Cooling tower windage loss

The rate of evaporation from a cooling tower is approximately 1 per cent of the circulation rate for each 5°C drop in temperature across the tower, or about 7 liters/h per ton of refrigeration. Windage losses will obviously depend on the prevailing wind conditions and the design of the tower with regard to spray elimination but, typically, these are about 0.2 per cent of the circulation rate. 

Evaporation losses are 1 % of the circulation for every 100 F of cooling range. Windage or drift losses of mechanical draft towers are 0.1-0.3%. Blowdown of 2.5-3.0% of the circulation is necessary to prevent excessive salt buildup. 

The makeup water requirement is, in turn, the sum of the evaporation rate plus water losses. Water losses are drift or windage (which, in modern cooling towers, is minimized by efficient drift eliminators), deliberate bleed or purge of water (normally to maintain cycles of concentration within a specified range), and any leaks. Thus for any reasonably accurate calculation concerning the quantity and costs of chemical treatment, it is essential to have factual information on the makeup and/or evaporation rate of a cooling system and also on the cycles of concentration, in addition to various water analyses. 

Make appropriate assumptions about windage and evaporation losses and set out and solve an equation for blowdown. Windage losses will be about 1.0 to 5.0 percent for spray ponds, 0.3 to 1.0 percent for atmospheric cooling towers, and 0.1 to 0.3 percent for forced-draft cooling towers for the forced-draft towers in this example, 0.1 percent can be assumed. As for evaporation losses, they are 0.85 to 1.25 percent of the circulation for each 10-degree drop in Fahrenheit temperature across the tower it is usually safe to assume 1.0 percent, so E = AT/10, where AT is the temperature drop across the tower. Therefore, in the present case,... 

Evaporation losses are 1% of the circulation for every 10°F of cooling range. Windage or drift losses of mechanical draft towers... 

For industrial water cooled by recirculation through a spray or tray-type tower, chromates are the most reliable from the standpoint of efficient inhibition. However, the critical concentration is high and as the sulfate and chloride concentrations increase through evaporation of the water, chromates tend to cause pitting or may cause increased galvanic effects at dissimilar metal couples. Windage losses (loss of spray by wind) must be carefully avoided because chromates are toxic. Toxicity also makes it difficult to dispose of chromate solutions whenever it becomes necessary to reduce the concentration of accumulated chlorides and sulfates. 

If chromate cannot be used because windage losses or disposal cause problems due to the toxicity of chromate, industrial cooling towers can be inhibited with about 50 ppm of sodium hexam-etaphosphate. Addition of a soluble zinc salt often improves inhibition by polyphosphates. Pitting and excessive scale formation are prevented by maintaining the pH in a range 6 to 7. 

---