Fog is an alternative evaporative cooling method to the fan and pad system. Its advantages include more uniform cooling throughout the greenhouse, higher efficiency and lower maintenance costs. It also provides a method for cooling greenhouses that don’t have fans.
What is fog?
Fog particles are generally considered to be less than 50 microns (0.002”) in diameter. The particle size typically used in high pressure greenhouse fog systems is about 10 microns. Mist, on the other hand, is particles from 50 to 100 microns. As a comparison, human hair is about 0.004” diameter that equals 100 microns. Breaking one gallon of water into 50-micron droplets will produce about 68 billion droplets of fog.
Injected into the air, these tiny water droplets remain suspended until they are evaporated. The smallest particles vaporize almost instantaneously. The larger ones are carried by air currents, gradually becoming smaller until they are vaporized. Mist size particles are much heavier and take much longer to evaporate. These are more likely to fall out and wet the plant surface or saturate the growing medium. If they don’t evaporate before nighttime, the potential for disease increases.
How fog works for cooling
Evaporative cooling uses the heat from the air to change the water droplets into vapor. In this process, large amounts of heat are required. The resultant air is therefore cooler and more humid, an advantage to plant growth. For each gallon of water that is evaporated, over 9,400 Btu of heat are absorbed. Ventilation air is then used to carry the warm, moist air out of the greenhouse. This is replaced by drier outside air.
The potential for cooling is measured by the difference between the dry bulb and wet bulb temperature. A sling psychrometer or battery-powered hygrometer are low cost instruments to get these measurements.
If you're looking for an alternative evaporative cooling method to fan and pad systems, you may want to consider fog.
Although evaporative cooling is most effective in the Southwest where the air is dry and the difference between dry bulb and wet bulb is near 40° F on a summer day, it can also be quite effective across the rest of the U.S. In most other sections of the country including the humid Southeast, there is a potential for 10 to 20° F cooling below ambient during the warmest part of the day when the stress on the plants is the greatest.
Several methods are used to produce fog. A typical system uses a high-pressure pump, distribution piping and nozzles that break the water stream into very fine droplets. Piston pumps are needed to develop the 800 to 1200 psi pressure to get the 10 to 20-micron size droplets. Most systems available from irrigation equipment suppliers and labeled as fog systems operate at 50 to 100 psi and create a droplet size larger than 50 microns. They are really mist systems.
Copper, stainless steel and re-enforced flexible hose are used for piping. Diameter is frequently ¼” or 3/8” as water supply required is only 1 to 2 gallons/hour/nozzle. Distribution piping for cooling is placed above the air intake louvers or vents and a few lines of pipe are placed above the crop.
Plastic, ceramic and stainless steel are used for nozzles. Nozzles should have anti-drip check valves to prevent dripping after the system shuts off. An integral 5-micron filter will remove particulates. Calcium deposits that coat the inside of the pipe and reduce flow can be reduced by the use of rain, bottled or reverse osmosis water.
Fog can also be produced by a system using a high-speed fan with water channeled to the tip of the blades. The shearing action as the water exits the blades produces a fine fog. The fan distributes the fog above the crop canopy.
Water at household pressure, injected through a nozzle into a stream of compressed air, will also produce a fine fog. Each nozzle requires both a water and air supply. Different flow rates and droplet sizes can be achieved by adjusting the water and air pressure. Distribution can be through ducts, HAF fans or nozzles evenly spaced over the crop.
Fog systems frequently operate with a controller or computer that measures vapor pressure deficit (VPD). The difference between saturation water vapor pressure and ambient water vapor pressure is the VPD and represents the evapotranspirational demand of the surrounding atmosphere as well as the proximity to the dew point.
Although expensive to install, fog systems have worked well for cooling and can also be used for propagation.