DESIGN OF AIR DISTRIBUTION SYSTEMS FOR CLOSED GREENHOUSES

In order to maintain uniform environmental conditions within a greenhouse, and hence uniform crop production, it is important that the air distribution system be correctly designed. This is more critical in a closed greenhouse, specifically designed for carbon dioxide enrichment, because low natural air infiltration can result in significant vertical and horizontal stratification of temperature, humidity, and carbon dioxide concentration.

The use of perforated polyethylene ducting to distribute air within greenhouses has become increasingly common. Ducting can be used for general air circulation, to eliminate stratification, and to deliver warm, cool, and CO₂ enriched air to precise points within the greenhouse. With careful design the greenhouse engineer should be able to ensure that all plants within a greenhouse receive optimal environmental conditions.

The designer of perforated ducting is faced with a number of choices regarding the size and layout of the system: the number of ducts to be used and their position within the greenhouse relative to the crop rows, the length and diameter of the duct, the number, size, shape, and spacing of the discharge holes, and the static pressure within the duct. Other factors that will affect the selection are the required air circulation rate and air velocity within the plant canopy to enhance heat and mass transfer without causing damage, and the temperature of the air being delivered. Once a duct has been selected the designer would like to know its system characteristics so that the most efficient inflation fans can be selected.