Page 10 - Uninterruptible Power Supplies

Standby Power Generating Sets
8 Chapter One

requires an increase of fuel, which in turn requires an increase of com-
bustion air, but this is not available until the compressor has attained an
increased speed. This is similar to the situation encountered with tur-
bocharged diesel engines as previously described.
The efficiency depends on the turbine parameters, for a single-shaft
machine at full load it may approach 25 percent which leads to a fuel
usage of 0.45 liters per kWh or 1 liter per 2.2 kWh. The efficiency will
drop off rapidly at lower loads due to the constant air flow characteris-
tic. The constant air flow results in a constant compressor loading and
the surplus combustion air at low loads leads to a lower operating tem-
perature. At 50-percent loading the efficiency may drop to 15 percent,
the manufacturer should be consulted for a realistic figure. For standby
generation purposes the efficiency is of less importance than for con-
tinuous running applications although a low efficiency leads to a large
fuel storage requirement. It is possible, however, to design a gas tur-
bine to run on almost any fuel, it may therefore be possible to make use
of boiler fuel or whatever is available on site. It is of course possible to
run on gas but it is not easily stored and the supply may not be reliable.
Gas turbines do not have any reciprocating masses and have mini-
mal vibration due to dynamic unbalance. Cooling is achieved mainly by
the combustion air flow, a simpler arrangement than the jacket water
cooling for diesel engines. The main bearings are cooled by the lubri-
cating oil which passes through an oil/air or oil/water heat exchanger.
Agas turbine will be very much smaller than a diesel engine of the same
power rating but it is not easy to quantify the difference between instal-
lations because the gas turbine will require more space for its air inlet and
exhaust systems and will probably require additional acoustic treatment.
Any restriction of the flow of the inlet air or of the exhaust gases has a sig-
nificant effect on the performance of the gas turbine. There will be an air
inlet filter and air inlet and exhaust acoustic attenuators. In order to
achieve low pressure drops these will require large cross-sectional areas,
leading to large inlet and exhaust ductwork. The ductwork is normally
designed as part of the gas turbine package and the air inlet pressure
drop is likely to be of the order of 1 kPa (100-mm water gauge). The
exhaust pressure drop may be a little less at say 0.75 kPa (75-mm water
gauge).
Gas turbines require specialized maintenance and the necessary skills
are not so widely available as they are for diesel engines.

Standards Relating to Gas Turbines

There is no international standard applicable to gas turbine–driven
generating sets, nothing equivalent to ISO 8528. There are two standards
of particular interest, ISO 3977—Gas Turbine Procurement and ISO

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