Heat Exchange and Water Evaporation

When water evaporates or changes from a liquid to a vapor, it absorbs heat from the atmosphere and cooler air results. That is the prin­ciple behind two greenhouse cooling systems: fan and pad and fog evaporative.

Fan and pad cooling uses exhaust fans and continuously wet pads of excelsior (a fibrous porous material), cross-fluted cellulose, aluminum fibers, or glass fibers. Through a recirculating water system, the pads are kept wet at one end of the greenhouse while fans at the opposite end of the house pull outside air through the pads into the greenhouse. The system goes by other names as well: wet-pad cooling or washed air cooling. Regardless of name, the principle is the same (Figures 19-16A and 19-16B).

Fog evaporative cooling uses a high-pressure pump to create a fine mist. The water droplets are so tiny that they remain in suspension in the air, then evaporate. As the fog disperses through the greenhouse, the evaporation of the tiny droplets causes the desired cooling without getting the plants wet.

Both fan and pad cooling and fog evaporative cooling systems can reduce greenhouse temperatures from 10° to 30° F below the outside air.

figure 19-16A. In this pad and fan cooling system, continuously wet pads of excelsior cool the air drawn through them into the greenhouse. (Delmar/Cengage Learning. Photo by Jack Ingels.)
figure 19-16B. These fans at the opposite end of the greenhouses pull the outside air through the pads. (Delmar/Cengage Learning. Photo by Jack Ingels.)