Compact ( Plate-Fin ) Exchangers
Compact exchangers are used primarily for gas service. Typically they have surfaces of the order of 1200 m2/m3, corrugation height 3.8~11.8 mm, corrugation thickness 0.2~0.6 mm, and fin density 230~700 fins/m. The large extended surface permits four times the heat transfer rate per unit volume that can be extended surface permits about four times the heat transfer rate per unit volume that can be achieved with shell-and-tube construction. Units have been designed for pressures up to 80 atm or so. The close spacing militate against fouling service. Commercially, compact connection with gas turbines. For mobile units, as in motor vehicles, compact exchangers have the great merits of compactness and light weight. Any kind of arrangement of cross and countercurrent flows is feasible, and three or more different streams can be accommodated in the same equipment. Pressure drop, heat transfer relations, and other aspects of design are well documented.
Air Coolers
In such equipment the process fluid flows through finned tubes and cooling air is blown across them with fans. The economics of application of air coolers favors services that allow 25~40℉temperature difference between ambient air and process outlet. In the range above 10Mbtu/hr, air coolers can be economically competitive with water coolers when water of adequate quality is available in sufficient amount.
Double-Pipe Exchangers
This kind of exchanger consists of a central pipe supported within a larger one by packing glands. The straight length is limited to a maximum of about 20 ft; otherwise the center pipe will sag and cause poor distribution in the annulus. It is customary to operate with the high pressure, high temperature, high density, and corrosive fluid in the inner pipe and the less demanding one in the annulus. The inner surface can be provided with scrapers as in dewaxing of oils or crystallization from solutions. External longitudinal fins in the annular space can be used to improve heat transfer with gases or viscous fluids. When greater heat transfer surfaces are needed, several double-pipes can be stacked in any combination of series or parallel.
Double-pipe exchangers have largely lost out to shell-and-tube units in recent years. They may be any combination of series or parallel.
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