制冷与空调技术基础.ppt

* * A cooling tower is a device commonly used to cool condensing water. In this design, warm water is sprayed over fill in the cooling tower while a propeller fan draws outdoor air upward through the fill. The movement of air through the spray causes some of the water to evaporate, a process that cools the remaining water. This cooled water then falls to the tower sump to be returned to the condenser. The final temperature of the water leaving the tower is determined, in part, by the humidity of the outdoor air. If the outdoor air is dry, the final water temperature can be considerably lower than the ambient dry-bulb temperature. If the outdoor air is humid, however, the final temperature will be near the ambient dry-bulb temperature. While a cooling tower can reclaim much of the condensing water, it cannot reclaim it all. The evaporation process uses up water to dissipate heat contributed by the cooling load plus the heat of compression. In addition, as the water evaporates, the dissolved minerals and water treatment chemicals become concentrated in the sump. To prevent this solution from becoming concentrated and possibly corrosive, water is periodically bled from the sump and an equal amount of fresh water is added. In the past, some water-cooled condensers used water from either a municipal or a natural water supply as the condensing water. After rejecting the condenser heat to this water, it was dumped into the sewer or back into the body of water. Environmental and economic restrictions have made this method uncommon. Finally, a geothermal well system can be used to reject the heat from the condenser by circulating the condensing water through a series of underground pipes. This method takes advantage of the naturally-cool ground temperatures. * The major components of a vapor-compression refrigeration system include the compressor, condenser, expansion device, and evaporator. The latter three will be discussed in this clinic—the compressor is discussed in a s