Effect of Screen Mesh Size on Vertical Temperature Distribution in Naturally Ventilated Tropical Greenhouses

Nets and screening materials are often used for greenhouses in the tropics to exclude insects. However, these nets have a direct influence on the microclimate inside the greenhouse. A wide range of such cladding materials are available with different mesh sizes. Experiments were conducted in four greenhouses with a tomato crop inside, to study the spatial distribution of air temperature in the vertical direction under natural ventilation. Both, absolute and relative temperature gradients were observed along the central vertical Z axis of a 3 m by 6 m greenhouse structure. A real-time comparison was made between greenhouses with 53, 34, 33 and 19% porosity screens for two plant growth stages (young and mature) and two plant density levels (single and double). Younger plants occupied 5% of the eaves height, while the mature plants occupied 50%; plant density was doubled from an average 1·7 plants/m2 to 3·3 plants/m2 with three and five rows kept lengthwise, respectively. The highest temperature value was obtained at the points nearer to the roof, which was about 5 °C higher than the coolest point in the vertical direction. The lower 60% of the height profile registered only 86-92% of the maximum temperature value, while the top 40% registered 92-100%. The observed air temperature gradients were up to 14% higher with vegetation. A decrease in porosity increased the vertical gradients from 5 to 10%. These vertical temperature gradient patterns were found to vary with both vegetative condition inside and with the insect screen used to clad the greenhouse, and the data can be used in the development of more accurate mathematical models of air-exchange through natural ventilation.