Cooling System Design Guidelines
Greenhouse cooling requirements vary greatly depending upon
the crop grown and the climate at the greenhouse location. Some general design
guidelines work in most situations:Cold weather ventilation:
Allow approximately 1 to 1.5 cubic feet per square foot of greenhouse floor area
per minute for cold weather ventilation (when it is near or below freezing
outside). This ventilation air must be uniformly distributed and mixed
throughout the greenhouse, before it comes in contact with your crop.
Cooling without evaporative cooling:
Allow for 10 to 12 cubic feet per square foot of greenhouse floor area per
minute for summertime cooling. Crops with large transpiration rates (tomatoes)
may get by with lower ventilation rates as long as they are healthy and full
grown during the hottest times of the year. In effect, the crop is cooling the
greenhouse by transpiration. Empty greenhouses require the highest ventilation
rates.
Cooling with evaporative cooling:
The most efficient evaporative cooling systems will cool the incoming air to
close to the outside wet bulb air temperature, and subsequently cool the air
inside the greenhouse as it is reheated by the sun. The added advantage of
evaporation cooling systems is the reduction of air VPD (Vapor Pressure Deficit)
which reduces water stress in the plant. The air exchange rate required depends
on the ability of the incoming air to absorb the moisture produced by
evaporative cooling beyond what is required to cool it down to the desired
temperature. In very dry climates, even very hot outside air can be cooled down
a great deal and still have additional evaporative cooling capacity left. In dry
climates we often see greenhouses with greatly reduced peak ventilation rates
that still achieve quite good cooling results - as little as 4 to 5 cubic feet
per square foot of greenhouse area per minute. For climates with humid summers,
the theoretical limit to evaporative greenhouse cooling is the wet bulb
temperature, and the practical limit is a few degrees warmer than this. This
usually means that an evaporative cooled house may not be able to cool itself
below the outside air temperature and may only be a few degrees cooler than a
greenhouse without evaporative cooling.
This description is intended to provide a basic explanation
of the evaporative cooling process and its limitations. An understanding of
Enthalpy and the use of psychrometric tables or charts along with good estimates
of evapotranspiration and greenhouse energy gains is required to accurately
calculate cooling system performance. Luckily, simple empirical observation
usually provides a sufficiently accurate estimate of performance. (Copy another
greenhouse with good performance for the same crops in the same climate.)
Remember, a large improvement in the cooling system performance usually results
in only a small reduction in greenhouse temperature. Conversely, a substantially
undersized system may only result in temperatures a few degrees higher. Other
factors, like shading, VPD, crop type, pruning, watering and feeding, may have
more impact than ventilation on crop performance.
Mechanical Cooling:Because
of the inherent limitations of evaporative cooling systems, Conservatories,
Research greenhouses and other specialty crops that require low temperatures or
low humidities should seriously consider Mechanical Air Conditioning (heat pump)
systems instead of evaporative cooling systems. When used in conjunction with
selective glass coatings and shading (to reduce heat load), both the capital and
operating costs can be kept reasonable for this type of application. The system
can be designed to rely on conventional cooling whenever outside conditions
permit, to further save on operating costs. Mechanical refrigeration systems
also make low RH, low temperature environments possible, while evaporative
cooling systems always raise the RH, often to unreasonably high values.
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