Fog and Fan Method for Greenhouse Cooling

Abstract

Fog and fan method, a new evaporative cooling method of greenhouse has been developed.For evaporative cooling, not only the volume of water to evaporate but also the voiume of fresh air wihch is taken into the greenhouse is one of the most important factors to be consdered. In pad and fan method, water spray and resulting evaporation are occured at the pad installed at the greenhouse wall which forms a large resistance for the air flow through the pad. In the present method, fine water droplets are sprayed in the greenhouse directly and evaporation takes place simultaneously in the greenhouse. Therefore, there is no resistance for intake air flow and a fan with considerably smaller capacity will be able to produce the same flow rate. Furthermore, horizontal variation of air temperature will be smaller.In the present method, droplets must de small enough to evaporate completely before they reach the floor, when nozzles are arranged at 3m height dry bulb temperature is 30.0°C and relative humidity is 80% in the greenhouse, droplets whose diameters are smaller than 88 microns can evaporate completely before they reach the floor. In order to generate fine droplets, the authors developed a new impaction nozzle. This nozzle is made of plastics and designed to be used at rather high pressures, 5-20kg/cm2.The generated droplet diameter becomes smaller as the pressure increases and a mode of diameter distribution exists in the range between 30 and 40 microns at the pressure of 10kg/cm2. Relative frequency of droplet diameters at three different pressures is shown in Fig. 2.A field experiment has been conducted using a seven-spanned greenhouse (49×42). Air space under the single span of the east end is separated by the polyethylene film from the other and is installed by 30 nozzles and 2 fans. The other is installed by six pairs of pad and fan system, although their capacities are too small to cool the rest greenhouse space. Results of cooling are shown in Figs. 4, 5 and 6. Air temperature in the fog and fan compartment is always lower than the outside air temperature, and is lower about 5°C than that of pad and fan compartment at noon. The horizontal variation of air temperature in the fog and fan compartment is within 1.1°C. It is remarkably small comparing with that of pad and fan compartment. Total volume of sprayed water is 3.0kg/min, and the floor area of the compartment is 280m2. Then, the volume of sprayed water per unit floor area is 10.7g/m2min.This fog and fan system can be installed easily and inexpensively at conventional greenhouses.