Presentation on theme: "Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:"— Presentation transcript:
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 mmH 2 Og) pressure compared to “low velocity” in the air ducts with fans generating about 520 mbar (50 mmH 2 Og).
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 chilled( 使冷, 变冷 ) water cooled type.
When cooling is by direct expansion, a separate steam heater coil is fitted in the unit for winter heating.
With brine or water coolers, a central heater is used so that the same coil serves for summer or winter.
Thermostatic control is provided sensing air delivery temperature itself, the temperature of the room, or the return air temperature.
All types of thermostats are found in air conditioning systems, direct acting, pneumatic and electrical.
In themselves, they are all satisfactory instruments, but the results they achieve are dependent on the correct sitting of their sensing elements.
Even the site for a direct acting thermostat to control one single berth( 铺 位 ) cabin must be chosen with care—if it is masked behind curtains, or too far away from the air inlet control will be too sluggish( 不灵敏的、迟缓的 ).
The correct location for a thermostat to control a block of cabins is more difficult to find.
One can pick on a “typical” cabin—but if the occupant opens his porthole ( 舷 窗 ) he can upset the whole block.
Another possibility is to site the thermostat in the alleyway ( 通道 ) of the block of cabins.
This position may be affected more by an open door or draught ( 气流 ) in the alleyway than by the temperature of the cabins.
Yet another possibility is to site the thermostat in the recirculation air trunk, carrying air back from the accommodation to the unit.
If the recirculation grill( 再循环格栅口 ) is close to an outside door, this position too can be affected by outside air temperature when the door is open, rather than by cabin temperature.
Reading Material A. General operation of the air conditioning installation
The first essential in operating the air cooling appliances( 设备 ) throughout the ship is to have all thermostats correctly set and correctly functioning.
In extreme weather conditions, either hot or cold, control of the plant usually presents few difficulties.
The capacity of many installations is such that under tropical conditions nearly all control valves move to the fully open position.
Although automated control has been lost, internal conditions are by and large acceptable.
Control difficulties arise in intermediate weather conditions when there is a call for only a small amount of cooling.
The worst case is when part of the ship, say inboard cabins( 内侧舱室 ) against the engine room, require cooling and other parts, say exposed upper cabins, require warming.
For this intermediate condition, thermostats must be correctly set by trial and error( 逐次逼近法、反复试验 ).
It is found that a uniform setting of say 21 ℃ throughout the ship is not satisfactory, but slight variations of a few degrees up or down are needed to suit particular regions of the ship.
Unfortunately, these variations in thermostat setting are not always the same for the cooling and heating condition and frequent resetting may be needed for a ship repeatedly passing from cold to warm weather.
The control problem is eased if the chilled brine (or water) of systems using chilled liquid circulation is held at about 13 ℃ in the intermediate weather conditions and lowered progressively to about 5 ℃ as tropical weather conditions are approached.
When air cooling is in use it is good practice to keep all portholes, windows and doors shut.
On passenger ship, some public announcement requesting that this be done is worthwhile.
A wise precaution for an engineer to take is to go through accommodation and public rooms periodically recording wet and dry bulb temperatures( 干湿球温度 ).
Keeping a log of these readings then serves to identify any malfunctioning of the installation as soon as it arises.
The quantity of cooled air delivered by an air conditioning unit should balance the sum of the quantity of air recirculated to the unit and the quantity mechanically exhausted.
The correct balance between supply and exhaust fans should be checked periodically.
Even with filters fitted ducts can become partially blocked and fan performance can fall off to upset the balance.
On older ship, temperature maintenance can be made easier by increasing the ratio of recirculated to fresh air( 循环空 气与新鲜空气的比率 ).
Most air conditioning units have dampers ( 风门 ) for adjusting this ratio and the effect of these can be extended after they have reached full travel by partially blocking fresh air inlets.
Care must be taken not to reduce the fresh air so that stuffiness( 不通风，闷 ) or smell arise.
Cleaning or renewal of filters is necessary at about 3 monthly intervals, the time varying according to location on the ship.
Disposable( 一次性的 ) filters can be vacuum cleaned so that in fact two or three ‘live’ are obtained before they need to be thrown away.
In addition to normal mechanical attentions, such as lubrication of bearings, and adjustment of fan belts and cleaning of motors, careful greasing of linkages of automatic controls is necessary.
Cooled air ducts should be examined to see that the insulation vapour seal remains in good order.
If a plastic film vapour seal becomes damaged, condensation forms within the film.
As well as making the insulation wet and ineffective, the condensation may become serious enough to cause drips( 滴水 ) and damp patches( 湿的斑纹 ) on ceilings.
B. THERMOSTATIC EXPANSION VALVES ( 热力膨胀阀 )
Thermostatic Expansion Vales This is the regulator through which the refrigerant passes from the high pressure side of the system to the low pressure side:
The pressure drop causes the evaporating temperature of the refrigerant to fall below that of the evaporator.
Thus the refrigerant can be boiled off by an evaporator temperature of –18 o C, for example, because the pressure drop brings the evaporating temperature of the refrigerant to say –24 o C.
The liquid refrigerant leaves the condenser with a temperature just above that of the sea water inlet, say 29 o C.
As it passes through the expansion valve the evaporating temperature decreases to –24 o C and
and some of the liquid boils off taking its latent heat( 潜热 ) from the remainder of the liquid and reducing its temperature to below that of the evaporator.
capillary bellows Push pins bulb Equalizing line
Equalizing line bellows Bulb Evaporator coil Push rod Outlet p1p1
The aperture( 孔, 口 ) in the expansion valve is controlled by pressure variation on the top of a bellows( 感压箱, 波纹管, ( 真空 ) 膜盒 ). Bellows manometer 波纹管式压力表
This is effective through the push pins and tends to open the valve against the spring.
Spring pressure is set during manufacture of the valve and should not be adjusted.
The pressure on the bellows is from a closed system of heat sensitive fluid( 感温 液体 ) in a bulb and capillary( 毛细管 ) connected to the top of the bellows casing.
The bulb is fastened to the outside of the evaporator outlet so that temperature changes in the gas leaving the evaporator are sensed by expansion or contraction of the fluid.
Ideally the gas should leave with 6 or 7 o C of superheat. This ensure that the refrigerant is being used efficiently and that no liquid reaches the compressor.
A starved condition in the evaporator will result in a greater superheat which through expansion of the liquid in the bulb and capillary, will cause the valve to open further and increase the flow of refrigerant.
A flooded evaporator will result in lower superheat and the valve will decrease the flow of refrigerant by closing in as pressure on the top of the bellows reduces.
Saturation temperature is related to pressure but the addition of superheat to a gas or vapour occurs the latent heat transaction( 交换 ) has ended.
The actual pressure at the end of an evaporator coil is produced inside the bellows by the equalizing line and this is in effect more than balanced by the pressure in the bulb and capillary acting on the outside of the bellows.
The greater pressure on the outside of the bellows is the result of saturation temperature plus superheat.
The additional pressure on outside of the bellows resulting from superheat, overcomes the spring loading which tends to close the valve.
A hand regulator is fitted for emergency use.
It would be adjusted to give a compressor discharge pressure such that the equivalent condensing temperature shown on the gauge at the compressor outlet was about 7 o C above the sea water inlet temperature and
and the suction gauge showed all equivalent evaporating temperature about the same amount (7 o C) below the evaporator.