Air cooled exchangers are usually composed of rectangular bundles containing several rows of tubes on a triangular pitch. Heat transfer is generally counter current, the hot fluid entering the top of the bundle and air flowing vertically upward through the bundle.
Since air is a universal coolant, there are numerous applications where economic and operating advantages are favourable to air-cooled heat transfer equipment. However, applications are limited to cases where the ambient air-dry bulb temperature is below the desired cooling or condensing temperature.
Where expensive or insufficient water supplies are encountered or where cooling water pumping or treating costs are excessive, it is often found that air-cooled units are desirable for several services. The adverse conditions of high relative humidity or excessive space requirements occasionally create high costs or installation difficulties for cooling towers. In some of those cases, air-cooled heat transfer equipment offers a satisfactory solution.
Full consideration should be given to adequate winter protection of air-cooled units installed in cold climates. It is essential that all possibilities of freeze-up be eliminated and external recirculation of hot air is necessary for severe winter conditions when the unit is subject to freezing and heating coils provided for protection against freeze-up shall be in a separate bundle and not part of the process tube bundle.
If the fluid being handled is subject to wide variations in viscosity over the range of atmospheric temperatures encountered, provisions must be made to control the extent of cooling at the lower ambient air temperatures.
Bundles may be fabricated in widths to 3.65 m (12 ft) and depths to 8 rows. Standard bundles are available in lengths of 2.44 m (8 ft), 3.05 m (10 ft), 4.57 m (15 ft), 6.07 m (20 ft), 7.31 m (24 ft), 10.36 m (34 ft) and 12.2 m (40 ft). Usually, the maximum dimensions are dictated by shipping requirements. Bundles may be stacked, placed in parallel, or in series, for a given service. Also, several small services may be combined in one bay.
In general, the longer the tubes and greater the number of tube rows, the less expensive the surface on a square meter basis.
In moderate climates, air cooling will usually be the best choice for minimum process temperatures above 65°C, and water cooling for minimum process temperatures below 50°C. Between these temperatures a detailed economic analysis would be necessary to decide the best coolant. It is recommended vendors consider installation of air fan coolers on pipe racks