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TYPES OF HEAT EXCHANGER
(Note: Following are the 'Types' of heat exchanger used in industry and may be used for any of the purposes listed previously in exchanger names).
1.FIXED HEAD or FIXED TUBE-SHEET EXCHANGER:
This is a 'Shell & Tube' type exchanger in which the tube-sheet ends are fixed (rigid), with no allowance for expansion. They are mainly used for low temperature heat exchange with low differential temperature (.T), between the heat exchange fluids.
The Tube-sheets, one at each end, consist of metal plates into which a bundle of tubes are fixed by 'Rolling' or 'Welding'. The length of the tubes is prevented from bending by 'Transverse Baffles', each of which hold half of the bundle. These baffles also provide channels for the shell-side fluid direction to constantly change and flow across the tubes a number of times.
(See Figure: 21)
2. FLOATING HEAD EXCHANGER:
In this type, also a shell & tube exchanger, one end of the tube-sheet (the 'Bell-end'), has provision made for expansion and contraction of the tubes. They are generally used for high .T between the fluids.
(See Figure: 22)
3. THE ' U ' - TUBE HEAT EXCHANGER:
Here the tubes are formed into a horizontal ' U' shape and held by a single tube- sheet. This gives the fluid passing through the tubes a 'Double Pass' through the exchanger.
(See Figure: 23)
4. THE DOUBLE-PIPE EXCHANGER:
This is simply two pipes, one placed inside the other. The outer pipe will carry one of the process streams and the inner pipe carries the other stream. The pipes may be fitted with 'Fins' to increase the heat transfer rate.
5. THE AIR-FIN (OR FIN-FAN) HEAT EXCHANGER:
An air-fin exchanger is, in principle, exactly the same as the radiator of a car engine.
The fluid to be cooled passes through the tubes while air is blown (forced draught), or pulled (induced draught), over the tubes by a fan or fans causing cooling of the fluid. The tubes may be plain or finned depending upon process requirements. Cooling control can be either by variable pitch fan blades or by variable speed or on/off motor, all of which will vary the amount of air flow across the tubes.
Photo of an Air-Fin Cooler as used in Industry
In the operation of a heat exchanger, the type of fluid flow through the equipment plays a big part in the heat transfer rate. For better heat transfer, a turbulent flow is preferred which will prevent a 'skin effect' - a layer of cooler, slower moving fluid on the inside surface of the tubes. With laminar (streamline) flow, such a layer will decrease the heat transfer rate.
(See Figure: 26 below)
Another factor in the heat transfer rate, is the number of 'Passes' made by the fluids both in the shell side and the tube side.
In the shell side, as explained earlier, the 'Transverse Baffles' will cause the shell fluid to pass across the tubes a number of times. In some exchangers, shell-side passes can be increased by 'Longitudinal' baffles. The baffles increase the time taken by the fluid to pass through the shell and, at the same time, will cause turbulent flow. The heat transfer rate is therefore increased.
In the tube side, the number of passes is increased by Baffle plates in the 'Channel Head'.
(See Figure: 27)
A further consideration is the flow of each fluid. Generally, the most efficient heat transfer is achieved by 'Counter-current Flow' - i.e. the two fluids are flowing in opposite directions through the exchanger.
However, certain operating requirements may call for the fluid flows to be 'Co-current' - i.e. the two flows are in parallel and in the same direction through the exchanger.
(Figures : 28, 29 & 30)