Articles
Vent, fog and fan, a cooling system for large greenhouses in hot weather with low humidity
Article number
1107_7
Pages
61 – 66
Language
English
Abstract
Vent, fog and fan (VFF) is an effective cooling system being used for large glasshouses in Australia.
Natural ventilation systems in large greenhouses are unable to exchange cooled greenhouse air with outside air when outside air temperatures are considerably higher than the desired (cooled) greenhouse air temperatures, because the greenhouse air is so much denser than the hot buoyant outside air.
Fog cooling becomes ineffective if the greenhouse air approaches saturation, as it would with no ventilation.
Effective cooling can be obtained by using exhaust fans with fogging, given low humidity in the outside air.
Ventilation air is drawn in through roof ventilators and exhausted at ground level by exhaust fans in the walls.
This system provides even temperatures within the greenhouses, without the temperature gradient associated with pad and fan cooling.
Installing fans is an additional capital cost, but their operating cost is minimal as they only need to be used when the outside air temperature is much higher than the required greenhouse temperature.
This system cools the air at the top of the crop evenly over the whole area of the greenhouse.
All other cooling systems for these operating conditions cool the air at the point of entry into the greenhouse and then absorb the heat by mixing cool air with warm greenhouse air, resulting in temperature and humidity gradients between inlets and exhausts.
Horizontal temperature gradients are very small with VFF. The vent, fan and fog system is described with experiences and results from a 1 ha glasshouse growing tomatoes.
Control problems are discussed with reference to the need for commercial software development.
Natural ventilation systems in large greenhouses are unable to exchange cooled greenhouse air with outside air when outside air temperatures are considerably higher than the desired (cooled) greenhouse air temperatures, because the greenhouse air is so much denser than the hot buoyant outside air.
Fog cooling becomes ineffective if the greenhouse air approaches saturation, as it would with no ventilation.
Effective cooling can be obtained by using exhaust fans with fogging, given low humidity in the outside air.
Ventilation air is drawn in through roof ventilators and exhausted at ground level by exhaust fans in the walls.
This system provides even temperatures within the greenhouses, without the temperature gradient associated with pad and fan cooling.
Installing fans is an additional capital cost, but their operating cost is minimal as they only need to be used when the outside air temperature is much higher than the required greenhouse temperature.
This system cools the air at the top of the crop evenly over the whole area of the greenhouse.
All other cooling systems for these operating conditions cool the air at the point of entry into the greenhouse and then absorb the heat by mixing cool air with warm greenhouse air, resulting in temperature and humidity gradients between inlets and exhausts.
Horizontal temperature gradients are very small with VFF. The vent, fan and fog system is described with experiences and results from a 1 ha glasshouse growing tomatoes.
Control problems are discussed with reference to the need for commercial software development.
Authors
R.A.J. White
Keywords
forced ventilation, above crop cooling, horizontal and vertical gradients, pad and fan
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