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the ground and in low speed ight. Therefore this system is typically used as a standalone cooling system for equipment which is operated only during ight.
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7.5.6 Vapour Cycle Systems
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The vapour cycle system is a closed loop system where the heat load is absorbed by the evaporation of a liquid refrigerant such as Freon in an evaporator (NB
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the trade name Freon is a registered trademark belonging to E.I. du Pont de Nemours & Company (DuPont)). The refrigerant then passes through a compressor with a corresponding increase in pressure and temperature, before being cooled in a condenser where the heat is rejected to a heat sink. The refrigerant ows back to the evaporator via an expansion valve. This system is illustrated in Figure 7.16.
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Evaporator
Heat Load
Vapour cycle cooling system
Although vapour cycle systems are very ef cient, with a coef cient of performance typically ve times that of a comparable closed loop air cycle system, applications are limited due to problems such as their limited temperature range and heavy weight compared to air cycle systems. The maximum operating temperatures of many refrigerants are too low, typically between 65 C and 70 C, signi cantly less than the temperatures which are required for worldwide operation. It should be noted that chloro uorocarbons (CFC) endanger the ozone layer and are the subject of much debate calling for a limitation in their use.
7.5.7 Liquid Cooled Systems
Liquids such as Coolanol are now more commonly being used to transport the heat away from avionics equipment. (NB COOLANOL is a registered trademark of Exxon for Silicate Ester dielectric heat transfer uids.) Liquid can easily replace air as a transport medium owing through the cold wall heat exchanger.
Cooling Systems
A typical liquid loop consists of an air/liquid heat exchanger which is used to dump the heat load being carried by the liquid into the air conditioning system, a pump and a reservoir as illustrated in Figure 7.17.
Avionic Modules Avionic LRU Air Air/Liquid Heat Exchanger
Cold Wall
Reservoir
Pump
Liquid Loop
Example of a liquid cooling system
The advantages are that it is a more ef cient method of cooling the heat source, and the weight and volume of equipment tends to be less than the air conditioning equipment which would otherwise be required. The disadvantages are that it is expensive, and the liquid Coolanol is toxic. Self sealing couplings must be provided to prevent spillage wherever a break in the piping is required for maintenance purposes. The Boeing AH-64C/D Longbow Apache attack helicopter uses such a vapour cooling system to cool the extended forward avionics bays.
7.5.8 Expendable Heat Sinks
An expendable coolant, typically water, can be carried to provide a heat sink by exploiting the phenomenon of latent heat of vaporisation. A simple system is shown in Figure 7.18. The liquid refrigerant is stored in a reservoir which supplies an evaporator where the heat load is cooled. The refrigerant is then discharged overboard. This type of system can only be used to cool small heat loads (or large loads for a short time), otherwise the amount of liquid refrigerant that must be carried on board the aircraft would be too large.
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Overboard
Heat Load
Simple expendable heat sink system
7.6 Humidity Control
Passenger comfort is achieved not only by overcoming the problems of cooling and cabin pressurisation, but also by controlling humidity in the passenger cabin. This is only a problem on the ground and at low altitudes, since the amount of moisture in the air decreases with increasing altitude. There is a particular dif culty in hot humid climates. For example, in Northern Europe the typical air moisture content can be 10 grams of water per kg of air, but in some parts of the Far East moisture contents of more than 30 grams per kg can be encountered. In a hot, humid climate the cabin inlet air supply temperature needs to be cold to keep the passengers and aircrew comfortable. Without good humidity control this can result in a wet mist being supplied to the cabin. In addition to the aim of ensuring passenger comfort, humidity levels must be controlled to prevent damage to electrical and electronic equipment due to excessive condensation. Humidity control also reduces the need for windscreen and window de-misting and anti-misting systems. The ne mist of water droplets in the cold cabin inlet supply must be coalesced into large droplets that can then be trapped and drained away. Two types of water separator are in common use with air cycle refrigeration systems: a centrifugal device and a mechanical device. In the centrifugal devices a turbine is commonly used to swirl the moist air. The relatively heavy water droplets are forced to the sides of a tube, where the water and a small amount of air is trapped and drained away, thus reducing the water content of the air downstream of a water separator. The mechanical water separator, which consists of a coalescer, a relief valve and a water collector, achieves the same result by forcing the moist air to ow through the coalescer where large droplets are formed and blown onto collector plates. The water runs down the plates and is then drained away. The relief valve opens to allow the air to bypass the water separator if ice forms. Simple water collection devices can be used in vapour cycle refrigeration systems to reduce humidity levels since the air is cooled to its dew point as it ows through the evaporator. Water droplets collect on the heat exchanger surfaces and can be simply trapped and drained away. Chemicals can also be used to reduce moisture content. In civil aircraft the air gaps between two plates of the passenger windows are commonly vented