Temperature check

Follow these 8 tips for better fog cooling.

Photos courtesy of John W. Bartok Jr.

Fog cooling is usually a better choice than fan and pad for most greenhouses. It provides more uniform temperature and humidity conditions at a lower cost per square foot and provides a 3° to 5°F lower temperature due to smaller droplet size. A review of the basics and types of systems was covered in the article “Cooling Your Greenhouse with Fog” in the June 2018 issue of Greenhouse Management magazine. This column provides a few tips on installation and maintenance.

Droplet size is critical – Fog droplets are best in the 10 to 25 micron size (25 microns = 0.001”). Larger droplets take longer to evaporate and may wet the plant or soil. To get tiny droplets requires a high pressure pump that can generate 800 to 1200 psi and nozzles with a orifice diameter of 0.0045 – 0.15 inch.

Select hardened stainless steel or ruby orifice nozzles for reduced pressure drop and less wear.

The orifice in brass and plain steel nozzles wears resulting in the flow rate and droplet size that increases and spray pattern changes over time. Anti-drip nozzles will keep water from damaging plants below.

Clean, mineral free water is required.

With the tiny orifice, any particulate matter, debris or dissolved minerals could block or distort the flow. Solids can be removed by filters with a one micron mesh. Hard water with calcium or magnesium ions greater than 120 ppm should be treated to remove the hardness. This can be done with a water softener, reverse osmosis or deionizing equipment. The reverse osmosis equipment will remove suspended solids such as rust, bacteria and algae and dissolved solids.

Naturally ventilated greenhouses with shade screens require special consideration.

To get good heat removal, open the roof vent on the leeward to get the suction from the wind and open the side vents for air intake air. The movable shade screen should have an open weave or it should be cracked open to allow the warm moist to be drawn out.

Large naturally ventilated houses may require some fan assist.

Research in Australia has shown that under conditions where the outside air temperature is considerably higher than the desired greenhouse temperature, fog cooled, dense greenhouse air is difficult to replace with the hot, light outside air. In this situation, exhaust fans at plant level could be used to remove the air. Cracking the roof vents open slightly will provide a uniform intake that cools the top of the crop evenly over the whole area of the greenhouse. Using traditional intake shutters at one end and fans at the other results in large temperature and humidity gradients. Although there is an initial cost for the fans, the operating cost is low as the fans are only needed when the outside air temperature is high.

Design the system to fit the local climate and crops grown.

Components such as pump size, number and output of the nozzles, pipe size and controls need to be matched to have an efficient system. This can best be done by the equipment supplier using computer software. Depending on maximum outdoor temperature, water use will be about 0.2-0.4 gallons/minute/1000 sq ft of greenhouse floor area. This should be increased for houses with greater than 10’ gutter height due to the increased air volume that has to be cooled.

Adequate nozzle-to-crop- space is necessary.

To allow the fog droplets adequate time to evaporate, a minimum space of at least three feet above the crop is necessary. This prevents wetting of the leaf surface and the potential for disease development.

Frequent inspection of the system is necessary.

Fog systems require minimal maintenance as there are few moving parts. Checking that the nozzles are all working properly is the most important. The small size of the nozzle orifice and the potential for blockage from particulate matter and chemical residue are the main concern. Also frequent cleaning of the filters may be required.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com

July 2021
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