Fog vs Fan-and-Pad

Fog vs Fan-and-Pad

by Ann Reily
Greenhouse Manager

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.