Natural Ventilation of Parral Greenhouses

A study was carried out at the Experimental Station Las Palmerillas of Cajamar to characterise the natural ventilation of the parral greenhouse. An 882 m2, five span, polyethylene film covered parral greenhouse was fitted with either a rolling or a flap ventilator attached to one side of each ridge and rolling ventilators in two 38 m sidewalls. Ventilation rates for different configurations of the ventilators were measured using the dynamic tracer-gas method (decay rate method), with nitrous oxide (N2O) as the tracer gas. With roof ventilators, the highest ventilation rates per unit ventilator area were obtained when flap ventilators faced the wind (100%), followed by flap ventilators facing away from the wind (67%). The lowest rates of roof ventilation were obtained with the rolling ventilators (28%); in this case the ventilation rate was independent of wind direction. The ventilation rates per unit area of rolling ventilator were highest for sidewall ventilation (42%), followed by roof and sidewall ventilation (37%). With the flap ventilators there was a non-linear increase in ventilation with increasing ventilator angle. The global wind pressure coefficient was found to be ventilation system dependent. Theoretical predictions of natural ventilation agreed well with the measured values for roof ventilation with root mean square and mean deviations of less than 1·2 m3/s and ±2%, respectively. However, agreement was less good for sidewall and roof with sidewall ventilation. Theory showed that the stack effect made a significant contribution to sidewall and roof with sidewall ventilation. A deficiency in the method of estimating the stack effect was identified that results in its failure for ventilators in horizontal roofs. Measurements showed that an insect screen with a porosity of 0·39 under the rolling ventilator in the roof, gave a 35% reduction in the ventilation rate. Using this result together with published data, it was shown that the ratio of the ventilation rate with a screen to the value obtained with the same ventilator without a screen could be represented by the quantity ε (2-ε) where ε is the screen porosity.