lesson 21 Air conditioning system 轮机英语教学课件.ppt

Lesson 21 Air conditioning system Air conditioning systems fall into two main classes: individual unit systems(单体式系统) in which each room contains its own small refrigeration plant and fan and air cooler; and central systems, where larger refrigeration machinery units are installed and their output distributed about the ship by a variety of means. Self-contained units are noisier than central systems, require more maintenance and have been found to have a relatively short life (about 7 years.) The single duct system only allows for adjustment of temperature in each room by the occupant manually controlling the air volume admitted. It is thus less flexible than any of the other systems, which allow individual temperature control, at least of sections of the ship if not individual rooms. With ducted systems, the modern tendency is to use “high velocity” in the air ducts with fans generating up to 2550 mbar (250 mmH2Og) pressure compared to “low velocity” in the air ducts with fans generating about 520 mbar (50 mmH2Og). This tendency helps installation as the size of ducts is reduced and prefabricated(预制) standard ducts can be used, but it incurs(招致) the heavier running costs of more powerful fans. Air terminals lined with sound insulation material are necessary to reduce the noise passing into the room with high velocity systems. In a typical marine pattern self-contained unit, air circulation is usually effected by means of a centrifugal fan, for quiet running, and a direct expansion cooler(直接蒸发式冷却器) served by a hermetic(密封的, 气密的) compressor. Water cooled condensers are used. As these contain small water passages, choking(堵塞) develops rapidly with direct sea water circulation and a better method is circulate with fresh water, itself cooled in a sea water/fresh water heat exchanger. Control is on/off by a thermostat sensing the temperature of air returning to the unit. The cooling coil of the central unit may be of the direct expansion, brine(盐水) or chill