Aerial coolers are often used to cool a hot fluid to near ambient temperature. They are mechanically simple and flexible, and they eliminate the nuisance and cost of a cold source. In warm climates, aerial coolers may not be capable of providing as low a temperature as shell-and-tube exchangers, which use a cool medium. In aerial coolers the tube bundle is on the discharge or suction side of a fan, depending on whether the fan is blowing air across the tubes or sucking air through them. This type of exchanger can be used to cool a hot fluid to something near ambient temperature as in a compressor interstage cooler, or it can be used to heat the air as in a space heater.
When the tube bundle is on the discharge of the fan, the exchanger is referred to as “forced draft.’1 When the tube bundle is on the suction of the fan it is referred to as an “induced draft” exchanger. Figure 3-14 shows a typical air cooled exchanger, and Figure 3-15 shows a detail of the headers and tube bundle. In Figure 3-15 the process fluid enters one of the nozzles on the fixed end and the pass partition plate forces it to flow through the tubes to the floating end (tie plate). Here it crosses over to the remainder of the tubes and flows back to the fixed end and out the other nozzle. Air is blown vertically across the finned section to cool the process fluid. Plugs are provided opposite each tube on both ends so that the tubes can be cleaned or individually plugged if they develop leaks, The tube bundle could also be mounted in a vertical plane, in which case air would be blown horizontally through the cooler.
Forced-air exchangers have tube lengths of 6 to 50 ft and tube diameters of % to IM-in. The tubes have fins on them since air is non-fouling and it has a very low heat transfer efficiency. The fins increase efficiency by effectively adding surface area to the outside surface of the tubes. Some of the typical sizes of air cooled exchangers are shown in Table 3-5.
In a single aerial cooler there may be several fans and several tube bundles as shown in Figure 3-16, which defines bay width, tube length, and number of fans. Typically, on a compressor cooler there may be many tube bundles—one for cooling the gas after each stage, one for engine cooling water, one for lube oil, etc.