A Guide to Evaporative Cooling

An evaporative cooling system, otherwise known as a “wet wall” or a “fan and pad” system, is an easy-to-install structural component used in horticultural production to reduce the temperature in a greenhouse using a cellulose pad, fan ventilation and a water supply.

These systems are simple to use because they run largely on their own, and can be less expensive and rely on less energy than HVAC components, says Brad Gaddy, market manager for SCHAEFER/Pinnacle Climate Technologies.

“The way the system works is, you have water that’s pumped up through a tank, and then the water falls down into a trough [into the cellulose pad wall installed in an opening of the structure] and goes back into the tank so it’s recycled,” he adds. “The way we move air through the system is exhaust fans at the opposite end of the greenhouse or structure, and that [circulates] the air.”  

While an evaporative cooling system can be effective in just about any structure that needs to be cooled, the lower the humidity in a geographic region, the better this system will work for a grow structure. “For instance, in a place like Phoenix, Ariz., where the humidity can be 8 to 12 percent, these pads can cool up to 30 degrees,” Gaddy says. 

Evaporative cooling been used in ornamental greenhouses for decades, but it’s also used in other applications, such as dairy farming or poultry. “As a matter of fact, [in] the state of Arizona, if you drive around and look at the houses, you’ll see these things on top of the roofs. They’re not air conditioners; they’re evaporative cooling systems because the humidity is so low that they can cool it with water instead of an excessive air conditioning system,” Gaddy says. 

Gaddy says that the majority of growers who install evaporative cooling systems can work directly with Pinnacle/Schaefer’s dealers to find a custom solution that will fit their structures. “My customers have dedicated tech service officers where they know how to size the equipment,” he says. 

However, to put an installation in perspective, the University of Massachusetts’ Greenhouse Crops & Floriculture Program provides an example calculation to determine the size of an appropriate system:

“Assume a 30' x 100' greenhouse with pads to be located at one end and the fans at the opposite end wall.  Total fan capacity, based on 8 cfm [cubic feet per minute] per square foot of floor area is 24,000 cfm (30' x 100' x 8 cfm/sq. ft. = 24,000 cfm). This could be supplied by two 42", ½ horsepower fans.  Using a 4" thick cellulose pad with airflow through the pad of 250 cfm/minute, the total pad area needed is 96 sq. ft. (24,000 cfm ÷ 250 cfm/sq. ft. = 96 sq. ft.). A pad measuring 4' high x 24' long would meet this requirement.

Minimum sump capacity required for the system is 72 gallons (96 sq. ft. x ¾ gallon/sq. ft. = 72 gallons). Recommended pump size is 12 gpm [gallons per minute] (24 ft. x ½ gpm/ft. = 12 gpm).    The size of the supply piping over the pad will depend on the design of the installation. If the water is piped to one end, it will have to be a minimum 1-1/4" diameter or if supplied from the center, it can be 1". The water supply to compensate for the evaporated water should be a minimum of 1 gpm.”

Gaddy says that proper equipment maintenance can help the system last longer.

^An outdoor view of an evaporative cooling system. Image courtesy of SCHAEFER/Pinnacle Climate Technologies.

“What we recommend is that after every growing season … clean the pads,” he says. “We don’t recommend pressure washing because that will tear them up, but try to get some of the debris out and let them dry. And if [cultivators] can store them somewhere, where they’re not exposed to the outside air, that’s optimal. … We also recommend that while using the system, dependent of how hard the water is, that you bleed the system off every so often to get some of the particulate out of the water, so that you’re introducing water that doesn’t have all that stuff built up. Because what happens is, as the water evaporates, the solids in the water do not, and they get caught in the pads. … [And] you might see your electrical bill go up because fans are trying harder to move the air.”

However, if taken care of, it’s likely that your system will last you up to eight years, he adds.