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The air flow velocity through the pad normally remains in the range of 0.5-3.0 m/s. They have good saturation efficiency, about 85-90 per cent, thus, yielding exit air temperature close to the wet bulb temperature (WBT). Most commercial pads are made of cellulose sheets, structured to give a bee-hive look and air flow channels across the width of the pad. Warm outside air is drawn through the pores and channels of the wetted pad when the fan is in operation and, as a result, the warm air loses its heat due to the evaporation of water.įigure.1: Fan-pad evaporative cooling systemĬooling pads of varied size and geometry are available. A pump is used to circulate water over and through the wetted surfaces of the pad. An exhaust fan is fitted on one end of the greenhouse while a wetted pad is placed on the opposite end. During evaporation, water takes heat from the warm air, thus, reducing the dry bulb temperature.įan-pad evaporative cooling (Figure 1) has been in use for over half a decade and is the most common system in closed greenhouses.
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All these are direct evaporative cooling methods as water is directly applied into the air by means of spray or wetted surface. Thus, evaporative cooling is more suitable for greenhouses in hot and dry climatic regions.Įvaporative cooling methods which are normally used for greenhouse cultivation are fogging, misting and fan-pad cooling. Dry air naturally has greater cooling potential than moist air. As a result, air temperature decreases, the extent of cooling depends on the incoming air RH and the saturation efficiency of the cooling process. The sensible heat of air is taken up by water, which in turn evaporates and thus, increases the moisture content of the air. In evaporative cooling of air, heat as well as mass exchange occurs between air and water. It is to be noted that, while shade nets are quite effective in restricting solar gain of a greenhouse, it also decreases the light transmittance, decreasing plant growth rate. But natural ventilation fails to extract excess thermal load in high radiation periods and in order to maintain conducive microclimate inside the greenhouse, fan ventilation is resorted to. When temperature is not at detrimental level, natural ventilation with appropriate shading could be employed. It becomes necessary to remove excess heat load from the greenhouse and enhance RH during hot summer and thus evaporative cooling gets due consideration. Growth and quality of most crops and flowers get affected when temperature exceeds 30✬ and RH drops below 50 per cent.
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But traditional open-field cultivation cannot support this potential business in floriculture, particularly in export market where stringent quality requirements are to be met. India has a very high growth potential in floriculture. A proper greenhouse, fully or partially covered, is therefore a need, particularly when commercial scale cultivation is attempted for plants that are sensitive to environmental parameters like radiation, temperature and relative humidity (RH). Mere shading does not serve the purpose and the shade-houses fail miserably in summer. Radiation intensity puts substantial load on a greenhouse for good part of a climatic cycle. Most parts of Indian peninsular region witness hot summer, characterised by high ambient temperature and high solar radiation.