Power Plant Engineering

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

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POWER PLANT ENGINEERING
pressure cylinder normally uses double flow). This type of unit is completely balanced against the end
thrust and gives large area of flow through two sets of bladings. This also helps in reducing the blade
height as mass flow rate becomes half as compared to single flow for the same conditions.
Reversed Flow. Reversed flow arrangement is sometimes used in h.p, cylinder where higher
temperature steam is used on the larger sets in order to minimise differential expansion i.e. unequal
expansion of rotor and casing. The use of single, double and reversed flow is shown in the layout
Fig. 6.5(c).
(G) On the Basis of Number of Shaft
(i) Tandem compound, (ii) Cross compound
(i) Tandem Compound. Most multi-cylinder turbines drive a single shaft and single generator
Such turbines are termed as tandem compound turbines.
(ii) Cross Compound. In this type, two shafts are used driving separate generator. The may be
one of turbine house arrangement, limited generator size, or a desire to run shafting at half speed. The
latter choice is sometimes preferred so that for the same centrifugal stress, longer blades may be used,
giving a larger leaving area, a smaller velocity and hence a small leaving loss.
(H) On the Basis of Rotational Speed
(i) constant speed turbines
(ii) Variable speed turbines
(i) Constant Speed Turbines. Requirements of rotational speed are extremely rigid in turbines
which are directly connected to electric generators as these must be a-c unit except in the smallest sizes
and must therefore run at speeds corresponding to the standard number of cycles per second and gov-
erned by the following equation :
N = 120 × Number of cycles per second = 120 f/p
Number of poles
The minimum number of poles, in a generator is two and correspondingly the maximum possible
speed for 60 cycle is 3,600 rpm; for 50 c/s of frequency, the speeds would be 3,000, 1500 and 750 rpm
for 2, 4 and 8 poles machines respectively.
(ii) Variable Speed Turbines. These turbines have geared units and may have practically any
speed ratio between the turbine and the driven machine so that the turbine may be designed for its own
most efficient speed. Such turbines are used to drive ships, compressors, blowers and variable frequency
generators.