Gas Turbine Handbook : Principles and Practices

48 Gas Turbine Handbook: Principles and Practices
KE = C2
2gcJ (4-7)
where HP is horsepower, C is the velocity of the air entering the compressor
or air and combustion products leaving the turbine, and g c
is
the gravitational constant 32.17 ft lb m
/lb f
sec 2 .
The engineering thermodynamics can be summarized with the
following relationships:
Gas Turbine Horsepower Output
HP = W
0.707 h 1 – h 2 + KE 1 – KE 2 (4-8)
where 0.707 (more exactly 0.7068) converts Btu/sec to horsepower.
Gas Turbine Efficiency
Thermal efficiency (η t
) of a gas turbine, considering the compression
and expansion processes as being irreversible, is defined as the
work output divided by the fuel energy input. The work output is the
total turbine work minus the work on the compressor (note compressor
work is negative). Therefore,
HP =
W
0.707 h 1 – h 2 + KE 1 – KE 2 (4-9)
η t =
2540
Btu/HP – Hr = 3600
KJ/kW – Hr = 3414
Btu/kW – Hr (4-10)
where KJ is kilojoules, kW is kilowatts, and Hr is hours.
This expression is the most used tool in comparing one gas
turbine with another. As used here, this expression represents the
simple cycle gas turbine efficiency. This equation is also used to demonstrate
that a particular engine is (or is not) deteriorating with use.
If the overall engine simple cycle efficiency is deteriorating then an
examination of each component is necessary to determine the cause
of the problem.