Physical properties of covering materials for naturally ventilated tropical greenhouses

R. Kamaruddin, B. J. Bailey and M. P. Douglass

Abstract

In the tropics, covering materials such as transparent polyethylene film and insect screens are commonly used for greenhouse roofing and sidewall respectively. The importance of covering material properties is exhibited in the greenhouse design and also in quantifying natural ventilation rate. This paper discusses how measurements of physical properties of roofing and sidewall materials are made. Light transmission of polyethylene film and insect screens was found to be almost similar, that is more than 75%. This will give similar light transmission to be received by the crop. Coefficient of discharges of the screens N50, N32 and N24 was found to be 0.411, 0.520 and 0.547 respectively. The smallest screen had the lowest value coefficient of discharge. When air flowed through the screen, the pressure drop increased linearly with the square of approach airspeed. It also increased with increasing screen hole. The relationship also showed that the pressure drop across the screen had increased with increase in apparent airspeed according to Forchheimer’s flow regime or a quadratic law.

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