Greenhouse Ventilation and Cooling
Forced-air Ventilation in Greenhouses
Many greenhouse operations rely on forced-air ventilation fans to move air into and out of the greenhouse. Forced-air ventilation includes fans on one end of the greenhouse and motorized air inlets (See Figure 3.4), or shutters, at the opposite end. The opening or openings can be located in the side of the greenhouse wall opposite the fans too (See Figure 3.5). When inside temperatures exceed the desired level, a thermostat opens the shutters and starts the exhaust fan(s). As the fans exhaust the heated air, a slight vacuum is created that draws in cooler outside air through shutters. When the desired temperature has been reestablished, the thermostat shuts off the exhaust fans and closes the motorized shutters.
Forced-air Ventilation Rates
Forced-air ventilation systems for greenhouses are normally designed to exchange the air volume inside the greenhouse with fresh outside air about once every minute. Changing the air once a minute (one air volume per minute) keeps the air exiting the greenhouse to within 8 to 10 degrees F of the air entering the greenhouse. The fan system should be sized to provide a maximum one-volume air exchange per minute, which corresponds to roughly 8 cubic feet per minute (cfm) of fan capacity for every square foot of floor area or 2.5 cubic meters per minute (cmm) for every square meter.
NGMA Air Exchange Rate Standards
The National Greenhouse Manufacturers Association (NGMA) indicates in its 2010 standards for ventilating and cooling greenhouses that a rate of removal of 8 cfm per square foot (2.5 cmm/m2 of greenhouse floor is sufficient. This applies to a greenhouse under 1,000 feet (305 m) in elevation, with an interior light intensity not in excess of 5,000 foot-candles (fc) (53.8 klux) and a temperature rise of 7 degrees F (4°C) from the pad to the fans.
Fan staging is an effective ventilation management tool for providing the proper amount of incoming air for good temperature control in the greenhouse. A stage is the percentage of the total fan capacity being used at one time. There should be sufficient ventilation stages so that transitions from stage to stage do not result in large indoor temperature swings.
Selecting Fans for Forced-air Ventilation
The capacity of the exhaust fan needs to be correlated and matched to the intake of air through louvers or vents. Fan capacity is a measure of the volume of air a fan can move and is expressed in cubic feet per minute (cfm) or cubic meters per minute (cmm) for every square meter. The amount of air a fan moves depends on the blade diameter, blade shape, fan speed (revolutions per minute, rpm), motor horsepower, and the shape of the housing. The two most common measurements used to describe the characteristics of a fan are blade diameter and motor horsepower.
Fan and Louver Location for Forced-air Ventilation
Fan systems work best if the draw (distance from the inlet to the fan) is less than 150 feet in the greenhouse. For most greenhouses this means installing the fans on one end wall with louvers placed in the opposite end. In greenhouses 150 feet long or longer, the cooling efficiency diminishes. The incoming air heats up as it travels across the greenhouse. With a long distance between air intake and exhaust, the air temperature increase is too high for cooling the entire greenhouse. In greenhouses longer than 150 feet, the fans should be located along the sidewalls so that they can draw air in through louvers in both ends.
Thermostat Selection and Placement
The operation of the fans may be controlled by a thermostat or preferably by a computerized climate control system. The control range of a thermostat should be from 45 to 90 degrees F (7 to 32°C). A smaller control range is not recommended because the fans will cycle on and off more often than is necessary.
Air Distribution with Forced-air Ventilation
Air distribution within the greenhouse is important if near optimal levels of temperature and humidity and normal or above normal levels of carbon dioxide are to be maintained near the growing plants. Since there is very little natural air movement in a closed greenhouse, temperature variations between the roof and plant surfaces will exist as warm, lighter air rises toward the roof, while cool, heavier air is pushed toward the growing plants.
Horizontal Air Flow Fan System
The horizontal airflow (HAF) system uses 16- to 24-inch diameter fans suspended about halfway between the roof and plant level to continuously move air throughout the greenhouse (See Figure 3.5). HAF fans should be installed approximately 7 to 8 feet above the floor and placed about a quarter of the way along the length of the greenhouse. Fans should be spaced 40 to 50 feet apart along the length of the greenhouse about one-fourth of the house width from each sidewall.
The overhead perforated sleeve system, more commonly called a polytube, combines a heating, air circulation and partial ventilation system in one package. Its main function is to convey and distribute air, as uniformly as possible, from a high-velocity fan at one end of the ventilated or heated space through a pressurized tube that has holes located along its length.
Maintenance of Forced-air Ventilation Fans
Maintenance should be done on a regular basis. This includes cleaning blades, removing grass or weeds in front of shutters and adjusting fan belts. Dirty, automatic shutters which do not fully open greatly restrict, airflow and drastically lower a fans ventilating efficiency. If these shutters do not close tightly when the fan is off, they will significantly increase the winter heating bill by allowing the infiltration of the cold outside air.
Click on the following topics for more information on greenhouse ventilation and cooling.