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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;.
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Lesson 21 • Air conditioning system
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 unitsare 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 theoccupant 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 terminalslinedwith 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. Asthese contain small water passages, choking(堵塞) develops rapidly with direct sea water circulation anda better method is circulate with fresh water, itself cooled in a sea water/fresh water heat exchanger.
Control is on/off by a thermostatsensing 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 theroom, 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 recirculationgrill(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 largeacceptable.
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) againstthe 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 precautionfor 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.
