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A combining cooling system is proposed and analyzed to achieve higher cooling efficiency in hot and humid climatic conditions such as in Saudi Arabia. The system was investigating and tested under the weather conditions of Al-Ahssa, Saudi Arabia. In this paper, the results of a study on combining cooling coil unit (CCU) and direct evaporative cooling (DEC) have been discussed.The cooling effectiveness of the cooling coil unit and direct evaporative cooling systems are experimentally investigating on small greenhouse model.Total solar radiation entering the greenhouse, temperature and relative humidity of outdoor air, dry and wet-bulb temperatures of air just leaving the cooling coil unit CCU), and dry and wet-bulb temperatures of air just leaving the direct evaporative cooling system (DEC) were measured and recorded on a data-logger to analyses their correlation with the effectiveness of each cooling system. The obtained data showed that, the hourly average solar radiation flux incident outside and transmitted into the greenhouse was 555.7 and 298.7 W/m2, respectively, consequently, the effective transmittance of the covering material was on the average 53.75%.  The temperature and relative humidity of outdoor air, respectively, were 37.3oC and 21.8%, dry and wet-bulb temperatures just leaving the cooling coil were 29.4oC and 18.8oC, respectively, and dry-bulb temperatures of air just leaving the direct evaporative cooling were 22.8oC. The obtained results also revealed that, the overall effectiveness of the combining cooling system was more than 100%. Thus, this system environmentally clean and energy efficient system, which considered as an alternative to the mechanical, vapour compression systems. This combining cooling system provided and maintained optimal microclimatic conditions that achieved vapour pressure deficit (VPD)  *Assistant Professor, Department of Agricultural Systems Engineering, King Faisal University, Hofuf, Saudi Arabia.   of 0.76 and 0.40 kPa inside the greenhouse model during daylight and at night, respectively. It can also conclude that, this combining cooling system can use in various climatic conditions as an environmentally clean and energy efficient system.

A METHODE TO IMPROVE EVAPORATIVE COOLING SYSTEM IN GREENHOUSES UNDER HOT AND HUMID CLIMATIC CONDITIONS