Ventilation

Ventilation – A 2003 survey of New York state dairy farms concluded that 22% of the electrical energy is used for ventilation on either tiestall or freestall housing.

Exhaust Fans

Electrical energy for exhaust fans can be reduced by purchasing higher efficiency fans. The difference between a standard fan and an energy efficient fan can result in a 20-30% energy savings. Generally larger fans are more energy efficient. You can’t see the difference but independent third party test results are available at the BESS lab web site so more informed decisions can be made. The chart below illustrate the potential energy savings and the recommended minimum efficiency (4th column) to look for when purchasing an exhaust fan in units of cubic feet per minute (cfm) per watt or air moved per unit of energy.

Energy efficiency for exhaust fans.
Fan Diameter (inches) Standard efficiency cfm/watt @ 0.05″ SP High Efficiency cfm/watt @ 0.05″ SP Recommended Min. Efficiency cfm/watt @ 0.05″ SP
12 7.3 9.1 7.5
14 8.6 13 12
16 9.9 13.6 12.5
18 10.1 13.4 13
20 9.7 12.1 11.5
24 10.9 16.5 15
30 11.7 14.6 14
36 16 20.4 19
42 16.5 21.7 19.5
48 18.5 23.3 21
50-53 20.3 25.1 23
54 20.5 26.7 24

Refer to UW Extension bulletin A3784-06 “Ventilation and Cooling Systems for Animal Housing”

Circulation / Mixing Fans

Freestall barns and loose housing in Wisconsin have typically been built as naturally ventilated barns with the option of installing circulating fans for the hot days. If fans are added, it is common to install one or two rows of high speed fans per group of animals; one row along the feed alley blowing over the animal’s backs and one row over the cow beds in the center of the group. Usually, 36″ or 48-50″ fans are used installed every 30 or 40 feet, respectively. New technology can reduce the energy cost.

High-Volume, Low-Speed (HVLS) Fans

HVLS fans entered the market about 8 years ago for use in high ceiling, wide-spaced pole buildings for circulating air. These are large paddle fans with diameters of 8 to 24 feet in diameter. They use a 1 or 2 horsepower motor per fan and move 140,000 to 300,000+ cubic feet of air per minute (CFM), depending on model, compared to a 48″ high speed fan that uses a one horsepower motor and moves approximately 29,000 CFM. Twenty-four foot HVLS fans are typically installed every 60 feet down the center of a freestall barn over the feed alley. If 50″ high speed fans were installed in this same area, the equivalent of 6 fans would be used which will consume 940 watts per fan for a total of 5640 watts. The HVLS fan would consume 1650 watts per fan for a savings of 3990 watts or 3.99 kWh per hour of use at full speed. For a typical Wisconsin farm fans would be need about 1500 hours in a typical year for a savings of 5985 kWh per year or about a $658 savings.

HVLS fans use a variable speed controller which allows the fan speed to be varied from full speed, about 45 RPM for a 24 foot fan, to about one third of full speed. As the fan speed is reduced, the energy consumption is also reduced. At half speed the fan will only use 15% of the energy used at full speed. Because of the low power consumption, some dairies use fans during the winter to reduce stratification of the air and control moisture.

Cows tend to stay dispersed and don’t bunch in front of fans, however the velocities of air are lower and one study shows that cow body temperatures are higher than with high speed fans during heat events. Users have indicated dryer floors, less bird traffic and fewer flies.

The following papers provide comparisons of HVLS fans to conventional fan systems.

High-Speed Circulating Fans

High Speed circulating fans can be used in many different sizes of buildings to increase the air velocity and reduce stagnation. Increase air velocity aids animal in controlling body temperatures during hot weather. The University of Illinois Bioenvironmental and Structural Systems (BESS) Laboratory has developed a testing method for the energy efficiency of circulating fans. This enables the comparison of fans from different manufacturers and different designs of fans. Test results are available at the BESS lab web site. In general, the larger the fan, the more energy efficient it will be. Fan guards cause a decreased in energy efficiency by 15% or more but are necessary when fans are mounted in reach of animals or people. The centerline velocity of a fan is not a reliable predictor of the efficiency. The following chart is the recommended minimum energy efficiency ratings for selecting energy efficient fans based on the fan test data from BESS Lab.

Energy efficiency for high-speed circulating fans.
Fan Diameter (inches) Avg. Energy Efficiency (lbf/dW) Min. Recommended Rating (lbf/kW) Number of fans tested
Standard High Eff. w/guards w/o guards
20″ 7.9 10.7 10 — 3
24″ 11.6 16.7 12 13 6
36″ 15.2 19.1 15 18 18
48″ 18.2 24.1 19 22 7
50-52″ 17.8 22.4 18 21 11

Maintenance is the key factor to keeping all fans operating efficiently:

  • Loose belts can reduce fan output by 30%. It is recommended that all new fans should have automatic belt tensioning devices. Depending on the type and age of your current fans, you may be able to retrofit them with automatic belt tensioning devices. This will help to ensure the fans run efficiently during the busy spring and summer seasons when field work demands are high.
  • Dirty shutters can reduce fan output by 40%. Cleaning guards and shutters regularly and lubricating the shutters with a dry graphite lubricant will keep fan output maximized.
  • If you are in the market for new fans remember, not all fans are created equal. The efficiency of fans, cubic feet per minute / watt, varies greatly even among the same size fan. Do not rely on manufacturer’s data unless it has been independently tested by  BESS Lab at the University of Illinois (http://bess.illinois.edu/) or other certified testing lab.

If you have questions about the information on this site, please contact
Scott Sanford, Distinguished Outreach Specialist, University of Wisconsin, sasanford@wisc.edu.