Fan care for efficient ventilation
Professor and Extension Engineer
April 26, 2011
Fans are an important tool for minimizing dairy cow heat stress. Fans are used to ventilate tunnel-ventilated barns and cross-ventilated barns. They are also used to mix air and provide cooling in naturally ventilated freestall barns and compost barns. Now is a good time to inspect your fans so that they are ready to run efficiently this summer.
The number of fans in dairy barns has increased in recent years. Cross-ventilated barns can have 30 to 90 or more large fans depending on the barn size. Tunnel ventilated fans can have 4 to 40 large fans. Naturally ventilated barns can have more than 50 axial mixing fans, depending on the barn size. Whether you have 10 fans or 100 fans, proper care will help them run efficiently.
Cross-ventilated and tunnel-ventilated barns commonly use 48 to 56-inch diameter fans with 1 to 2 horsepower (hp) motors. Axial mixing fans in naturally ventilated barns are commonly 36 or 48-inch diameter with ½ to 1 hp motors.
If you have 50 fans with 1 hp motors running continuously, you can easily use between 42 and 63 kilowatt-hour (kWh) or more of electrical power depending on the motor efficiency. Assuming 10 cents per kWh, it can cost $4.20 to $6.30 per hour to run the 50 fans or $3,000 to over $4,500 per month if running continuously all month. The lower electrical cost assumed a 90% efficient motor and the higher electrical cost assumed a 60% efficient motor. Purchase efficient fans and motors to reduce electrical operating costs.
The amount of airflow delivered depends on fan performance. You can find performance information on many agricultural fans on the Bioenvironmental and Structural System (BESS) Laboratory website at http://bess.illinois.edu/. The laboratory is operated by the Agricultural and Biological Engineering Department at the University of Illinois. They conduct fan performance tests on agricultural fans and report airflow rates and airflow per watt (cubic feet per minute (CFM) per watt) at several static pressure conditions. This information can be used to find efficient axial agricultural fans.
Numerous 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. The reduced fan performance in actual barns is attributed to slipping belts on belt-driven fans, corroded or dirty louvers that do not open fully when the fans run, insufficient or plugged fresh air inlets, or other airflow obstructions.
When loose belts slip, the motor runs and uses electricity but the fan blades do not turn at the desired revolutions per minute (rpm) and the airflow is less than expected. Tighten loose belts but avoid over tightening them.
Ventilation fans in tunnel-ventilated and cross-ventilated barns have shutters that open when the fans run and close when they do not run. Dirty, corroded or damaged shutters that are hard to open or do not open fully will reduce fan airflow. If the shutters restrict a fan's airflow other fans will turn on and run to provide the airflow needed to ventilate the barn. Electricity used to run additional fans is an extra expense that could be avoided by cleaning and lubricating shutters. Use graphite to lubricate the shutters; oil will collect dust and dirt. Repair or replace broken or damaged shutters.
Tunnel-ventilated and cross-ventilated barns need sufficient air inlet openings to match the airflow being blown out by the fans. As temperatures rise and more fans turn on to increase airflow, either more inlet area is needed or the air velocity through the inlets must increase. Insufficient inlets and excessive inlet air velocities restrict airflow and starve the exhaust fans for air. In this case, the fans run and use electricity but they cannot efficiently exhaust air because the inlets are restricted. Inlets in tunnel ventilated barns are sized to provide 2.5 square feet (ft2) of inlet opening per 1000 CFM of airflow. Check summer inlets and make sure that they can open fully before warm weather hits.
Many cross-ventilated barns have evaporative cooling pads that serve as air inlets on one side of the barn opposite the fans. They are sized to provide 5 ft2 of pad per 1000 CFM. Evaporative pads can become dirty or plug with algae. The pads can be inspected to find plugged areas. Plugged areas can be either cleaned or replaced. Algae growth can be managed by adding chemicals to the water that wets the evaporative cooling pad. Follow pad supplier recommendations.
For safety, fans should have guards or shutters on both the inlet and exhaust side to prevent children, workers and animals from access the rotating blades. Turn off the power to fans before you service or repair them.
Exhaust fans on a cross-ventilated barn.