Factors Affecting Performance

Cooling Range and Approach From chapter 1, range is defined as the temperature difference between water entering (CW return) and leaving (CW supply) the tower, while approach is the difference between the temperature of the water leaving and the wet-bulb temperature of air entering. The cooling tower efficiency is simply expressed as,Cooling tower efficiency = (CW return temperature - CW supply temperature) / (CW return temperature - Air wet bulb temperature) × 100%

Consequently,Cooling tower efficiency = Range / (Range + Approach) × 100%

From these, it can be seen that a cooling tower with a smaller approach is more efficient. Cooling towers usually have a 5 to 10⁰F approach. While a small approach is desired, investment cost may be impractical since the size of the cooling tower increases exponentially as the approach is being lowered.

Usually, the range and cooling water flow rate are the parameters being balanced. This is because the heat load is already given from consumer demand, and ambient air wet-bulb temperature may not be manipulated. Increasing the range will make the cooling tower efficient. This can be done by increasing the cooling water return temperature, or by lowering the cooling water supply temperature. In either of the cases, usually, one temperature is constant due to the requirement of end users. Of these two options, increasing the cooling water return temperature is more practical since the temperature difference between air and water in contact will be much larger. The larger the temperature difference, the more heat can be dissipated.

If the only option is to lower the cooling water supply temperature, the result will also lower the approach. In turn, the design will require a much larger tower.

Wet-bulb Temperature: This is a significant parameter for cooling towers relying on evaporative cooling. Design wet-bulb temperatures depend on existing site conditions. Thus, careful site surveys must be conducted, especially during summer months when the ambient temperature and relative humidity are high. A designer must consider publications from engineering and scientific organizations such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the National Oceanic and Atmospheric Administration for the unique, worst-case design conditions for a given location.

From the previous point, it is seen that a high ambient wet-bulb temperature will decrease the approach. Thus, at locations where there are high wet-bulb temperature conditions present, larger cooling towers are required for a given cooling load.

Consumer Heat Load: The size and cost of a cooling tower is proportional to the heat load. Cooling towers are usually designed using the maximum consumer heat load or cooling demand. They will then be the rated capacity of the cooling tower. However, there are times when there is low demand for cooling. In these cases, the tower will operate at a lower efficiency.

In order to save energy, one method is to use fan speed control. Heat transfer rate is increased by higher air velocity. If not much heat transfer or evaporative cooling is needed, fan speeds can be reduced. This can be done by using variable speed drive motors, two-three speed fan motors, and adjustable pitch fan blades. Another option is to design a multi-cell cooling tower. In this case, one cell may be on standby during off-hours or at times of low demand.