TITLE.PM5

678 ENGINEERING THERMODYNAMICS

dharm

\M-therm\Th13-5.pm5

T

s

4 ′

4

3

2 ′

2

1

10

5

7 ′

7

6

9

9 ′

8

p^2

px

p^1

Fig. 13.51. T-s diagram for the plant.
Operation 10-11 : Air cooled to T 1 by the cooler before entering the L.P. compressor.
The energy balance for the whole plant is as follows :
Q 1 + Q 2 – Q 3 – Q 4 = W
In a closed cycle plant, in practice, the control of power output is achieved by varying the
mass flow by the use of a reservoir in the circuit. The reservoir maintains the design pressure and
temperature and therefore achieves an approximately constant level of efficiency for varying
loads. In this cycle since it is closed, gases other than air with favourable properties can be used ;
furthermore it is possible to burn solid fuels in the combustion heaters. The major factor respon-
sible for inefficiency in this cycle is the large irreversible temperature drop which occurs in the air
heaters between the furnace and circulating gas.
Note 1. In a closed cycle gas turbines, although air has been extensively used, the use of ‘helium’ which
though of a lower density, has been inviting the attention of manufacturers for its use, for large output gas turbine
units. The specific heat of helium at constant pressure is about ‘five times’ that of air, therefore for each kg mass
flow the heat drop and hence energy dealt with in helium machines is nearly five times of those in case of air. The
surface area of the heat exchanger for helium can be kept as low as 1/3 of that required for gas turbine plant using
air as working medium. For the same temperature ratio and for the plants of the same output the cross-sectional
area required for helium is much less than that for air. It may therefore be concluded that the size of helium unit
is considerably small comparatively.

2. Some gas turbine plants work on a combination of two cycles the open cycle and the closed cycle. Such
   a combination is called the semi-closed cycle. Here a part of the working fluid is confined within the plant and
   another part flows from and to atmosphere.