Common Cooling Tower Problems – Interference, Recirculation and Legioellosis

Interference and Recirculation:

A number of cooling towers may have to be used if the hot water load is large. The tower are very often installed at one place in the plant for convenience. Interference and recirculation are two common problems with cooling towers placed in proximity. Some of the moist air leaving a tower may be sucked into another tower installed “downwind” in the vicinity of the former [Figure 1(a)] affecting the performance. This is called “interference”. Sometimes a part of the moist air from a tower may be sucked into itself by the “forced-draft fan”. This is “recirculation” [Figure 1(b)]. The ratio of the “plume velocity” to the “wind velocity” is an important factor to determine the extent of recirculation. If this ratio is small, a forced-draft tower becomes more susceptible to recirculation [Figure 1(c)]. Both the phenomena of interference and recirculation reduce the enthalpy driving for cooling and adversely affect the performance of cooling towers.

Figure 1 (a) Interference (b) and (c) recirculation.

The Problem of Legioellosis:

A cooling tower offers an environmental congenial for the growth of a rod-shaped bacteria called legionella pneumophila that causes respiratory disease. Infection may be caused by mist of drift from a cooling tower. The disease is reported to have claimed lives in many places. Cooling tower water must be tested periodically for the presence of this bacteria. Dosing of ozone, hydrogen peroxide, or the use of a UV source have all been found to be useful to control the growth of this bacteria. Also, a cooling tower must be cleaned periodically and stagnant pockets of water must be avoided. In some countries a code of practice (COP) for cooling tower operation has been introduced to check the problems of bacterial growth, plume, etc.

Figure 2 Legionella Pneumophila.

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