Chapter 3

Greenhouse Ventilation and Cooling

Greenhouse Mist and 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 mist and fog cooling. Mist and fog systems utilize high-pressure nozzles to form fine water droplets for cooling the greenhouse (See Figure 3.11). Mist consists of particles from 50 to 100 microns (µm) whereas fog particles are generally considered to be less than 50 µm in diameter. Injected into the air, tiny water droplets of mist or 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. Mist size particles are heavier and take much longer to evaporate. These are more likely to fall out and wet the plant surface or saturate the growing medium. Therefore fog is more desirable for evaporative cooling whereas mist is usually better for plant propagation. Mist (or fog) systems do not cool as effectively as a conventional evaporative cooling pad system. Fog and mist systems can be used with or without mechanical ventilation to cool the interior of the greenhouse, but they still require controlled air exchange to maintain the correct humidity/air temperature balance.

Fog Systems

Fog systems generate fog containing water droplets with a mean size of less than 10 µm. These droplets are sufficiently small to stay suspended in air while they are evaporating and extracting heat from the air. The speed of evaporation of water and, consequently, the rate of cooling of air increase proportionately as water droplet size decreases. If the evaporative conditions are such that the moisture being added to the air evaporates readily, cooling takes place with little or no moisture deposited on the leaves. If too much moisture is added, moisture may be deposited on the leaves, leading to disease.


The fog cooling system can be used in greenhouses that are cooled by ventilators alone. Fog nozzles are spaced above plants throughout the greenhouse. Fog comes on intermittently to cool air that has entered the greenhouse through side-wall ventilators.

Water Quality

The greatest problem associated with fogging systems is nozzle clogging from particulate and chemical matter. Particles of sand or clay can clog the fog nozzles.

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. apart.

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