新版轮机英语unit18 air conditioning system

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However, some air is “consumed” by humans and some machinery so there is a requirement for renewal（补充）.
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Public rooms and accommodation (住处) will operate with a reduced percentage of air renewal since the conditioning cost of 100% renewal would be considerable.
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All types of thermostats（温度 调节装置） are found in air conditioning systems, direct acting, pneumatic and electrical.
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In themselves, they are all satisfactory instruments （ 器 械 ） , but the results they achieve are dependent on the correct sitting of their sensing elements（传感元件）.
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Another possibility is to site the thermostat in the alleyway(通 道) of the block of cabins.
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This position may be affected more by an open door or draught( 通 风 , 气 流 ) in the alleyway than by the temperature of the cabins.
The single duct system(单风管 系 统 ） only allows for adjustment of temperature in each room by the occupant（居 住 者 ） manually controlling the air volume admitted.
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Even the site for a direct acting thermostat to control one single berth(卧铺) cabin must be chosen with care— if it is masked behind curtains（窗 帘）, or too far away from the air inlet control will be too sluggish(不 灵敏的、迟缓的).
Lesson 18 Air conditioning system
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Ships travel the world and are therefore subject to various climatic(气候） conditions.
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The crew of the ship must be provided with reasonable conditions in which to work regardless of the weather.
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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.
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but it incurs(招致) the heavier running costs( 运 行 成 本 ） of more powerful fans.
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Air terminals lined with sound insulation material are necessary to reduce the noise passing into the room with high velocity systems.
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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
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The correct location for a thermostat to control a block of cabins is more difficult to find.
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One can pick on a “typical” cabin（典型舱室）—but if the occupant( 居 住 者 ) opens his porthole(舷窗) he can upset （扰乱） the whole block.
Self-contained units are noisier than central systems, require more maintenance and have been found to have a relatively short life (about 7 years.)
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a better method is circulate with fresh water, itself cooled in a sea water/fresh water heat exchanger.
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Control is on/off by a thermostat ( 温 度 传 感 器 ) sensing the temperature of air returning to the unit.
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and central systems, where larger refrigeration machinery units are installed and their output distributed about the ship by a variety of means.
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and a direct expansion cooler( 直 接 膨 胀 式 冷 却 器 ) served by a hermetic(气密的) compressor. Water cooled condensers are used.
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As these contain small water passages, choking( 堵 塞 ) develops rapidly with direct sea water circulation and
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With brine or water coolers, a central heater（中央换热器） is used so that the same coil serves for summer or winter.

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Thermostatic control(控制温度) is provided sensing air delivery temperature itself, the temperature of the room, or the return air(回风） temperature.
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Air conditioning systems fall into two main classes:
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individual unit systems(独立空 调系统) in which each room contains its own small refrigeration plant and fan and air cooler;
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Relative humidity, expressed as a percentage, is the ratio of the water vapour pressure in the air tested, to saturated（饱和的） vapour pressure of air at the same temperature.
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The fact that less water can be absorbed as air is cooled and more can be absorbed when it is heated is the major consideration in air conditioning system design.
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Yet another possibility is to site （地点，场所） the thermostat in the recirculation air trunk, carrying air back from the accommodation to the unit.
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Other factors are the nearness（靠 近） of heat sources, exposure to sunlight, sources of cold and the insulation （ 绝 热 ） provided around the space. (Fig.18-1)
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The cooling coil（冷却盘管） of the central unit may be of the direct expansion, brine(盐 水) or chilled(使冷, 变冷) water cooled（冷水式） type.