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A h e t ~... V G M U inc . o ni c here are tw pri obj in fur op o 3. A s c f i syste H resp is ina ufor co ef to ab 9 aTc i
g el. 0
Furnaces operat rated with ith but un unstable. Careful
I
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low ex excess-air instrumentation
wide widely ly
t-,... I
tiontion-fue fuell effic efficie iency ncy and safet safety y while while maint maintain aining ing the the desi desire red d flow flow,, temp temper erat atur uree and and pres pressu sure re cond condiition tions. s. The The compl complex ex inte intera ract ctio ions ns betw betwee een n the the proc proc-ess ess vari variab able les, s, and and the the mult multip iple le simu simult ltan aneo eous us mani manipu pula la-tion tionss need needed ed to meet meet the the obje object ctiv ives es,, make make auto automa mati ticc contr control ol essen essentia tial. l. SplitSplit-se secon cond d decis decisio ions ns requi required red to preprevent vent an expl explos osio ion n haza hazard rd in a fue fuell-ef effi fici cien entt furn furnac acee (low (low exces excess-a s-air ir opera operatio tion) n) are best best left left to aut automa omatic tic protec protectio tion n
used used with with fire fired d
ery ery
sys system tem
and
the
waste aste base based d
are fue fuel-e l-effi fficient and co control
is base based d
temperature
Brie Brieff
revi review ew
furn furnac acee
on flu fluegas egas of the
of the the heat heat-r -rec ecov ov--
tempe empera ratture, ure,
stream
that
dewp ewpoin oint,
picks
Broa Broadl dly y
spea speaki king ng,,
heat heat-r -rec ecov over ery y
1. Proc Proces esss-st stre ream am
2. Stea Steam m
heaters
are
syst system emss
heat heatin ing g
in conv convec ecti tion on
widely
used
up t he he
should uld
be
can can incl includ ude: e:
sect sectio ion. n.
gene genera rati tion on..
t he he
most
heat-recovery
and and are are inst instru rume ment ntal al
system
in boo boost stin ing g
the the
a system.
The
oper operat atio ions ns..
the
of fund fundam amen enta tals ls
The The facto factors rs that that dete determ rmin inee heat heater er 1. Flueg Fluegas as exit exit tempe temperat ratur ure. e. 2. Exces Excess-a s-air ir for combu combust stion ion..
1--I
Desi Design gn
beat eato The dec decisi ision to use use such such equi equip pmen ment on a tech techno noec econ onom omic ic feas feasib ibil ilit ity y stud study. y.
for for fire fired d heat heater ers, s,
ling ling conti continu nuou ouss
heat heater ers. s.
efti eftici cien ency cy are: are:
3. Type Type oí fue fuel. l. 4. Heate Heaterr casi casing ng loss loss.. Improv Improveme ement nt in heater heater effic efficie iency ncy is usuaI usuaIly ly realiz realized ed by incor incorpor porati ating ng:: a heat-r heat-reco ecove very ry syste system; m; impro improved ved instru instru-mentat mentation ion and and contr control; ol; moremore-eff effici icien entt burne burners; rs; improv improved ed insula insulatio tion; n; effici efficien entt soot soot blower blowers; s; reduce reduced d air air leaks. leaks.
control
and
instrumentation
impo import rtan antt proc proces esss vari variab able less with with air air preh prehea eati ting ng are: are: Fuel Fuel flowrate flowrate..
of
for for cont contro roll of a fir fired ed heat heater er
.
.. ..
. Operat Operating ing
exce excessss-ai air. r.
Proc Proces esss
flui fluid d
flow flowra rate te..
Process Process
fluid fluid temperat temperature. ure.
Furnac Furnacee draft. draft. Flam Flamee
cond condit itio ion. n.
. Combustibles.
such
-.
Tamparatura-controllad
"" ""
chambar
,..'
"
Pipa siza (nominal)
unracovarad
(i.a.,
prassura
loss) *
B . co th to p p t G cont requ goo m oo dev Th foc ea flow m how pos so eas pro sin vent yiel a l pres ow dro but is co tivel expe and m req a d tra fro ay uc rrangi aEsse ecta c to cros irc Th A n an A ele has cha nn si el f p o t b i c w 1% of valu bas on 95 ith of test poi Itge E, la [ g E r in sa g T ( = Be its o i v t s tio to th lo oc Mounting plata' \ \ \
4 5 6 8 10 12 14 16 18 20 24 30 36 42 48 60
I I
Support
plata
Aspiration-air
lina ,/
Aspirator-air inlet --'
oritice platas pressure (rula
11 9 21 16 13 11 10 9 8 7 6 5 4 3 3 2
is approximately ot thumb)
60%
ot the
c/
d itterential
Choosing a transducer
as to which cheaper
is preferable,
but
oxygen
analyzers
are
and are' more widely used. The zirconium-oxide
-"
\
Flow
measurement
Fuel-oil,
and
process-fluid,
flowrate
measurement
can
be
ferred. It offers several advantages ayer the other existing types-such as reduced maintenance requirements,
satisfactorily achieved by orífice meters. The fuel-gas flowrate can be corrected by using a densitometer. Airan orífice produces
element
a high,
has be en found
permanent,
suitable
pressure
for this
loss. The
content-and is readily adaptable for use as a probe type or extractive type. The extractive type can be used in
housed
joto
in a temperature-controlled
chamber)
and is dis-
charged back to the furnace (Fig. 1). The sensor output signal is determined with respect to
ilar to the pitot tube, but with vastly improved accuracy,
is further reported to have a 0.1 %-of-value repeatability, based on an average of various differential-pressure readings. The permanent pressure loss in the Annubar is comparable to that of the venturi tube; hence, it is much less energy-intensive compared with the orífice plate (Table I). Further, the Annubar is relatively insensitive to surface wear or abrasion on edges or sensor parts, ensuring
long-term
accuracy.
However,
overall
and flow range
tial-pressure meters, etc.).
secondary instrumentation (transmitters, Hence, care must be exercised in the selec-
Excess-air
The
differen-
instruments.
used
and carbon
[02]Reg.
monoxide
low-excess-a ir
meters.
Controversy
trim
are
exists
= Concentration of oxygen in reference of oxy-
the cell has to be maintained at a constant temperature. Since the resulting output signal is inversely propor-
excess-air
to c ontr ol
gas
gas
gas = 0.209 for air; [02Jsample gas = Concentration
tration.
estimation
indicators
oxygen
64 '-'
secondary
by the
sys-
tem accuracy
tion of the
are limited
the
[02Jsample
Therefore, (see
Table
greater
accuracy,
reliability
and'r
II).
It is advisable to locate the analyzer installation at the heater bheching, where errors due to air leakage are
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