\A Look at Fog Cooling Systems
by Jana Weed
Greenhouse Manager, October 1989

Figure 1. Cooling by evaporation Each gram of water that evaporates will absorb 539 calories of heat. For example: Aquafog operating at high output may be able to achieve the complete evaporation of 20 gallons of water per hour. This amount of evaporation would absorb 39,118,464 calories of heat, equivalent to 155,232 BTU's or 12.9 tons of cooling.

Fog cooling systems add humidity to the air and reduce greenhouse temperatures by producing clouds of water droplets which remove heat from the air as they evaporate.

Ideally, the cloud should remain suspended in the air long enough to completely evaporate and not condense on surface areas.

The clouds are made up of tiny water droplets, which are generated by passing water under great pressure, usually from 400 pounds per square inch to as high as 2,000 psi, through tiny holes or nozzles.

Fog cooling can be used in both mechanically cooled or naturally ventilated greenhouses year-round. In the summer, the fog evaporates, cooling and humidifying in the air. In the winter, it maintains humidity levels to prevent plant dehydration caused by heating systems.

Manufacturers say that fog cooling can be used with any type of crop. It also can be used in propagation areas, and is particularly good for plug production because it adds humidity to the environment without saturating the medium and inducing disease problems.

Fog cooling systems can be used for foliar feed applications as well as for freeze protection.

Proponents of fog cooling systems say fog helps produce a better quality of crops, increases production and saves on energy costs.

High-pressure fog cooling systems include pumps, control systems to activate the pumps, filtrations systems, chemical injection systems, pipes or hoses, and a network of nozzles.

Other fog cooling systems, often called humidifier systems, are self-contained systems that operate without pumps. Generally they are portable systems that connect to standard water hoses and use motor-driven fans rather than nozzles to disperse fog droplets.

When choosing a system and its components, whether it's a high-pressure or humidifier product, a grower needs to consider his or her existing ventilation system, type of crop, regional climate and water quality.

Cooling in a Fog
by Ann Reily
Greenhouse Manager, June 1989

Fog vs. fan-and pad

The argument has long gone back and forth over whether fog cooling is more effective than fan-and-pad cooling. Part of the answer goes back to basic physics and depends on the temperature and relative humidity of the outside air. Relative humidity is measured with the use charts based on wet bulb and dry bulb readings. As relative humidity increases, the wet bulb temperature increases. It is not possible with evaporative cooling of any type to lower the temperature below the wet bulb reading.

For example, in Texas, if it is 90°F outside and the air has a relative humidity of 80 percent, no evaporative cooling method will result in a temperature lower than 85°F. In Arizona, where the relative humidity would be lower, it would be possible to lower the temperature by as much as 30 degrees.

Mee explained that fan-and-pad cooling is about 80 percent efficient when pads are new; that figure can drop to 50 percent as pads deteriorate. Fan-and-pad cooling can at best cause a reduction of temperature by 80 percent of the difference between the dry and wet bulbs reading, where fog cooling can lower temperature to the wet bulb reading, or close to it.

Herbert Hincks of Agritech of Broadway, N.C., doesn't fully agree. He said he believes that no system will cool to less than 2 degrees above the wet bulb temperature. He does agree that, except for a mechanical air conditioner, fog cooling will lower the temperature further than will any other method of cooling.

Mee also explained that fog cooling can be used in conjunction with fan-and-pad cooling and is especially useful as the efficiency of the pad decreases. The pads will lower the temperature of the incoming air; the fog will further cool the air. Without the fog, the temperature of the air would rise as it got nearer to the fans. When the pads completely deteriorated, they could be replaced, or more fog nozzles could be added and the pads eliminated.

Jaybird Manufacturing, Inc. | Woskob Industrial Park | 2595-B Clyde Avenue | State College, PA 16801 USA
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