Enhancing fog evaporation rate using an upward air stream to improve greenhouse cooling performance

Fogging systems are still not widely used for greenhouse cooling mainly owing to their low cooling efficiency and high risk of pathogen expansion caused by excess wetness of plant foliage. To increase the cooling efficiency and to keep the foliage relatively dry, the ratio of evaporated fog to generated fog (i.e. evaporation ratio) needs to be increased. In this study, a vertically installed conventional nozzle with two small fans (100 mm by 100 mm) to provide an upward air stream for enhancing fog evaporation was compared with a vertical nozzle and an approximately horizontal nozzle, both without any fan. To minimise the effect of other factors, the study was conducted under indoor conditions. With fans, a downward cooled air stream with fog was rarely observed by an image-enhancing camera. On the other hand, without fans, a strong downward cooled air stream with fog was observed, indicating that most of the fog descended without evaporation. The nozzle with fans provided a 1.5 times better evaporation ratio and three times wider cooling area than did the nozzle without fans under similar conditions. The nozzle with fans produced a lower and more uniform air temperature. The system employing a nozzle with small fans can achieve a higher evaporation ratio, resulting in an increased greenhouse cooling efficiency and a decreased possibility of pathogen expansion. (C) 2005 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd.