Understanding fan efficiency

In many greenhouses it is the fans that use the most electricity. Exhaust fans generally operate about 2,000 hours per year. Horizontal air flow (HAF) fans frequently operate 4,000 to 8,000 hours per year depending on the length of the cropping season. Fans on furnaces and unit heaters may run as much as 2,000 hours per year.

Therefore, when purchasing new or replacement fans, it’s important for growers to select those that have been tested in accordance with AMCA (Air Movement and Control Association) standards. Fan output for a particular diameter varies considerably between manufacturers and models. For example, for a 36-inch diameter fan with ½ horsepower motor delivering 10,880 cfm is 30 percent more efficient than a similar size fan delivering 7,770 cfm. This can have a significant influence on the number of fans needed to provide the volume of air that is needed for ventilation.

Another factor to consider is the ventilating efficiency ratio (VER), which is the rating system for fans that was developed during the 1970s. The VER is a measure of the number of cubic feet of air moved per minute (cfm) per watt of input energy. It generally varies between eight for the least efficient fan to 25 for the most efficient. Motor efficiency, number, shape, and pitch of blades, shroud and guard design, shutters, and cone shape influence performance.

Testing of fans is done by AMCA and the BESS Lab (Bioenvironmental and Structural Systems Laboratory at the University of Illinois. Fans are subject to a standard test and results are published. Not all fans have been tested to these standards. Some are rated by the manufacturer by combining the rating of the motor, blade, and shutter. These can give misleading results.

One area in which only limited testing has been done is that of HAF fans. The recommended design for HAF fan installations is two cfm/square foot of floor area. Although they have small motors, the many fans required can add up to considerable electricity use as they usually operate 24 hours a day.
 

The table is a comparison of some of the HAF fans available, which includes data from testing labs and from manufacturers’ literature. Also included is the operating cost for each fan that will provide 1,000 cfm for 24 hours. Knowing the operating cost is important when selecting fans.

Finally, maintaining the fan is as important in reducing energy costs as buying the most efficient model. Poor maintenance can reduce efficiency by 50 percent or more.

 

Bartok is a regular contributor to Greenhouse Management and an agricultural engineer and emeritus extension professor at the University of Connecticut. He is an author, consultant and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.

Have a question? You can write John at jbartok@rcn.com.