Page 30 - Uninterruptible Power Supplies

Standby Power Generating Sets
28 Chapter One

Water-Cooled Diesel Engines
It is necessary to estimate the heat losses into the engine room; these will
originate from the engine casing, the generator, and the exhaust system.
The generator efficiency will be of the order of 95 percent and the heat
balance figures from the previous section may be rearranged as:

Useful energy (kWe) appearing at the generator 36.5 percent
terminals
Engine-driven radiator fan 1.5 percent
Heat lost in exhaust gases 30 percent
Heat lost from radiator 25 percent
Heat lost from engine casing 5 percent
Generator losses 2 percent

Estimated values for the engine casing and generator losses may be
derived from the above heat balance. The exhaust system losses will
vary widely depending on the length of internal pipework, whether
there is an internal silencer, and the amount of thermal insulation. In
the following example a loss of 6 percent of the generating set kWe is
assumed from the exhaust system.
If the generating set kWe power rating is indicated by P the losses
may be listed:
■ Losses from the engine casing (0.05 0.365) 0.137 P kW
■ Losses from the generator (assumed to be 5%) 0.05 P kW
■ Losses from the exhaust system (assumed to be 6%) 0.06 P kW
■ Total losses to room 0.247 P kW

The standard reference conditions specified in ISO 8528 allow a max-
imum air temperature of 25°C and knowing the losses into the engine
room, and the permissible rise in temperature of the ventilating air, it
is possible to calculate the air flow required from the formulae:

Air mass flow (kg/s)
Heat losses into engine room (kW)
(1.2)
Temperature rise (°K) Specific heat capacity of air (kJ/kg °K)
Air mass flow (kg/s)
3
Air volume flow (m /s) (1.3)
3
Density of air (kg/m )
As an example the probable ventilation requirements for a 100 kW
generating set may be estimated in the following fashion. (For the final

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