FUEL SUPPLY
OFF
OFF ONON
WARM AIR OUTLETS
COLD AIR OUTLETS
RAM AIR
EXHAUST
COMBUSTION CHAMBER DEMISTER
FLOW CONTROL VALVE ENGINE DRIVEN AIR BLOWER
AIR SUPPLY
ON OFF
Page 4-4 B1 Mod 11.04 Issue 30 Jan 2003
JAR 66 CATEGORY B MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION
uk
engineering
4.3 COOLING
When bleed air is used as the air supply, the air tapped off the engine compressor can reach a temperature in excess of 300 degrees Celsius.
This is obviously far too hot to be fed directly into the air-conditioned areas, so it must first be cooled down to around 20 degrees Celsius.
There are two main methods of cooling; Air Cycle and Vapour Cycle cooling systems.
4.3.1 Air Cycle Cooling
Air cycle cooling relies on three basic principles; surface heat exchange, expansion and energy conversion.
Surface heat exchange, provides cooling by passing the air tapped from the engine compressor (charge air) across some form of heat exchanger. The charge air is subjected to the effect of a colder cross flow, normally ambient air, scooped by an intake and passed across the heat exchanger as the aircraft moves forward (ram air). Although 90% of heat is given up in this way, the charge air temperature can never be reduced below the ram air temperature by this method alone.
Expansion, provides cooling when the pressure of the charge air is reduced by increasing its velocity and expanding it across the turbine of a so-called Air Cycle Machine (ACM) or Cold Air Unit (CAU). In this way, the temperature of the charge air can be rapidly lowered to zero degrees Celsius, irrespective of the ram air temperature
Energy Conversion, cools by making the hot air do work. This is achieved by using the charge air to drive a turbine, which is connected by a shaft to the compressor or fan within the cold air unit, thus converting heat energy into kinetic energy. This method will also help to reduce the charge air to zero degrees Celsius.
Issue 1 - 20 March 2001 Page 4-5
Turbo Compressor Figure 3
4.3.1.1 HEAT EXCHANGERS
These are components within the air conditioning system that transfer heat from one gas stream to another. Ram air is used as the cooling medium to cool the very hot charge air ducted from the engine compressor or the gearbox mounted air compressor or blower.
Depending on where they are placed within the air conditioning system, heat exchangers are often described as;
A ‘Pre-cooler’ or ‘Primary Heat Exchanger’
An ‘Inter-cooler’ or ‘Secondary Heat Exchanger’
The basic construction is a sealed unit containing a series of cooling passages; through which the charge air flows and over which the ram air is directed. Between these passages are thin corrugated strips, that also serve to dissipate heat as the ram air passes over them.
TEMPERATURE CONTROL VALVE
COMPRESSOR TURBINE
SECONDARY HEAT EXCHANGER
RAM AIR TO CABIN MIXER UNIT PRIMARY HEAT EXCHANGER
HOT AIR INLET
Page 4-6 B1 Mod 11.04 Issue 30 Jan 2003
JAR 66 CATEGORY B MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION
uk
engineering
4.3.1.2 AIR CYCLE MACHINE (ACM) OR COLD AIR UNIT (CAU)
The ACM/CAU is the primary component in an air cycle cooling system. A number of different types can be found including;
The turbo-compressor, the brake turbine and the turbo-fan.
All three use the charge air to drive the turbine and the major differences between each type, relates to the overall weight for a given mass flow, the size and method of dissipating the power output of the turbine.
Turbo Compressor Cold Air Unit Figure 4
The turbo-compressor type consists of a turbine driving a centrifugal compressor and operating in conjunction with an inter-cooler connected between the compressor and turbine stages.
Its basic construction consists of two main casings, the turbine volute and compressor volute casings. The two casings are connected together and enclose a bearing housing with two bearing assemblies, supporting a shaft upon which the turbine and compressor wheels are mounted.
BLEED AIR TO INTERCOOLER FROM INTERCOOLER TO DISTRIBUTION SYSTEM COMPRESSOR DIFFUSER NOZZLE BLADES
TURBO COMPRESSOR
Issue 1 - 20 March 2001 Page 4-7 The turbine wheel revolves within a nozzle ring and the compressor wheel rotates within a diffuser ring. The very hot charge air from the engine compressor bleed and routed via the pre-cooler, enters the eye of the ACM/CAU compressor. It becomes compressed on passing through the diffuser ring, increasing its temperature and energy.
From the compressor, the hot air is directed across the inter-cooler matrix over which ram air passes and is then directed into the turbine volute nozzle ring, where it drives the turbine. The resultant expansion and energy conversion, rapidly lowers the air pressure and temperature.
It is then directed towards the passenger cabin. (See Fig 3)
The ACM/CAU compressor and turbine wheels rotate at extremely high speeds, often in excess of 80,000 rpm, so efficient bearing lubrication is essential to ensure smooth and trouble-free running.
Two lubrication methods are used; Integral wet sump arrangements, or pressurised air bearings that need no oil lubrication.
The wet sump type normally has a sump containing oil and a means of metering it to the bearings usually by the use of integral ‘wicks’ or with an ‘oil slinger’ that pumps an optimum oil/air mix to the bearings. This ensures the correct amount of oil at the bearings at all times. Oil replenishment is critical however, as too much oil will lead to the charge air being oil contaminated and too little oil, may result in a premature seizure of the rotating shaft.
The air bearing type uses a pressurised air supply to support the shaft in a similar manner to the ‘hovercraft principal’. As the rotor ‘floats’ on a thin layer of air, it is essential that this type is kept clean and dry and completely free from oil and grease.
Brake Turbine Cold Air Unit Figure 5 RAM AIR TO CABIN MIXER UNIT HEAT EXCHANGER CONTROL VALVE AMBIENT AIR INLET
COMPRESSOR TURBINE
BLEED AIR
Page 4-8 B1 Mod 11.04 Issue 30 Jan 2003
JAR 66 CATEGORY B MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION
uk
engineering
The brake-turbine type of ACM/CAU, has its charge air routed directly from the pre-cooler to drive the turbine. The air expands across the turbine as before, resulting in a large temperature and pressure drop. Since this layout dispenses with the need for an inter-cooler, it results in a greater efficiency due to weight saving. To safeguard against the turbine rotating too fast, it is coupled with a compressor, which rotates in ambient air and consequently acts as a braking medium. Additionally, the slower rotation of the shaft further improves turbine output efficiency. (See Fig 5)