84-11: Determination of Fan Flow and Water Rate Adjustment for Off-Design

The determination of the performance of a mechanical draft-cooling tower requires that the air mass flow through the tower be known. Since this flow is not measured, it has been customary to use the manufacturer's design airflow and adjust it by the on-third power of the ratio of the design fan horsepower to test fan horsepower. This basic adjustment is inadequate for today's technology and needs to be improved. The most nearly correct approximation of airflow through a tower can be obtained by incrementally moving through the tower from air inlet to outlet while calculating mass flows, energy balances, and pressure drops for each increment and then utilizing fan curves to determine volumetric and mass flows. This procedure would account for changes in hair humidity and density through the tower, evaporation of water, and effect of water rate on air pressure drop, and changes in fan characteristics. Unfortunately, this is an interactive numerical process that requires considerable computer capabilities, a data bank of the pressure drop characteristics of tower components, and fan characteristics. These type calculations maybe within the capabilities of all in the near future, but for now a more elementary approach is recommended, using a calculator. This approach depends on assumptions that are acceptable if the tower test is conducted within CTI code requirements. The fan must be considered a constant suction volume blower for a given blade pitch. The total pressure at the fan, a function of volumetric flow, must be assumed to be unaffected by other considerations and the fan horsepower must be assumed to change only as mass flow and wet air density changes.