Page 37 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 35

Exhaust Silencers
The open outlet end of the exhaust system is a source of considerable
noise, a prominent frequency being the (low-frequency) engine firing rate,
and depending on the acoustic limitations of the site, one or two silencers
will be included in the system, the primary silencer being mounted close
to the engine while the secondary may be outside of the engine room.
If one silencer only is installed it will probably include reactive and
absorptive sections, if two silencers are used, the primary silencer is
likely to be reactive for attenuation of the lower frequencies, and the sec-
ondary absorptive for attenuation of the higher frequencies. Standard
industrial silencers are available which provide an attenuation between
20 and 40 dB(A) at the exhaust outlet.

Discharge of Exhaust Gases
To comply with the Clean Air Act Memorandum (see the next subsec-
tion) the discharge of exhaust gases should be vertical, and to prevent
the entry of rain it is usual to terminate the pipe obliquely and to pro-
vide a gravity-closed weather flap that is opened by the flow of the
exhaust gases. If the end is open and is likely to allow excessive entry
of rain, leading to internal corrosion, it would be prudent to provide one
or more drain plugs at suitable points within the engine room. Test
runs would boil off this water but if test runs are expected to be infre-
quent, provision should be made for drain plugs.
The open outlet of the exhaust system is a source of noise and nox-
ious gases, its location has important environmental consequences and
demands some attention at the planning stage. The following criteria
should be taken into account:

■ The minimum height of the outlet will be determined by The 1956
and 1993 Clean Air acts. The actual height of the outlet is subject to
the approval of the local planning authority. The Clean Air acts are
discussed in the next subsection
■ The outlet should not be near any inlet grilles or opening windows
that would allow the gases to enter any building.
■ Exhaust backpressure reduces the engine output; the pipe run
should be as short as practicable and the number of bends should be
kept to the minimum. Bends should be of long radius type. If the
length exceeds 10 m a larger bore may be necessary.
■ The location of the outlet should take into account the effect of both
noise and gases on the occupants of any nearby buildings, which may
be offices, commercial or industrial premises, public buildings, or res-
idential premises.

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