GAS TURBINES

• Gas turbines are used mainly for electric power generation and also in jet engines of aircrafts and in turbochargers of internal combustion (IC) engines.

• It  has  limited  application  in  marine  engines.  Gas  turbines  have  the  unique advantage of using any type of fuel, i.e. solid, liquid or gas.

• Gas turbines operate either on an open cycle or in a closed cycle.

Working of an Open Cycle Single Stage Gas Turbine

• A simple open cycle gas turbine is represented in Fig. 17.3(a).

• It consists of a compressor, a combustion chamber and a turbine.

• The compressor and turbine co by a common shaft with a suitable flange.

• Air from the atmosphere is taken and compressed to a pressure ratio ranging from 2-8 before assing to the combustion chamber where the fuel is injected.

• The fuel burns and the temperature is raised at constant pressure. Then, it passes to the turbine where it expands to its original pressure before being exhausted to atmosphere. Fig. 17.3 (b).

Advantages of Gas Turbines

1. Possibility to use any type of fuel.

2. Compact size, less weight and low space requirement.

3. Simple foundation and low installation cost.

4. Less requirement of lubrication oil, water, etc.

5. Vibration is less.

Disadvantages of Gas Turbines

1. High operating temperature in the combustion chamber and in the turbine. So we need special high temperature alloys.

2.  Thermal  efficiency  is  very  low  in  the  case  of  simple  gas  turbine  due  to  high temperature of about 450° C in the waste exhaust gases.
and also for cooling.

Methods to Improve the Thermal Efficiency of a Single Stage Gas Turbine

1.By using a regenerator to heat the compressed air before entering the
combustion chamber as in Fig. 17.4 thereby making use of the heat in the exhaust gases before
leaving to the atmosphere.

2.   By   using   a   multistage   compressor   with   intercooling   to   reduce   the   work   of compression.

3.  By  using  a  multistage  turbine  to  reduce  the  temperature  of  exhaust  gases  before leaving the turbine.

Closed Cycle Gas Turbine

The closed cycle plant can use some stable gas with a higher specific heat as the working medium.

Instead   of   burning   the   fuel  directly   in   the   air   steam,   an   externally   fired combustion  chamber  or  furnace  is  used  and  heat  is  transferred  to  the  working medium through a heat exchanger.

Intercooler is also provided to improve the overall efficiency of compressions.

As  a  multistage  turbine  is  used,  the  temperature  of  exhaust  gases  leaving  the turbine is considerably reduced resulting in a higher thermal efficiency.

The regenerator preheats the gas before entering the furnace. By these provisions, the thermal efficiency is further increased to about 30 per cent.

The closed cycle has the following advantages:

1. Flexibility as to the type of fuel.

2. Uncontaminated working medium, and hence maintenance is easier.

3. Possibility of using a gas having better thermal properties as the working medium. By using an inert gas with high specific heat, the unit will become compact.