how to calculate cooling tower fan cfm

Cooling Tower Fan Airflow (CFM) Calculation Guide

In the design and operation of cooling tower systems, * * fan air volume (CFM, Cubic Feet per Minute) * * is an important indicator for measuring air flow capacity. Reasonable calculation of fan air volume can not only ensure efficient heat exchange in cooling towers, but also save energy consumption and extend equipment life.

The concept of CFM

CFM represents the volume of air that a fan can deliver per minute. For cooling towers, the fan CFM directly determines the flow rate and cooling efficiency of air passing through the packing. If the air volume is insufficient, the water temperature cannot be lowered; Excessive air volume can increase energy consumption and may lead to an increase in floating water.

Basic formula for calculating CFM of cooling tower fan

The calculation of CFM for cooling tower fans is mainly based on the required air volume and heat transfer load of the cooling tower. The commonly used formulas are as follows:

CFM=3.17×Q×10⊃3;/ΔT×Pair

Among which

CFM: Fan Airflow (cubic feet per minute)

Q: Cooling water flow rate (gallons per minute, GPM)

Δ T: Water temperature drop (℃ or ℉, corresponding to the formula unit)

Pair: Air density (lb/ft ⊃3;)

3.17: Unit conversion factor, used to unify the units of water flow, temperature difference, and air flow

Simplified calculation method

In engineering practice, to quickly estimate the CFM of cooling tower fans, the following empirical formula can be used:

CFM≈1.08×GPM×ΔT

Among which

GPM: Circulating water flow rate (gallons per minute)

ΔT: Water temperature drop (℉)

1.08: Empirical coefficient, including specific heat of water and air and unit conversion

Calculation precautions

1.Consider changes in air density:

In high temperature, high humidity, or high-altitude areas, changes in air density can affect CFM and require correction.

2. Fan efficiency: 

The actual efficiency loss of the fan (usually 70-85%) needs to be considered when selecting.

3. Wind resistance and pressure drop: 

The resistance of the tower fill, float eliminator, and air duct will increase the fan load, and CFM needs to be adjusted according to the total pressure.

4. Safety margin: 

It is recommended to increase the margin by 5-10% on the basis of calculating CFM to cope with environmental changes and fan aging.