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.
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.
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
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
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.