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Fan selection and maintenance

Kevin A. Janni, Professor and Extension Engineer
April 26, 2014

Do you know how much airflow your hot weather fans move? Independently rated 48 inch diameter fans can move between 14,600 and 26,800 cubic feet per minute (CFM) at 0.10 inches of water static pressure (in SP). Fan diameter does not determine airflow rate. If you have a tunnel ventilated barn with 50 dry cows that needs 52,800 CFM of airflow (30 ft wide x 8 ft tall x 220 ft per minute = 52,800 CFM), you need four of the fans that move 14,600 CFM. You need only two of the fans if they move 26,800 CFM.

When selecting agricultural fans for use in dairy barns there are several factors that you and your equipment supplier need to consider including: the airflow rate between 0.10 and 0.15 in SP, ventilation efficiency ratio (VER), dealer service and support, fan reliability, useful life, purchase price and operating cost.

The University of Illinois BESS lab conducts independent third party testing of agricultural fans. Test results are available online at Reports give the make, model and manufacturer as well as a complete description of guards, shutters and discharge cones used during the fan test. Reports also give airflow rates in cubic feet per minute (CFM), motor speed in revolutions per minute (RPM), electricity use in watts (W), and VER in cubic feet per minute per watt (CFM/W) at different static pressures.

Static pressure describes the resistance to airflow that a fan must overcome. Things that produce resistance to airflow include wind blowing at the fan outlet, inlets, baffles in tunnel or cross ventilated barns, fan shutters and guards. The goal is to manage the airflow resistances and select fans that can overcome the expected resistances and provide the airflow needed for the animals in the barn. The general recommendation is to select fans, tested with shutters, guards and discharge cones in place, based on their airflow at 0.125 in SP. That static pressure allows the barn to have between 0.05 and 0.10 in SP between inside and outside the barn and to overcome the resistance to a 10 mph wind blowing against the fan. A 10 mph wind can produce about a 0.05 in SP resistance. Higher winds blowing against a fan can produce greater flow resistance and reduce airflow.

I was at a mechanically ventilated barn on a very windy day and saw the wind close the shutters while the fan was running. The fan was running and using electricity but the fan was not providing any ventilation. On that particular day, that barn was ventilated by fans on the other side but the barn was not performing as desired. Deflectors can be built to reduce the wind effect if wind blowing against exhaust fans is a major problem.

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.

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