Turbocharger Construction

An example of turbocharger construction is provided by ABB Turbo Systems' (Brown Boveri) VTR design which remained unchanged for many years (Figure 7.8).

Figure 7.8 Cross-section of Brown Boveri type VTR501 turbocharger

The diesel engine exhaust gases enter through the water-cooled gas inlet casing (50), expand in the nozzle ring (30) and supply energy to the shaft (20) by flowing through the blading (21). The gases exhaust to the open air through the gas outlet casing (60), which is also water cooled, and the exhaust piping. The charge air enters the compressor through an inlet stub (82) or through the silencer filter (80). It is then compressed in the inducer and the impeller (25), flows through the diffuser (28) and is fed to the engine via the pressure stubs on the compressor casing (74).

Air and gas spaces are separated by the heat insulating bulkhead (70). In order to prevent exhaust gases from flowing into the balance channel (2) and the turbine side reservoir, barrier air is fed from the compressor to the turbine rotor labyrinth seal via channel X.

The rotor (20) has easily accessible bearings (32, 38) at both ends, which are supported in the casing with vibration damping spring elements. Either roller or plain bearings are used but for the most common construction using roller bearings a closed loop lubrication system with an oil pump directly driven from the rotor is used (47, 48). These pumps are fitted with oil centrifuges to separate out the dirt in the lubricating oil. The bearing covers are each fitted with an oil filter, an oil drain opening and an oil gauge glass. On models with plain bearings, where the quantity of oil required is large, these are fed from the main engine lubricating oil system.

A key feature of this VTR design was the modular construction to match a wide range of diesel engine types. The separate modules of the turbocharger, as shown in Figure 7.8, are: the silencer filter (80); the air inlet casing (82); the compressor housing (74); the gas outlet casing (60); the outlet casing feet (680); and the gas inlet casing (50). The fixing screws are placed so that the radial position of all the separate casings can be arranged in any position relative to the other.

Compressor wheels for MAN B&W turbochargers are almost exclusively milled from an aluminium forged blank. Cast aluminium alloys were once used as well but superseded as mechanical strength requirements increased. Titanium alloys may also be exploited for applications imposing high air outlet temperatures at high pressure ratios. Radial turbine wheels are purchased as casting blanks of a nickel base alloy. 'Built' rotors are used only for axial turbine wheels (the cast or forged individual blades are retained in the disk by a fir-tree foot). For smaller diameter ranges, progress in casting technology has yielded axial turbine wheel castings incorporating the blades (integral wheels).

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