This section is concerned with towers where waste heat is removed from cooling water by direct contact with atmospheric air. Such towers are described in Section 3.12 of HEDH, which also gives the basis of thermal design. Operational problems are discussed below, for large natural draught towers, described in Section 328, and for the usually smaller mechanical draught towers, described in Section 329.

A. Testing

Some general problems in the testing of heat exchangers are listed in Section 367J. Problems specific to water cooling towers are discussed in #%SECTION_3.12.6_A%#, which also deals with acceptance testing. The most difficult problem is to measure the velocity and temperature of the air at various locations, as it leaves the tower. Similar problems arise in the testing of air-cooled heat exchangers, as described in Section 368G, but with direct-contact cooling, the problem is made worse by the fact that the air at outlet is saturated with water vapour and contains droplets. In order to obtain a heat balance and check the thermal performance, it is necessary to determine the temperature and the increase in the enthalpy of the air at many points across the area where the air leaves the tower. In view of these difficulties, complete tests on water cooling towers are rare.

Water cooling towers are designed to give their specified performance under conditions when the wet and dry-bulb temperatures of the ambient air are so high that they are likely to be exceeded on only a few days per year. The manufacturer should be asked to supply performance curves which show the mean recooled water temperature at various flow rates and inlet temperatures of the water, with a wide range of atmospheric conditions. The performance of the tower can then be checked in Winter and Spring, and, if it is below what is expected, the cause can be investigated and rectified before the hot Summer weather reduces the performance of the tower. The most likely causes of poor performance are disintegration of the packing, maldistribution of the water and maldistribution of the air; the 2nd and 3rd of these are discussed in the next sub-sections.

B. Maldistribution of water