The airflow through mechanical draft cooling towers can determined/specified in two different ways:
- Firstly, the fan curve can be entered into the software by specifying up to a sixth order polynomial. This case is used when the fan is known. The polynomial (equation) that gives the fan static pressure vs volumetric flow can be entered into the software. The fan curve should be given by the fan manufacturer. The fan curve is only valid for one specific fan configuration, i.e. if the number of fan blades or the blade angle changes then a new curve should be entered into the software for the new configuration. Some fan manufacturers provide software with their fans which generate fan curves for all possible fan configurations. The polynomial that represents the fan curve should be determined by the user by using a curve fitting procedure. Microsoft Excel is commonly used for curve fitting. By using the fan curve the software will determine the operating point of the cooling tower by iteratively solving the draft and energy equations. The fan speed or other cooling tower parameters, like the fill height, can then be varied to achieve the required cooling load.
- The second approach is to specify the air flow through the cooling tower that will achieve the required cooling load. The pressure drop through the cooling tower is calculated by the software. The pressure drop and flow rate can then be used to specify a fan that will meet all the geometrical and operational requirements