Theturbo-compressor type consists of a turbine driving a centrifugal compressorand operating in conjunction with an inter-cooler connected between thecompressor and turbine stages.
Its basicconstruction consists of two main casings, the turbine volute and compressorvolute casings. The two casings are connected together and enclose a bearing housingwith two bearing assemblies, supporting a shaft upon which the turbine andcompressor wheels are mounted. The turbinewheel revolves within a nozzle ring and the compressor wheel rotates within adiffuser ring. The very hot charge air from the engine compressor bleed androuted via the pre-cooler, enters the eye of the ACM/CAU compressor. It becomescompressed on passing through the diffuser ring, increasing its temperature andenergy. From thecompressor, the hot air is directed across the inter-cooler matrix over whichram air passes and is then directed into the turbine volute nozzle ring, whereit drives the turbine. The resultant expansion and energy conversion, rapidlylowers the air pressure and temperature. It is thendirected towards the passenger cabin.
(See Fig 3) The ACM/CAU compressorand turbine wheels rotate at extremely high speeds, often in excess of 80,000rpm, so efficient bearing lubrication is essential to ensure smooth andtrouble-free running. Two lubrication methods are used; Integral wet sumparrangements, or pressurised air bearings that need no oil lubrication.The wet sump type normally has a sump containing oil and ameans of metering it to the bearings usually by the use of integral ‘wicks’ orwith an ‘oil slinger’ that pumps an optimum oil/air mix to the bearings. Thisensures the correct amount of oil at the bearings at all times. Oilreplenishment is critical however, as too much oil will lead to the charge airbeing oil contaminated and too little oil, may result in a premature seizure ofthe rotating shaft. The air bearing type uses a pressurised air supply tosupport the shaft in a similar manner to the ‘hovercraft principal’.
As therotor ‘floats’ on a thin layer of air, it is essential that this type is keptclean and dry and completely free from oil and grease.