Introduction to Heat Exchangers Course objectives What are exchangers for? Exchanger types How are they specified? The design task
Objectives B y the en d of the th e cou cou r se you you wil wi l l
• be familiar with the main main exchanger types • know which is likely to be the best type for a given application • understand what are the key factors in exchanger design • be able to estimate estimate the size and cost cost of key exchanger types • have the background necessary to start using commercial exchanger design software • be an informed purchaser of heat exchangers
Lecture series • Introduction to heat exchangers • Selection of the best type for a given application • Selection of right shell and tube • Design of shell and tube
Q=UA
T
Contents • Why we need heat exchangers • The basics of their design • Some general features of exchangers • Different types of exchanger • The design process
Example of an exchanger
Bundle for shell-and-tube exchanger
What are heat exchangers for? • To get fluid streams to the right temperature for the next process – reactions often require feeds at high temp.
• To condense vapours • To evaporate liquids • To recover heat to use elsewhere • To reject low-grade heat • To drive a power cycle
Feed-effluent exchanger
Feed-effluent exchanger
Exothermic reaction
Heat recovery
Distillation Reflux condenser
Top product Feed
Column Reboiler
Bottom product
Naphtha and gases
Typical crude oil distillation Top pump around
E2
Desalter Top pump around
E2
Bottom pump around
Heavy gas oil
E3
E5
Kerosene r e w o t n o i t a l l i t s i D
Light gas oil
Heavy gas oil
Light gas oil
Kerosene
E4 E1
Bottom pump around
E5 Storage
Furnace
E6 Reduced crude
Reduced crude
Power cycle
Steam turbine
Boiler
Feedwater heater
Condenser
Q = U A T yw Thot
Tcold
We have thermal resistances in series
1
U
1
cold
r cold
yw
w
r hot
1
hot
Heat utilities • Hot utilities – Boiler generating service steam (maybe a combined heat and power plant) – Direct fired heaters (furnace) – Electric heaters
• Cold utilities – Cooling tower (wet or dry) providing service cooling water – Direct air-cooled heat exchanger
Thermal integration or process integration
• Reducing the hot and cold utility needs by interchanging heat between process streams • If the plant needs are primarily heat, thermal integration is usually by “pinch technology” - Software HX-Net • If the plant is concerned with heat and work, pinch technology is supplemented with “exergy analysis”
Local and mean values •
means from the hot side to the cold side including all resistances “Overall”
• However it is still at a particular point in the exchanger: i.e. it is local • Hence you can have a local, overall coefficient LOCALLY FOR WHOLE EXCHANGER
q U T Q U A T
Integrating over the exchanger area Local equation q
dQ dA
UT
dQ
Rearranging dQ
T and integrating
UdA
dQ
T UdA
Q T
AT
dA
Total area AT
Definitions of mean values From previous slides
Q T
U m AT
T m dQ
Comparing the two sides 1
Tm
1
Q T
dQ
T
Q
T UdA
QT
AT
U m
1 AT
UdA AT
Special case where Ts are linear with Q • Eqn. integrates to give log. mean temperature difference - LMTD
Ta T b Tm T LM ln( Ta / T b )
e r u t a r e p m e T
T a
Q T b
Multipass exchangers T 1
• For single-phase duties, theoretical correction factors, F T , have been derived • F T values are less than 1 • Do not design for F T less than 0.8
Tm FT T LM
T
. 2 p t 2 m e T
t 1
Q
Typical
F T correction
factor curves
For shell and tube with 2 or more tube-side passes
Curves are for different values of R P
t2
T1
t 1 t 1
; R
T1 t2
T 2 t 1
T , t = Shell / tube side 1, 2 = inlet / outlet
Thermal effectiveness Stream temperature rise divided by the theoretically maximum possible temperature rise
T 1 ,in
T 2 ,out
T1,in T 1,out T1,in T 2 ,in
T 1 ,out
T 2,in
Compactness • Can be measured by the heat-transfer area per unit volume or by channel size • Conventional exchangers (shell and tube) have channel size of 10 to 30 mm giving about 100m2/m3 • Plate-type exchangers have typically 5mm channel size with more than 200m2/m3 • More compact types available
Compactness 10
60
Hydraulic diameter, mm 1
0.1 Human lungs
Special Car radiator Plate fin Plate Shell-&-tube 100
1000
2/m3
10 000
Main categories of exchanger Heat exchangers
Recuperators Wall separating streams
Regenerators Direct contact
Most heat exchangers have two streams, hot and cold , but some have more than two
Recuperators/regenerators Recuperative
Has separate flow paths for each fluid which flow simultaneously through the exchanger transferring heat between the streams Regenerative
Has a single flow path which the hot and cold fluids alternately pass through.
Rotating wheel
Double Pipe Simplest type has one tube inside another - inner tube may have longitudinal fins on the outside
However, most have a number of tubes in the outer tube - can have very many tubes thus becoming a shell-and-tube
Shell and Tube Typical shell and tube exchanger as used in the process industry
Shell-side flow
Complete shell-and-tube
Plate and frame • Plates hung vertically and clamped in a press or frame. • Gaskets direct the streams between alternate plates and prevent external leakage • Plates made of stainless steel or higher quality material • Plates corrugated to give points of support and increase heat transfer
Plate types
Corrugations on plate improve heart transfer give rigidity Many points of contact and a tortuous flow path Chevron
Washboard
General view of plate exchanger “Plate exchanger” normally refers to a gasketted plateand-frame exchanger
Flow Arrangement within a PHE Gaskets arranged for each stream to flow between alternate plates
Alternate plates (often same plate types inverted)
Air-cooled exchanger • Air blown across finned tubes (forced draught type) • Can suck air across (induced draught)
Finned tubes
ACHE bundle
Plate-fin exchanger
• Made up of flat plates (parting sheets) and corrugated sheets which form fins • Brazed by heating in vacuum furnace
Can have many streams 7 or more streams are typical
Typical plate-fin
Spiral (plate)
Good for streams with large solids
Cooling Towers • Large shell with packing at the bottom over which water is sprayed • Cooling by air flow and evaporation • Air flow driven by forced or natural convection • Need to continuously make up the cooling water lost by evaporation
• Used for batch heating or cooling of fluids • An agitator and baffles promote mixing • A range of agitators are used • Often used for batch chemical reaction
Agitated Vessel
Proprietary types • Types described so far are generic types • These can be made by any company with necessary skills (no real patent protection) • There are now many special, proprietary exchangers made by one company or a small number of companies under licence • One example is the “printed circuit exchanger” by Heatric
Printed circuit heat exchanger • Plates are etched to give flow channels • Stacked to form exchanger block • Block diffusion welded under high pressure and temperature • Bond formed is as strong as the metal itself
Printed circuit exchanger
Note that “compact” does not mean small but means large surface area per unit volume
Distribution of types in terms of market value in Europe
Cooling Towers 9%
Waste Heat Boilers 5%
Other Heat Recovery 10%
Air Coolers 10% Other Proprietary 2% Other Plate 4% Plate & Frame 13% Other Tubular 5%
Shell & Tube 42%
Preliminary points on selection • Tubes and cylinders can withstand higher pressures than plates • If exchangers can be built with a variety of materials, then it is more likely that you can find a metal which will cope with extreme temperatures or corrosive fluids • More specialist exchangers have fewer suppliers, longer delivery times and must be repaired by experts • S&Ts cannot normally give high thermal effectiveness,
Design sequence • Design the process flow flow-sheet • Specify the heat exchanger requirements • Select the best exchanger type for the job • Thermal design of exchanger • Mechanical design of exchanger Looping back may be necessary at any stage but can be difficult because of the project timetable
Who does what? • Design the process flow flowsheet
Processor/ end user
• Specify the heat exchanger requirements • Select the best exchanger type for the job
Contractor
• Thermal design of exchanger • Mechanical design of exchanger Manufacturer
Exchanger specification • Heat load (duty) along with the terminal temperatures of the streams • Maximum pressure drop each streams – liquids - 0.5 bar – gases/vapours below 2bar - 10% of inlet pressure
• Design pressures and temperatures • Size/weight constraints • Standards to apply – General standards like ISO, TEMA, ASME etc – Companies own standards
• Other requirements
