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Lesson 21 Air conditioning system

Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

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Page 1: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

Lesson 21• Air conditioning system

Page 2: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Air conditioning systems fall into two main classes:

Page 3: 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;

Page 4: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• and central systems, where larger refrigeration machinery units are installed and their output distributed about the ship by a variety of means.

Page 5: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Self-contained units are noisier than central systems, require more maintenance and have been found to have a relatively short life (about 7 years.)

Page 6: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The single duct system only allows for adjustment of temperature in each room by the occupant manually controlling the air volume admitted.

Page 7: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 8: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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).

Page 9: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 10: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Air terminals lined with sound insulation material are necessary to reduce the noise passing into the room with high velocity systems.

Page 11: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 12: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 13: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Control is on/off by a thermostat sensing the temperature of air returning to the unit.

Page 14: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 15: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• When cooling is by direct expansion, a separate steam heater coil is fitted in the unit for winter heating.

Page 16: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• With brine or water coolers, a central heater is used so that the same coil serves for summer or winter.

Page 17: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Thermostatic control is provided sensing air delivery temperature itself, the temperature of the room, or the return air temperature.

Page 18: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• All types of thermostats are found in air conditioning systems, direct acting, pneumatic and electrical.

Page 19: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• In themselves, they are all satisfactory instruments, but the results they achieve are dependent on the correct sitting of their sensing elements.

Page 20: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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 ).

Page 21: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The correct location for a thermostat to control a block of cabins is more difficult to find.

Page 22: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• One can pick on a “typical” cabin—but if the occupant opens his porthole(舷窗 ) he can upset the whole block.

Page 23: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Another possibility is to site the thermost

at in the alleyway(通道 ) of the block of cabins.

Page 24: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• This position may be affected more by an open door or draught(气流 ) in the alleyway than by the temperature of the cabins.

Page 25: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Yet another possibility is to site the thermostat in the recirculation air trunk, carrying air back from the accommodation to the unit.

Page 26: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 27: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

Reading Material

• A. General operation of the air conditioning installation

Page 28: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The first essential in operating the air cooling appliances(设备 ) throughout the ship is to have all thermostats correctly set and correctly functioning.

Page 29: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• In extreme weather conditions, either hot or cold, control of the plant usually presents few difficulties.

Page 30: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The capacity of many installations is such that under tropical conditions nearly all control valves move to the fully open position.

Page 31: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Although automated control has been lost, internal conditions are by and large acceptable.

Page 32: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Control difficulties arise in intermediate weather conditions when there is a call for only a small amount of cooling.

Page 33: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 34: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• For this intermediate condition, thermostats must be correctly set by trial and error(逐次逼近法、反复试验 ).

Page 35: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 36: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 37: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 38: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• When air cooling is in use it is good practice to keep all portholes, windows and doors shut.

Page 39: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• On passenger ship, some public announcement requesting that this be done is worthwhile.

Page 40: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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 ).

Page 41: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Keeping a log of these readings then serves to identify any malfunctioning of the installation as soon as it arises.

Page 42: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 43: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The correct balance between supply and exhaust fans should be checked periodically.

Page 44: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Even with filters fitted ducts can become partially blocked and fan performance can fall off to upset the balance.

Page 45: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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 ).

Page 46: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 47: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Care must be taken not to reduce the fresh air so that stuffiness(不通风,闷 ) or smell arise.

Page 48: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Cleaning or renewal of filters is necessary at about 3 monthly intervals, the time varying according to location on the ship.

Page 49: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 50: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 51: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Cooled air ducts should be examined to see that the insulation vapour seal remains in good order.

Page 52: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• If a plastic film vapour seal becomes damaged, condensation forms within the film.

Page 53: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 54: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• B. THERMOSTATIC EXPANSION VALVES

• (热力膨胀阀 )

Page 55: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

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:

Page 56: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The pressure drop causes the evaporating temperature of the refrigerant to fall below that of the evaporator.

Page 57: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Thus the refrigerant can be boiled off by an evaporator temperature of –18oC, for example, because the pressure drop brings the evaporating temperature of the refrigerant to say –24oC.

Page 58: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The liquid refrigerant leaves the condenser with a temperature just above that of the sea water inlet, say 29oC.

Page 59: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• As it passes through the expansion valve the evaporating temperature decreases to –24oC and

Page 60: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 61: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

capillarybellowsPush

pins

bulb

Equalizingline

Page 62: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

Equalizing linebellows

BulbEvaporator coil

Pushrod

Outlet

p1

Page 63: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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 manometerBellows manometer波纹管式压力表波纹管式压力表

Page 64: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• This is effective through the push pins and tends to open the valve against the spring.

Page 65: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Spring pressure is set during manufacture of the valve and should not be adjusted.

Page 66: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 67: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 68: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• Ideally the gas should leave with 6 or 7oC of superheat. This ensure that the refrigerant is being used efficiently and that no liquid reaches the compressor.

Page 69: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 70: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 71: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 72: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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.

Page 73: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The greater pressure on the outside of the bellows is the result of saturation temperature plus superheat.

Page 74: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• The additional pressure on outside of the bellows resulting from superheat, overcomes the spring loading which tends to close the valve.

Page 75: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• A hand regulator is fitted for emergency use.

Page 76: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• 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 7oC above the sea water inlet temperature and

Page 77: Lesson 21 Air conditioning system. Air conditioning systems fall into two main classes:

• and the suction gauge showed all equivalent evaporating temperature about the same amount (7oC) below the evaporator.