1. HEAT EXCHANGER By Farhan Ahmad Department of Chemical Engineering,
University of Engineering & Technology Lahore
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2. Criteria for the selection of heat exchanger
Suitable on the grounds of operating pressure and temperature, fluid-material compatibility, handling, extreme thermal conditions
Estimating the cost of those which may be suitable
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3. General considerations
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 less suppliers, longer delivery times and must be repaired by experts
The last point means that specialist exchangers are not favoured in less developed parts of the world
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4. Double pipe heat exchanger
Normal size
Double-pipe heat exchangers are competitive at duties requiring ft2
Built of carbon steel where possible
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5. Advantages/disadvantages of double-pipe HE
Easy to obtain counter-current flow
Can handle high pressure
Modular construction
Easy to maintain and repair
Many suppliers
Disadvantage
Become expensive for large duties (above 1MW)
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6. Scope of double pipe HE Maximum pressure
300 bar(abs) (4500 psia) on shell side
1400 bar(abs) (21000 psia) on tubeside
Temperature range
-100 to 600oC (-150 to 1100oF)
possibly wider with special materials
Fluid limitations
Few since can be built of many metals
Maximum ε = 0.9
Minimum ΔT = 5 K
It should be noted that the ranges and limits quoted above are a guide as to what is normal today. This limits are being extended. Also, with care in design and with specialist manufacture, it is possible to extend the limits, although this may be at additional cost.
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7. Shell and tube heat exchanger
Size per unit ft2 ( m2)
Easy to build multiple units
Made of carbon steel where possible
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8. Advantages/disadvantages of S&T
Extremely flexible and robust design
Easy to maintain and repair
Can be designed to be dismantled for cleaning
Very many suppliers world-wide
Disadvantages
Require large plot (footprint) area - often need extra space to remove the bundle
Plate may be cheaper for pressure below 16 bar (240 psia) and temps. below 200oC (400oF)
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9. Scope of shell and tube (Essentially the same as a double pipe)
Maximum pressure
300 bar(abs) (4500 psia) on shell side
1400 bar(abs) (21000 psia) on tubeside
Temperature range
-100 to 600oC (-150 to 1100oF)
possibly wider with special materials
Fluid limitations
Few since can be built of many metals
Maximum ε = 0.9 (less with multipass)
Minimum ΔT = 5 K
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10. Plate and frame heat exchanger
Plates pressed from stainless steel or higher grade material
titanium
incoloy
hastalloy
Gaskets are the weak point.Made of
nitrile rubber
hypalon
viton
neoprene
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11. Advantages of plate and frame HE
High heat transfer - turbulence on both sides
High thermal effectiveness possible
Low ΔT - down to 1K
Compact - compared with a S&T
Cost - low because plates are thin
Accessibility - can easily be opened up for inspection and cleaning
Flexibility - Extra plates can be added
Short retention time with low liquid inventory hence good for heat sensitive or expensive liquids
Less fouling - low r values often possible
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12. Disadvantages of plate & frame HE
Pressure - maximum value limited by the sealing of the gaskets and the construction of the frame.
Temperature - limited by the gasket material.
Capacity - limited by the size of the ports
Block easily when solids in suspension unless special wide gap plates are used
Corrosion - Plates good but the gaskets may not be suitable for organic solvents
Leakage - Gaskets always increase the risk
Fire resistance - Cannot withstand prolonged fire (usually not considered for refinery duties)
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13. Scope of plate & frame HE
Maximum pressure
25 bar (abs) normal (375 psia)
40 bar (abs) with special designs (600 psia)
Temperature range
-25 to +1750C normal (-13 to +3500F)
-40 t C special (-40 to +3900F)
Flow rates
up to 3,500 m3/hour can be accommodated in standard units
Fluid limitations
Mainly limited by gasket
Maximum ε = 0.95
Minimum ΔT = 1 K
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14. Principal Applications
Gasketed plate and frame heat exchangers have a large range of applications typically classified in terms of the nature of the streams to be heated/cooled as follows:
Liquid-liquid.
Condensing duties.
Evaporating duties.
Gasketed units may be used in
refrigeration
heat pump plants and
extensively used in the processing of food and drinks.
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15. Comparison with Shell and Tube Heat Exchangers
In quantitative terms, 200 m2 of heat transfer surface requires a plate and frame heat exchanger approximately
3 metres long,
2 metres high and
1 meter wide.
For a tubular heat exchanger achieving the same effect, some 600 m2 of surface would be required in a shell
5 metres long and
1.8 metre in diameter,
plus the extra length
needed for tube bundle removal.
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16. Welded plates heat exchanger
Wide variety of proprietary types each with one or two manufactures
Overcomes the gasket problem but then cannot be opened up
Pairs of plates can be welded and stacked in conventional frame
Conventional plate and frame types with all-welded (using lasers) construction have been developed
Many other proprietary types have been developed
Tend to be used in niche markets as replacement to shell-and-tube
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17. Principal Applications
As for gasketed plate and frame heat exchanger, but extended to include more aggressive media.
Welded plate heat exchangers are used for the evaporation and condensation of refrigerants such as ammonia and hydrochlorofluorocarbons (HCFCs), and for different chemicals.
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18. Comparison with Shell and Tube Heat Exchanger
As for gasketed plate and frame units.
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19. Plate Fin Exchangers
Formed by vacuum brazing aluminium plates separated by sheets of finning
Noted for small size and weight. Typically, 500 m2/m3 of volume but can be 1800 m2/m3
Main use in cryogenic applications (air liquifaction)
Also in stainless steel
As a rough guide, a plate fin would be a fifth the size of a shell and tube for the same duty. Of course, a shell and tube exchanger is often not suitable for many plate-fin applications involving many streams and small temperature differences.
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20. Scope of plate-fin exchanger
Max. Pressure 90 bar (size dependent)
Temperatures to 150oC in Al
Up to 600 with stainless
Fluids Limited by material
Duties Single and two phase
Flow configuration Cross flow, Counter flow
Multistream Up to 12 streams (7 normal)
Low ΔT Down to 0.1oC
Maximum ΔT 50oC typical
High ε Up to 0.98
use only with clean fluids
The standards of ALPEMA (Brazed Aluminium Plate-fin Exchanger Manufacturers Association) may be downloaded free of charge from the ALPEMA web site -
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21. Principal Applications
The plate-fin heat exchanger is suitable for use over a wide range of temperatures and pressures for
gas-gas,
gas-liquid and
multi-phase duties.
Typically, these involve
Chemical and petrochemical plant:
Hydrocarbon off-shore applications:
Miscellaneous applications:
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22. Comparison with Shell and Tube Heat Exchanger
A plate-fin heat exchanger with 6 fins/cm provides approximately 1,300 m2 of surface per m3 of volume. This heat exchanger would be approximately 10% of the volume of an equivalent shell and tube heat exchanger with 19 mm tubes.
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23. Spiral heat exchangers
The classic design of a spiral heat exchanger is simple
the basic spiral element is constructed of two metal strips rolled around a central core forming two concentric spiral channels.
Normally these channels are alternately welded, ensuring that the hot and cold fluids cannot intermix
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24. Operating Limits
Maximum design temperature is 400oC set by the limits of the gasket material.
Special designs without gaskets can operate with temperatures up to 850oC.
Maximum design pressure is usually 15 bar, with pressures up to 30 bar attainable with special designs.
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25. Applications
It is ideal for use in the food industry as well as in brewing and wine making.
Spiral heat exchangers have many applications in the chemical industry including TiCl4 cooling, PVC slurry duties, oleum processing and heat recovery from many industrial effluents.
Spiral heat exchangers also provide temperature control of sewage sludge.
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26. Comparison with Shell and Tube Heat Exchanger
Spiral designs have a number of advantages compared to shell and tube heat exchangers:
Optimum flow conditions on both sides of the exchanger.
An even velocity distribution, with no dead-spots.
An even temperature distribution, with no hot or cold-spots.
More thermally efficient with higher heat transfer coefficients.
Small hold up times and volumes.
Removal of one cover exposes the total surface area of one channel providing easy inspection cleaning and maintenance.
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27. PLATE AND SHELL HEAT EXCHANGERS
The plate and shell heat exchanger combines the merits of shell and tube with plate heat exchangers
Current plate and shell heat exchanger models accommodate up to 600 plates in a shell 2.5 m long with a 1 m diameter
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28. maximum working pressure is 100 bar
Operating Limits
The maximum operating temperature of a plate and shell heat exchanger is 900oC
maximum working pressure is 100 bar
handle flow rates of 11 litres per second on the shell side.
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29. Principal Applications
The principal applications for plate and shell heat exchangers are:
· Heating including district heating.
· Cooling including cryogenic applications.
· Heat recovery.
· Combined exchanger/reactors vessels.
· Condensation/evaporation
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30. Comparison with Shell and Tube Heat Exchanger
For heat exchangers of equivalent area and capacity, plate and shell designs are smaller due to the higher ratio of heat transfer area and specific volume. It is claimed that the plate and shell heat exchanger will occupy only 20 to 30% of the footprint of equivalent capacity shell and tube types.
The maximum operating pressure of the plate and shell unit will also be higher.
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31. Stream Location (Rules of thumb)
more corrosive fluid goes tube-side
saves costs when using alloys, cheaper to construct tubes from alloys rather than the shell and tubesheet
higher pressure stream goes tube-side
small diameter tubes handle stress better than large diameter shells.
more severely fouling fluid goes tube-side
easier to clean tube-side using high pressure water lance, brushing, chemical cleaning, etc.
fluid with lower film coefficient goes shell-side
allows use of finned tubing to increase Aoho
fluid with low ΔPmax goes shell side
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