TURBOCHARGER

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

Turbocharger - General Description:

The construction of a typical turbocharger can be seen in Figures below. The main body is built up from casings - a gas inlet casing, a gas outlet casing and the compressed air outlet casing. The air inlet casing can be of the side entry type. To this ducting is attached which draws cool clean air through a filter compartment which communicates with the outside atmosphere at deck level. This arrangement has the advantage of ensuring that the compressors and air coolers are relatively free from contamination. However, it is expensive and it is more usual to have a combined air filter/silencer fitted to the turbocharger, drawing air directly from the engine room.

The compressor wheel generally consists of two parts: the main impeller and the inducer, both made from a light alloy and keyed on to the shaft. A labyrinth air seal on the rear face of the impeller rotor, together with grooves in the heat shield form an effective seal between the compressor and the exhaust side of the turbine. The designed seal clearances should be maintained for, if they are excessive, the loss of air into the exhaust can affect the efficiency of the turbocharger.

The turbine end of the turbocharger consists of water-cooled exhaust casing, which house the nozzle-ring turbine wheel and blading etc. These, together with the rotor shafts, are of heat resisting alloy steel (nickel-chrome- alloy) to withstand continuous operation at temperatures in excess of 450*C. Some degree of air-cooling is given by controlled leak-off past the labyrinth seal between back of impeller and volute easing. An air seal is also provided at the labyrinth gland next to the rotor bearings by air bleed from the delivery side of the blower to prevent air/gas and oil tracking across the bearings.

The rotor shaft is supported at both ends by ball and roller, or plain bearings. In the former case, one end of the rotor bearing assembly consists of a doubter ball-and-roller bearing, mounted in pulsation-damping springs, whilst the other consists of a single-row roller bearing in a similar resilient mounting.

The ball-and-roller bearings have to be changed after prescribed periods, which depend upon the service speed of the turbocharger. The periods recommended by the manufacturers are based upon empirical data, which has been statistically interpreted. Quick analysis by audio or visual means to assess the further use of old bearings is not recommended.