Introduction to Heat Ex Changers
Transcript of Introduction to Heat Ex Changers
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Introduction to Heat ExchangersCourse objectives
What are exchangers for?
Exchanger types
How are they specified?
The design task
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ObjectivesBy the end of the course you will• be familiar with the 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 the size and cost of key exchanger
types• have the background necessary to start using
commercial exchanger design software• be an informed purchaser of heat exchangers
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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 = U A T
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Contents• Why we need heat exchangers
• The basics of their design
• Some general features of exchangers
• Different types of exchanger
• The design process
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Example of an exchanger
Bundle for shell-and-tube exchanger
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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
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Feed-effluent exchanger
Feed-effluentexchanger Exothermic reaction
Heat recovery
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Distillation
Bottom product
Feed
Top product
Reflux condenser
Reboiler
Column
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Typical crude oil distillation
E2
E1
E3
E4
E5 E6
E2
E5
Storage
Kerosene
Desalter
Top pumparound
Top pumparound
Naphthaand gases
Kerosene
Furnace
Reduced crude
Lightgas oil
Heavygas oil
Reducedcrude
Heavy gas oil
Light gas oil
Bottom pumparound
Dis
till
atio
n to
wer
Bottompumparound
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Power cycle
Boiler Condenser
Steam turbine
Feedwaterheater
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Q = U A T
We have thermal resistances in series
Thot
Tcold
1 1 1
Ur
yr
coldcold
w
whot
hot
yw
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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
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Thermal integrationor 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”
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Local and mean values• “Overall” means from the hot side to the cold
side including all resistances
• 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
mTmT TAUQ
TUq
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Integrating over the exchanger area
Local equation
Rearranging
and integrating
qdQ
dAU T
dQ
TUdA
dQ
TUdA
Q AT T
dQ
dA
Total area AT
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Definitions of mean values From previous slides
Comparing the two sides
Q
TU A
dQ
TUdA
T
mm T
Q AT T
1 1
T Q
dQ
Tm T Q
UA
UdAmT AT
1
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Special case where Ts are linear with Q
• Eqn. integrates to give log. mean temperature difference - LMTD Ta
T TT T
T Tm LMa b
a b
ln( / ) Tb
QT
empe
ratu
re
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Multipass exchangers
• For single-phase duties, theoretical correction factors, FT, have been derived
• FT values are less than 1
• Do not design for FT less than 0.8
Q
Tem
p.
T1
T2
t1
t2
T F Tm T LM
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Typical FT correction factor curvesFor shell and tube with 2 or more tube-side passes
T, t = Shell / tube side 1, 2 = inlet / outletP
t t
T tR
T T
t t
2 1
1 1
1 2
2 1
;
Curves are for different values of R
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Thermal effectiveness
T T
T Tin out
in in
1 1
1 2
, ,
, ,
Stream temperature rise divided by the theoretically maximum possible temperature rise
T1,in T1,out
T2,outT2,in
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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
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Compactness
m2/m3100 1000 10 000
Hydraulic diameter, mm60 10 1 0.1
Shell-&-tube
Plate
Plate fin
Car radiator
Special
Human lungs
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Main categories of exchanger
Heat exchangers
Recuperators Regenerators
Wall separating streamsWall separating streams Direct contact
Most heat exchangers have two streams, hot and cold, but some have more than two
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Recuperators/regenerators
RecuperativeRecuperative
Has separate flow paths for each fluid which flow simultaneously through the exchanger transferring heat between the streams
RegenerativeRegenerative
Has a single flow path which the hot and cold fluids alternately pass through.
Rotating wheel
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Double PipeSimplest 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
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Shell and TubeTypical shell and tube exchanger as used in the process industry
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Shell-side flow
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Complete shell-and-tube
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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
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Chevron Washboard
Plate types
Corrugations on plateimprove heart transfergive rigidity
Many points ofcontact and atortuous flow path
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General view of plate exchanger
“Plate exchanger” normally refers to a gasketted plate- and-frame exchanger
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Flow Arrangement within a PHE
Alternate plates (often same plate types inverted)
Gasketsarranged foreach stream toflow betweenalternate plates
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Air-cooled exchanger
• Air blown across finned tubes (forced draught type)Air blown across finned tubes (forced draught type)
• Can suck air across (induced draught)Can suck air across (induced draught)
Finned tubes
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ACHE bundle
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Plate-fin exchanger
• Made up of flat plates (parting sheets) and corrugated sheets which form fins
• Brazed by heating in vacuum furnace
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Can have many streams
7 or more streams are typical
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Typical plate-fin
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Spiral (plate)
Good for streams with large solids
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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
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Agitated Vessel• 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
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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
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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
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Printed circuit exchanger
Note that “compact” does notmean small but means largesurface area per unit volume
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Distribution of typesin terms of market value in Europe
Shell & Tube42%
Other Tubular5%
Plate & Frame13%
Other Plate4%
Other Proprietary2%
Air Coolers10%
Cooling Towers9%
Waste Heat Boilers
5%
Other Heat Recovery
10%
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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,
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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
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Who does what?
• 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
Processor/end user
Contractor
Manufacturer
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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
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The designer must supply an exchanger which
• Meets the stated specification
• Has reasonable initial costs and operating costs (most exchangers are bought on the basis of the cheapest tender)
• Has a reasonable lifetime– no damaging vibration– no thermal fatigue– no unexpected fouling or corrosion