HEAT EXCHANGER

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HEAT EXCHANGER

• By
• Farhan Ahmad
• Department of Chemical Engineering,
• University of Engineering Technology Lahore

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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

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

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Double pipe heat exchanger

• Normal size
• Double-pipe heat exchangers are competitive at
  duties requiring 100-200 ft2
• Built of carbon steel where possible

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Advantages/disadvantages of double-pipe HE

• Advantages
• 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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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 e 0.9
• Minimum ?T 5 K

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Shell and tube heat exchanger

• Size per unit 100 - 10000 ft2 (10 - 1000 m2)
• Easy to build multiple units
• Made of carbon steel where possible

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Advantages/disadvantages of ST

• Advantages
• 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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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 e 0.9 (less with multipass)
• Minimum ?T 5 K

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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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Advantages of plate and frame HE

• High heat transfer - turbulence on both sides
• High thermal effectiveness - 0.9 - 0.95 possible
• Low ?T - down to 1K
• Compact - compared with a ST
• 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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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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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 t0 2000C 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 e 0.95
• Minimum ?T 1 K

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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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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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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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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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Comparison with Shell and Tube Heat Exchanger

• As for gasketed plate and frame units.

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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

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Scope of plate-fin exchanger

• Max. Pressure 90 bar (size dependent)
• Temperatures -200 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 ?T Down to 0.1oC
• Maximum ?T 50oC typical
• High e Up to 0.98
• use only with clean fluids

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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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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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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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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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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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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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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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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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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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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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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 ?Pmax goes shell side