Summer ventilation: Fan efficiency and maintenance

Fan efficiency is commonly measured as cubic feet per minute (cfm) of air per watt (W) of electricity consumed to give a cfm/W number.  Fan efficiency is affected by several factors including blade design, housing design, and motor efficiency.  Therefore it can be said that “not all fans are created equal” and you often “get what you pay for”. 
 
If you are looking to buy new fans make sure you take a look at the efficiency rating of the fans as you compare. Typically larger fan will have better efficiency than smaller fans.  If installing fans in a tunnel ventilation system, look for fans with a minimum efficiency rating of 20 cfm/W at 0.05-inches static pressure.
 
Efficiency ratings for circulation fans are also available.  However, it is given as a Thrust Efficiency Ratio in terms of pounds of force per kilowatt of power (lbf/kW). Once again, larger fans tend to have a better efficiency. When shopping for circulation fans you should be looking for fans with a minimum rating of 21 lbf/kW.
 
Possibly the best thing you can do to improve the efficiency of your ventilation system is to simply maintain your fans. Poor maintenance —mostly lack of cleaning — can reduce efficiency by as much as 40%! What this means is the electric bill stays the same, but less air is moving in the barn. Those squeaking bearings, flopping belts, and dirty blades and shutters are really just robbing your power. Accumulation of as little as 1/8 inch of dirt on the fan blades can significantly reduce fan performance. Monthly fan maintenance and cleaning would be best, but at a minimum it should be done before start-up and twice during the summer.
 
To keep cows happy, healthy, and productive requires ventilation. In the summer this means lots of air to help in cow cooling, which leads to lots of fans. Making sure you choose well built and efficient fans and then regularly maintaining those fans goes a long way in helping to keep energy costs in check on the dairy.
 
For more about managing ventilation in your dairy barn, view the video at: http://www.das.psu.edu/dairy-alliance/education/technology
 
- By John Tyson, agricultural engineer, Penn State Extension Dairy Team, jtt107@psu.edu, 717-248-9618