , 5 3, 11
LIGHT CASTABLE AP G r ee n K a s t -O -L i t e 2 5 AP G r ee n 4 5-L AP G r e e n C a s t a b l e 22 AP G r e e n C a s t B l o c k M i x
1 42 5 1 37 0 1 20 0 87 0
0. 51 9 0. 38 9 0. 24 5 0. 08 6
0 . 50 5 0 . 37 5 0 . 27 4 0 . 14 4
0 . 5 34 0 . 3 61 0 . 3 17 0 . 2 16
0. 5 77
13 78 11 37 84 9 35 2
8 96 3 1 7 2 37 1 89 6 1 38
8 3, 6 , 7 , 8 3, 8 8
GUN & RAM MIXES P r e m i e r 85 R AM H S H -W Tu f f S h o t L I AP G r ee n K a s t -O -L i t e 2 6-L I G R
1 65 0 1 42 5 1 42 5
3 . 13 0 0. 89 4 0. 54 8
2 . 03 4 0 . 95 2 0 . 53 4
1 . 8 17 0 . 9 66 0 . 5 62
1. 7 16 0. 9 81 0. 6 20
1. 74 5 0. 99 5
28 0 3 19 06 14 74
3 4 4 74 2 3 7 87 1 3 7 90
5, 9 6, 11
CERAMIC FIBER Th e r m a l C e r a m i c s S a f f l F i b e r f r a x D u r a b l a n k e t 26 00 F i b e r f r a x D u r a b l a n k e t 26 00 F i b e r f r a x 55 0 P a p er Th e r m a l C e r C er a b l a n k e t Th e r m a l C e r C er a b l a n k e t
1 53 5 1 42 5 1 42 5 1 26 0 117 5 117 5
0. 05 5 0. 07 4 0. 07 4 0. 0 6 1 0. 07 2 0. 05 3
0 . 10 1 0 . 16 6 0 . 14 1 0 . 10 8 0 . 14 4 0. 11 5
0 . 1 44 0 . 3 14 0 . 2 61 0 . 1 92 0 . 2 45 0 . 2 16
0. 2 16 0. 5 13 0. 4 27 0. 3 06 0. 3 75 0. 3 03
0 . 33 2
48 96 12 8 19 2 96 12 8
Max Service Temp, °C (Note 1)
260
FIREBRICK H -W K a r u n d a l X D H -W U FAL A AP G r e e n K X-9 9 AP G r e e n E m p i r e S
1 81 5 1 65 0 1 37 0 1 31 5
INSULATING BRICK AP G r ee n G r ee n l i t e 3 0 Th e r m a l C e r a m i c s K -2 5 Th e r m a l C e r a m i c s K -2 3 Th e r m a l C e r a m i c s K -2 0
Products
0. 3 61
BLOCK & BOARD F i b e r f r a x D u r a b oa r d L D 1 26 0 0. 08 1 0 . 12 3 0 . 1 79 0. 2 50 F i b e r f r a x D u r a b oa r d H D 1 26 0 0. 08 1 0 . 12 6 0 . 2 12 0. 2 32 Th e r m a l C e r a m i c s TR -2 0 1 09 0 0. 0 9 2 0. 111 0 . 1 33 U S G K -F AC 19 1 03 5 0. 07 4 0 . 12 8 S c h u l l e r Th e r m o 12 35 0 0. 06 6 0 . 09 2 P AR TE K P a r o c 12 12 35 0 0. 06 1 0 . 14 1 S c h u l l e r 10 00 S p i n G l a s 45 5 0. 07 2 NOTES 1. Maxim um Service Temperat ure listed has no safety factor included. 2. 9090-94%A 94%Alumina lumina product for extreme extreme t emperat ure or high velocity velocity service. 3. Ca st propert propert ies listed, gunning product ava ilable but propert propert ies will be different. different. 4. Low silica product. product. 5. For For burn er blocks blocks and severe service. service. 6. Can be used as one shot (single (single layer) lining. 7. High performan ce medium weight lining. 8. May be used as back-up back-up insulat ion in 2 layer lining. 9. 85%Alumina 85%Alumina ra mmin g plastic for for burner blocks, etc. 10. For For externa l insulat ion only. only. 11. May be used as hot face lining lining in dua l layer system. 12. 12. Dia tomaceous tomaceous eart h ba se. 13. Minera Minera l wool base. 14. Calcium silicate base. 15. Fiber Fiber gla ss base.
8-3
Cold Density, Crush 3 kg/m Strength, kPa
25 6 41 6 40 0 29 6 24 0 19 2 48
Notes
2
4
3 45 4 83 1 38 6 2 28 110 3
12 10 , 1 3 10 , 1 4 10 , 1 3 10 , 1 5
FIG. 8-4
6
Gr
Heat Transfer Constants2 for Eq 8-4 (Natural or Free Convection) Configuration
D or Y (Y = Gr • Pr)
C
m
1.36 0.55 0.13
0.20 0.25 0.33
10 < Y < 2(10 ) 2(107) < Y <3 (10 10) 5 10 3(10 ) < Y < 3(10 )
0.54 0.14 0.27
0.25 0.33 0.25
D < 0.1 0.1 < D < 0.5 0.5 < D
0.53 0.47 0.11
0.25 0.25 0.33
0.49 0.71 1.09 1.09 0.53 0.13
0.00 0.04 0.10 0.20 0.25 0.33
4
Vertical Plates or Cylinders
Y < 10 4 9 10 < Y < 10 9 10 < Y
Horizontal Plates: Facing Up Facing Up Facing Down Long Horizonta Horizonta l Cylinders L >D
5
7
–5
Short Horizontal Cylinders L = D < 8 in in .
Y < 10 -5 –3 10 < Y < 10 -3 10 < Y < 1 1 < Y < 10 4 4 9 10 < Y < 10 9 10 < Y
or
=
C (G r • P r )
Nu
=
h • D o k
or
Nu
=
h • L k
Eq 8-7
Tfilm
=
88 + 45.6 2
Tfilm
=
66.8 66.8 ° C
do
= 88.9
mm
= 0.47 (G r • P r )0.25 0.25 d o3 • ρ2 • g • β • ∆T • C p h • d o = 0.47 1000 • k 1000 • µ • k 3 ρ = 990 kg /m g = 9.807 m /s 2 β = 0.00063 1/° C ∆T = 88 − 45.6 ° C Nu
Eq 8-4
h • d o 1000 • k
µ
From Eq. 8-4, 8-5a, 8-6a and 8-7, and Fig. 8-4: 8-4 :
Where
=
Cp • k
Solution Steps
Natural or free convection — occurs when the only force force promoting promoting th e fluid flow results from tempera tu re differences ences in the fluid. fluid. Un der these conditions conditions the heat tra nsfer coefficient is obtained from the Nusselt equation. Nu
=
Eq 8-6b
Example 8-2 — What What is the hea t t ra nsfer coeffi coeffici cient ent for na tural convection around a 75 mm NPS Sch 40 pipe surrounded by wat er at 88° 88° C? Assume To for th e pipe is 45.6 45.6°° C.
Eq 8-3
m
Pr
ρ2 • g • β • ∆T µ2
The coeffic coefficient ient of t herm al expansion β for low pressure gas (i.e. (i.e. idea l ga s) equa ls 1/(T, K). It is left in th is form for u se in G r a nd is n ot convert convert ed to 1/° C for dimensional consistency. consistency.
Newt on’s on’s law of coo cooling ling a pplies pplies to convective convective heat tr a nsfer h • A • ∆T
=
106 • L 3 •
Eq 8-6a
Nu, Gr, and P r ar e dimensio dimensionless nless when the units indicat indicat ed in Fig . 8-1 8-1 a re used in equa tions 8-4 8-4 th rough 8-7. 8-7. The The physical properties properties are those of the fluid at the film temperatur e, which which is often often a ssumed to be the a verage of the solid solid surface and bulk fluid tempera tempera tures. Fluid propertie propertiess ma y ha ve to be evalua evalua ted at an assumed film temperature, and this assumption then confirm confirm ed from t he r esults — see Exa mple 8-4. 8-4.
Heat tra nsfer betw betw een een a solid solid and a n ad jacent jacent fluid occurs occurs by movement of the fluid molecules. molecules. Hot molecules molecules leave t he surfa ce of the solid solid a nd a re replaced by cold cold ones. Most Most of the resistance to this form of of heat tra nsfer occurs occurs in a t hin film or or layer next to the solid surface. This layer exists even if the bulk fluid flow flow is violently tur bulent.
=
Gr
ρ2 • g • β • ∆T d o3 • ρ2 • g • β • ∆T = 1000 • µ2 µ2
The constant constant s C a nd m d epend epend on on th e shape an d size of of the solid surface, the orientation of the surface to the fluid, wheth er the solid solid is hotter tha n t he fluid fluid or vice vice versa, an d the magn itude of (Gr (Gr • P r). A b rief summ ar y of C and m for for some usual situa tions is given given in Fig. 8-4. 8-4 .
Convection
Q
=
3
10 • D o •
Eq 8-5a Eq 8-5b FIG. 8-5
Heat Transfer Constants for Equation 8-8 2, 3 Forced Convection Configuration F l a t p la la t e p a r a l le lel t o f lo low
Characteristic Length
Re 3
P l a t e l en en g t h
10 < Re < 10
Cylinder axis perpendicu- Cy linder dia meter lar to flow
Pr 5
0.648
0.50
1 < Re < 4
> 0.6
0.99
0.33 0.33
4 < Re < 40
> 0.6
0.91
0.39
> 0.6
0.68
0.47
> 0.6
0.193
0.62
> 0.6
0.0266
0.81
0.023
0.80
3
4(10) < Re < 4(10) 4
4(10) < Re Outside ban k of tubes
4
Inside pipe dia meter
10 < Re
St agger ed outside tube diam eter In line
4
0.7 < P r < 700
3
2(10 2(10)) < Re
> 0.6
0.33
0.60
3
> 0.6
0.26
0.60
2(10) 2(10) < Re
8-4
m
> 0.6
40 < Re < 4000
Inside pipes
C
4.187 kJ /(kg • ° C ) = 4.187 µ = 0.63 0.63 mP a • s k = 0.632 W/(m • ° C ) The properties of water (ρ, β, µ, k) Cp
Where:
are based on the film
temperature.
ho
=
0.25
•
ρ2 • g • β • ∆T • C p 1000 • µ • k
µ =
=
= 697.3
2
W/(m 2 • ° C )
=
Nu
C • R e • P r
0.33
=
Nu
C • R e • P r
Re
=
ρ
µ
=
0.3537 • M D•µ
=
=
0.3537 • q l • D•µ
ρ
=
q gs • R D 55.4 • D • µ
=
=
Eq 8-8b
d • V •
ρ
1000 • L • V •
Eq 8-9a
µ
353.7 • q l • d•µ
18.05 • q gs • R D d•µ
ρ
hi
3.06 (19 ) (10−3) 0.052 2 = 1971 W/(m • ° C )
U • A • ∆TLM
Eq 8-10
=
1 1 ho
+
Do D i • h i
+
1 ho
+
do d i • h i
+
D o • ln (D o /D m ) D o • ln (D m /D i) D + + r f o + r f i • o 2 • k o 2 • k i Di 1 d o • ln (d o /d m ) d o • ln (d m /d i ) d + + r f o + r f i • o 2000 • k o 2000 • k i di
Example 8-4 — Find t he overa overa ll heat t ra nsfer coeffic coefficient ient for a 75 mm NP S Sch 80 pipe pipe submerged submerged in an 88 ° C wa ter ba th. 280 000 Sm 3/da y of 13 800 kP a (abs) nat ura l gas is to be heated from 16 °C to 29 °C. Refer to Examples 8-2 and 8-3; note that the heat flow flow t hrough the pipe is similar similar to Example 8-1. Solution Steps Use h i , h o, an d th e conductio conduction n t hrough the pipe wall to find U . Then check the heat fluxes flux es to see tha see tha t t he right right film temtempera pera tures w ere used. used. From Fig. 8-8, 8-8 , k = 45.3 W/(m •°C) for the pipe wa ll at 48° C. Assume clean clean pipe.
31 ° C
hi
From E q 8-7, 8-7, 88-8a, a nd 8-9a 8-9a a nd Fig. 88-5 5: Nu
=
When t here is only one solid solid la yer, delete delete t he fourth term in Equation 8-11 and change the subscript m to i and delete the subscript subscript on k in the third t erm.
Solution Steps
di
= 0.023 (R e ) (P r ) 0.8
C p • µ k
Eq 8-11
Example 8-3 — Find the heat transfer coefficient for 280 000 Sm 3/da y of 0.6 0.6 relat relat ive densit densit y na tur al ga s flowing flowing a t 13 800 800 kPa (a bs) in in a 75 mm NP S Sch 40 pipe pipe wh en th e pipe pipe wa ll and ga s temperatures a re 40 40 °C an d 22 ° C, respectively respectively..
=
0.33
=
=
Eq 8-9b
As before, bef ore, Re is dimensionless when the units indicated in Fig. 8-1 ar e used in equa tions 8-9a 8-9a a nd 8-9b. 8-9b. The The const const a nts C an d m depend on the configuration configuration a nd t he type of fluid fluid flow flow — lamina r, intermediat intermediat e, or or t urbulent — wh ich is cha racterized by th e magn itude of the Reynolds Reynolds number. Fi g. 8-5 8-5 lists lists values for the more common common situa tions.
40 + 22 2
= 0.60
U must be based on some specific area. Considering all the resistances to hea hea t tr ansfer t hrough a hollo hollow w cylinder cylinder w hose hose wa ll is made of two different different ma terials (i.e. (i.e. meta l pipe pipe an d a layer of insula insula tion), tion), the overall heat tra nsfer based on the insulation outside diameter is:
ρ
µ
=
6 3 = 0.28 10 S m /d a y
hi
Q
Eq ua tion 8-8b 8-8b need be used only for high viscosity viscosity f luids such a s glycol. glycol.
Tfilm
q gs
So far, only only th e individual or local local heat tra nsfer coeffic coefficients ients have been considered. As discussed in Section 9 “Shell and Tube Heat Exchangers” t he individual heat tra nsfer coeffi coeffi-cients cien ts are combined combined into an overa overa ll heat tra nsfer coeffi coefficcient. See Fig. 9-3 f 9-3 for or calculat ion of of ∆TLM .
or
=
kJ /(kg • ° C )
Overall Heat Transfer Coefficient
Uo
Re
= 3.06
0.33
353.7 • M d•µ
=
Cp
0.8
Where 1000 • D • V •
W/(m • ° C )
0.023 (1000 ) 0.052 18.05 (0.28) (106 ) 0.6 −3 73.7 (73.7) (19) (10 )
0.14
µb • µw
• s
=
hi
Eq 8-8a
0.33
) m P a
0.023 • (1000 ) • k 18.05 • q gs • R D di di • µ
U tilizat ion ion of a viscosity viscosity correction correction t erm gives th e Sieder-T Sieder-Ta te correlation: m
−3
0.8
cent to a solid is promoted by external force, e.g., pumping, agita tion, etc. etc. The The result is a substa ntia l increase increase in the hea t tra nsfer ra te. The The D ittus-B ittus-B oelter oelter correlation correlation is: m
19 (10
= 0.052
RD
Forced convection — occurs when the fluid flow adja-
mm
k
0.25
0.47 k (88.9 ) (990 ) (9.807)(0.00063 )(42.4)(4.187) 1000 • (0.63)(0.632 ) 88.9 1000 3
ho
= 73.7
di
3 0.47 • 1000 • k d o do
0.33
0.8
18.05 • q gs • R D C p µ = 0.023 k di • µ
h • d i 1000 • k
0.33
k
8-5
= 1971 1971 a nd 73.7 a nd = 73.7 = 45.3 W/(m
= 697.3 W/(m 2 • ° C ) d o = 88.9 mm • ° C ) ho
FIG. 8-6 Fin Efficiency Chart 4
From E q 8-11 8-11
FIG. 8-7 Fin Tip Temperature5
Uo
= =
1 ho
+
1 do d • ln (d o /d i ) + o d i • h i 2000 • k 1
1 697.3
Ao
= 448.4 π • d o =
To
=
+
+
88.9 ln ( 88.9 /73.7) 2000 • 45.3
W/(m 2 • ° C )
1000
88
88.9 73.7 • 1971
Ti1
= 0.2793 =
16
m2
Ti2
= 29
°C
From Fig. 9-3
∆TLM =
(To − Ti1 ) − (To − Ti2 ) (88 − 16 ) − (88 − 29 ) = To − Til 88 − 16 ln ln 88 − 29 To − Ti2
°C
65.28 ° C ∆TLM = 65.28 Q = U o Ao ∆TLM = 448.41(0.2793 )(65.28 ) 1 = 8176 W per linea r m To confirm confirm th e film film t emperat ures a nd th e validity of the individual heat transfer coefficients, the heat fluxes outside, through, a nd inside the pipe must be compared compared w ith t he overovera ll heat flu x. Consider Consider one linear meter of pipe. At th e avera ge gas temperature of 22.5°C, calculate the fluxes for one linear meter of pipe. Qo
8-6
=
h o • Ao • ∆To
FIG. 8-8 Thermal Conductivity of Ferrous Materials4
8-7
FIG. 8-9 Normal Total Emissivity of Various Surfaces 3 A. Meta ls and Their Their Oxides Surface
T, °C*
Emissivity*
2 25-575
0. 03 9-0. 0 57
Aluminum H ig ig h l y po pol i s h ed pl pl a t e , 98 98. 3 % p u r e P ol i s h e d p l a t e
23
0. 04 0
R ou gh pl a t e
26
0. 05 5
2 00-60 0
0 . 11-0. 1 9
38
0. 21 6
O xi d i z ed a t 59 9° C Al u m i n u m -s u r f a ced r oof i n g Ca lorized lorized surfaces, heated at 600°C: C o pp er S t ee l
6 2. 4 % Cu C u , 3 6. 8 % Zn Zn , 0. 4% P b, b, 0. 3% 3% Al Al 8 2. 9% C u , 1 7. 0 % Zn C h ro rom iu iu m; m; (s (s ee ee Ni N i ck ck el el Al Al lo loy s fo for N ii-C r st s t ee eel s) s)
Commercial, scraped scraped shiny but not
Emissivity*
S t ee l , ox i d i z e d a t 59 9° C
199 -5 99
0 . 79
I r on ox i d e
499 -1 199
0. 85 -0. 89
24
0. 80
24
0 . 82
C a s t p l a t e: e: S m o ot h
23
0 . 80
R ou g h
23
0. 82
38-24 9
0 . 95
S h e et s t e el , s t r on g r o u g h ox i d e l a y e r D en s e s h i n y ox i d e l a y e r
0 . 18-0. 1 9
C a s t i r on , r ou g h , s t r on g l y o xi d i z ed
2 00-600
0. 52-0. 5 7
Wr ou g h t i r on , d u l l ox i d i z ed
21 -3 60
0 . 94
S t ee l p l a t e, r ou g h
38 -3 71
0. 94 -0. 97
19 9-599
0. 41 -0. 46
20
0. 11
P la la t e, ox i d i z ed b y h e a t i n g a t 59 9° C
199 -5 99
0. 37 -0. 48
N i ck e l ox i d e
649 -1 254
0. 59 -0. 86
52 -1 034
0 6 4-0 76
24 7-357
0. 02 8-0. 03 1
257 -3 77
0 . 03 3-0. 0 37
277
0. 0 30
3 8-538
0. 08 08-0. 26 26
Copper Commercial, emeried, emeried, polished, polished, but pits r em a in in g
T, °C*
2 00-60 0
Brass Highly polished: polished: 7 3. 2% C u , 26 . 7% Zn
Surface
189
0. 03 0
High t emperature alloy steels (see Nickel Alloys) M on el m e t a l , ox i d i z e d a t 59 9° C Nickel E l e ct r op l a t ed on p i ck l ed i r o n , n ot p ol i s h e d
Nickel alloys C h r om n i ck el
FIG. 8-9 Normal Total Emissivity of Various Surfaces 3 A. Meta ls and Their Their Oxides Surface
T, °C*
Emissivity*
2 25-575
0. 03 9-0. 0 57
Surface
Aluminum H ig ig h l y po pol i s h ed pl pl a t e , 98 98. 3 % p u r e P ol i s h e d p l a t e
23
0. 04 0
R ou gh pl a t e
26
0. 05 5
2 00-60 0
0 . 11-0. 1 9
38
0. 21 6
O xi d i z ed a t 59 9° C Al u m i n u m -s u r f a ced r oof i n g Ca lorized lorized surfaces, heated at 600°C: C o pp er S t ee l
6 2. 4 % Cu C u , 3 6. 8 % Zn Zn , 0. 4% P b, b, 0. 3% 3% Al Al
199 -5 99
0 . 79
499 -1 199
0. 85 -0. 89
24
0. 80
24
0 . 82
C a s t p l a t e: e: S m o ot h
23
0 . 80
R ou g h
23
0. 82
38-24 9
0 . 95
D en s e s h i n y ox i d e l a y e r
0 . 18-0. 1 9
C a s t i r on , r ou g h , s t r on g l y o xi d i z ed
2 00-600
0. 52-0. 5 7
Wr ou g h t i r on , d u l l ox i d i z ed
21 -3 60
0 . 94
S t ee l p l a t e, r ou g h
38 -3 71
0. 94 -0. 97
19 9-599
0. 41 -0. 46
20
0. 11
P la la t e, ox i d i z ed b y h e a t i n g a t 59 9° C
199 -5 99
0. 37 -0. 48
N i ck e l ox i d e
649 -1 254
0. 59 -0. 86
52 -1 034
0. 6 4-0. 76
21
0. 2 62
216 -4 90
0. 44 -0. 36
216 -5 27
0. 62 -0. 73
NCTNCT-3 a lloy (20%Ni; 25%Cr), br own, s pl ot ch ed , ox i d i z e d f r om s e r v i ce
216 -5 27
0. 90 -0. 97
NCT-6 alloy (60%Ni; 12%Cr), smooth, black, firm adhesive oxide coa t f r o m s er v i ce
271 -5 63
0. 89 -0. 82
24
0 . 043 & 0. 06 4
2 27-32 7
0. 0 45-0 . 053
24 7-357
0. 02 8-0. 03 1
257 -3 77
0 . 03 3-0. 0 37
277
0. 0 30
3 8-538
0. 08 08-0. 26 26
High t emperature alloy steels (see Nickel Alloys) M on el m e t a l , ox i d i z e d a t 59 9° C Nickel E l e ct r op l a t ed on p i ck l ed i r o n , n ot p ol i s h e d
Copper Commercial, emeried, emeried, polished, polished, but pits r em a in in g
189
0. 03 0
Commercial, scraped scraped shiny but not m i r r or -l i k e
22
0. 07 2
P ol i s h e d
117
0. 02 3
P late, heat ed long long tim e, cove covered red with t h i ck ox i d e l a y er C u p r ou s o xi d e
S t ee l , ox i d i z e d a t 59 9° C
2 00-60 0
8 2. 9% C u , 1 7. 0 % Zn C h ro rom iu iu m; m; (s (s ee ee Ni N i ck ck el el Al Al lo loy s fo for N ii-C r st s t ee eel s) s)
25
0. 78
79 9-109 9
0 . 66-0. 5 4
Nickel alloys C h r om n i ck el Nickeli n (18-32 (18-32 Ni; 55-68 Cu; 20 Zn), g r a y o xi d i z ed KA-2S KA-2S a lloy steel – (8%Ni; 18%Cr), light silvery, silvery, rough, brown, a f t er h ea t in g
Iron and st eel
Af t e r 42 h r. h ea t i n g a t 98 0° F
Metallic surfaces (or very thin oxide layer): C a s t i r on , p ol i s h e d
200
0. 2 1
G r ou n d s h e et s t e el
93 8-109 9
0 . 55-0. 61
S m o ot h s h ee t i r on
8 99-103 8
0 . 55-0 . 60
Oxidized surfaces: I r on p l a t e, p i ck l ed , t h en r u s t e d r e d C o m pl e t el y r u s t e d R o l l ed s h e et s t e el O xi d i z ed i r on C a s t i r on , ox i d i z ed a t 599° C
Qo
π • 88.9 = (697.3) • • (88 − 45.6) 1000 = 8257.3 8257.3 W per linear m
Qp
=
Qo
Qp
=
8496.2 8496.2 W per linear m
Qi
=
h i • Ai • ∆Ti
Qi
0 . 61 2 0. 68 5
21
0. 65 7
100 1 99-599
Zinc C om m e r ci a l,l, 99. 1 % pu r e, p ol i s h e d
0. 73 6 0. 6 4-0. 7 8
O x i d i z ed b y h e a t i n g a t 7 50° F
39 9
0 . 11
G a l v a n i z e d s h e et i r on , f a i r l y b r i g h t
28
0. 2 28
G a l v a n i z e d s h e e t i r o n , g r a y o x i d i z ed
24
0. 2 76
The a bove refers to clean pipes. pipes. Fouling occurs occurs wit h continued use. Sometimes, fouling f a ctors a re specified; specified; e.g., 0.000 0.0002 2 interna lly and externally. externally. These These are correc correction tion factors tha t a re a dd ed t o 1/U o. So,
(2) • (3.14) • (1) • (45.3) • (45.6 − 40 ) 88.9 ln 73.7
=
Qi
20 19
Ti n – b r i g h t t i n n ed i r o n s h ee t
2 • π • L • k • ∆T d o ln d i
Qp
Emissivity*
I r on ox i d e S h e et s t e el , s t r on g r o u g h ox i d e l a y e r
Brass Highly polished: polished: 7 3. 2% C u , 26 . 7% Zn
T, °C*
1
U of
=
U of
= 374.34
1 + 0.0002 + 0.0002 • 88.9 Uo 73.7 W/(m 2 • ° C )
Finned Tubes When combustion combustion gases flow externally a nd hea t a liquid liquid in a pipe, pipe, there is a signific significant ant disparity between h i [usually over 850 85 0 W/(m 2 • K)] and h o [usua lly less th a n 60 W/(m2 •K)]. To overcome this and make better use of a given length of pipe, the externa l surfa ce is finned. When When th e gas flow is norma l to the pipe axis, helical fins — ty pically pically 1.25 1.25 to 3 mm t hick, 12.5 12.5 t o 40 40 mm h igh, a nd 80 to 240 fins/m — ar e used. The The resu lt is an increase increase of up to tenfold tenfold in the externa l ar ea of the pipe. pipe.
π • 73.7 = (1971) • • (40 − 22 ) 1000 = 8214.4 8214.4 W per linear m
The tota l external surfa ce ar ea of a finned pipe pipe and the cross cross sectiona sectiona l or or projected projected area restr icting norma l gas flow per linlinear foot foot a re:
The a greement is close close enough for purpose of this exa mple.
8-8
FIG. 8-9 (cont’d) Normal Total Emissivity of Various Surfaces 3 B. Refractories, Refractories, Bu ilding ilding Materials, Pa ints, and Miscell Miscellaneous aneous Surface
T, °C*
Emissivity*
Boa rd
23
0.96 0.96
P a per
38-371 38-371
0.93-0.945 0.93-0.945
Surface
Asbestos
T, °C*
Paints, lacquers, varnishes
Brick Red, rough, but no gross irregula rities Silica, ungla zed, rough
21
0.93
1000
0.80
Silica, gla zed, rough
1100
0.85
G rog brick, glaz ed
1100
0.75
See Refractory Ma terials below.
Snowh ite enamel varnish on rough iron plat e
23
0.906
Bla ck shiny lacquer, sprayed on iron
24
0.875 0.875
Bla ck shiny shellac on tinned iron sheet
21
0.821 0.821
B lack ma tt e shella c
77-146 77-146
0.91
B la ck la cquer
38-93 38-93
0.80-0.95 0.80-0.95
Fla t black lacquer
38-93 38-93
0.96-0.98 0.96-0.98
White la cqu er
38-93 38-93
0.80-0.95 0.80-0.95
100
0.92-0.96 0.92-0.96
10%Al, 22%la 22%la cquer body, on rough or smooth surfa ce
100
0.52
100
0.3
100
0.27-0.67 0.27-0.67
21
0.39
Oil pain ts, sixteen different, all colors colors
Carbon T-car -car bon (Gebr. Siemens ) 0.9%a 0.9%a sh (this started w ith emissivity at 125° 125° C of 0.72, 0.72, but on heating cha nged to values given)
127-627
Aluminum paints and lacquers
0.81-0.79
Ena mel, whit e fused, on iron
19
0.897 0.897
26%Al, 27%la 27%la cquer body, on rough or smooth surfa ce
G lass , smooth
22
0.937
Oth er Al paint s, var ying age an d Al content
G ypsum, 0.02 in. thick on smooth or blackened plate
21
0.903
Ma rble, light gra y, polished
22
0.931
Oak, plan ed
21
0.895
Oil layers on polished nickel (lube oil)
20
Al lacqu er, var nish bind er, on rough plat e
P olished surfa ce, alone
Al pain t, aft er heat ing to 327°C Pla ster, rough lime Por celain, glazed Quar tz, rough, fused Refractory mat erials, 40 different different poor poor ra diators
0.045
+ 0.0010.001-in. oil
0.27
+ 0.0020.002-in. oil
0.46
+ 0.0050.005-in. oil
0.72
In finitely thick oil layer
0.82
Oil layers on aluminum foil (linseed oil) Al foil
100 100
0.561
+ 2 coat coat s oil
100
0.574
=
π • d o n • t n • π d f2 − d o2 • 1 − + 6 1000 1000 2 10
Acs
=
do 1000
+
n • t • (d f − d o) 10
6
0.35
10-88 10-88
0.91
22
0.924
21
0.932 0.932
599-999 0.65-0.75 0.70 0.80-0.85 0.85-0.90
Roofing paper
21
0.91
Ha rd, glossy plat e
23
0.945
Soft, gra y, rough (reclaimed)
24
0.859
0-100
0.95-0.963
Rubber
Wa ter
* When When t wo temperatu res and tw o emissivities emissivities are given, th ey correspo correspond, nd, first to first a nd second second t o second, second, and linear int erpolat erpolat ion ion is permissible.
Ao
149-316 149-316
good good rad iators
0.087†
+ 1 coat coat oil
a nd
Emissivity*
† Although Although th is value is probably probably high, it is given given for comparison comparison with th e data , by the sa me investigator, to show t he effect effect of oil oil layers. See Aluminum, Aluminum, pa rt A of this table.
Solution Steps
Eq 8-12
Abscissa in Fig. 8-6: 8-6 : Hf
Eq 8-13
t
The surface area of the fins is not not a s efficient efficient a s the external surface of of t he pipe bec because ause h eat absorbed absorbed a t the fin surfa ce must be conducted to the fin base before it can pa ss thr ough ough th e pipe wall. Fin efficiencies efficiencies ar e given in Fig. 8-6. 8-6 . These eff icienc ciencies ies are a pplied pplied to the tota l externa externa l ar ea.
X
= 31.75
= 2.67
= 0.045
mm
√ ( ) √ ( )
Hf
= 0.045
Another important consideration is fin tip temperature. This can be obtained from the fin efficiency and Fig. 8-7. 8-7 . Fig. 88-8 8 gives gives t he therma l conductivitie conductivitiess a nd ma ximum recrecommended fin tip tempera tur es for for t he more common common ferrous ferrous construction construction ma terials.
mm
ho k f • t
22.2 43.3 2.67
31.75
= 0.626
Example 8-5 — Calculate the external surface and fin efficiency ciency for 100 100 mm NP S S ch 80 pipe with t he following following finning: 118 118 fins/m, fins/m, 31.75 mm h igh, 2.67 2.67 mm th ick. Assume:
do
= 114
df
= d o + 2 H f = = 177.5 mm
df do
h o = 22.2 22. 2 W/(m 2 • ° C) k f = 43.3 W/(m • ° C)
=
mm
177.5 114
114 + 2 (31.75
= 1.557
From Fig. 8-6, 8-6 , fin efficien cy is 87%. 87%.
8-9
)
FIG. 8-10
FIG. 8-12
Partial Pressure of CO 2 Plus H2O
Gas Emissivity 10
FIG. 8-11 Beam Lengths for Gas Radiation 8 Dimension Ratio
Mean Beam Length, L
Rectangular Furnaces, Length-Width-Height, In Any Order
}
1-1-1 1-1-1 t o 1-1-3 1-2-1 1-2-1 t o 1-2-4
(2/ (2/3) (Fur na ce Volum e)
1-1-4 1-1-4 t o 1-1-∞
(1) (Sm (Sm a llest D imension)
1-2-5 1-2-5 t o 1-2-∞
(1.3) (Smallest Dimension)
1-3-3 to 1-∞-∞
(1.8) (Smallest Dimension)
⁄ 3
1
using Eq 8-14, 8-14, it it is recommended recommended 6 tha t F not be less less tha n 0.67. 0.67. Also, lso, a ll temperatures a re a bsolute. bsolute. Fig. 8-9 gives the emissivities of common common construction met als, oxides, refractories, and insulation materials. The emissivity of combust combust ion gas es is more complex complex beca beca use it d epends on t he temperature and the product (P • L). See Figs. 8-10 8-10,, 8-11,, a n d 8-12 8-11 8-12..
Cylindrical Cylindrical Furna ces, ces, Diameter-Height 1-1 1-1
(2/3) (Dia me t er )
1-2 1-2 t o 1-∞
(1) (Diameter)
Example 8-6 — Wha Wha t is th e radia nt h eat flux to a 0.9 m length of a 0.6 m ID firetube when the combustion gases inside the tube are at 1540°C 540°C a nd the firetube wall is at 150°C? 150°C? Assume 20%exc 20%excess ess a ir is used.
Fr om Eq 8-12: 8-12:
Solution Steps
= 118 per m e t e r π • d o n • t n • π (d f2 − d o2) Ao = 1 − 1000 + 2 • 106 1000 11 42) π • 114 118 • 2.67 118 • π (177.52 − 114 • 1 − + • = 2 1000 1000 106 = 3.676 m 2 pe r linear linear m
n
cu r ve v e d s u rf r f a ce ce a r e a total surface area
D
=
0.6
F
=
F
σ • F (T1 − T2 ) 1 + 1 − 1 ε1 ε2 4
L
π • D • L
=
0.9
•L
+2•
π•D
2
=
4
π (0.6 ) (0.9 ) π • 0.6 2 π (0.6) (0.9 ) + 2 4
= 0.75
From Fig. 8-10, 8-10, P = 0.24 .24 atm
An estimate of the radia nt hea t flux between two surfaces is :
=
=
π • D
Radiation
Q A
F
From Fig. 8-11, 8-11, L = D, L = 0.6 0.6 so P • L = 0.14 0.144 4 atm • m
4
or P • L = 14.6 14.6 kPa • m
Eq 8-14
From Fig. 8-12, 8-12, From Fig. 8-9, 8-9 ,
The geometric or or view fa ctor, ctor, F, is the fra ction of the surfa ce ar ea tha t is expose exposed d to and a bsorbs bsorbs rad iant heat. The The equa equa tion for for F must be determined from an a nalysis of the geometry. geometry. In
ε1 = ε2 =
0.12 0.12 0.79 0.79 (steel, oxidized oxidized at 600°C 600°C )
Eq ua tion 8-14 8-14,,
σ = 5.67 (10−8) 8-10
W/(m 2 • K 4)
FIG. 8-13
T1
Combination Convection and Radiation Film Coefficients for Air in Contact with Vertical Walls or Surfaces 11
T2
= 1540 + 273 K = 150 + 273 K σ • F • [ (T1)4 − (T2)4] = 1 + 1 − 1 ε1 ε2 − 8 (5.67)(10 ) 0.75 [(1813)4 − (423)4] = 1 + 1 − 1 0.12 0.79 = 53 271 W per squa re m
Q A
Note tha t T is in K.
Heat Losses Hea t losses from from equipment equip ment surfaces surf aces occur occur primarily by r a diation and convection. Fig. 8-13 8-13 gives the combined heat transfer coefficient, h c+ h r , in terms of the wind velocity and the temperat ure difference difference between the surfa ce an d the surrounding a ir. ir. Example 8-7 — How much heat can be saved per linear linear meter by covering a 200 mm NPS Sch 40 steam header, carrying 100 100 kPa (ga) stea m at 120°C 120°C , with a 25 mm thick layer of block block insula tion? Assume a mbient conditions conditions a re -1°C wit h a 24 km/hr win d. Solution Steps Using Fig. 8-13 the heat loss from the bare pipe is: h cr L
FIG. 8-14
h cr
Effect of Fuel/Air Ratio on Flue Gas Analysis for 41 283 kJ/Sm 3 Natural Gas (0.63 Gas Relative Density) Containing 83% CH 4 and 16% C2H61
Do Tp Ta Q
= h c + h r Combined convection convection a nd ra dia tion coefficie coefficient nt = 1m = 33.2 = 0.219 m = 120 ° C = −1 ° C = h cr • Ao (Tp − Ta ) = 33.2 • π • 0.219 (120 − (−1)) = 2764 W/per linear m
For the insulated pipe, assum e the outside outs ide surface of the insula tion is at 10 ° C. Then Then from Fig. 8-13: 8-13 : h cr Q
= 25.5 = 25.5 • π (0.219 + 2 • 0.025)[10 − (−1)] = 237 W/per linear m
This agrees closely enough with the heat flow through the insula tion — Exa mple 8-1. 8-1. Heat saved = 276 2764 − 237
=
2527 W/m
COMBUSTION Combustion is the rapid chemical reaction between oxygen an d a combustible combustible ma terial tha t releases releases heat a nd light. light. Usually the combustible material is a hydrocarbon and ambient air supplies the oxygen. Complete combustion occurs when ther e is sufficient sufficient oxygen to convert convert a ll of of the ca rbon to carbon dioxide dioxide a nd a ll of of the hy drogen to wa ter. Incomplete Incomplete combuscombustion means th at there is eith eith er unburned or or part ially reacted fuel, i.e., ca ca rbon monoxide, hydr ogen, ogen, etc.
NOTE: NOTE: Values for rich mixtures depend depend somewha t on combustion cham cham ber design. The average values shown are within 1 ⁄ 2%of corr corr ect for H 2, C O , a n d C O 2, but ma y be as much as 2%low for other other constituents constituents . Some external external heat is u sually required for mixtures with less tha n 70%aerat ion (dotted (dotted lines). lines). Da shed lines lines show the trends with poor poor mixing or quenching. quenching. G as constituent percentages are on a dry basis to allow comparison with gas analyzer readings, that measure the gas volumes after water vapor has condensed out of th e sample. With t he correct correct am ount of air (10.56 (10.56 m 3), each cubic meter of th is fuel gas pr oduces 2.14 2.14 m 3 H 2O, 1.15 m 3 CO 2, 8.39 m 3 N 2; so %CO 2 = 100 100 × 1.15 m 3 C O 2 ÷ (1.15 + 8.39) m3 dry flu e gas = 12.1%.
Methane is the main constituent of natural gas. It reacts with oxygen oxygen to form carbon dioxide dioxide and w at er. er.
8-11
C H 4 + 2 O 2
FIG. 8-15a
C O 2 + 2 H 2O
This stoichiometry stoichiometry is typical of all hyd roca roca rbons. One a tom of carbon requires one molecule of oxygen and four atoms of hydr ogen ogen r equire one molecule molecule of oxygen. The The t heoretical a ir is that needed for complete combustion of the carbon and hydrogen , i.e., tw o molecules molecules of oxygen for one molecule molecule of meth an e. Excess Excess air is tha t supplied supplied in a ddition ddition to wha t is required. For example, 20%ex 20%exce cess ss air mean s th at the a ir supplied is is 1.2 times the stoichiometric amount.
Standard Cubic Meters of Dry Air Needed per Standard Cubic Meter of Hydrocarbon for Complete Combustion (CH4, C 2H6, C3H8) 13 s a G l e u F f o r e t e M c i b u C r e p r i A y r D f o s r e t e M c i b u C
→
The following following rea ction ction r epresents t he complete combust combust ion ion of an arbitrary carbon-based fuel compound.
(0.7, 0.2, 0.1)
12
hy o + c + 4 − 2 + s O 2 → h y n i (c) C O 2 + 2 H 2O + (s ) S O 2 + 2 N 2 C cH hy O oS s N ni
(0.7, 0.3, 0)
11
Fig. 8-14 s hows how the composition of the flue gases depends on the a mount of combust combust ion air.
(0.8, 0.2, 0)
Air Requirements Requirements
(0.9, 0.1, 0)
10
Fig . 8-15a 8-15a shows shows the m 3 of of dry a ir needed per per scf of of para ffinic hydr oca oca rbons for complete complete combust combust ion ion in term s of the specific specific gravit y of the fuel. In using t his figure any inert compo components nents in the fuel, e.g., nitrogen, carbon dioxide, etc., must be excluded. Fig. 8-15b shows the ma ss of of humid air r equired equired per ma ss of dry air a t 760 mm H g a nd percent percent relative humidity. humidity.
(1.0, 0, 0)
9 0.5
0.6
0.7
0.8
Relative Density of Fuel Gas (Air = 1.0)
Air is a bout bout 20.9%oxygen 20.9%oxygen on a dry basis, hence 4.77 4.77 mols (or m 3) of air supply 1.0 mol (or m 3) of of oxygen. Applying th is to met ha ne, 9.54 mols (or (or m 3) of air a re needed for every mol 3 (or (or m ) of metha ne.
FIG. 8-15b Mass of Humid Air Per Mass of Dry Air At 760 mm Hg and Percent Relative Humidity
8-12
FIG. 8-16
The effect effect of wa ter va por por in th e air is relat ively small a t low and moderate moderate temperat temperat ures. ures. Sa turat ed air at 15°C conta conta ins 1.75% 1.75%wa wa ter. St ill this s hould be considered considered a nd 2-3% 2-3%more more a ir is usually a dded if exact exact calculat calculat ions ions a re not ma de. The The wa ter content in saturated air increases rapidly with temperature; e.g., e.g., at 38° 38° C sat ura ted air conta conta ins about 6.5%wat 6.5%wat er, er, and a t 46° 46° C it cont cont a ins a bout 10%.
Effect of Ambient Temperature and Barometer Pressure on Air Actually Delivered
Some situations may result in a higher amount of water vapor coming from the combustion air and fuel gas. Consider the complete complete combustion combustion of of 1 mol of wa ter sa tur at ed metha ne at 38° 38° C, 103.4 103.4 kPa (ga) with 20%exc 20%excess ess air wit h a ir a lso wa ter sa tur a ted at 38° 38° C. This This intr oduces oduces 0.79 0.79 mol of of wa ter from the a ir an d 0.032 0.032 mol of of wa ter from the ga s. Addit Addit iona iona lly, lly, 2 mols mols of wa ter from the methan e co combustion is added wat er resultresulting in approximately 21%water in the flue gas of which 30% is from from th e air a nd ga s humidity. humidity. Also Also,, steam or w at er a ddition ddition for NO x control introduces more water vapor to the flue gas. These situat ions ions increase th e wet bulb temperature of th e flue ga s. Wa Wa ter condensa condensa tion should be considered considered in mass a nd energy bala nces nces a nd excess excess a ir calculat calculat ions. ions. E rrors in considconsiderat ion ion of wat er vapor co content a nd air t emperat emperat ure may cancel a 10%exc 10%excess ess a ir calculat ion, ion, resu lting in incomplete incomplete fuel comcombustion. Designs a nd opera opera tions should consider consider local local wea ther conditio conditions ns and seasonal chan ges. The theoretical air requirement of an arbitrary carbonbased fuel compound, in mols of air per mol of fuel, can be calculated calculated w ith t he follo following wing equa tion. AO
= 4.77 • c + h y − o + s 4 2
Eq 8-15
Analysis of the flue gases provides useful useful informa tion about the a ctua l excess excess air a nd t he efficienc efficiencyy of fired fired equipment . The The following equations provide the excess air percentage for a sulfur a nd oxygen free, ca ca rbon-bas rbon-bas ed fuel combustion combustion wit hout soot soot formation. Analysis Analysis must be molar molar a nd on a dry ba sis.
FIG. 8-17 Gross Thermal Efficiency for a Gas with HHV = 37.3 kJ/Sm 3
EA
=
=
F l o C O 2o • 100 AO C O 2 + C O
F lo O 2o • 100 AO 20.9 − O 2 + C O
Eq 8-16
Many forced-draft burners supply a fixed volume of air. Fig. 8-16 shows shows t he effec effectt of ambient temperatur e and ba rometric pressure pressure on the a mount of air actua lly delive delivered. red.
Heating Value The heating va lue of of a fuel is is the a mount of heat r eleased eleased during complete combustion with the stoichiometric amount of dry a ir. ir. This This involve involvess a heat balan ce (the (the usua l dat um or reference temperature is 15°C). Hydrogen in the fuel fuel burns to wat er and w hen the flue gases gases a re cool cooled ed to 15° 15° C, t he physical st a te — either either v apor or liquid — of of this w a ter must be a ssumed. So the lat ent heat of va va poriporizat ion ion of the wa ter ma y or may not be co considered to be part of the h eating value. The The result is tw o definitio definitions ns for th e heat ing va lue. The The higher or gross hea ting valu e, HH V, includes the heat of co condensat ion ion a nd th e lower lower or net hea ting va lue, lue, LHV, LHV, assumes the wa ter remains in the vapor vapor sta te. Fig. 23-2 i n t he “P hysical Pr operties” operties” sectio section n gives the net an d gross heat ing va lues lues of most pure hydroca hydroca rbons. For For mixtures calculate the molar, or volume, average.
8-13
FIG. 8-18 Typical Enthalpy of Combustion Gases for a Dry Natural Gas Fuel and 20% Excess Dry Air
8-14
Thermal Efficiency
Also
The two ways to express the heat released during combustion result in tw o definitions definitions for the t herma l efficiency efficiency,, gross and net.
Hav
GTE
NTE
= =
U HT GHI
RD R D = 34.16 • H s • P B • g − a + Ta Tg 2 1.7955 • Tg M • 4 • f • H s + 1 • + f D s P B • R D g • D 4s 10 000 Since ρg is less than ρa , the first term in Eq 8-18,
Eq 8-17a
UHT NHI
which is the tota l a vailable driving force force,, is alw ays negative. This This must be decrea decrea sed by add ing the losses from friction friction a nd resista nce in th e seco second t erm.
Eq 8-17b
Therefore NTE NTE exceeds exceeds G TE. There is a tend ency to use t he gross heating value with the net thermal efficiency even though th e bases a re inconsis inconsistent tent beca beca use the numerical numerical va lues are higher tha n the correspo corresponding nding net heat ing value and gross t herma l efficiency efficiency..
Burners Four types of burners are commonly used in direct fired heaters:
•
Inspirating pre-mix burners . The The pa ssa ge of fuel gas through a venturi pulls in the combustion air. These burners ha ve short short dense flames tha t a re not not a ffec ffected ted by wind gusts. • Raw gas burners. Some of the a ir requir ed for combuscombustion is pulled pulled in by a venturi. The The rest of the a ir is a dmitted th rough a seco seconda ry a ir register. register. These These burners have larger tu rndown ra tios, require require lower lower ga s pressures, a nd are a lso quieter. quieter. • Low NOx burners. The The addition of a t ertiary air r egister egister reduces the amount of nitrogen oxides in the flue gas. This type also can be operated with less excess air than the a bove bove types. • Combination gas and oil burner . An oil burner is added to the gas spider so that fuel oil can also be used. One-tent One-tent h kilogram of steam per kilogra kilogra m of fuel fuel is usually required to atomize the oil. B urner a ir registers are sometimes used to control control the excess cess a ir. Ten t o fifteen percent excess excess a ir is a compromise compromise for best therm al efficiency efficiency and lowest NO x levels. levels. Air Air lea ka ge into the heater around sight door openings, header box gaskets, terminal penetrations, etc. should be minimized.
Fig. 8-17 shows how t he gross therma l efficie efficiency ncy can be determined from the exce excess ss air a nd sta ck gas t empera empera ture. Especially for insulated heaters or furnaces, the combustion efficiency is close to the gross thermal efficiency. The difference ence is the heat lost lost t hrough the wa lls to the surroundings. The typical enthalpy of the combustion (or stack) gases w hen w hen natural natur al ga s is burned wit h 20%ex 20%exce cess ss dry a ir is shown in Fig . 8-18 8-18..
Draft Combustion Combustion air is obta obta ined by na tura l, force forced, d, a nd induced draft. Natural draft uses the buoyant effect of the hot flue gases in t he sta ck to draw air into the combustion combustion zone. zone. Forced Forced dra ft is the result of an a ir blower blower or fan. In duced duced draft refers refers to a blower blower in t he sta ck. The draft a vailable (P (P a) from from a sta ck is is the na tura l (or (or static) draft less the frictional and exhaust gas velocity losses. As defined defined below below the dra ft is a lway s negative. The The ava ilable dra ft should be sufficient sufficient t o overco overcome me the hea d losses in in t he air inlet registers, convection section tube coils, baffles, damper, and an y w ast e heat recov recovery ery devices. devices. Also Also,, it should conta conta in a mar gin of safety to allow the damper to be in an intermediate position position to deal w ith sit e conditions conditions such as a tm ospheric pressure changes, humidity changes, and temperature changes (daily a nd sea sona sona l var iations). iations). Also, Also, the draft should should be sufficie ficient nt to obta obta in a negat ive pressure pressure along the entire heater fire side flo flow path .
Unmuffled burners have 100 to 110 dBA noise levels. Requirement s for 85 or 90 dBA noise noise levels, levels, mea sured one met er from the heater, require noise noise a ttenua tion plenums plenums a nd orifice orifice mufflers. Most Most burners ha ve a continuous continuous pilot pilot flame tha t r eleases eleases up to 15kW. The The pilot pilot is lit by ha nd or w ith a spa rk plug. The The pilot pilot should be left left on wh en:
In r eferences eferences 35 a nd 36 informa tion can be found found a bout pressure losses in various devices such as coils, duct transitions, etc. Cha nges in velocity velocity h ead sh ould be considered considered in hea d loss calculations because of the low densities of the air and flue ga s. The The absolute roughn ess of of the inner st a ck depends depends on the ma teria l, constru constru ction, ction, a nd lining, if an y. The The absolute roughness ma y ra nge from 0.3 to 9 mm. The The equipment ma nufa cturer should be consulted for more precise values of stack roughness and for other factors which may influence total head loss. loss.
=
H s • g
(ρg − ρa ) +
V 2• ρg Hs + 1 4 • f f • 2 Ds
• • • • • •
The fur na ce will not overheat during no-flow no-flow conditions. conditions. The fuel is etha etha ne or or hea vier gas es. The furna ce is used int int ermitt ently, e.g., regenera regenera tion ga s heaters. The pilot heat release does not affect furnace turndown ratio. The refractory must rema in dry for fast sta rt-up. rt-up. Nuisance shutdowns are una voidable. voidable.
Gas Burner Performance Several factors influence gas burner performance, such as the gas pressure, temperature, and composition that affect heating value, gas density and combustion characteristics; also, the aera tion, part part ial heat load, load, loca loca l altitude, etc. etc. Some typical problems tha t ca n occur occur as a consequ consequ ence of inco incorrect rrect burn er selection, selection, or from cha cha nging opera ting conditions, or using non-intercha non-intercha ngea ble gas include flash ba ck, yellow yellow t ipping, ipping, flame lifting, lifting, sooting, sooting, an d inadequa te heat input.
Refer Refer t o Fig. 17-2 t 17-2 t o determine the Fanning friction factor (f f) using the Reynolds number determined from Eq 8-9a and the r elat ive roughness (dimensionless) (dimensionless),, which is the qu otient of the absolute roughness roughness an d th e diameter, diameter, both both in t he same units. Hav
Eq 8-19
Eq 8-18
8-15
The change in gross heat input thr ough a gas burner orifice rifice caused by cha nges in the opera opera ting condit condit ions ions a nd ga s compocomposition sition can be estima ted wit h the following following equa tion, where the pressure and temperat ure a re a bsolute. bsolute. The The equat ion ion can also be used for the lower heating value. The term in parenthesis is called the Wobbe Index, which is a gas interchangeability parameter.
G HI2 G HI1
Y2 •
= Y1 •
√ √
P g 2 • ∆P 2 H H V2 • Tg2 R D 2 √
DIRECT FIRED HEATERS Direct fired hea ters va ry in size from 0.15 MW sma ll package regeneration gas heaters to 300 MW steam hydrocarbon reformer heaters. In the gas processing industry, the usual ra ng e is 0.3 to 6 MW. MW.
Types There are t wo bas ic configura configura tions: cylindr cylindr ica ica l a nd cabin, see Fig. 8-19. 8-19 . The The simplest design is vert ical-cylindrical ical-cylindrical wit h only only ra diant tubes. The The NTE NTE is about 60%an 60%an d the sta ck gas temperature is 650°C or more. The burner in the floor fires upward. A stainless steel baffle slows the exit flow of the hot gases and reradiates heat back to the top part of the tubes. There is a short short sta ck th at usually ha s no da mper. mper. The The design design is low cost cost a nd su ited for low cost cost fu el.
Eq 8-20
P g 1 • ∆P 1 H H V1 • Tg1 R D 1 √
In E q 8-20 8-20 the temperatu re an d pressure are absolute, absolute, an d th e expa expa nsion fa ctor (Y) (Y) is a fun ction ction of the burner n ozzle char a cteristics and t he fuel inlet inlet a nd outlet pressures. For low low inlet gas pressure the expan expan sion sion fa ctor ctor is a pproximately pproximately 1, and for small chan ges in in t he fuel gas pressure the expansion expansion fa ctors Y 1 a n d Y 2 have similar values and may be ignored. In other circumstan circumstan ces ces consult consult the burn er manu facturer for dat a.
Adding a convect convect ion section improves t he NTE NTE t o about 80%. The ra diant section section ma y be either cylindrical cylindrical or cabin, cabin, a nd t he coil coil configura configura tion eith er helica helica l or serpentin e. These These heat ers cost cost more tha n t he all-ra all-ra diant type but th ey use less less fuel fuel for an y given given duty. B y cool cooling ing th e combustion combustion ga ses to about 150°C 150°C , the NTE NTE can be increased to over 90% 90%. This This r equires eit her a combuscombustion air preheater using exhaust gas or a n a dditional dditional convec convec-tion section. These units have the highest capital cost and lowest fuel requirement for any given duty.
Flue Gas Condensation Flue gas water condensation may produce corrosion problems caused by acid gases such as SO 2 when present in the flue beca beca use of the sulfur content of some some fuels, an d th erefore erefore temperature control above the SO 2 dewpoint of flue gases should be considered. However, highly efficient fired equipment or w ast e heat recove recovery ry equipment burning sw eet eet fuels an d/or with acid resista resista nt d uct mat erials may utilize flue flue gas wa ter condens condensation ation ta king advanta ge of of its latent heat.
NOx Control
Design modifications are used when the tube material is expensive. A bridge wall is installed down the center of the cabin. The The ra diant tubes ar e placed placed above the bridge wa ll so tha t they a re, in effec effect, double fired. fired.
Cylindrical or Cabin? Vertical or Horizontal Horizontal Tubes?
The ma in fa ctors influencing NO x forma forma tion are flame temperature, excess air in the flame, time in which combustion gases ar e at fla me tempera tempera ture, an d fuels fuels conta conta ining nitrogen nitrogen compounds.
Cylindrical Cylindrical heat ers have the follo following wing a dvant ages:
• • •
In na tura l draft heat ers the most most common common means of cha nging NO x emissions is through t he use of low low NO x burn ers. The The premix a nd ra w ga s burners produce produce NOx levels levels greater th an 0.056 g NO x /MJ of burner heat release. These These ar e genera lly used wh en no NOx r equirement is specified. specified. The The upper end of the low NO x burner design uses a partially staged raw gas bur ner t o achieve levels of 0.034 0.034 to 0.052 0.052 g NO x /MJ . The The mid ra nge low low NO x burner design uses st a ged gas t o achieve levels levels of 0.022 to 0.034 g NO x /MJ . F or low er NO x requirements staged gas tips and internal flue gas recirculation are combined t o produce produce NO x emissions a t 0.013 0.013 g NO x /MJ an d below. These numbers are based on clean fuel gas (not oil burning) an d a re dependent dependent on firebox firebox temperatu re an d excess excess air.
• • • • • •
A sta ged-air ged-air burner is a low low N O x burner in which a portion of the combustion air is injected downstream of the burner block block to mix wit h th e combustion combustion products products a nd unbur ned fuel from the primary combustion zone.
They require the sma llest llest plot ar ea for a given given dut y. The cost is usua lly 10%to 10%to 15%lower 15%lower in t he lar ger sizes. They can accommoda accommoda te more para llel pass es in the process coil. For large duties, a cylindrical cylindrical heater ha s a ta ller ller firebox firebox and more nat ural draft a t the burner. burner. The flue ga s velocity velocity is usua lly higher in t he convection convection section, section, hence, the flue ga s film coeffic coefficient ient is h igher. Fewer expensive tube supports or guides are requir ed in th e convection convection section. section. The noise plenums plenums or preheat ed combust combust ion ion a ir plenums a re smaller. smaller. Fewer soot blowers are required in the convection section. Soot blower blower s a re not needed for ga seous seous fuel. If coil drainage is a problem, a helical coil may be used when there is only one one pass.
Cabin heat ers have the follo following wing adva nta ges:
• •
The process process coil coil ca ca n a lwa ys be dra ined. Two-pha wo-pha se flow problems a re less severe. (Slug flow can generally be avoided.)
A sta ged-fuel ged-fuel burner is a low low NO x burner in w hich hich a portion portion of the fuel is mixed mixed w ith a ll of of the combustio combustion n a ir with in the burner block while a second portion of the fuel is injected downstream of the burner block to provide delayed combustion.
•
Cabins can accommodate side-firing or end-firing burners instea d of only vertically upwa rd firing. This This permits the floor floor of t he hea ter t o be clo closer ser to t he ground. (Some burner m anufa cturers prefer prefer t o fire liquid liquid fuels horizonhorizontally.)
The injection of water or steam is also used to reduce the N O x formation by reduction reduction of the peak flam e temperature.
•
A smaller capital investment is required when t he duty is less t ha n 3 MW. MW.
8-16
FIG. 8-19 Example Cylindrical and Cabin Direct Fired Heaters
a nd for cabins a good good ra tio of of widt h to height t o length length is 1:2:4 1:2:4.. The firebox shell is reinforced steel plat e. The The insu la tion behind the tubes is usually 125 mm of 1:2:6 lumnite (cement), haydite (aggregate), vermiculite (insulation) castable. The floor floor is a t lea st 150 mm of 1:2:6 1:2:6 ca ca sta ble, often often w ith a firebrick surface. The bridge wall is always firebrick.
Radiant Section The ra dia nt section section or firebox should:
•
Obta in complete complete combust combust ion of of the fuel with a rea sona ble a mount of excess a ir, i.e., 10%to 15%.
• • •
Conta in the flame a nd a void void impingement impingement on the tubes. Distribute Distribute the radiant heat flux.
By far the most common tube material is A-106B carbon steel. The nominal size range is 50 to 200 mm. with 75 and 100 mm the most prevalent. Short radius return bends are standard and the tubes are usually 1.5 nominal diameters from the refractory wall. For these arrangements, the maximum h eat flux directly directly facing the flame is 1.9 times times the a verage flux. (W (With long rad ius return bends th e maximum hea t flux is 1.45 1.45 times t he a vera ge.) The flux to th e front 60° of the tube is 1.8 times times the avera ge and the front h alf-tube alf-tube flux is 1.5 1.5 times the a verage. Any Any flux ma ldistribution ldistribution due to tall nar row
Cool the combustion ga ses to 800800-1000 1000°° C to protect t he convection section.
The proport proport ions of the firebox a re th e key t o good good performa nce. Genera lly the fla me length should be 60%o 60%off the firebox length and t he cle clear ar ance betw betw een een the flame an d tubes at least 0.5 m. For sma ll cylindrical cylindrical h eat ers, the t ube circle circle should should be equal t o the length of the firebox. firebox. For small cabin heat ers, the width , height, height, a nd tu be length length should be equal. For large heaters the height of a cylindrical heater is twice the tube circle,
8-17
FIG. 8-20 Chart to Estimate the Fraction of Total Heat Liberation That is Absorbed in the Radiant Section of a Direct Fired Heater
fireboxes fireboxes or short fla mes, usua lly less tha n 15%, must be added to this. For double firing, the circumferential maldistr ibution is reduced from 1.8 to 1.25. 1.25.
flue ga ga s, and fuel and a tomizing tomizing steam, a ll hea hea t contents contents r eferred to a d at um of 15 15° C. Fig . 8-20 8-20 provides an estimate of the fraction of the total heat libera libera tion tha t is absorbed in in the ra diant s ection ection in in terms of the a llowa llowa ble heat f lux to the tu bes. The The kg a ir /kg fu el fired el fired is needed needed a nd t his can be obta obta ined from either either Fig . 8-21 8-21 if the LH V of of t he fuel is known , or by st oichio oichiometry. metry.
Eq ua tions 8-21 8-21 a nd 8-22, 8-22, a s well a s Fig. 8-20, 8-20 , ma y be be used to obta obta in an estimat e of of the a bsorbed bsorbed heat in the ra diant secsection of a fired heat er, er, expresse expressed d a s a fra ction ction of the tota l net heat liberat liberat ion, ion, in terms of the a verage heat flux to the tubes, the a rra ngement of the tubes (c (circumferential), ircumferential), and t he air t o fuel weight ra tio. These These equa tions a re solutions of of the WilsonWilsonLobo and Hottel equat ion. ion. B
R
= =
0.317 • d o • n • S • a 1
−
π • I
• G
Fig. 8-20 is 8-20 is for fired heaters with one row of 200 mm NPS pipes, spa spa ced ced t wo pipe nominal sizes (NPS ). Correction factors for other other designs, to be multiplied by by G prior to gra ph rea ding, ar e shown shown in the figure.
2
√ (B 2 + 70.56 • 10 6 • B ) − B 35.28 • 10 6
Eq 8-21
Example 8-8 — Est imat e the radia nt t ube area for a 3000 3000 kW regenera regenera tion gas heat er. er. To To avoid overheating overheating the t ubes, ubes, a ra dia nt flu x of 30 000 W/ W/m 2 is specified. The design calls for 100 mm N P S S ch 80 tu bes on a 2400 mm t ube circle. The The fuel is 0.61 0.61 relat ive density ga s w ith LH V of of 37 260 260 kJ /m 3. Us e 20% 20% excess excess a ir.
Eq 8-22
where "a" is a constant depending on the arrangement of tubes. The The "a " va lue is: is: No. of rows 1 2
Tube spacing 2 • NPS 3 • NP S 0.88 0.73 0.99 0.91
Fuel gas a nd combustio combustion n a ir a re supplied supplied at 15° 15° C. The The heat er NTE NTE is 80% 80%. The The t ubes a re a rra nged in one row a t 200 mm spacing.
Solution Steps
The tota l heat libera tion consist consist s of the lower heat ing value of the fuel and t he sensible heat in combustion air, recirculat recirculat ed
r
8-18
= 1500
kg flue gas
/(MW • h r ) (Fig. 8-21) 8-21 ).
= r • LHV
417 kg flue ga s /( k J •10
= 417 • 37 260 =
6
)
R
=
1
−
√ [( 7.603 • 10 ) + 70.56 • 10 • (7.603 • 10 )] − (7.603 • 10 6) 6 2
6
35.28 • 10
15.537 15.537 kg flue ga s /m 3 fuel gas
=
Mas s of 1 m 3 fuel gas
(1)(0.61)(29) 23.68
=
R = 0.747 kg Q
Mas s of combust combust ion a ir = 15.53 15.537 7 – 0.747 0.747 = 14.79 14.79 kg G = 14.79/0.747 = 19.8 kg air /kg fuel
=
0.525 0.525 (Note: (Note: Fig . 8-20 8-20 yields R = 0.535, 0.535, so use th e a vera ge of of R = 0.530 0.530.. UHT • R N TE
=
3000 • 0.530 0.80
Radiant heat tra nsfer nsfer area
=
Correction fa ctor for 100 mm t ubes is 1.02 (Fig . 8-20 8-20 )
Total tube length =
R = 0.535 0.535 (Fig. 8-20) 8-20 ) E q 8-21 8-21
B
(0.317)• (114.3 ) • (1) • (3.14) • (30 000) • (19.82) = (200 ) • (0.88 )
B
= 7.603 • 10 6
Tube length =
=
1
−
=
1988 • 1000 30 000
= 66.3
66.3 0.359
=
184.7 m
184.7 37
= 4.992
m
Convection Section The purpose of of t he convection convection section section is to t ra nsfer a s much heat as possible possible from the combustio combustion n ga ses lea lea ving the ra diant section. As always there is the trade-off between capital cost, i.e., adding more tubes, a nd operat ing cost, cost, i.e., improved improved ther mal efficiency.
E q 8-22 8-22 R
Qr I
kW
There are 37 vertical vertical tubes in a cylindric cylindrical al heat er with a 2400 2400 mm d iamet er tube circle wh en th e tubes a re 200 200 mm center t o center. center.
0.317 • d o • n • π • I • G 2 S • a
=
= 1988
The sur face a rea of 100 100 mm NP S pipe is 0.35 0.359 9 m 2/m
G corr corr ected = 19.8 19.8 (1.02 (1.02)) = 20.0 kg air /kg fuel
B
6
6
√ (B 2 + 70.56 • 10 6 • B ) − B 35.28 • 10 6
FIG. 8-21
FIG. 8-22
Flue Gas Rates 9
Flue Gas Convection-Coefficients for Flow Across Staggered Banks of Bare Tubes 9
8-19
The constru constru ction ction is similar t o tha t for the rad ian t section, section, a steel pla pla te shell with internal cast able or ceramic ceramic fiber fiber insulation. The tubes are staggered, and the space between the sidewa sidewa ll and the t ube is fille filled d with “corbe “corbels” ls” to prevent prevent t he flue gases from bypassing th e end tubes.
Assume tha t t he sett ing loss of of 2% or 75 kW occurs occurs in th e rad iant section. section. The heat content content ra te of the combustion combustion gases leaving radia nt section: Q radiant exit 3750 –1988 –1988 – 75 = 1687 1687 kW exit = 3750 3 = 6073.2 6073.2 (10 ) kJ /h r
The first tw o rows of th e convection convection section section a re called shock tubes a nd th ey “see” the firebox flame. The The first row receives receives the full radia nt h eat flux an d also some some conve convective ctive heat tra nsfer. It has the highest heat transfer flux in the heater and is a lwa ys ba re tu bes. The The second second sh ock ock row r eceives eceives about onethird of the radiant flux as well as convective heat transfer from the flue gas. It is also bare tubes. If long radius return bends are used, the third row will receive receive rad iant heat and it too should should be bar e tubes.
The entha lpy of the exit exit ga s from ra diant section: section: H = 6073.2 6073.2 (103)/5625 = 1080 kJ /k g Tg = 918°C 918°C (Fig. 8-18, 8-18, Flue G as – LHV)
Convection Section: Area for gas flow
Helical fins, sometimes sometimes serra ted to increase turbu lence, lence, are used a s soon soon a s possible, possible, i.e., i.e., when the fin t ip temperat ure is not excessive, e.g., e.g., 540° 540° C for carbon st eel, see Fig. 8-8. 8-8 . Typically when natural gas is the fuel, the fins are 25 mm high, 1.5 mm t hick an d up t o 240 240 fins per per linea r met er. er. For oil fired fired hea ters w here soot soot deposition is possible, possible, the fins a re 25 mm high, 2.7 mm thick and not more than 120 fins per meter. Often the first finned row row has few er, er, shorter, shorter, and thicker thicker fins to reduce th e fin tip tempera tur e. Where ash an d soot soot fouling a re expected, expected, a lan e is left left every four or five rows for soot blowers. These These are t ubes equipped equipped with nozzles nozzles tha t direct direct steam a gainst the tu bes. Soot Soot blowing blowing is intermittent an d is seldom seldom used more than once every shift.
= (no. of tubes ) (L ) (spacing − D ) = (6) (2.4 ) (0.2 − 0.114) = 1.24
m2
G g = 5625/(1.24) (3600) = 1.23 kg /(s • m 2)
First shock row. Assume the average gas temperature is 885°C 885°C a nd tube wa ll temperat temperat ure is 260 260°° C. Tg mean
260 + 885 2
=
= 573°C
2 8-22 ) h o = 21.6 W/(m • ° C) (Fig. 8-22)
A
=
0.359 m 2 per linear m
(Exam ple 8-8) 8-8)
Atubes = 14.4 (0.359) (0.359) = 5.17 m 2
The fins compensa compensa te for the low low flue gas h eat tr a nsfer coefcoefficient. Typicall Typicall y, th e heat flux in th e convection convection section is 6.312.6 12 .6 k W/m 2 of fin ned su rf a ce or 38-76 38-76 kW/m 2 on a bare tube basis.
Qc
Cast iron tube supports can be used below 425°C and 25% chrome – 12% 12% nickel is good good up to 1100 1100°° C. With With high va na dium or sodium levels in t he fuel oil, 50%chrome 50%chrome – 50%nickel 50%nickel must be used.
=
h o A ∆T
= (21.6) (5.17) (885 − 260 )
=
69.795 kW 2
Fl ux = Q/A = 30 000 W/m (Exa mple 8-8) 8-8) Q r = (Q/A) (A) (A) = 30 000 (5.17) = 155.1 kW Q c + Q r = (69.7 (69.795 95 + 155.1) 155.1) = 224.9 224.9 kW
The distance between supports for horizontal tubes should be th e lesser of 35 outside tu be diam eters or 6 m. The The dista nce between supports on vertical tubes should not exceed either 70 tube diameters or 12 m. Usually the retur n bends ar e exexternal to the tube sheets. This prevents flue gases from bypassing th e tube fins. fins.
Q exit gases = (1687 (1687 – 224.9) = 1462.1 kW H exit gases = (5263.6 (5263.6 (103) kJ /hr )/5625 k g/h = 935.7 k J /kg Tg
Fig. 8-22 shows 8-22 shows a pproximate pproximate externa l heat tra nsfer coe coeffifficients for 75, 100, and 150 mm NPS. steel pipe arranged in sta ggered ggered rows a nd surrounded by combustion combustion gases.
exit
= 820° 20° C (Fig. 8-18, 8-18 , Flue Gas – LHV)
Second shock row is analogous except that the radiant heat flux is one third of that for the first row, i.e., 10 000 00 0 W/m 2
Example 8-9 — Desig n t he convection section for th e 3000 3000 kW regeneration gas heater of Example 8-8. The heat loss is assumed to be 2%of the hea t r elease. elease. Use six 100 100 mm NP S Sch 80 tubes on 200 mm center-to-center spacing with 2400 2400 mm effective length length in each r ow. ow. After tw o rows of bare shock t ubes us e finn ed pipe, 118 118 fins/m, 32 mm h igh, 2.7 mm th ick. Assume Assume pipe wa ll tempera tur es of 9090-240°C 240°C a cross cross t he finned pa rt of th e convec convection tion section section a nd a vera ge values of 250 250 a nd 260° 260° C for the t wo shock shock rows.
Q r = (10 000) (5.17) (5.17) = 51.7 kW
Solution Steps Fig. 8-23 summar izes izes the design of of both both t he radia nt a nd conconvection sections. A trial and error solution for assumed temperatures is required. Details follow for the converged solution. Q t o t a l = du t y/G TE = 3000/ 3000/0.80 = 3750 kW
Tg
With h o = 21 W/(m 2 • ° C ) Q c = (21) (5.17) (796 – 250) = 59.3 kW Q c + Q r = 59.3 59.3 + 51.7 = 111.0 111.0 kW Q exit gases = 1462.1 1462.1 – 111 111 =
4864 (10 (10 3) /5625 =
H exit gases = exit gases
1351.1 1351.1 kW 864.7 kJ /kg
= 762°C 762°C (Fig. 8-18, 8-18 , Flue Ga s – LHV)
Finned rows. The combustion combustion ga s mass velocity velocity increases beca beca use of the increased cross sectiona sectiona l ar ea of finned pipe. From Eq 8-13.
=
114 1000
Gg
=
5625 [14.4(0.2 – 0.134)(3600)]
r = 1500 kg flue ga s/(MW • hr) (Fig (Fig . 8-21 8-21 ) Flu e ga ses flow ra te = 3.75( 3.75(150 1500) 0) = 5625 5625 kg/hr
8-20
118 (2.7 ) (178 − 114 )
Acs
+
10
6
=
0.134 m 2/linear m
= 1.64
kg /(s • m 2)
FIG. 8-23 3000 kW Regeneration Gas Heater
Section
Gas Heat Content Rate kW (LHV) In Out
Heat Transfer kW
Exit Gas Enthalpy kJ/kg
Exit Gas Temperature °C
R a d ia n t
375 0
1 6 8 7*
1 9 88 * *
1080
918
S h ock R ow 1
1687
1462. 1
224 . 9
935 . 7
820
S h ock R ow 2
14 62. 1
1351. 1
111
86 4. 7
762
F i n n ed
1 351. 1
675
676. 1
4 32
395
* Heat lo losses sses are 75 kW ** From Example Example 88-8
FIG. 8-24 Natural Draft Profiles
8-21
Q f = 3000 – 1988 – 224.9 – 111 = 676.1 kW
Re = 138 164 For an absolute roughness of 0.6 mm, the relative roughness is 0.001 th en
Q(exit ) = 3750 – 3000 – 75 = 675 kW H g(exit) = 2.43 (106)/5625 = 432 kJ /kg
f = 0.005 0.005
Tg (exit) = 395° 395° C (Fig. 8-18, 8-18, Flue Ga s – LHV)
From E q 8-18 8-18
Assuming tha t H HV is 10%mo 10%more re tha n LH V, the gross heater efficiency effici ency is 80%/1.1 80%/1.1 = 72.7% 72.7%. Note th a t th is a grees closely closely with Fig. 8-17 for 20%exc 20%excess ess air an d 390° 390° C.
Hav
Tp1
=
90° 90° C;
Tp2
=
240° 240° C;
Tg1
=
762°C 762°C ; Tg2
=
395°C 395°C ; Tg
Tp
= 165°C
av
av
578.5° C = 578.5°
V
=
ho
=
25 W/(m 2 • ° C ) (Fig. 8-22) 8-22 )
Qf
=
h o Ao ∆TLM (Eq 8-10) 8-10)
Ao
= 676.1 (10 3 )/[25 (405 )] = 66.78
π • D 2 4
V
m
2
Hav
Hav
Typical dra ft profiles profile s for direct fired natural draft heaters a re shown shown in Fig . 8-24 8-24 . There There a re tw o way s to contr contr ol th e flow flow of combust combust ion a ir: sta ck da mpers or or combustion combustion air registers. There should should be a slight va cuum in in a na tura l draft h eater to prevent prevent leakage of the flue gases. There There is usua lly an increase increase in pressure of 8 Pa per meter of firebo firebox x height a nd several mm pressure d rop across t he convection convection section.
Example 8-10 — Find the a vailable draft in a 0.6 m ID by 6 m long sta ck att ached to the t op of the convection convection section section for the 3 x 10 6 W regeneration gas heater of examples 8-8 and 8-9. Assume dry air a t 15° 15° C a nd 101. 101.4 4 kP kP a.
Fig . 23-21 23-21
From E q 8-9 8-9 5625 kg/hr
D = 0.6 0.6 m = 0.024 mPa • s
Re
=
0.3537 • (5625 ) 0.6 • (0.024)
• 0.746
=
6 • 9.8067 • (0.746 − 1.227 ) 2
• 0.746
2
• 4 • 0.005 •
6 0.6
+ 1
= −3.739 P a
This is a mixtur e of lumLHV Castable Refractory — This nite (cement), haydite (aggregate), and vermiculite (insulation) in 1:2:6 proportions. This low cost, concrete-type insula tion ha s a density of 880 880 kg/m 3 a low coefficient of expan expan sion, sion, a nd n egligibl egligiblee shrinka ge. It is h eld eld to t he vertical vertical wa lls with bu llhorns — V-sha ped steel wir e anchors welded to the outer casing. If a h igh sulfur (more tha n 1%) fuel is burned, to prevent prevent the sulfur tr ioxide ioxide in in t he flue gas from at ta cking the iron in t he lumnit e, use a low-iron low-iron (1% (1%) cement. cement. L HV casta ble wit hst a nds high (45-60 45-60 m/s) gas fa ce veloc velocities. ities. Ra in or snow entering through the stack will not hurt the concrete. B efore efore sta rt-up, rt-up, cure cure and dry out the refra ctory to avoid “spalling” or flaking caused by unequal t hermal expansion. construction of this ma teCeramic Fiber — A san dw ich construction rial in tw o densities densities is sometimes sometimes used. Because the ceramic fiber is porous, a protective coating is normally applied first to the steel casing, for protection against sulfur oxides in the flue ga s. A 50 or or 75 mm la yer of 64 kg/m 3 ceramic fiber is impaled over over t hin sta inless inless steel studs w elded elded t o the casing. Over t his is pla ced a 25 mm la yer of 128 kg/m 3 material. Stainless steel wash ers twisted onto th e studs hold the ceram ceram ic fiber fiber in place. This light weight material is much lighter than LHV and is suitable for convection sections. Advantages include ease of applic applicat at ion ion in freezing weather, when w at er in refractory is a problem; and no need to dry out or cure the ceramic fiber wh en it is first a pplied. pplied. For cyclic cyclic opera opera tions, less heat is stored in th e fiber fiber wh ich ich reduces wa rm-up time. Ra in or snow entering the heater during shutdown may drench the fiber, causing it to tear away from the studs on a vertical wall. It cann ot wit hst a nd ga s fa ce veloc velocities ities a bove 15 15 m/s. If soot blow-
Solution Steps
0.3537 • M D •µ
π • 0.6 2
Insu lat ion ion protects the heat er shell from the hot combust combust ion ion gases a nd usua lly reduces reduces the heat losses losses to less less tha n 2%of the hea t relea se. Three Three common common types of insulat ion are:
The stack draft must overcome the gas friction loss in the convection section, burner, and stack. The stack diameter is often s ized for 4.6 to 6.1 m/s st ack g a s velocity. The The st ack is normally bar e car car bon bon st eel eel but must be lined if the flue gas temperature exceeds 400°C or if the fuel has high sulfur content. All wall temperatures should be above the dew point of the flue gas.
=
ρg
Insulation
Stack Draft
Re
ρg Hs 4 • f f • D s + 1
5625 3600
+ 7.408
With 14.4 linear linear m per row this is 1.45 1.45 rows. U se 2 rows.
µa from
V2 • 2
V = 7.408 m /s
L pipe = 66.78/[(0.87) [(0.87) (3.676)] = 20.88 m
=
=
•
4
This is th e theoretic theoretically ally required surfa ce ar ea. From E xample 8-5, 8-5, the fin efficienc efficiencyy is 87%a 87%a nd t he externa l surfa ce a rea of the finned pipe is 3.676 m 2/line a r m .
µg
(ρg − ρa ) +
M
372° C = (165 + 578.5)/2 = 372°
Tfilm
µ
H s • g
ρg a n d ρa from the ideal gas la w. ρg = 0.746 ρa = 1.227
∆TLM = [(762 − 240 ) − (395 − 90 )]/ ln (522 /305) = 405°C
M =
=
Calculate
Pipe and gas temperatures are :
Assume
(From Fig. 17-2) 17-2)
8-22
FIG. 8-25
ing or steam lancing of finned finned t ubes is required, required, LHV casta ble refractory must be used or the fiber must be covered with a 3 mm st a inless steel erosion erosion sh ield. The The erosion erosion shield is a lso needed needed for gas t urbine exha exha ust hea t recovery recovery units.
Example Direct Fired Reboiler
Insulating Firebricks (Ifb) — These are used for bridgewa lls a nd floors. Ifb a re qu ite dense, ca. 2400 2400 kg/m 3, are shipped loose, and field installed. They must be dried out slowly. After proper installation, firebricks are sturdy and resist deteriora deteriora tion from weath er an d high ga s veloci velocities. ties. External Insulation — In su lfur plan ts a nd in cold cold climates, external insulation is frequently used to maintain a minimum sta ck t emperat emperat ure to prevent prevent condensa ondensa tion within the sta ck.
Other Design Considerations Film temperature — Tempera Tempera tures above 260°C 260°C will cause ma ny h ydrocar bons to decompo decompose se an d coke coke layers t o deposit posit in the t ube. This This increases the film and metal temperatures and leads to tube failures. Actual decomposition temperature is highly dependent upon the fluid characteristics. Snuffing — — If a tube ruptur es, fire fire could could break out in t he firebox. Connections are needed to admit CO 2 or a steam or water spray to snuff out the fire. The velocity of this vapor should b e kept below 24 m/s to a void erosion of th e refra ctory. firebox may contain an explosive mixture of gases. Before attempting to relight the h eater, this mixture must be purged.
excess excess air a t t he burner may differ from that in the sta ck due to leakage of ambient a ir thr ough ough t erminal holes, holes, box box gaskets, cracks in th e cas cas ing, etc.
Sampling — — To contr contr ol the excess a ir, sam ples of flue ga s are needed from various points of the heater. The amount of
Flue gas temperature — To To determine th e therm a l efficiency ciency of of the hea ter, the sta ck temperatu re must be monitored. monitored.
Purging — If the pilot flame or electric spark fails, the
FIG. 8-26 Heater Alarm/Shutdown Description N o te te :
Al a r m s a n d s h u t d ow ow n s a s s h o w n a r e n ot ot t o b e c o on n s i de de r ed ed a s m e et et i n g a n y m i n im im u m s a f et et y r e q u ir ir e m en en t b u t a r e s h ow ow n a s r e pr pr e senta tive of types used for for control control syst ems.
Basic Criterion – The failure of any one device will not allow the heater to be damaged.
Schematic Label
Alarm/Shutdown Description
Regeneration Gas Heater
TS H -1
H i g h S t a ck Tem p er a t u r e
S ee N o t e 1
TS H -2
H i g h O u t l e t Te m p e r a t u r e
S ee N ot e 1
Hot Oil Heater and Direct Fired Reboiler S e e N ot e 1
FSL
L o w Ma Ma ss ss F l ow Th Th r o u g h Tu b es
S ee N o t e s 2 & 4
S e e N ot es 3 & 4
BSL
F l a m e F a i l u r e D e t e ct i o n
S ee N o t e 5
S e e N ot es 5 & 6
P SL
L ow F u e l P r e s s u r e
P S H -1
H i gh F u el P r ess u r e
S ee N ot e 7
S e e N ot e 7
P S H -2
H i g h C a b i n P r es s u r e
S ee N o t e 8
N ot a pp l i ca b l e , i f n a t u r a l d r a f t
S e e N ot e 6
Notes: 1. A direct immersion immersion jacketed jacketed th ermocouple ermocouple is preferred preferred because because th e response response is ten times fa ster tha n a gr ounded thermocoupl thermocouplee in a w ell. A filled filled bulb system is a poor poor th ird choice. choice. The The high stack ga s temperature sh utdown should be set approximat approximat ely 110°C 110°C above normal operation. 2. An orific orificee plate signal should be backed backed up by a low pressure shutdown shutdown to ensure adequate process process stream flow under falling pressure conditio conditions. ns. 3. The measurement should be on on the hea ter inlet to avoid errors errors due to two-phase two-phase flow. flow. 4. Differential pressure pressure switches mounted directly directly across across an orifice orifice plate plate a re not satisfactory due to switch hysteresis. An analog differential differential pressure tra nsmitter w ith a pressure switch switch on the output is recommended. recommended. The ana log signal signal should be brought brought t o the shutdown pan el so tha t the flow level can can be readily compared compared with t he shutdown point. 5. The flame scanner should be aimed at th e pilot pilot so tha t a flameout signal will be generat generat ed if the pilot pilot is not not large enough to ignite the main burn er. er. 6. If the heater design precludes precludes flame flame scanners, a low fuel gas pressure pressure shutdown should be installed to prevent prevent unintent ional ional flam eout. This shutdown should detect detect ga s pressure at th e burner. 7. Either burner pressure or or fuel control control valve diaphragm pressure may be used. used. This shutdown sh ould be be used used whenever large load load changes are expecte expected. d. It prevents the hea ter from overfiring overfiring wh en the temperatu re controller controller drives the fuel valve valve wide open open to increase heat output with insufficient insufficient air. 8. This shutdown should block block in all lines to the heater because, when activated, a tube has probably ruptured. Ga s is probably probably burning vigorously vigorously outside outside the heater.
8-23
• • •
Process coil thermowells — The The firing ra te is controlled to maint ain t he correct correct process process stream outlet outlet t emperat emperat ure.
Draft gauges — These are needed to set the stack dam per per or burner air registers. registers.
The fuel ga s cont cont rol valve is w ide open. open. The fuel ga s composition composition or pressure va ries widely. The tubes in the heater a re not not st ra ight.
Options to Improve the Thermal Efficiency
Soot blowers — In liquid fuel syst ems these can be used intermittently to reduce fouling of the finned tubes.
Option I. Add Convection Surface
Controls
Effects: 1. 2. 3. 4.
Sta ck temperature temperature is reduc reduced. ed. Furna ce effic efficienc iencyy is increased. increased. Heat release release is is decreased. decreased. Flue gas pressure drop in the convec convection tion section section is increased. 5. D r a f t i s d e cr cr ea ea s e d. d. 6. Tube side pressure pressure drop drop is increased. increased. 7. N O x is reduced.
Fig.8-25 shows a n example contr contr ol system system for a fired heater. Fig.8-26 l Fig.8-26 lists ists th e shutdown/ala rms a s shown with some comcomments a bout bout t he proper proper installat ion ion a nd use of these part icular icular devices. devices. The The control system a s depicted depicted by these figur es should not be considered comple complete te but only representa tive of the conditions to be carefully considered considered in designin g a contr contr ol system for fired equipment. The following following ar e some indicat ions ions of possible tr ouble: ouble:
• • • • • • •
The burner fla me is not symmetr ica ica l, pulsat pulsat es or or breat hes, is unusua lly long long or lazy , lifts off off th e burner, etc. The burn er is not a ligned a nd/or th e flam e is too close close to the t ubes. ubes. Lack of negative pressure at the top of the firebox. The sta ck gas is smoky. The gas in the firebox firebox appears hazy. There are unequa l temperat temperat ures, more more tha n 6°C d ifferifference, on th e process process pas s outlets. The sta ck temperat temperat ure increases increases stea dily with n o cha cha nge in the process process heat dut y.
Things to consider: 1. Increasing Increasing stack height. height. 2. More More weight from added convec convection tion tubes. 3. Check structure and foundation to see if added weight can be supported. 4. If not, design adjacent structure to house co convection nvection tubes and support sta ck. 5. Consider Consider increased pumping pumping cost cost for process process stream. 6. If fuel is t o be cha cha nged, some some existing existing convec convection tubes ma y ha ve to be removed to accommodat accommodat e soot soot blowers. blowers.
FIG. 8-27 Convection Heater
8-24
FIG. 8-28 Water Bath Indirect Heater
6. N O x increases increases unless burners a re changed.
Option II. Add Economizer for Waste Heat Recovery
Things to consider: 1. Induced Induced draft a nd forced forced draft blowers blowers must be installed. installed. 2. Burners must be replace replaced. d. 3. Check Check if tube suppo supports rts a nd refractory refractory will withstand higher tempera tempera tures. 4. Plot space space near near furna ce must must be available. available. 5. System is selfself-contained. 6. Conside Considerably rably more instrumentation instrumentation must be installed. installed.
Waste heat options: Steam generation Steam superheating superheating B oile oilerr feedwa feedwa ter heat er
Effects: 1. 2. 3. 4. 5. 6.
Sta ck temperature temperature is reduc reduced. ed. Furna ce effic efficienc iencyy is increased. increased. Flue gas pressure drop in in th e conve convectio ction n section section is increased. increased. D r a f t i s d e cr cr ea ea s e d. d. No cha cha nge in in proce process ss stream operat operat ion. ion. N o ch a n g e i n N Ox.
Convection Heaters Heat ers in wh ich ich a ll the heat tra nsfer is conve convective ctive (there is no radiant section) are unique modifications of direct fired heaters. Because all aspects of the operation, e.g., fuel combustion, bustion, combustion combustion gas t empera empera ture, tube w all t empera empera ture, etc., are cont cont rolled, rolled, these hea ters a re ideally suited for offshore platforms and oth er applic applicat ions ions tha t deman d a h igh degree degree of safet y.
Things to consider: 1. Study increased increased load load on structure and foundation foundation as in Option I. 2. Will a dded boiler boiler ca ca pacity lower efficiency efficiency of of existing existing boilers? 3. Check poss possibil ibility ity of a temperatu re cros cross. s.
Fig. 8-27 is a sketch of a convection heater, with recycle regulating the heat flux. The cylindrical steel shell is internally insulated with light castable or ceramic fiber, and the combustion combustion and heat excha excha nge sections sections are separa ted by a n insulated w all. The The fuel gas an d a ll of of the combustion combustion air a re fed to t he short -flam e, pre-mix pre-mix burn ers. The The combust combust ion gases mix ra pidly pidly w ith r ecycl ecycled ed sta ck ga ses to produce produce the inlet inlet ga s to the h eat exchange section. This This ga s a t 650650-815°C 815°C flows under the dividing dividing wa ll and th en upwar d across the finned finned tubes of the pr ocess ocess coil. coil. P a rt of the cooled cooled ga s a t 175175-26 260°C 0°C leaves as sta ck ga s a nd t he remainder is recycle recycled. d. The The flow flow in the process process coil coil is downw a rd a nd count count er-current er-current t o th e hot gases.
Option III. Install Air Preheat System Effects: 1. Sta ck temperature temperature is reduc reduced. ed. 2 . N a t u r a l d r a f t i s d ec ecr ea ea s e d. d. 3. Furna ce effic efficiency iency is increased. increased. 4. Firebox Firebox temperatur e is increased. increased. 5. Heat flux rat es are inc increased. reased.
8-25
FIG. 8-29
FIRETUBE HEATERS
Methane Pressure-Enthalpy Diagram
Firetube hea ters r a nge in dut y from 17.6 17.6 kW glycol glycol reboilers reboilers to 3500 kW oil or gas pipeline heaters. The design, controls, an d operat operat ion ion of firetube heaters va ry widely from those used used in simple, simple, una tt ended “wellhead “wellhead ” equipment to t hose used used in complex, well-instrumented, gas plants.
Water Bath Heaters Fig. 8-28 is a sketch of of an indirect fired fired wa ter bat h heat er. er. This d esign is t ypical of a ll indirect fired vessels. The The firet ube is in the ba th in the lower half of the vessel an d th e proce process ss coil is in the upper half of the vessel. The heat transfer medium, in this case water, fills the vessel. A fill hatch, drain, wells for for thermosta ts, a nd a coil coil to preheat preheat the fuel are stan da rd. S izes ra nge from 600 mm OD by 1.5 m long long t o 3600 3600 mm OD by 9 m long. Only one fir etu be is needed below 1465 kW. kW. Some small heaters are uninsulated. More frequently 25 to 50 mm of wea therproof cera cera mic fiber fiber or or light cas ta ble insulation is placed ar ound th e cylindrical cylindrical shell. The The ends a re left ba re so a s not to impede access access t o the coil, coil, burner, or sta ck. As As sh own in Ex a mple 8-7, 8-7, insula tion ca n redu ce hea t losses up to 75%. A long lazy yellow flame increases the fire tube life and increases creases the radiant flame flame a rea. Almost every coil bundle requires a unique design to meet th e requirements of heat d uty, working pressure, corrosio corrosion n a llowance, sour gas service, NACE MR-01-75, and governing codes, codes, usua lly ASME S ection ection VII or ANSI B 31.3. 31.3. U se of a P -H diagra m w ill simplif simplifyy calculat calculat ions ions wh en both both sensible sensible and latent heat cha nges occur occur in the process process stream .
Recycling some of the stack gas controls the inlet gas temperature to th e heat exchan exchan ge sectio section, n, and in tu rn th e maximum film temperature of the process stream. The combined volume of of th e combust combust ion and recycled recycled gases is much grea ter tha n t ha t in most direct fired heaters. This This results in higher gas veloc velocitie ities, s, higher externa l heat tra nsfer coeffi coefficcients, a nd a smaller process coil surface area. The thermal efficiency is high because pre-mix burners operate satisfactorily at 10% excess excess air an d the sta ck gas t empera empera ture can be as low low a s 38°C 38°C a bove the inlet process strea m tempera tur e. The The price paid for this is t he elec electricity to run the recirculation recirculation fan; t his is usua lly lly more tha n offset offset by t he fuel saving. saving.
Example 8-11 — What heat duty is required to vaporize 10 m 3/hr of liquid propan e at 15° 15° C a nd 1600 1600 kPa (ga ) and superheat th e vapor by 10 10° C? Solution Steps Refer to Section Section 24 “Thermodyna “Thermodyna mics” and th e P -H dia gra m for propane. At 15°C and 1701 kPa (abs) the enthalpy of propan e is 547 547 kJ /kg. The The exit enth a lpy at 61°C (10° (10° C a bove the dew point) an d 1701 1701 kP a (a bs) is 950 950 kJ /kg. The The liquid dens ity is 510.2 5 10.2 k g/m 3. 10m 3/hr
q l
=
M
= (10 ) (510.2) = 5102
UHT
kg /h r
= (M / 3600) (H 2 − H 1)
FIG. 8-30 Typical Bath Properties for Firetube Heaters
Heater
Bath Temp. °C
Outside Coil Bundle ho W/(m2 • K)
Firetube Flux Q/A Q/A kW/m 2
Stack Temp. °C
Firetube Efficiency NTE %
Wa ter B a th
82–91 82–91
910 910
32–41
400–480
76–82
50 % E th ylene G lycol
91–96 91–96
650 650
25–32
425–480
76–80
Low P ressure St eam
118–1 118–121 21
5680 5680
47–57
425–480
76–80
H ot Oil
149–28 149–288 8
230
19–25
480–590
71–76
Molten S a lt
204–42 204–427 7
1135
47–57
535–650
68–74
TE G Reboiler
177–204 177–204
–
19–25
425
75–80
Amine Reboiler
118–132 118–132
–
21–32
480
75–80
8-26
FIG. 8-31 103 kPa (ga) Steam Bath Heater
= (5102 / 3600 ) (950 − 547 ) = 571.1
P ath CDE expands expands the gas immediatel immediatelyy and th en heats heats it. This results in the smallest coil area because the largest log mean temperat ure differenc differencee between between the wat er bat bat h an d the metha ne is available. But the expans expans ion ion crosses crosses the hydrat e line and t he gas will freeze. freeze.
kW
Usua lly long long ra dius return bends connec connectt t he passes in the serpentine coil; coil; however, however, short ra dius retur n bends sometimes “fit” t he coil coil bundle into the shell. The heat duty a nd pressure drop determine determine the pipe diameter a nd t he number of para llel llel flow flow path s a nd passes in t he process process coil. coil. Often Often th e internal process process stream heat tra nsfer coeffi coefficcient is much larger t han the ext erna l wa ter ba th coefficie coefficient, nt, e.g., Exa mple 88-4. Two para llel llel flow flow pa ths of four four passes ma y be an a lternat ive heat tr a nsfer design t o one one flow flow pa th of eight pa sses. The The effect effect on the process stream pressure drop is significant. Because the pressure drop for turbulent flow is proportional to the 1.83 power of the velocity and the pipe length per pass has been halved, the two flow path pressure drop approaches one seventh of tha t for one one pass.
Path CHE supplies all the heat needed and then expands the ga s. This This is feasible, but not d esirable beca beca use the lowest temperature difference requires the largest coil area. P ath CFG E first heats the methan e so tha t the expansio expansion n just touches touches the hydra te line, expands expands it, a nd heat s to the exit temperatur e. This This is t he minimum coil coil ar ea t ha t correspo corresponds nds to an operable path. S o: P rehea t duty : (970 (970 – 900) 900) = 70 kJ /kg P osthea t dut y: (104 (1040 0 – 970) 970) = 70 kJ /kg In practice, some penetration of the hydrate line is possible; and the bala nce between “preheat” “preheat” and “postheat “postheat ” passes is such such tha t t he lowest lowest temperatur e is 5 to 8°C below below the hydra te line. line.
In ma ny w ellhead ellhead a pplic pplicat at ions ions t he coil coil contains contains a choke that th rott les the ga s from well pressure to processing processing or pipeline pipeline pressure. This divides the coil into preheat and postheat passes. The The ra pid expansion expansion is isentha isentha lpic lpic and, if t he ga s temperature fa lls lls t oo low, low, hydra tes form.
Freezing of the water bath is a potential problem. If the heat er is insula insula ted, a cont cont inuous pilot pilot suffices. Severa l different antifreeze additives have been tried and all have shortcomings:
Example 8-12 — Find t he optimum optimum dist ribution between preheat and postheat duty for expanding expan ding met hane from 20 000 kPa (abs) and 25°C (see po point C in Fi g. 8-29 8-29 ) to 7000 7000 kPa (abs) and 35° 35° C.
• •
Solution Steps
•
Refer to Fig. 8-29 which is a P -H diagr am for CH 4 on wh ich ich the line AGB for for hydra te forma te forma tion is superimposed. super imposed. (This (This is a combination of Figures 20-15 a nd 24-23 24-23.) .) Consider th e folfollowing lowing three alterna tive pat pat hs.
8-27
Methyl a lcohol lcohol is volatile a nd ha s to be replenished. replenished. It is also a fire hazard. Ca lcium lcium chloride chloride and rock salt in concentra concentra tions tha t a re effective effective a re very corrosive. corrosive. Glycols are generally accepted as the safest and most trouble-free additive. The decomposition products are a cidic; cidic; it it is recommend recommend ed tha t corrosion corrosion and r ust in hibitors be u sed concurrent concurrent ly. ly.
FIG. 8-32 Typical Physical Properties of Hot Oil
FIG. 8-33 Salt Bath Heater
8-28
FIG. 8-34 Amine Reboiler
Stack Press. Press. Gauge Conn. Conn. Relief Valve Conn.
Vapor Outlet Fill Conn.
Weir
LLC
Water Level
LLSD
Gauge Glass Conn. Burner Front
Firetube Lean Amine Outlet
Removable Firetube
Drain
Rich Amine Inlet
Glycols reduce reduce the heat tr a nsfer coeffic coefficient ient of the bat h significant ly. ly. For exa mple, a 50%by 50%by w eight solution of ethylene glycol glycol reduces the firetube flux by 20% 20%a nd the extern al bath heat heat transfer coefficient for t he pr ocess coil by 40% 40%. (See Fig . 8-30 8-30)
Manufactured heat transfer oils ar e “blended” “blended” for about a 110°C operating range. For example, Fig. 8-32 g ives typical hea t t ra nsfer properties for a 150 150 to 290 290°° C polyphenyl ether. The advantages of hot oils are:
Fig. 8-30 compares the bath properties of fire tube heaters. The adva nta ge of using wat er as the heat tra nsfer medium medium is appa rent. The The relatively low low ba th temperature r esults esults in t he lowest lowest sta ck tempera tempera ture an d t he highest firetube firetube efficienc efficiencyy. Note that the firetube efficiency does not account for heat losses. For large well insulated heaters the overall process NTE NTE ma y exceed exceed 80% 80%. But for small uninsu lat ed heat ers wit h interm itten t operat ion ion t he process process NTE NTE ma y be as low low a s 60% 60%.
• • • •
Low vapor pressure pressure at ambient temperat ure Alwa ys liquid liquid and eas y to handle B lended lended for a specifi specificc temperatur e ran ge Higher specific specific heat tha n n ormally occ occurring h ydrocar ydrocar bons
The disadvantages include:
Low Pressure Steam Heaters When a process stream temperature of 70 to 100°C is needed, needed, a 104 104 kPa (ga) steam heater can be used to reduce reduce the required size of the tube bundle. Construction, shown in Fig. 8-31, 8-31 , is un der ASME ASME Section Section I V co code. Stea m outlet a nd condensat condensat e return return connectio connections ns are sta nda rd so tha t th e steam ma y be used in externa l excha excha ngers if desired. The The condensing condensing steam has a n externa l proce process ss coil coil heat tra nsfer coeffi coefficcient of 4540-68 45 40-68 00 W/(m 2 • K). K). It is importa importa nt t o vent all air a t sta rt-up. rt-up. With in sula tion a nd controls, th e NTE NTE can be a s much a s 80%, wh ich is close close to the efficiency efficiency for a f iretube.
• •
Escaping vapors ar e environmentally environmentally undesirable
• •
U sually a n ANSI Cla ss 300 300 flange design design is required
• •
Low heat tra nsfer prope properties, rties, see Fig. 8-30. 8-30. (The firetube flux is half that of a water bath heater and the external process process coil coil heat tr a nsfer coeffic coefficient ient is a bout one qua rt er.) er.) When overheated, the oils will oxidize and coke on the firetube. Also, Also, th ey can be ignit ed Et hers a re expensiv expensivee The ethers a re hygr osco oscopic a nd mu st be kept dry
Molten Salt Heaters
Hot Oil Heaters
Molten Molten salt bat hs operate a t 200 200 to 425°C. 425°C. They a re mixtures of sodium sodium nitra te and the nitrites of sodi sodium um a nd pota pota ssium. The advantages include:
These heaters furnish a heating bath to 315°C or higher, wh ich is hot hot enough for dry desiccant or hydr oca oca rbon recovery recovery regeneration gas. Another less severe application is heavier hydr oca oca rbon va porizat ion ion prior to injection injection into a gas pipeline to raise the heating value.
•
8-29
Good heat tra nsfer properties, properties, see see Fi g. 8-30 8-30 . The The fir etube flux is as high as that for a low pressure steam heater. Note the process coil heat transfer coefficient of
1135 113 5 W/(m 2 • K) is partially due to the small diameter tubes that usually comprise the coil bundle.
• •
0.14
h
• P r
Thermally stable to 540°C.
The sa lt is h ygroscopic. ygroscopic. If w et, it decrepita tes on melting and is ha zardous. zardous.
Because of the higher operating temperature there are no flan ge joi joi nts on the shell exce except pt for the sa lt loading ha tch, see Fig. 8-33. 8-33 . On lar ge heaters the expansion of of the firetube may be suffici sufficient ent t o warr ant an expan expan sion sion joint joint for the sta ck. The The shell is insula ted t o protect protect personnel.
Direct Fired Reboilers Ga s treat ing and dehydrat ion ion frequently employ employ direct direct fired reboilers. More detailed proc pr ocess ess descriptions a re given in S ections 20 and 21. 21. Fig. 8-34 is a sketch of a typical direct fired reboiler. The rich fluid containing the sour gas or water is boiled boiled in th e reboiler reboiler to remove the sour ga s or the w at er. er. The The lean fluid then is used to treat or dehydrate t he proce process ss gas stream again. Surge tanks for the lean fluid may be integral with the reboil reboiler er as shown, or may be mounted as a separa te vessel beneath th e reboiler reboiler..
Example 8-13 — What is the firetube flux when the combustion gases are at 1540°C and the firetube wall is at 150°C. Assume that the fuel is natural gas and the heat release is 1.2 MW wit h 10%excess 10%excess air in a 0.6 m I D pipe. Refer t o examp le 8-6. 8-6. The The r a dia nt hea t f lux is 53.27 kW/ kW/m . The convective convective heat flux must be added. r = 1400 1400 kg /MW hea t release (Fig. 8-21) 8-21) Combu st ion ga s f low is (1.2)( (1.2)(1400) 1400) = 1680 1680 kg/hr Fr om Eq 8-7, 8-7, 8-8b 8-8b an d 8-9a 8-9a a nd Fig. 8-5 0.14
h • D k
•
(Fig. 8 −5) 0.14
= 0.023 • R e
0.8
• P r
0.33
•
µb µw
Troubleshooting The following following problems ca ca n occur occur w ith fir etube hea ters.
•
k = 0.066 W/(m • ° C) at 950°C 950°C C p = 1.04 kJ /(kg • ° C )
µb µW
Even t hough firetube failure is is ra re, it is advisable to prevent m ovement or flexing flexing w ith r estraining bars. This This prevents weakening of the weld joints at the end plate. In addition, wh en th e fuel is is oil, oil, include include a dr ain w ith plug in t he bottom of the firetube leg leg between the end plate a nd th e burner flange. Then the firetube can be drained if any oil accidentally gets past t he burner. burner.
Fig . 8-35 8-35 s hows hows a n example e xample control system for an indirect fired heater. Fig. 8-36 lists the shut down/ down/ala rms a s shown with some some comments comments about th e proper proper installat ion ion a nd use of these particular devices. The control system as depicted by these figures should not be considered complete but only representa tive of the conditions to be carefully considered considered in designing a control system for fired equipment. The controls probably probably va ry more tha n t he design of of the hea ter. For For example, a wellhead wellhead line heater or glycol glycol dehydrat or ma y ha ve no more than an on-off thermostat for the main burner and a small continuous pilot. A line heater, hydrocarbon vaporizer, or a mine reboiler reboiler ma y ha ve all of of the controls listed listed in Fig. 8-36. 8-36 .
2
µb µw
This is a maximum firetube flux a nd is typical only for for w at er bat h or low low pressure steam hea ters.
Controls
Solution Steps
0.33
Tota l hea t flux = 53 271 + 10 355 = 63 626 W/m 2
An a lternat ive to the burner front front shown in the equipment equipment sketches is a flame arrestor, the element of which provides man y sma ll tortuous tortuous pat hs between rolled rolled sheets of of th in corcorrugated aluminum. Sucking the combustion air through the element element is a n a dditiona dditiona l pressure pressure drop for for th e stack draft to overcome. The term arrestor is somewhat of a misnomer because an y fire in the firetube or around th e burner is conta conta ined rather than extinguished. The passage through the arrestor cools cools the ga s so tha t extern a l combust combust ion ion does not occur occur..
Firetubes typically range from 150 to 750 mm ID and from 1525 to 9140 mm long. Normally the burner flame extends ha lfway down down the first leg. A mitered joi joint nt return bend is used to reduce the resista nce to flow of the combustion combustion gas es.
• P r
k D
Inspirat ing partia l prepre-mix mix burners burners ar e used used in the vast ma jority of firetube heaters. The gas is preheated before expansion, flow control, and flow through the burner. While the burner dra ws t he prima prima ry air into the firetube, it is the stack dra ft, usually less tha n 25 mm H 2O, tha t overcomes overcomes the pressure drop of of th e combustio combustion n ga s flow flow and admit s the seconda seconda ry a ir. The The sta ck height is 3 t o 6 m.
Firetubes, Burners, Stacks
= 0.023 • R e
•
Q/Ac = 7.45 (1540 - 150) = 10 355 W/ W/m 2
One of of th e requirements requirements for heat ing regenera regenera tion gas is a very low pressure dr op across th e coil coil bundle. To To realize t his and still obtain good heat transfer, the coil bundle consists of many, small diameter, U-tubes in parallel. These are welded into a tube sheet sheet t hat is at ta ched ched to a cha nnel header. To reduce thermal stresses caused by the cold inlet gas, the inlet ha lf of of the tube sheet is insulated.
Nu
•
µb µw
(0.6 )(1680 ) = 0.023 π 0.6 2 0.045 3600 4 1000 0.33 0.14 (1.04)(0.045) 0.045 0.0665 • 0.6 0.066 0.023 = 7.45 W/(m 2 • ° C )
More difficult difficult to han dle, large lumps must be broken broken up.
0.8
0.33
0.8
The sa lt will not ignit e.
The disadvan ta ges are:
• •
= 0.023 • R e
0.8
= 0.04 0.045 5 mP a • s = 0.02 0.023 3 mP a • s
8-30
Bath level loss can be the result of too too high high a bat h temperature. This is often caused by the temperature contr oller oller on t he process process str eam . Fouling of the pr ocess ocess coil, coil, interna l an d/or externa l, mea mea ns a hotter bat h is needed to a ccomplish ccomplish the sa me hea t t ra nsfer. The The coil coil should be removed removed a nd cleaned. cleaned.
FIG. 8-35 Indirect Fired Heater
FIG. 8-36 Bath Heater Alarm/Shutdown Description N o te te :
Al a r m s a n d s h u t d ow ow n s a s s h o w n a r e n ot ot t o b e c o on n s i de de r ed ed a s m e et et i n g a n y m i n im im u m s a f et et y r e q u ir ir e m en en t b u t a r e s h ow ow n a s r e pr pr e senta tive of types used for for control control syst ems.
Schematic Label
Alarm/Shutdown Description
Line Heater
Hydrocarbon Low Pressure Reboiler Steam Heater
TS D H -2
H i g h B a t h Tem p er a t u r e
No Note 1, 2
Ye s
Hot Oil or Salt Heater
Glycol Reboiler
Amine Reboiler
No
Ye s
Ye s
Ye s
LS L
L o w B a t h L ev e l
No Note 2
No
Ye s
Ye s Note 3
No Note 2, 3
Yes Note 2, 3
P SL
L ow F u e l P r e s s u r e
Ye s Note 4
Ye s
Ye s
Ye s Note 4
Ye s
Ye s
P SH
H i gh F u el P r e ss u r e
Ye s Note 4
Ye s
Ye s
Ye s Note 4
Ye s
Ye s
BSL
F l a m e F a i l u r e D et e ct i o n
No Note 2, 5
Yes Note 5
No Note 2
Yes Note 5
No Note 2
No Note 2
P SH
H i g h Ves s e l P r es s u r e
No
No
Ye s Note 6
No
No
Ye s Note 6
Notes: 1. When the process process stream is oil, a high bath tempera tempera ture shut down precludes precludes the danger of coking. coking. 2. This instrumentat ion is for for heat ers located located in gas processing processing sections. sections. Wellhead ellhead u nits ha ve a minimum of controls. controls. 3. Low ba th level protects protects both the firetube a nd bat h wh en it will coke (hot oils, glycol, am ine) or decompos decomposee (molten (molten salt ). 4. Low/high fuel pressure pressure is alwa ys used when the fuel gas is taken from the exit process process stream. 5. Optical Optical U V scan scan ners or flame rods should should be used because because of the speed speed of response. response. 6. Code requirements, ASME S ection ection IV or VIII. VIII.
8-31
FIG. 8-37 Methods to Increase Firetube Heat Transfer
Flame Arrestor
Firetube Economizer
End View of Heat Exchanger
TC
Firetube Turbulator
If fouling of the coils coils is not th e problem, problem, wa ter losses can be reduced with a vapor recovery exchanger mounted on top of the heater shell. It consists of thin tubes that condense the water vapor. Vapor losses can also be reduced by altering the composition composition of of th e heat medium or, in dra stic cases, by cha cha nging the heat medium.
•
Shell side corrosion is cau sed by d ecompo ecomposition sition of th e ba th. (The decompositio decomposition n products of am ines a nd glycols a re corrosive.) corrosive.) Some decomposition decomposition an d corrosion corrosion is inevitable; however, excessive decomposition is usually due to overheating near the firetube. Corrosion inhibitors are commonly added. There are numerous reasons for overheat ing th e ba th: localized localized ineffective ineffective hea t transfer caused by fouling, excessive flame impingement , etc. An An impr oper oper fla me can s ometimes be modified modified without system shut down. Fouling, however, however, requires requires removal of the firetube.
8-32
•
Inadequate heat transfer ma y r esult esult from from improper improper flame, under-firing, firetube fouling, coil fouling, poor shell fluid dyna mics, too sma ll a firet ube or coi coil, l, etc. If it is not improper design, then it is most likely fouling or a n improper flame. The The solution solution ma y be a simple burn er adjustment to correc correctt t he air to fuel mixture. mixture.
•
High stack temperature can be the result of an improper proper a ir to fuel mixture. A leak of combust combust ible mat erial from the process side to the firetube is also a cause. It can a lso be th e result of excessive excessive soot soot d eposition eposition in t he firetube.
•
Firetube failure is most commonly caused by localized overheating and subsequent metallurgical failure. These “hot spots” are caused by hydrocarbon coking and deposition on the ba th side. Firetube corrosi corrosion on is ca ca used by burn ing a cid cid ga ses for fuel. The The most d a ma ging corrosio corrosion n occurs occurs in th e burner a ssembly. ssembly. There There is little tha t can be done done ex-
FIG. 8-38 Example Hot Oil System
•
cept to change the fuel and this may be impractical. Proper metallurgy is essential when burning acid gases. High or low fuel gas pressure can have a dramatic effect effe ct on on t he operat operat ion ion of a firetube heater. Burners a re typically typically ra ted a s heat output at a specifie specified d fuel pressure. pressure. A significantly lower pressure means inadequate heat release. Significan tly higher pr essure ca ca uses over-f over-firing iring and overheating. The most common causes of a fuel gas pressure problem are the failure of a pressure regulator or an unacceptably low supply pressure.
to circulate circulate the ba th t hrough the economize economizerr or an actua tor to position position th e dam per, per, is needed. With good good control of t he excess a ir, i.e., i.e., 5%to 5%to 10%an 10%an d a sta ck tempera tur e of 200 200°° C, t he NTE NTE a pproaches 90%. However, the pressure drop drop across the firetube incre increaa ses and the sta ck dra ft decreases. decreases. This This means t ha t a forced forced draft burner ma y be required. The The econo economics mics are usua lly favora ble and short pa yout periods periods for t hese modificat modificat ions ions a re common. common.
HOT OIL SYSTEM
Improved Thermal Efficiency Economic Economic incentives ha ve promoted the d evelopment evelopment of devices vices t o improve the t herma l efficiency efficiency,, i.e., reduce the excess air a nd the sta ck tempe temperat rat ure.
A simplifi ed sche mat ic of of th e ma jor jor component component s of a hot oil system is given in Fig. 8-38. 8-38 . The heat transfer medium is pumped through a fired heater to the heat exchangers and retu rns t o the pump suction surge vessel. vessel. In some ca ca ses a fired heat er is replaced replaced by a wa ste heat source, source, such such a s the exhaust stack of a fired turbine. The slip-stream (typically less than 5%) filter is optional but it w ill help to reta in th e performan ce chara cteristics cteristics of of the hea t medium.
Control of the flow flow of a ir into the firetube or the ga s flow in the sta ck with da mpers is sensitive sensitive because because the relatively weak sta ck dra ft is easily influenced influenced by an a dditional pressure drop. drop. Severa Severa l designs designs ar e available: axially rotating vanes ar ound the burner, burner, a pivoting horseshoe ar ound the burner, a hinged plat e over over the air inlet duct, a rota ting plate in the sta ck, etc. etc.
P roper roper design of of th e heat er is critical critical for satisfa ctory ctory opera opera tion. The The hea t tr an sfer fluid must h ave sufficient velocity, velocity, gengenera lly 1.2 to 3 m/s, t o avoid excessive excessive film tempera tur es on th e heat er tubes. Hot spots spots can lead lead to tube failure and fluid degrad at ion. ion. Design an d capacity of a hea ter should be limited limited so tha t t he maximum film temperature does not not excee exceed d th e maximum recommended operating temperature of the fluid.
Methods Method s to increase firetube heat transfer are shown in Fig. 8-37. 8-37 . An economizer (end view) consisting of longitudina lly finned finned tubes inserted into the return leg leg of the firetube adds heat transfer surface; a turbulator increases the heat transfer coefficient; and internal fins both add area and increase crease t urbulence urbulence.. Often a dditional dditional equipment, equipment, e.g., e.g., a pump
8-33
The surge vessel is provided provided with b lan ket ga s an d vent connections. Expansion room for the hot oil from ambient to operating conditions must be provided. On small systems the surge ta nk ma y be sized sized to hold all of the heat medium. Two piping arrangements are used for the surge vessel; flow thr ough ough t he vessel or the vessel vessel as a surge riding on on t he pump suction line.
solids regeneration, heat supply to absorption chillers, combined heat and power systems, and mechanical power as in turbochargers. Heat recovery equipment introduces additional head losses losses in the flue gas pa th, w ith t he consequenc consequencee of possible changes in required stack height, fan power, and damper position. In the case of combustion engines and gas tur bines, there is a loss of of power.
P ump head r equirements equirements a re usually 275 275 to 55 550 kPa . Frequently th e pump is spared a nd provided provided with isolating va lves for servicing. A slip-stream should pass through the off-line pump to mainta in the off-li off-line ne pump at operat operat ing temperature. Selection of pumps should consider the high operating temperature and its effect on seals, packing rings, and gaskets. Slight leaka ge may occ occur a t sta rtup; the pump gland should not be tightened until th e system reaches operating operating t empera empera ture. Suction strainers should be used used during sta rtup.
Exha ust ga ses from from equipment equipment with high excess excess air, such such a s ga s tu rbines w hich conta conta in 13 to 17%molar 17%molar volume of oxygen, oxygen, temperatures between 455 and 565°C and pressures up to 2.5 kP a (ga) ma y be used to supply th e oxygen oxygen for the combustion of fuels fuels in fired hea ters.
REFERENCES 1 . N or or t h Am e r ic ic a n M a n u f a c t u r in in g C o r p .,., C o m b u st st i on on H a n d b o ok ok , Second E dition, 1978.
P iping iping design a nd insta llat llat ion ion mu st include consid considerat erat ion ion to minimize vapor tra ps an d to relieve relieve expansion expansion a nd contra contra ction str esses. The The num ber of flan ges in a h ot oil oil system should be minimized; ANSI ANSI Cla ss 300 300 lb flan flan ges will aid in minimiz ing lea lea kage.
2. La uer, B. E., Oil & G as J ourna l, Series 88-18-52 18-52 to 1111-22-53. 53. 3 . P e r r y, y, R . H . a n d C h i l t o n, n, C . H . , E d i t or or s , C h e mi mi ca ca l E n g i n ee ee r i ng ng Handbook, Fifth Edition, 1973. 4 . E s c oa oa F i n t u b e C or or p . E n g i n ee ee r i ng ng M a n u a l , 1 97 97 9. 9.
During initial sta rtup, the system may conta conta in wa ter, which must be slowly vaporized and removed. The surge vessel is sometimes sometimes the high point in t he system to a id in th is operat operat ion. ion. On shut down, fluid should be kept circula circula ting t o dissipat e residual refractory heat. Planned maintenance should include an alysis of the heat tra nsfer medium, inspectio inspection n of insulation, an d inspection inspection of of heat er an d mechanical equipment. equipment.
6. But hod, A. P. and Ma nning, W. W. P., University of Tulsa, Personal Communication.
Consult Cons ult suppliers supplie rs of hot oils to obtain design information such as Fig. 8-32 or suggestions such a s reference 12. 12.
9. Wimpress, impress, P. H., H ydrocarbon ydrocarbon P rocess rocess & Petroleum Ref., VolVolume 42, No. 10, 117, 1963.
5 . K e n t u be be C o m pa pa n y , D e si si g n B r o ch ch u r e , 19 197 3. 3.
7. McAdams, McAdams, W. W. H., H eat Tran Tran smission, smission, Third Third E dition, dition, 1954. 1954. 8. Kern, D. Q., Q., Process Process Heat Tran Tran sfer, sfer, 1950 1950..
10. Lobo, W. E. an d Eva ns, J . F., F., AIC AIC hE Tra Tra ns., Volume Volume 35, 35, 743, 743, 1939 1939.. 11. Nelson, Nelson, W. L., Oil Oil & Ga s J ournal, J anua ry 4, 194 1947, 7, p. p. 77. 77.
WASTE HEAT RECOVERY Economi Economical cal a nd environmental considerations considerations ma y lead to the use of wa ste hea t recovery recovery systems. Flue gases from fired equipment , combustion combustion engines and ga s turbines a re common common hea t sources for for considerat ion ion to use wa ste hea t recovery. recovery. The The recove recovered red hea t m ay be used used in the sa me equipment equipment to increase increase its th ermal efficie efficiency ncy an d to supply heat t o other other equipment. Fired equipment also ma y combine combine the hea ting of several process stream s a t different different temperatur es. Typic Typical al applicat applicat ions ions include combustion combustion air a nd/or fuel preheat ing in t he sam e equipment . B urn ers ca ca n be self-recuperat self-recuperat ive a nd selfself-regenerative. Some applications include additional heat input by refiring. Heat t ransfer systems systems may have:
12. 12.
Dow Corning Corp., Corp., “Heat “Heat Transfer Transfer System Design Design Checklist,” Checklist,” Version 1.2, Form 24-250-85.
13. 13.
API R P 11T 11T, “Installation and Opera Opera tion of of Wet Wet Steam G enerators,” USA.
14. 14.
API S PE C 12K, “Indirec “Indirectt-T Type Oil Oil Field Field Heaters,” USA.
15. 15.
API RP 12N, “Ope “Opera ra tions, Maintena nce and Tes Testing ting of Firebox Firebox Flame Arrestors,” USA.
16. 16.
API RP 530, 530, "Calculation of Heater Tube Tube Thick Thickness ness in Petroleum Petroleum Refineries," USA.
17. 17.
API RP 531M 531M,, “Measurement “Measurement of Noise Noise from Fired Fired Pr ocess ocess Heaters,”USA.
18. 18.
API RP 532, 532, "Measurement of Thermal Thermal E fficie fficiency ncy of of Fired Fired Pr ococess Heaters," US A.
1 9. 9.
AP I R P 5 33 33 , “ Ai Ai r P r e h e a t S y s t e m s f or or P r o ce ce ss ss H e a t e r s , ” U S A. A.
2 0. 0.
AP I S TD TD 5 3 4, 4, “ H e a t R e co co vvee r y S t e a m G e n er er a t o r s , ” U S A. A.
2 1. 1.
AP I P U B L 5 3 5, 5, “ B u r n er er s f or or F i r e d H e a t e r s i n G e n e r a l R ef ef i n er er y Services,” USA
2 2. 2.
AP I R P 5 5 0, 0, " M a n u a l o n I n s t a l l a t i on on s of of R e fi fi n er er y I n s t r u m e n t s and Control Systems, Pa rt III, Fired Heaters Heaters an d Inert Gas Generators."
•
Str eams conta conta ct, as in bubbling bubbling flue gases in wat er hea hea ters.
•
No contact contact of streams, a s in shell a nd tube heat excha excha ngers.
•
An int int ermedia te heat t ra nsfer fluid by forced forced circulat circulat ion ion a s in pumped oil oil systems, or or by na tura l circ circulat ulat ion ion a s in thermosyphons. The intermediate fluid may have a phase change as in hea t pipes. pipes.
•
A solid solid heat heat tra nsfer media, such such as in rota ry or alterna ting regenera tive devices. devices.
23. 23.
API S TD 560," 560," Fired Heaters for for Genera l Refinery Refinery Services," Services," USA.
In s ome speci speciaa l devices, devices, such as combust combust ion ion engine a nd t urbine exhaus t ga s silencers, silencers, a fluid is circulat circulat ed for for h eat recovery. recovery.
24. 24.
API RP 573, 573, “Inspectio “Inspection n of Fired Boilers Boilers and Heat ers,” US A.
2 5. 5.
AP I P U B L 4 36 36 5, 5, “ C h a r a c t e r iz iz a t i o n of of P a r t i cu cu l a t e E m i s si si on on s from Refinery Refinery P rocess rocess Heaters and Boilers,” Boilers,” US A.
2 6. 6.
AI C h E , “ F i r e d H e a t e r s, s, A G u i d e t o P e r fo fo r m a n ce ce E v a l u a t i o n s ,” ,” USA.
Applications of waste heat recovery from fired equipment, combust combust ion ion engine a nd ga s tur bine flue ga ses include include process process strea m heating, wa ter heating, steam genera genera tion, liquids liquids and
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2 7. 7.
AG A , “G “G a s E n g i n ee ee r s H a n d b oo oo k, k, 1 st st E d i t i on on , ” I n d u st st r i a l P r e s s Inc., N.Y., USA.
34. 34.
NFPA 85G, 85G, " Pr evention evention of of Furnace Implosions Implosions in Multiple Multiple Burn er B oileroiler-Furnaces," Furnaces," U SA.
28. 28.
IGE /TM/2, “Internat ional Index of Sa fety Standa rds and C odes odes Relating to Ga s Ut ilisation in Industry a nd Commerce,” Commerce,” The The Institution of of Ga s Engineers, England.
35. 35.
Idelchick Idelchick,, I.E., Ha ndbook ndbook of Hydraulic Resistan Resistan ce, ce, 1960 1960,, Tra Tra nslated from Russian by Isra el Program for Scientific Scientific Tran Tran slations 1966, 1966, Reproduced Reproduced by U.S. D epart ment of Comm erce. erce. Russia.
36. 36.
Miller Miller,, D.S., Interna l Flow Flow S ystems, Vol Vol.. 5 in the BHR A Fluid Engineering S eries, eries, 1978, 1978, En gland.
29. 29. IGE COMMUNIC ATION 1507 1507,, “The “The Develop Development ment of Gr oundwater H eating a t P ressure Reductio Reduction n S tat ions,” ions,” The The Institution of Gas Engineers, England. 3 0. 0. S A L F OR OR D U N I VE VE R S I TY TY, C e n t r e fo fo r N a t u r a l G a s E n g i n ee ee r in in g , Ga s Utilisation Courses Notes, Notes, Dr. Robert Robert Pritchar d, England.
37. Nelson, Nelson, W.L., W.L., Petroleum Petroleum Refinery Refinery En gineering, gineering, 4th Edition, USA. 38. 38.
Pitt s, D.R. and Sissom, L.E., Heat Tra Tra nsfer Theory heory and P roblems, roblems, 1977, USA.
3 2. 2. N F P A 8 5B 5B , " P r e ve ve n t io io n of of F u r n a ce ce E x pl pl os os io io n s i n N a t u r a l G a s Fired Multiple Burner Boiler-Furnaces," USA.
39. 39.
Potter, Potter, J .P., .P., Power P lant Theo Theory ry and Design, Design, 2nd Edition, Edition, 1959 1959,, USA.
33. 33. NFPA 85D, 85D, "P revention revention of Furnace Explosio Explosions ns in Fuel OilOil-Fired Fired Multiple Multiple B urner Boiler-Furna Boiler-Furna ces," ces," US A.
4 0. 0.
P r i t c h a r d , R .,., G u y , J . J . , a n d C o n n or or , N . E . , I n d u s t ri ri a l G a s U t i l iisation, 1977, 1977, En gland.
31. 31. NFPA 85A 85A,, "P revention revention of Furna ce Explosio Explosions ns in Fuel Oil- and Natur al G as-Fired as-Fired Single Burner B oileroiler-Furna Furna ces," ces," U SA.
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