Controlled Environment Agriculture

Temperature Control

Many modern greenhouses use exhaust fans and air inlets to reduce and control air temperature and relative humidity. Air inlets are typically opened wide when fans start and close when fans stop. Surprisingly, this can create a problem. The problem is that, when wind is more than a slight breeze, it will create suction (negative pressure) zones on the downwind side of the greenhouse, and even along sides of the greenhouse parallel to the wind. Suction can draw air out the ventilation inlets, making inlets into outlets and negatively affecting total ventilation rate as well as air distribution patterns within in the greenhouse. The solution is to open vents only slightly (a few inches) so ventilation fans can maintain a slight vacuum inside the greenhouse, which prevents reverse air flow though the inlets. The inlets are controlled by a differential pressure gauge (available, for example, from Dwyer Instruments) which keeps vents closed sufficiently to maintain a slight vacuum. A Slight vacuum does not affect the ventilation rate, for fans can hold a pressure difference significantly greater than the vacuum required to resist wind. Interestingly, this aspect of mechanical ventilation has been applied successfully to animal house ventilation for many years. Of course, for this to work, the greenhouse must be relatively air tight, which it should be in any case for good air temperature control and uniformity. The following article describes the concept in greater depth.

Download Controlling Greenhouse Ventillation Through Pressure Differences (pdf)