08/28/2020

Evaporative Cooling for Greenhouses

Evaporative cooling is a natural process. We even see it in humans as we sweat. But it can also be utilized to naturally cool down your greenhouse. Learn more about evaporative cooling below.

 

How Evaporative Cooling Works

Evaporative pad and fan cooling systems use water-soaked cellulose pads that cool the outside air by evaporation as it is drawn through the pads by exhaust fans. Water is spread evenly over the top of the pads as a film to get uniform coverage. The excess water is collected in a trough at the bottom of the system to get recycled through again. A pump and valve add more water as needed. The pads are usually 1 foot wide by 4 or 6 inches thick with heights ranging from 2 to 6 feet. Custom sizes are available and systems can be stacked to get heights greater than 6 feet. 

Evaporation of water is an endothermic reaction that consumes heat. It’s why you feel cool when you get out of a swimming pool on a hot summer’s day. These systems are also known as “Wet Walls” or “Swamp Coolers”. 

 

Cooling Possibilities

An evaporative cooling system can provide 10 – 25 degrees Fahrenheit worth of cooling, with the possibility of a few more degrees in hot, arid parts of the country.  Air can only hold so much water, so humid areas will be on the lower end of that spectrum. These numbers are based on the hottest 5 days of the year for a given locality. A grower in Phoenix might see a cooling differential in the mid 20’s when it’s 110 degrees outside with low humidity. In parts of the country where humidity is typically high, a grower will not see as drastic of a temperature difference on hot/humid days. Sorry folks, it’s physics!

 

Sizing Your System

To size a system, determine the cubic footage (not square footage!) of your greenhouse. Divide that number by 375 to get the square feet of 6” thick pads needed to cool your house. Divide by 250 for 4-inch pads. You also use the cubic footage number to determine what size and quantity of fan you will need. Usually, base the number of fans needed on one air change per minute.  The total fan CFM should equal the number of cubic feet of the area you are trying to cool. Keep your house tight. An open door reduces the amount of air flowing through the pads creating an inefficiency within your system.

 

Best Practices

As with any piece of equipment, there are some best practices to maximize system performance. Including:

  • Let your pads dry out once every 24 hours.
  • Check the pads for dry spots. This usually means there is a clog in your spray pipe. 
  • Check with the pad manufacturer for approved chemical cleaners.
  • A water pH level between 6 and 8 is recommended. Exceeding these limits will deplete the resin content of the cellulose and shorten pad life.
  • Clean filters weekly and check the pump for debris.
  • Completely flush the system every 4 to 6 weeks.
  • Bleed off 3% to 5% of the water every day to avoid mineral build-up. The water evaporates, but minerals do not. They stay in the water and can build up on the pads, lowering the efficiency. They can also clog the emitter holes in your spray pipe.

Interested in learning more about evaporative cooling and how it can be beneficial for your greenhouse? Check out our product line and contact us! We’d be happy to discuss a system that will work for you.