COOLING TOWER CYCLES OF CONCENTRATION

As water is evaporated, minerals are left behind in the recirculating water. As evaporation continues, the water becomes more concentrated than the original make up water. This eventually can lead to saturated conditions. The term cycles of concentration compares the level of solids of the recirculating cooling tower, to the level of solids of the original raw make up water. If the circulating water has four times the solids concentration than that of the make up water, then the cycle of concentration (COC) is 4.

Since chlorides can be easily measured in a sample of water, usually the cycle of concentration is defined as,

Cycle of concentration = Chlorides in tower water / Chlorides in make-up water

The factors on which cycle of concentration depends are:

a) Solid concentration in the make-up water – this would be available from the make-up water chemical analysis.

b) Maximum recommended solid concentration in the cooling tower – cooling tower manufacturers recommend the upper limits of solid concentration depending on the type of cooling tower fill. There is no upper limit for cooling towers with splash-type fills. For cooling towers with film-type fills, the recommended concentration varies with the fill density.

c) Overall water balance of the plant and type of ash disposal system – it has been seen that for a wet-type ash disposal system, a COC of 3 to 4 is ideal from overall water balance point of view. However, if the plant has dry-ash disposal, a higher COC can be recommended.

d) Cost and availability of water – In areas where water is scarce, a higher COC may be economically more feasible, even with the associated higher chemical cost.

It may be noted, that the percentage of make-up water does not change significantly for COC beyond 4.5.

The increase in COC may adversely affect the overall heat transfer coefficient of the condenser tubes, leading to a decreased output, due to scaling. However, proper selection of chemicals and installation of on-line tube cleaning equipment can mitigate this effect.