Power Plant Engineering

STEAM TURBINE 199

difference between the pass-out steam and electrical load requirements. This type of turbine is suitable

where there is dual demand of steam-one for power and the other for industrial heating, for example

sugar industries. Double pass-out turbines are sometimes used.

(ii) Regenerative Turbine. This turbine incorporates a number of extraction branches, through

which small proportions of the steam are continuously extracted for the purpose of heating the boiler

feed water in a feed heater in order to increase the thermal efficiency of the plant. Now a days, all steam

power plants are equipped with reheating and regenerative arrangement.

(iii) Condensing Turbine. In this turbine, the exhaust steam is condensed in a condenser and the

condensate is used as feed water in the boiler. By this way the condensing turbine allows the steam to

expand to the lowest possible pressure before being condensed. All steam power plants use this type of

turbine.

(iv) Non-Condensing Turbine. When the exhaust steam coming out from the turbine is not

condensed but exhausted in the atmosphere is called non-condensing turbine. The exhaust steam is not

recovered for feed water in the boiler.

(v) Back Pressure or Topping Turbine. This type of turbine rejects the steam after expansion to

the lowest suitable possible pressure at which it is used for heating purpose. Thus back pressure turbine

supplies power as well as heat energy.

The back pressure turbine generally used in sugar industries provides low pressure steam for

heating apparatus, where as a topping turbine exhausts into a turbine designed for lower steam condi-

tions.

(E) On the Basis of Number of Cylinder: Turbine may be classified as

(i) Single cylinder and (ii) Multi-cylinder.

(i) Single Cylinder. When all stages of turbine are housed in one casing, then it is called single

cylinder. Such a single cylinder turbine uses one shaft.

(ii) Multi-Cylinder. In large output turbine, the number of the stages needed becomes so high

that additional bearings are required to support the shaft. Under this circumstances, multi-cylinders are

used.

(F) On the Basis of Arrangement of Cylinder Based on General Flow of Steam. (i) Single flow,

(ii) Double flow, and (iii) Reversed flow

Single Flow. In a single flow turbines, the steam enters at one end, flows once [Fig. 6.5(a)] through

Single flow

()a ()b ()c

Double flow Reversed flow

Fig. 6.5
the bladings in a direction approximately parallel to this axis, emerges at the other end. High pressure
cylinder uses single flow. This is also common in small turbines.
Double Flow. In this type of turbines, the steam enters at the centre and divides, the two portions
passing axially away from other through separate sets of blading on the same rotor Fig. 6.5(b). The low