Another key fan selection factor is the ventilation efficiency rating which give the airflow per watt of electrical energy used (CFM/Watt). VER values for 48 inch diameter BESS lab tested fans range from 12.7 to 23.4 CFM/Watt. VER is more important when selecting fans that run more hours.
For example, if you have the choice between two fans that move 21,500 CFM but with different VER values, either 13.3 or 20.6 CFM/W, and you expected the fan to run continuously for 90 days or 2160 hours in the summer, the lower VER rated fan would use $349 of electricity while the higher VER rated fan would use $225 of electricity (assumed $0.10 per kWh electricity). If the fans run more hours, the more energy efficient fans save more in electrical costs.
Another way to save money is to maintain and clean all ventilating fans monthly. Research studies that measured agricultural fan performance in dairy, poultry and swine barns report airflow rates in the field are between 40 to 80% of the BESS laboratory values. You cannot provide adequate hot weather ventilation with these poor performance ratings and more fans would be needed to provide the ventilating rate required. Reduced fan performance in the field is attributed to slipping belts on belt-driven fans, corroded or dirty shutters that do not open fully when the fans run, insufficient or plugged fresh air inlets, or other airflow obstructions caught in screens. Use graphite when lubricating fan shutters. Oil will catch dirt and lose its lubricating properties.
When loose belts slip, the motor runs and uses electricity but the fan blades do not turn at the desired RPM and airflow is less than expected. Tighten loose belts but avoid over tightening them. Turn off the power to fans before you service or repair them.
Fans should have guards or shutters on both the inlet and exhaust side to prevent children, workers and animals from access the rotating blades.
Exhaust fans on a mechanically ventilated barn.