Evaporative cooling, which uses the heat in the air to evaporate water from plants and other wetted surfaces can be used to cool the greenhouse as much as 10 to 20ºF below the outside temperature. Although evaporative cooling is most effective in dryer climates, such as the southwest, it can provide cooling anywhere in the U.S.
Changing water from the liquid to the vapor phase absorbs considerable heat. It takes 1 Btu of heat to raise the temperature of 1 pound of water 1ºF, but it takes 1,060 Btu's of heat to change the same amount of water to a vapor. In other words, for each gallon of water evaporated, about 8700 Btu's of heat are absorbed. Exhausting this heated air out of the greenhouse provides the cooling.
The chart shows the effect of the relative humidity of the outside air on the temperature of the air that leaves the evaporative cooling system. For example, at a 90ºF outside temperature, the air would be cooled to about 67ºF if the relative humidity is 30% but only to 82ºF if the relative humidity is 70%. Outside air with a lower relative humidity will result in greater greenhouse cooling.
The fan and pad system has been the standard system for evaporative cooling for more than 50 years. In this system, aspen or cellulose pads are mounted in one endwall or sidewall of the greenhouse. They are supplied with water from a pipe above the pads and excess water is collected in a gutter at the bottom. Air drawn through the wet pads by fans mounted in the opposite endwall or sidewall is saturated and cools the greenhouse. The pads are sometimes removed for the winter to allow more light to enter the greenhouse.
Pad size – The normal design airflow rate for cooling a greenhouse with fans is eight cubic feet per minute (cfm) per square foot of floor area. This has to be modified if the greenhouse is not shaded so that summer light levels inside are above 5,000 foot-candles, if the greenhouse is located at a high altitude above 1000 feet or the pad to fan distance is less than 100 feet. The airflow rate is the total fan capacity needed and should be supplied at 0.1" of water static pressure.
The total pad area should be sized to accommodate 150 cfm per square foot of pad area for aspen pads and 250 cfm and 350 cfm for 4" and 6" cellulose pads. The total pad area (square feet) is determined by dividing the airflow rate by one of the above values.
Water supply - To work effectively, the pads need an adequate supply of water to keep them wet but not so much that you have a solid curtain of water. This is usually 1/3 gallon per minute (gpm) per foot of pad length for aspen pads, ½ gpm for 4" thick and ¾ gpm for 6" thick cellulose pads. The pump needs to be large enough to provide the above rates based on the square feet of pad area. In larger systems several pumps may be required. A makeup water supply is also needed. It should be sized for one gallon per minute per 100 square feet of pad area.
Sump capacity - The water collection sump can be either a tank or with some of the modular units, a section of large diameter PVC pipe. It needs to be large enough to supply the pump. Normal recommendations are ½ gallon capacity per square foot for 2" thick pads, ¾ gallon for 4" and 1 gallon for 6" thick pads.
Controls – The fan and pad system is usually activated as the last stage of cooling where the fans can't maintain the desired greenhouse temperature. It can be controlled by a thermostat, controller or computer. As the controller and computer provide multiple stages of cooling, these are the preferred controls. To prevent excessive greenhouse humidity in warm weather, the pump on the evaporative cooling system could be controlled by a humidistat.
Example calculation – Assume a 30' x 100' greenhouse with pads to be located at one end and the fans at the opposite endwall. Total fan capacity, based on 8 cfm per square foot of floor area is 24,000 cfm (30' x 100' x 8 cfm/sq ft = 24,000 cfm). This could be supplied by two – 42", ½ horsepower fans. Using a 4" thick cellulose pad with airflow through the pad of 250 cfm/minute, the total pad area needed is 96 sq ft ( 24,000 cfm ÷ 250 cfm/sq ft = 96 sq ft). A pad measuring 4' high x 24' long would meet this requirement.
Minimum sump capacity required for the system is 72 gallons (96 sq ft x ¾ gallons/sq ft = 72 gallons). Recommended pump size is 12 gpm ( 24 ft x ½ gpm/ft = 12 gpm). The size of the supply piping over the pad will depend on the design of the installation. If the water is piped to one end it will have to be a minimum 1-1/4" diameter or if supplied from the center, it can be 1". The water supply to compensate for the evaporated water should be a minimum of 1 gpm.
Equipment – Fan and pad systems can be made up of individual components to fit a particular greenhouse. Pad height should be no less than 2' and no greater than 8' to get uniform pad wetting. Modular units are available that come with all the components assembled ready to be mounted to the wall. Several units may have to be installed to get the pad size needed.
Some growers have had good success with portable evaporative cooler units. These are mounted on wheels and only require a hose and electrical connection. They are placed near a door in the area at the end of the greenhouse opposite the fans. They are limited in capacity based on the amount of pad area and several units may be needed in hot weather to get adequate cooling.
Spray-on evaporative cooling systems utilize nozzles mounted outside the greenhouse to spray water onto the pads. These are non-recycling systems as the excess water drains away from the greenhouse.
Evaporative Air Coolers (Swamp Coolers) – these self-contained units have evaporative pads and a blower and are generally used in smaller free-standing greenhouses. They are usually mounted outside and blow the moist air in through an opening in the sidewall. The size in cfm is usually equal to 1.2 to 2.0 times the volume of the greenhouse. If the greenhouse is shaded, this value can be reduced somewhat. An exhaust opening such as a door, roof vent or shutter on the wall opposite the swamp cooler inlet has to be provided to remove the heated air.
Maintenance – Water that is returned to the sump should be screened (50 mesh) to remove pad fibers and other debris. Where the water contains high levels of salts, either a continuous bleed-off or periodic flushing of the sump is necessary. Valves or caps should be installed to allow easy flushing of the system components. Operation of the fan and pad system should be done with the doors closed to prevent short circuiting of the air. The sump and piping should be black to prevent algae growth. Operating the fans for a half hour after the evaporative cooling system has been shut off will help to dry it eliminating algae formation. The area around the pads should be kept free of weeds to reduce clogging.
Evaporative cooling can help to maintain a cool summer environment that doesn't stress the plants. The fan and pad system is the standard system in use for many years in greenhouses and other segments of agriculture.
John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, department of Natural Resources and the Environment, University of Connecticut, Storrs CT - 2013