Fan cooling is the major electrical cost for many growers during the summer. My observations of many greenhouse ventilation systems over the years has shown that there is room for considerable improvement. Many of the following suggestions are common sense, but a reminder is often necessary.
Preventive maintenance means higher efficiency
Clean the fan blades and motor frequently. The layer of dust reduces air flow over the blades. Dirt on the motor traps heat and leads to overheating.
If the belt squeals when the fan starts, it is either loose or worn. To check the belt for proper tension, press in the center between the pulleys. It should only move a distance equivalent to the belt’s width. A belt with cracks or frayed edges should be replaced.
Shutters should operate smoothly. Examine for bent or stuck-open shutters. Lubricate bearings and levers.
Select high-efficiency replacement motors
Although high-efficiency motors cost a little more, the payback is short. A 1-horsepower capacitor start motor-operating on 115 volts; draws 13.6 amps. A similar high-efficiency motor draws only 9.2 amps, a 32-percent savings in electricity.
Check voltage at the fan. Many fans are located at the opposite end of the greenhouse from the electric distribution box. Wiring should be large enough to keep voltage drop less than 2 percent, otherwise motor overheating can occur.
Location and calibration of thermostats is important
Thermostats and sensors should be located at plant height, near the center of the growing area, adjacent to the heating thermostats. Installation of all thermostats in an aspirated box (where air is drawn over sensing elements) will give more accurate control. Check the accuracy of the thermostat by placing an accurate thermometer next to it to compare readings.
Thermostats with +/- 1°F accuracy save electricity. The 4 to 6°F differential between on and off on a mechanical thermostat means the fan will operate for a considerable time after the set point is reached.
For larger operations, an electronic controller has the advantage of integrating both the heating and cooling systems.
Provide adequate fan capacity
The installed fan capacity should equal at least one greenhouse volume per minute. For hoophouses and older gutter-connected houses, one volume air change/minute may be adequate but for new gutter-connected houses with gutter height of 14 to16 feet, fan capacity should probably be higher especially in the southern U.S.
Fan location away from the prevailing wind increases efficiency
Design air flow is not reached if the fan has to exhaust into the prevailing wind. Fan output is decreased about 1 percent for each mile/hr of wind.
Shutters and guards reduce air flow
Shutters reduce air flow and efficiencies of a fan by 10 to 25 percent. The best location for a shutter is on the inlet side of the fan. Slant-leg design fans have increased efficiency due to the shutter being placed inside where it disrupts the flow pattern less. Winter heat loss is also less. Exterior cones also increase efficiency.
Location of the shutters and vents is important to uniform cooling
Intake shutter area should be at least 1¼ times the fan area. Shutters should be located on the opposite endwall or sidewall from the fans. Continuous sidewall vents will give more uniform cooling as the air is brought in over the whole growing area.
Select fans with a high Ventilating Efficiency Ratio (VER)
When purchasing new fans, select those that have been tested in accordance with Air Movement and Control Association (AMCA) standards. Compare the VER. This is the volumetric rate of air movement to the rate of energy consumption. It varies from about 10 cubic feet per minute of airflow (cfm/watt) for a poor fan to 20 cfm/watt for the best fans. Look for fans that have at least a 17 VER.
Stage the fans to meet ventilation needs
The installation of multi-speed fans or several fans allows different levels of ventilation that can meet different heat loads. Running all the fans when only a little cooling is needed results in overcooling and increased electricity usage. Control the fans with multi-stage thermostats, a controller or a computer.
Don’t overcool the greenhouse. The fans have to run much longer to get the last degree of cooling.
Keep doors and vents near the fan closed to prevent short circuiting of air.
Reduce summer fan operation time by applying shading to the outside of the glazing or a reflective shade material inside the greenhouse.
Keep the greenhouse full of plants to increase evaporative cooling from plant transpiration.
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.
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