Effect of insect-proof screens and roof openings on greenhouse ventilation

The object of this work was to determine and model the air exchange rate of greenhouses with insect-proof screens over the vents. Experimental tests were conducted on multi-span tunnel greenhouse with two types of ventilation openings, one with continuous roof vents and the other with an insect-proof screen which was able to replace a flexible roll-up plastic cover. Ventilation rates of the experimental greenhouses were measured using the tracer gas technique. Discharge coefficients were determined in a 1:3 scale model replica of the experimental greenhouses. A simple model that includes the global effect of the wind on the greenhouse ventilation was developed and compared with the measured air exchange values. The main results determine the reduction in the discharge coefficient when an insect-proof screen is incorporated. It was found that vents located in lateral spans have a higher discharge coefficient than those located in central spans. Some practical considerations for the design of more efficient ventilation systems can be derived from this fact. The global wind effect coefficient was found to be dependent on the type of vent, insect-proof screen and wind speed. Some results are not in agreement with others previously published, but obtained under different environmental conditions. This coefficient is derived from empirical measurements which makes its general application questionable. In spite of its simplicity, the theoretical model describes accurately the ventilation rate of the greenhouse with both types of vents. The roll-up roof vent provides a ventilation rate about three times greater than the continuous roof vent. It has an acceptable air exchange, while acting as a barrier against the entrance of undesirable pests. (C) 1999 Silsoe Research Institute.