The water-cooled condensers, cooling towers and evaporative condensers described in Sections 6.4, 6.5 and 6.6 all use water at a temperature which will promote the growth of the bacterium Legionella pneumophila. While cor­rect water treatment and other precautions are 100% safe against this trouble, users may prefer to avoid any risk by using a sealed water system. Such heat exchangers are generally termed dry coolers. They are of similar construction to air cooled condensers, illustrated in Figure 6.2.

The fluid is circulated through the tubes of the heat exchanger, while blow­ing air over the outside. The tubes will usually have an extended surface.

The use of dry coolers cannot take advantage of the lower cooling tempera­tures available by the evaporation of the cooling water, and are limited by the ambient dry bulb temperature, rather than the wet bulb. Higher power is there­fore required, and a given size compressor will perform less cooling duty.

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