Grower 101: Using Evaporative Cooling, Part I

On a bright, sunny summer day, a 30- x 100-foot greenhousewill gather about 32 million British Thermal Units (Btus) of heat. This isequivalent to burning 32 gallons of fuel oil or 320 therms of natural gas. Ifthe greenhouse is full of plants, about one-half of this heat is used fortranspiration and evaporation. The remainder of the cooling has to be conductedthrough ventilation. If the greenhouse is empty and closed, the temperature canexceed 150° F.

Understanding the basics

Shade on the outside of the greenhouse will keep some of theheat out. Shade on the inside, if it contains aluminum foil, will reflect someof the heat back out. Ventilation, either natural or fan, will remove aconsiderable amount of the heat that is collected. Still, on summer days, thetemperature may exceed the desired level that promotes good plant growth by 10-20ºF. Excessive temperature results in delayed flowering and internode stretching.Evaporative cooling may be the best choice under these conditions.

Evaporative cooling, which uses the heat in the air toevaporate water from leaves and other wetted surfaces, can cool the greenhouseto 10-20° F below outside temperature. It takes one Btu of heat to raisethe temperature of one pound of water 1° F, but it takes 1,060 Btus of heatto change the same amount of water to a vapor.

With an evaporated cooling system, humid air containing theheat that it picked up within the greenhouse is exhausted out through the ventsor fans, and cooler, drier air is brought in. Evaporative cooling works bestwhen the humidity of the outside air is low. For example, in Reno, Nev., theaverage summer dry bulb temperature is 96º F and the wet bulb is 61ºF. With an evaporative cooler having an efficiency of 80 percent, thetemperature would be cooled to about 68° F. These conditions are mostcommon in the dry Southwest, but even in the more humid sections of the UnitedStates, significant evaporative cooling can occur most days in the summer. Inhumid New Orleans, where the average summer dry bulb temperature is 93° Fand the wet bulb is 78° F, the cooled air would be about 81° F,acceptable for the production of most plants. Figure 1, right shows acomparison of air temperature leaving the evaporative cooling system atdifferent levels of relative humidity outside the system when the insidetemperature is 90° F.

Fan and pad system

Several evaporative cooling systems work well in commercialgreenhouses. The most common is the fan and pad system. It contains a cellulosepad, overhead water supply pipe, gutter to collect excess water, a sump tank,pump, piping and control.

The 4- or 6-inch-thick pad is treated with anti-rot saltsand stiffening and wetting agents. Pads are normally installed continuouslyalong the side or end of the wall opposite the fans. The amount of pad areaneeded is calculated by multiplying the floor area by 8 feet and dividing by250 for a 4-inch pad or 400 for a 6-inch pad. For example, a 30- x 100-footgreenhouse with a 4-inch pad would require 96 sq.ft. of pads (30 x 100 x 8÷ 250 = 96 sq.ft.)

The overhead water supply pipe should distribute the waterso the pad is wet uniformly. The minimum water flow rate is 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 is collected below the pad in a gutter andpiped to a sump tank. Tank capacity needs to be 0.8 gallon per sq.ft. of padfor 4-inch pads and 1.0 gallon per sq.ft. for 6-inch pads. Water returning tothe sump should be filtered to remove any debris. A make-up water supply andfloat valve keep the water level constant. In areas having water with a highmineral content, it is advisable to bleed 3-5 percent of the water to minimizesalt buildup. Algae growth in the re-circulated water can be controlled with abiocide.

Modular pad systems of 5 and 6 feet are now available. Theseare self-contained and come completely assembled and ready to bolt to the wall.Installation time is reduced considerably. Only water and electricalconnections have to be attached.

Next month, find out about swamp coolers, mist and fogsystems and fan-generated fog and how they can work for you.

John Bartok is an agricultural engineer and extension professor-emeritus in the Natural Resource Management and Engineering Department at the University of Connecticut, Storrs, Conn. He may be reached at jbartok@rcn.com.