Natural ventilation in insect screened single span greenhouses under warm weather

Abstract

The ventilation rates of different types of ridge vents in combination with insect-screened side vents were assessed in single greenhouses in terms of the difference in temperature and humidity inside and outside under tropical conditions. The A-frame (slanted roof) was comparatively advantageous over the conventional arch frame (curved roof) for keeping daytime temperature lower in single span greenhouse with insect-screened side vents (mesh size: 1 by 1 mm) and without roof vents. The inclusion of ridge vents further reduced the internal temperature and relative humidity (RH) in the A-frame greenhouse during the daytime. The opening area of the ridge vent within the range between 9.3% and 14% (of the floor area) did not significantly change the ventilation based internal temperature and RH when operated under low wind speeds (0.5±0.5 m s-1). Meanwhile the effect of ridge orientation, with respect to wind direction, on greenhouse ventilation was not obvious in terms of temperature or RH under inconsistent wind directions and low wind speeds. Greenhouse ventilation positively responded to low winds (0.25 m s-1) by reducing internal temperature as well as RH. However, the response to a further increase in wind speed from 0.25 to 0.5 m s-1 was not significant. Based on climate control characteristics an A-frame single-span greenhouse design with double sided alternate ridge vents and insect-screened side vents could be appropriate for tropical climates under low wind speeds and inconsistent wind directions as a cost effective and user-friendly greenhouse design. Particularly, it is highly applicable for the small-scale controlled environment vegetable production in mid and low elevations in the wet zone of Sri Lanka. Keywords: Alternate ridge vent, curved roof, continuous ridge vent, ridge orientation, wind effect.