Arbel, A., Institute of Agricultural Engineering, Agricultural Research Organization, Bet-Dagan 50250, Israel
Shklyar, A., Institute of Agricultural Engineering, Agricultural Research Organization, Bet-Dagan 50250, Israel
Barak, M., Institute of Agricultural Engineering, Agricultural Research Organization, Bet-Dagan 50250, Israel

A theoretical study was conducted to evaluate an evaporative cooling system for greenhouses in the following set-up: (a) evaporative cooling by installing uniformly distributed fogging nozzles in the space over the plant canopy; and (b) natural ventilation based on thermal effect using uniformly installed openings on the roof and on lower part of the side walls of the greenhouse. In accordance with this setup, a computer model was established in order to calculate the required ventilation opening area (on the roof and the side walls) as related to the desired conditions inside the greenhouse and to the ambient conditions. Usually, differences in temperature between inside and outside the greenhouse are used for calculating the required openings area. This computer model, however, is based on the difference in air density between inside and outside the greenhouse. Results indicate that the use of temperature difference can lead to a significant error in ventilation design. This error increases as the temperature difference decreases and the relative humidity difference increases.