THE INFLUENCE OF FANS ON ENVIRONMENTAL-CONDITIONS IN GREENHOUSES

This paper shows how the horizontal flow of air in a greenhouse is influenced by the number and position of air recirculation fans, and by the presence of a growing row crop of tomatoes. The three-dimensional distributions of air speed, temperature, humidity and carbon dioxide (CO2) concentration were measured with and without the fans in operation. The uniformity of the spatial distribution of the studied climatic variables decreased as the solar irradiance increased. In "still-air" conditions on sunny days (average values during the experimental period: outside solar radiation 445 W m-2, inside temperature 30·C, inside water vapour pressure 3·78 kPa, CO2 concentration 630 vpm (parts per million by volume)), spatial variations of 7 K in temperature, 1 Kpa in water vapour pressure and 150 vpm in CO2 concentration were detected. When the fans were used these variations were reduced to 1·6 K, 0·3 Kpa and 20 vpm respectively. The growing crop reduced the spatial uniformity of air speed; the ratio of the maximum to minimum speed was two in the empty greenhouse but rose to seven when the plants were 2·6 m high. Low air speeds occurred in the lower regions of the canopy when the crop was tall. Some damage to leaves and fruits was observed on plants immediately in front of the fans. © 1994 Silsoe Research Institute.