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 engineering-resource.com

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 engineering-resource.com

Double pipe heat exchanger

• Normal size Double-pipe heat exchangers are competitive at duties requiring 100-200 ft 2 • Built of carbon steel where possible engineering-resource.com

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) engineering-resource.com

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 600 o C (-150 to 1100 o F) – possibly wider with special materials

Fluid limitations

– Few since can be built of many metals Maximum ε = 0.9

Minimum ΔT = 5 K engineering-resource.com

Shell and tube heat exchanger

• • • Size per unit 100 - 10000 ft 2 (10 - 1000 m 2 ) Easy to build multiple units Made of carbon steel where possible engineering-resource.com

Advantages/disadvantages of S&T

• •

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 200 o C (400 o F) engineering-resource.com

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 600 o C (-150 to 1100 o F) – 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 engineering-resource.com

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 engineering-resource.com

Advantages of plate and frame HE

• • • • • • • • • High heat transfer - turbulence on both sides High thermal effectiveness - 0.9 - 0.95 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 engineering-resource.com

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) engineering-resource.com

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 +175 0 C normal (-13 to +350 0 F) – -40 t0 +200 0 C special (-40 to +390 0 F)

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 engineering-resource.com

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 engineering-resource.com

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 m 2 /m 3 of volume but can be 1800 m 2 /m 3 • Main use in cryogenic applications (air liquifaction) • Also in stainless steel engineering-resource.com

• • • • • • •

Scope of plate-fin exchanger

• • Max. Pressure Temperatures Fluids Duties Flow configuration Multistream Low ΔT Maximum ΔT High ε 90 bar (size dependent) -200 to 150 o C in Al Up to 600 with stainless Limited by material Single and two phase Cross flow, Counter flow Up to 12 streams (7 normal) Down to 0.1

o C 50 o C typical 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: engineering-resource.com

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 engineering-resource.com

Operating Limits

 Maximum design temperature is 400 o C set by the limits of the gasket material.

 Special designs without gaskets can operate with temperatures up to 850 o C.

 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 coefficients.

efficient with higher heat transfer Small hold up times and volumes.

Removal of one cover exposes the total surface area of one channel providing maintenance.

easy inspection cleaning and engineering-resource.com

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 engineering-resource.com

Operating Limits

• The maximum operating temperature of a plate and shell heat exchanger is 900 o C • 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 engineering-resource.com

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 A o h o fluid with low ΔP max goes shell side engineering-resource.com