The greenhouse environment can quickly become too hot for
healthy plant production even in the winter.
During the summer months extra cooling beyond what fan ventilation can
provide may be necessary. In a previous blog
and High Tunnel Cooling: Shading, the use of shade was discussed as an
additional cooling measure during warm weather. Evaporative cooling is another tool that can
be used to reduce greenhouse temperatures.
A "fan and pad" system uses exhaust fans to pull air through evaporative
cooling pads. This technique utilizes the cooling effect produced when water
evaporates and cools the air as it is pulled through the pad.
Cooling pads cool by creating large amounts of surface area that are exposed to air
being drawn into the greenhouse by exhaust fans. This large surface area allows for quick
evaporation. Common pad materials include cellulosic materials or aspen fiber pads. Cellulosic pad materials are the preferred choice by most growers as they last longer than aspen pads.
Evaporative cooling becomes more efficient as the relative humidity drops.
In Alabama we have exceptionally high humidity, especially along the
Gulf Coast. The ability to cool with evaporative cooling can be estimated by
taking the difference between wet and dry bulb temperatures. This difference is the Wet Bulb Depression (WBD). WBD represents how much the air can be cooled
through evaporative cooling. As humidity increases, the gap between wet and
dry bulb temperature decreases resulting in less efficient cooling.
Because of the high humidity in
Alabama the ability to cool with evaporative cooling is somewhat limited, but
may be significant depending on the situation.
Table 1. Summarizes WBD in one of the most humid areas of our state,
Mobile, AL. WBD was calculated for what
was considered the warmest time of day (11:00 AM to 3:00 PM). The data suggest that during 64% of the hours,
intake air temperature could be reduced by over 5°F during July, 2014. The reduction in air temperature only occurs
around the exhaust side of the cooling pad.
Even in a well-designed greenhouse, air can easily pick up over 8-9 F° as it
passes through the greenhouse. Cooling pads are not going to be able to
achieve 100% efficiency. For a
freestanding 30 x 96 ft. greenhouse, cooling pad equipment cost can be anywhere
from $1800-$2000. Depending on your
situation, the cooling capacity of a fan and pad system may or may not be
significant enough to justify that level of investment.
Fan and pad systems need to be
sized appropriately to achieve the maximum cooling efficiency. When a fan and pad system can not be justified,
in some situations passive ventilation with open sided greenhouses may provide
adequate cooling for a given crop without using any energy. In the case where biosecurity or insect exclusion is a priority a closed greenhouse with a fan and pad system may be
necessary. Greenhouse lettuce growers in
Baldwin County have had great success with cooling pads in combination with reflective
shade cloths. Floriculture growers
throughout Alabama have also had great success with passively ventilated
Cooling pads require regular
maintenance to insure they remain efficient.
Algae and mineral build up can reduce air movement and lower efficiency. Consult with the cooling pad manufacturer before applying any chemicals, as some can reduce the life expectancy of pad materials. The following links may provide some useful information for cooling pad care:
Kuul-pad approved chemicals
Handling mineral deposits
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