air conditioning and cabin pressurisation notes (1)
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air conditioning and cabin pressurisation
The atmosphere above10,000ft is too thin and cold for normal breathing.
Passenger carrying aircraft, operating above this height need an air conditioning and pressurisation system. The temperature of the air passing through the passenger cabin, flight deck and other compartments must be strictly controlled, as well as flow rate and level of humidity.
Cabin temperature will normally be maintained between 15 and 30 degrees Celsius. Additionally, a controlled amount of pressurisation is necessary, so that the air pressure in the passenger cabin and adjacent areas does not exceed the equivalent of the ambient air pressure at 8000ft.
Air conditioning is also essential for un-pressurised aircraft types.
A typical air conditioning and pressurisation system comprises eight principle sub-systems:
Air Supplies (Pneumatics ATA 36)
Mass Flow Control
The source of fresh air supply and arrangement of essential components will vary between aircraft type and each air conditioning system, but in general one of the following methods described in the following paragraphs will be adopted:Engine Bleed Air (compression)
This method is the most common and is installed on the majority of modern aircraft types. Very hot air is tapped from the main engine compressor stages and supplied to the cabin, flight deck and other areas. Before the air enters the cabin, it is passed through a temperature control system, which reduces its temperature and pressure. Additionally, a means of flow control is utilised and in some aircraft, humidity control forms part of the system. (See Fig 1)
In pressurised aircraft, the discharge of the conditioned air is regulated to maintain the cabin pressure at the selected pressure altitude.
Typical (Compression) Bleed Air System
Air Compressors or Blowers
This method is used on turbo-prop, piston engine or even turbo-jet aircraft where main engine compressor bleed is unavailable or unsuitable.
Normally the compressor or blower will be mechanically driven from the accessory gearbox of the main engine and its air supply routed via a temperature control system, in a similar manner to the engine bleed method.Auxiliary Power Unit (APU)
The APU is a small gas turbine engine, which can be connected into the main air supply system and provide an independent means of air conditioning and pressurisation, either on the ground or in flight, when the main engines cannot supply. It will utilise the engine bleed air principle outlined above.
This method is normally found as the primary ventilation system on un-pressurised aircraft. A ram air scoop placed directly into the airflow, will provide the means of air supply as the aircraft moves forward.
Since the air at altitude will be cold, the temperature control system through which it passes before entering the cabin, will normally be a form of heater.
A self-contained combustion type heater will be employed, or the some form of exhaust gas heater. The air conditioning ducting will be routed around the combustion heater casing or around engine exhaust duct to obtain convection heating.
On pressurised aircraft, a ram air system can be used as a means of emergency ventilation, following a complete loss of the main system.
Typical Combustion Heater System
This will be an independent means of heating or cooling the passenger cabin on the ground. It can be used on aircraft that do not have an APU. The trolley will be connected externally to the aircraft, via a purpose built inlet into the air conditioning system and normally employs a combustion type heater and the means to control the output of the air temperature from a control panel the cart.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.Air Cycle CoolingAir 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.
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.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
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.
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 5The 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 sl