Page 46 - Uninterruptible Power Supplies

Standby Power Generating Sets
44 Chapter One

Discharge of Exhaust Gases
The exhaust gases include a number of unpleasant pollutants and the
discharge should be at high level, above the top of the building or hous-
ing and well away from any air inlets associated with building ventila-
tion. In the United Kingdom the discharge should be in accordance
with the 1956 Clean Air Act which was discussed in a previous section.

Noise from Exhaust Discharge
The exhaust outlet will be a source of noise over the full range of audi-
ble frequencies, dominant frequencies being the engine firing rate and
its harmonics; a turbocharger will introduce higher frequencies. For a
1500-rpm six-cylinder four-stroke engine, the firing rate will be at 75 Hz,
so it is apparent that the noise from an engine exhaust will include a
wide spectrum of frequencies. The local authority will require informa-
tion on the level of noise expected to be emitted.
In order to evaluate the effect of the noise on the neighborhood, it is
necessary to obtain acoustic data relating to the emitted noise from the
exhaust system design organization, which is usually the generating
set manufacturer. The data should be in the form of acoustic sound
pressure levels at a specified distance from the outlet, for octave band
centre frequencies between 31.5 Hz and 8 kHz. The distance from the
outlet will probably be 2 m and it may be useful to know that in a free
field the noise is attenuated by 6 dB for each doubling of the distance.
Thus at a distance of 4 m the attenuation would be 6 dB and at 16 m
the attenuation would be 18 dB. The local authority will require infor-
mation on the level of noise expected to be emitted and will have rules
regarding noise levels which will take into account the type of nearby
buildings and their occupants.

Noise from Diesel Engine Cooling and
Ventilation Air
The radiator of a diesel engine will require a large quantity of cooling air.
For sets up to about 700 kW the radiator will probably be local to the
engine and the air passing through it will also provide ventilation for the
engine room. If the radiator is remote from the engine it will have its own
fan or fans which will be another source of noise and a supply of air will
be required to the engine room to remove the heat gained from the engine
casing. All these air inlets and outlets require openings in the enclosures
which allow the escape of noise. Acoustic treatment will usually be
required and the effect on the occupants of any nearby buildings will have
to be considered. As in the case of exhaust noise the local authority will
require information on the level of noise expected to be emitted.

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