Power Plant Engineering

9.3.4. GAS TURBINES
The common types of turbines, which are in use, are axial flow type. The basic requirements of
the turbines are lightweight, high efficiency; reliability in operation and long working life. Large work
output can be obtained per stage with high blade speeds when the blades are designed to sustain higher
stresses. More stages of the turbine are always preferred in gas turbine power plant because it helps to
reduce the stresses in the blades and increases the overall life of the turbine. More stages are further
preferred with stationary power plants because weight is not the major consideration in the design which
is essential in aircraft turbine-plant.
The cooling of the gas turbine blades is essential for long life as it is continuously subjected to
high temperature gases. There are different methods of cooling the blades. The common method used is
the air-cooling. The air is passed through the holes provided through the blade.
9.4 REGENERATION AND REHEATING
Generally, the thermal efficiency of the simple open cycle is only about 16 to 23% as lot of heat
energy goes waste in the exhaust gases. Moreover the cycle efficiency directly depends upon the tem-
perature of the inlet gases to the turbine. And as the metallurgical limitations do not permit the use of
temperatures higher than about 1000°C, a sizeable increase in efficiency cannot be expected through the
increased temperature of the gases. Of course, this efficiency handicap can be overcome by incorporat-
ing thermal refinements in the simple open cycle e.g. regeneration, reheating. But the plant will become
complex in contrast to the simple open cycle plant which is compact, occupies very little space, does not
need any water and can be quickly run up from cold. The thermal refinements can raise the plant effi-
ciency to over 30% and thereby obliterate the advantage of fuel efficiency possessed by diesel or con-
densing steam power plants. These refinements are discussed below: