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Understanding Evaporative Cooling

Balancing the many factor that will keep your plants in an optimum growing environment can be tricky. Most plants start to show signs of distress at 86 degrees Fahrenheit. Greenhouse growers know that temperatures inside a greenhouse can soar well above that during the hot summer months. Gothic Arch Greenhouses offers many options when it comes to cooling a greenhouse. Using shade cloth, fans, and ventilation are all common ways to keep greenhouses cool. But if you live in area with extreme temperatures, you may need to do more to keep your greenhouse plants safe. In many instances, we recommend installing an evaporative cooler in your greenhouse to combat high temperatures.

Evaporative cooling, which uses the heat in the air to evaporate water from leaves and other wet surfaces, can cool your greenhouse up to 20 degrees lower than the outside temperature. The amount temperature reduction you’ll be able to achieve with evaporative cooling depends on both the initial humidity of the air as well as the ambient temperature. Drier air has a greater capacity for water to be evaporated into it, so you’ll reach lower temperatures than in air that is already at a high humidity.

Evaporative coolers are:

  • Simple to install and use
  • Inexpensive as compared with other cooling methods
  • Environmentally friendly, because no chemicals are used
  • Effective in lowering the temperature in a greenhouse significantly

Two evaporative-cooling systems are commonly used in greenhouses: pad-and-fan and fog.

Fan and pad system

The most common evaporative cooling system for commercial applications is the fan and pad system. It contains a cellulose pad, overhead water supply pipe, a gutter to collect excess water, a sump tank, pump, piping, and control system. Warm air is pulled through damp evaporative cooling pads, then cooler air is expelled throughout the greenhouse.

Evaporative cooling pads are treated with anti-rot salts, stiffening agents, and wetting agents. The 4 inch or 6 inch pads are typically installed along the wall opposite to greenhouse fans. This installation ensures that incoming ventilation air travels through the pads to cool before entering the greenhouse.

Overhead water supply pipes should uniformly wet the pad a minimum flow rate of 0.5 gpm per sq.ft.for a 4-inch pad and 0.8 gpm per sq.ft. for a 6-inch pad. Excess water collects in the gutter and is sent to the sump tank. Filtering the water in your sump tank is important, and will remove any debris that may have collected.

In larger greenhouses, the temperature from one end to the other can vary as much as 10 degrees between the inlet side and where the air exits on the exhaust side. Cooled air will pick up heat inside the greenhouse structure; however, if you are cultivating multiple varietals, simply arranging plants and benching by heat tolerance can help.

Evaporative Cooling Pad Greenhouse Evaporative Cooling System

Fog System

Fog systems can be an excellent choice for greenhouse cooling, and are particularly effective in greenhouses with natural-ventilation systems. Natural-ventilation is not easily compatible with pad and fan systems, due to additional airflow resistance created by evaporative cooling pads. Fog systems can also provide more uniform cooling.

A high rate of water pressure is used to produce a quickly evaporating mist, cooling the air before reaching plants’ surfaces. The nozzles of a fog system are installed throughout the greenhouse, resulting in a more uniform cooling pattern compared to a pad-and-fan system. The recommended spacing is approximately one nozzle for every 50-100 square feet of growing area, using approximately 1-1.2 gallons per hour.

Important considerations

If you’re considering a pad and fan cooling system, you’ll want to make sure your greenhouse structure seals tightly when it is in use. It is very important to push entering air through the pads, as cooling depends on air moving through the cooling pads. All doors and vents should close with no gaps, or the air will be pulled through these openings rather than through the cooling pad.

If you’re considering a fog system, you’ll want to be sure that you can accommodate a high pressure water line for proper operation.

And with both methods of evaporative cooling, keep in mind the humidity tolerance of whatever crop you are growing. Plants can transpire a lot of water, and these cooling methods increase the humidity in the microclimate of your greenhouse. A high relative humidity (above 80-85 percent) should in many cases be avoided, because of increased the incidence of disease and reduction in plant transpiration. Research your plants’ preferences and your area’s relative humidity as you make your cooling choices.

Our experts can help you with this process. Call Gothic Arch Greenhouses today for advice and friendly service, 1-800-531-4769.