Heat Exchanger
- A device that facilitate the exchange of heat between two fluids that are at different temperatures
Applications of Heat Exchangers
- Chemical
reactors (Jackets)
- Preheating feeds
- Reboilers (Distillation column)
- Condensers (Distillation column)
- Air heaters for driers
- Crystallisers
- Heat transfer fluids
Types of Heat Exchangers
Direct contact heat exchanger:
o Two fluids are not separated
- Gas
bubbled through the liquid
- Liquid sprayed
in the forms of droplets into the gas
Indirect contact heat exchanger:
o Two fluids are separated
o Heat transfer through the metal surface from one fluid to another
Parallel /Counter/Cross Flow Heat Exchanger
Parallel Flow:
Both
the hot & cold fluids enter the heat exchanger at the same end and move in
the same direction
Counter Flow:
Hot
and cold fluids enter the heat exchanger at opposite ends and flow in opposite
directions
Cross Flow:
Hot and cold fluids usually move perpendicular to each other
Double Pipe Heat Exchanger
One fluid flows through the smaller
pipe
Other flows through the annular apace
Advantages:
o Simplest and Cheapest
o Used for high pressure applications
Disadvantages:
o Expensive for large duties
o Can’t be used in handling dirty fluids
o (Choking problem)(Used for only clean fluids)
Applications:
o Effluent cooling
o Pre heating
o Heat recovery
Compact Heat Exchanger
- Thin
plates or corrugated fins are attached closely spaced to walls separating
two fluids
- Commonly used in gas-gas and gas-liquid
Advantages:
o Large heat transfer surface area per unit volume
o Increase the H.T.C. of gas with increase surface area
o Lower cost
Disadvantages:
o Limited choice for high pressure
o Small passages likely to foul
Applications:
o Oil / water coolers
o Water / water coolers
o Condensers and evaporators
Shell & Tube Heat Exchanger
Advantages:
o
Extremely flexible & robust design
o
Easy to maintain & repair
o
Dismantled for cleaning
Disadvantages:
o
Relatively large size & weight
o
Not suitable for automotive &
aircraft
Applications:
o
Petrochemical: Processing &
Refining
o
Food & Beverage
o
Metals and Mining
o Pharmaceuticals
Plate & Frame Heat Exchanger
- Hot &
Cold fluids flow in alternate passages
- Suited
for liquid-to-liquid heat exchange
- Hot &
cold fluid streams at about same pressure
Advantages:
o Effective heat transfer (turbulence on both sides)
o Low cost because plates are thin
o Can easily be opened up for inspection and cleaning
o Less fouling
Disadvantages:
o
Gaskets may not be suitable for organic solvents
o
Usually not considered for refinery duties (Cannot withstand prolonged
fire)
Applications:
o
Ethanol and Corn Processing
o
Industrial Energy
o
Power Plants
o Food & Beverage
Regenerative Heat Exchanger
Static type:
o
Hot & cold fluid flow through the mass
alternatively
o
Heat transferred from hot fluid to the matrix
o
Heat transferred from matrix to the cold fluid
Dynamic type:
- Rotating drum & continuous flow of
hot & cold fluid
- Periodic passing of drum through:
- Drum
serves as medium to transport of heat
Advantages:
o Simple design
o Porous mass having large heat storage capacity
Disadvantages:
o Mixing of the fluid streams (separation problem)
o Constant heating and cooling puts a lot of stress cause cracking or breakdown of materials
Applications:
o Heat recovery from exhaust gases
o Air conditioning applications
o In food industry
o Temperature control of sewage sludge
Spiral Heat Exchanger
Composed of two concentric spiral
channels
Advantages:
o Applied where fouling and plugging are problems
o Ease of maintenance
Disadvantages:
o Higher initial cost
o Maximum design temperature is 400oC (Special designs: operate up to 850oC)
o Maximum design pressure is usually 15 bar (Special designs with pressures up to 30 bar
Applications:
o Food industry
o TiCl4 cooling
o PVC slurry duties
o Oleum processing
o Temperature control of sewage sludge
Fouling of Heat Exchanger
- Deposition
of undesirable substance on the heat transfer surfaces.
- Increases
the overall thermal resistance and lowers the overall heat transfer
coefficient of heat exchangers.
- Impedes
fluid flow, accelerates corrosion and increases pressure drop across the
heat exchanger
- Strangely more heat exchangers are opened for cleaning due to excessive pressure drop than for an inability to meet the heat transfer requirement.
References
1. Cengel, Y. A. Heat Transfer: A Practical Approach, 2nd Edition, McGraw-Hill.
2. Bergman,
T. L.; Lavine, A. S.; Incropera, F. P.; DeWitt, D. P. Fundamentals of Heat and
Mass Transfer, Seventh Edition, Wiley.
3. J. P.
Holman, Heat Transfer. Sixth Edition, McGraw-Hill Book Company.
4. McCabe W.
L.; Smith, J. C.; Harriott, P. Unit Operations of Chemical Engineering. Fifth
Edition, McGraw-Hill International Editions.
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