Greenhouse Ventilation and Cooling
Greenhouse Fog Systems
Fan and pad cooling systems are 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 (Figure 6.11). 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.
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 pounds per square inch (PSI) (6.9MPa) and possibly at pressures up to 1,500 PSI (10.3MPa). Copper, stainless steel, and reinforced 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.
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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