Page 29 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 27

or outlet ducting, louvers or acoustic attenuators are to be installed,
either a more powerful fan or an additional fan will be required.
Direct air–cooled diesel engines are simpler than radiator-cooled
engines: there are no corrosion or freezing problems and no leaking
from failing pipe work or from burst hoses.
In the preceding text some guideline figures are given for water-cooled
engine losses and efficiency. If, for simplicity, it is assumed that the effi-
ciency, the casing losses, and the exhaust losses of direct air cooled
engines are similar, the allocation of losses for direct air cooled sets may
be restated as:

Useful energy transferred to generator 38 percent
Energy used by engine driven cooling fan 2 percent
Heat lost in exhaust gases 30 percent
Heat lost to cooling air 25 percent
Heat lost from engine casing 5 percent

Gas Turbine Cooling
A gas turbine will have smaller losses from the turbine casing and
there is no radiator, most of the waste heat is in the exhaust gases.
Because of the high rotor speed the casing of a turbine is much smaller
than that of the equivalent engine and it is more easily thermally insu-
lated; this leads to the smaller casing losses. Heat is removed from the
rotor bearings by lubricating oil flow and the lubricating oil system will
include an oil cooler, probably of the oil/air type discharging air to out-
side of the building.

Engine Room Ventilation
The temperature within the engine room must be maintained at a rea-
sonable level, and the heat losses from the engine and generator have to
be removed by a flow of ventilation air. The heat losses will be mainly
from the engine, the losses from the generator will be only a few percent
(typically 5 percent) of its rating; the heat losses from a turbine and its
gearbox will be much less than from a diesel engine of comparable rating.
The quantity of air entering the room will always exceed the quan-
tity leaving; combustion air is drawn into the engine and is dispersed
in the exhaust gases. The combustion air requirement is small and for
3
diesel engines may be assumed to be 0.17 m /s for a 100 kW set and pro
rata for other ratings. A gas turbine requires more, typically twice as
much as a diesel engine (3.4 m /s for a 1 MW set).
3

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