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10 ENGINEERING THERMODYNAMICS

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and the rest is utilised in electrical energy. The gas turbines work on open cycle, semiclosed cycle

or closed cycle. In order to improve efficiency, compression and expansion of working fluid is

carried out in multistages.

1.4.5. Energy cycle for a simple-cycle gas turbine

Fig. 1.7 shows an energy-flow diagram for a simple-cycle gas turbine, the description of

which is given below :

Fuel in

Compressed

air

Compressor Turbine

Air in Exhaust

Power

gas

Combustor

Fig. 1.7. Energy flow diagram for gas-turbine unit.

— The air brings in minute amount of energy (measured above 0°C).

— Compressor adds considerable amount of energy.

— Fuel carries major input to cycle.

— Sum of fuel and compressed-air energy leaves combustor to enter turbine.

— In turbine smallest part of entering energy goes to useful output, largest part leaves in

exhaust.

Shaft energy to drive compressor is about twice as much as the useful shaft output.

Actually the shaft energy keeps circulating in the cycle as long as the turbine runs. The

important comparison is the size of the output with the fuel input. For the simple-cycle gas tur-

bine the output may run about 20% of the fuel input for certain pressure and temperature condi-

tions at turbine inlet. This means 80% of the fuel energy is wasted. While the 20% thermal

efficiency is not too bad, it can be improved by including additional heat recovery apparatus.

1.5. Refrigeration Systems

Refrigeration means the cooling of or removal of heat from a system. Refrigerators work

mainly on two processes :

1. Vapour compression, and


2. Vapour absorption.
   Simple Vapour Compression System :
   In a simple vapour compression system the following fundamental processes are completed
   in one cycle :


3. Expansion 2. Vapourisation 3. Compression 4. Condensation.