Greenhouse Ventilation and Cooling
Greenhouse Fog Systems
Fan and pad cooling is based on the principle that evaporating water takes heat from the air. This same principle is applied in fog systems used in greenhouses for cooling and humidifying. Fog systems utilize high-pressure nozzles to form fine water droplets for cooling the greenhouse (See Figure 5.9). Injected into the air, tiny water droplets of fog remain suspended until they are evaporated. The smallest particles vaporize almost instantaneously, and the larger droplets are carried by air currents; gradually becoming smaller until they are vaporized. Fog systems do not cool as effectively as a conventional evaporative cooling pad system. Cooling effectiveness of fog systems depends on ambient air having a relative humidity of less than 90 percent; however, operation of these systems causes air to rise above this level of humidity. Thus, these systems must be used in conjunction with careful monitoring of relative humidity and the use of exhaust fans to exchange the humid air with drier air from outside. If drier air from outside is not available, then a fog system will be ineffective as a cooling system. An alternative to fog systems is mist systems but they generally deliver larger water particles that will not evaporate completely before contacting a surface, thus wetting plants, benches, floors, and so on. Mist size particles are heavier and take much longer to evaporate. Therefore, fog systems are more desirable for evaporative cooling whereas mist systems are usually better for plant propagation.
Fog System Configurations
A fog system can be installed in any greenhouse configuration, such as with natural or mechanical ventilation. Greenhouses equipped with exhaust-fan cooling lend themselves well to fog cooling too. A line of fog nozzles is installed just inside the inlet ventilators. Exhaust fans on the opposite wall draw outside air in through the open ventilators and then through the fog, where the air is cooled.
Pumps and Nozzles
Pumps used to provide fog droplets typically operate at 1,000 psi (6.9 MPa) and possibly at pressures up to 1,500 psi (10.3 MPa). Copper, stainless steel and re-enforced flexible hose are used for piping. Most fogging systems are based on high-pressure nozzles (less than 50 microns in diameter). The water usage per nozzle is approximately 1 to 1.2 gallons per hour. Plastic, ceramic, and stainless steel are used for nozzles.
Fog Control Systems
Fog systems can be controlled with a time clock and timer, thermostat, light-operated interval switch (LOIS), humidistat, or controller. The time clock governs the time of day the system operates. The timer turns the mist on for several seconds every few minutes. Fogging can be controlled by a two-stage thermostat, which allows more water to be applied on excessively warm, bright days. Temperature settings should be 5 to 10 degrees F (2.8–5.6°C) apart.
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