Influence of Circulation Fans on the Distribution of Air Temperature and Air Velocity in a Greenhouse

This paper shows how airflow and air temperature distribution in a greenhouse are influenced by the number and position of circulation fans. As a first measure, the influence of air movement produced by a circulation fan on the distribution of airflow was examined in greenhouses with no plants and tomato plants at the harvest stage of development. The air velocity measured at 2 m from the center of the fan was 6.04 m s-1 in an empty greenhouse. In general, the air velocity decreased with an increase in distance from the fan. The air velocity measured at 22 m from the fan was 0.33 m s-1 in the empty greenhouse. Moreover, the fully developed tomato plants caused a large decrease in air velocity, for example the air velocity measured at 22 m from the fan was 0.04 m s-1. As a second measure, the influence of different combinations of fan positions and numbers of fans was investigated in a greenhouse with fully developed tomato plants. At the top of the tomato canopy, the air velocity increased with an increase in the number of fans. However, the air velocity decreased at the bottom of the tomato canopy. These results indicate a gradual decrease in airflow on the leeward side caused a non-uniform air temperature distribution in the greenhouse. On the other hand, when the fans were set at 5, 10 and 15 per 1000 m2, the measured average air velocities were 0.24, 0.36 and 0.44 m s-1, respectively. Therefore, we conclude that 10-15 fans per 1000 m2 are necessary to produce an average air velocity of 0.3 m s-1, when a fully developed crop is in the greenhouse.