The objective of this work was to experimentally investigate the influence of vent type (side, roof or both) and of an anti-aphid insect screen used to prevent insect intrusion on the ventilation rate of a round arch with vertical side walls, polyethylene covered greenhouse. The greenhouse was equipped with two side roll-up vents and a flap roof vent located at the University of Thessaly near Velestino in the continental area of Eastern Greece. Microclimate variables as well as the airflow rate were measured during summer. Two measuring methods were used for the determination of ventilation rate: (a) the decay rate 'tracer gas' method, using nitrous oxide N{sub}2O as tracer gas, and (b) the greenhouse 'energy balance' method. In order to study the effect of vent type on ventilation rate, in a greenhouse with an anti-aphid insect screen in the vent openings, airflow was determined during periods with ventilation being performed by: (i) roof, (ii) side or (iii) both roof and side vents. Furthermore, in order to study the effect of insect proof screen on airflow, measurements were also carried out during periods that ventilation was performed by side vents without a screen in the openings. A good correlation was found between the air exchange rate values calculated using the two methods, with the values obtained by the tracer gas method being slightly lower than those obtained by the energy balance method. Furthermore, the data of ventilation rate obtained by the tracer gas method fitted better to the model used for the prediction of ventilation rate. In addition, the use of anti-aphid screen in vent openings caused a 33% reduction in greenhouse ventilation rate. From greenhouse ventilation performance point of view, it was found that the most effective vent configuration was the combination of roof and side vents, followed by side vents only (46% reduction in ventilation), while the least effective was roof vent (71% reduction ventilation).
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