Grower 101: Using Evaporative Cooling, Part I

March 18, 2003 - 13:58

Find out what to use to keep your greenhouse ventilated and cool in the hot, humid summer.

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

Understanding the basics

Shade on the outside of the greenhouse will keep some of the
heat out. Shade on the inside, if it contains aluminum foil, will reflect some
of the heat back out. Ventilation, either natural or fan, will remove a
considerable amount of the heat that is collected. Still, on summer days, the
temperature may exceed the desired level that promotes good plant growth by 10-20º
F. 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 to
evaporate water from leaves and other wetted surfaces, can cool the greenhouse
to 10-20° F below outside temperature. It takes one Btu of heat to raise
the temperature of one pound of water 1° F, but it takes 1,060 Btus of heat
to change the same amount of water to a vapor.

With an evaporated cooling system, humid air containing the
heat that it picked up within the greenhouse is exhausted out through the vents
or fans, and cooler, drier air is brought in. Evaporative cooling works best
when the humidity of the outside air is low. For example, in Reno, Nev., the
average summer dry bulb temperature is 96º F and the wet bulb is 61º
F. With an evaporative cooler having an efficiency of 80 percent, the
temperature would be cooled to about 68° F. These conditions are most
common in the dry Southwest, but even in the more humid sections of the United
States, significant evaporative cooling can occur most days in the summer. In
humid New Orleans, where the average summer dry bulb temperature is 93° F
and 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 a
comparison of air temperature leaving the evaporative cooling system at
different levels of relative humidity outside the system when the inside
temperature is 90° F.

Fan and pad system

Several evaporative cooling systems work well in commercial
greenhouses. The most common is the fan and pad system. It contains a cellulose
pad, 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 salts
and stiffening and wetting agents. Pads are normally installed continuously
along the side or end of the wall opposite the fans. The amount of pad area
needed is calculated by multiplying the floor area by 8 feet and dividing by
250 for a 4-inch pad or 400 for a 6-inch pad. For example, a 30- x 100-foot
greenhouse 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 water
so 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 and
piped to a sump tank. Tank capacity needs to be 0.8 gallon per sq.ft. of pad
for 4-inch pads and 1.0 gallon per sq.ft. for 6-inch pads. Water returning to
the sump should be filtered to remove any debris. A make-up water supply and
float valve keep the water level constant. In areas having water with a high
mineral content, it is advisable to bleed 3-5 percent of the water to minimize
salt buildup. Algae growth in the re-circulated water can be controlled with a
biocide.

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

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

About The Author

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.

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