Page 127 - Mechanical Engineer's Data Handbook

116 MECHANICAL ENGINEER’S DATA HANDBOOK

3.7.3 Reheat factor and overall
efficiency

Referring to the ‘condition curve’ on the h-s diagram:
AhA =available stage enthalpy drop
Ah, = isentropic stage enthalpy drop
AhoA = available overall enthalpy drop
Aho, = isentropic overall enthalpy drop
Stage efficiency qs=- Ah,
Ah,

Overall efficiency qo = -
Ah01
v
Reheat factor RF=2
?,

h

3.8 Gas turbines

The gas turbine unit operates basically on the con- The output power increases with both compression
stant-pressure cycle, particularly in the case of the ratio and turbine inlet temperature.
‘closed cycle’. In the ‘open cycle’ air is drawn in from The effect of losses and variation in fluid properties
the atmosphere, compressed and supplied to a com- is shown on the basic cycle. The efficiency of the basic
bustion chamber where fuel is burnt with a large cycle can be greatly increased by incorporating a heat
amount of ‘excess air’. The hot gases drive a turbine exchanger between the compressor outlet and the
which drives the compressor and also provides useful combustion chamber inlet. It uses the exhaust gases
work. The efficiency increases with compression ratio. from the turbine to preheat the incoming air.