Grower 101: Using Evaporative Cooling, Part II

April 18, 2003 - 12:37

Find out more about how to keep your greenhouse ventilated and cool in the heat of summer.

The heat of the summer is right around the corner, and that
means more energy is being used to keep your greenhouse cool. As you learned in
Part I in the March issue of GPN, fan and pad systems are great for cooling,
but there are more options: swamp coolers, mist and fog systems, and fan-generated
fog units.

Swamp coolers

Typically known as "swamp coolers," self-contained
evaporative cooling units are mounted on a concrete pad adjacent to the
greenhouse. A unit consists of a metal enclosure that contains a blower or fan and
either cellulose pads and a water pump or a polyester belt that rotates in a
pan of water. The dry outside air drawn in through the pads or belt picks up
moisture before it enters the greenhouse. After the cool air is heated inside
the greenhouse, it is exhausted through the louvers or vents, taking the heat
with it. These units work well in naturally vented greenhouses.

Portable swamp coolers are also available. They are usually
placed inside the door of a greenhouse on the end opposite where the fans are
located. Air that enters through the door is conditioned and picks up heat as
it travels toward the fans. A hose connection and electric supply are needed.
One unit will cool up to 1,200 sq.ft. Measurements taken last summer in a
Connecticut greenhouse showed that air temperature in a shaded greenhouse with
swamp coolers was several degrees below outside ambient.

Mist and fog systems

Research has shown that fine mist or fog is more effective
than cooling pads or swamp coolers. Most mist and fog systems utilize
high-pressure nozzles to form fine water droplets. Mist contains droplets in
the 50- to 100-micron size (thickness of a human hair = 0.004 inches = 100
microns). Fog contains droplets of 0.05-50 microns. The smaller the droplet
size, the quicker it absorbs heat and changes to the vapor state. Larger
droplets take longer to change and can end up wetting the surface they fall on.
Therefore fog is more desirable for evaporative cooling whereas mist is usually
better for plant propagation.

A high-pressure pump is necessary to develop the force
needed to form mist or fog. Depending on the nozzle style, it takes 60-500 psi
to form mist droplets and 500-1,200 psi to form fog. High-pressure piping is
also necessary. Particular care has to be given to filtration to prevent nozzle
clogging. Because so little water is needed, (typical nozzle output is
0.5?1.0 gal. per hour) some growers use demineralized, bottle water or
rain water. Anti-drip nozzles are also installed to prevent draining the system
after the water is shut off.

Most fan systems are set up with many of the nozzles over
the intake louvers and vents, with a small percentage distributed throughout
the crop area. Building shelters over the intake end of the greenhouse and
mounted nozzles within this will restrict any dripping to outside the
greenhouse. In natural ventilation systems, the nozzles are uniformly
distributed throughout the greenhouse.

Fan-generated fog units

An alternative system utilizes a
fan with a special hub and blade assembly. Water is fed into the hub and
channeled to the four blades. As the water exits the end of the blade, it
encounters a shearing speed and atomizes into fine fog droplets. The fan
distributes the fog in the greenhouse. Sizes are available that will cool up to
several thousand square feet of growing area.

With all systems, adding
excessive moisture to the greenhouse can increase the incidence of disease.

Usually, evaporative cooling is
the last stage of cooling in the greenhouse. If the fan system or natural
ventilation cannot satisfy the environmental needs, the evaporative cooling
system activates. Control can be from a thermostat, controller or computer. If
you need to keep the temperature down for crop production this summer, try
evaporative cooling.

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.

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