Page 38 - Uninterruptible Power Supplies

Standby Power Generating Sets
36 Chapter One

Generating sets are sometimes installed at ground level in light wells
enclosed by buildings. In such circumstances there is no alternative to
running the exhaust pipe vertically and terminating it, in accordance
with the Clean Air Acts requirements, above the highest part of the
building. This may result in a pipework run much in excess of the 10 m
mentioned earlier, and in such cases it is essential that the engine man-
ufacturer should be consulted. The engine performance will be affected
and some adjustments may be required.

The 1956 and 1993 Clean Air Acts
The Clean Air acts apply to engine exhaust gases and the requirements
are set out in the Third Edition of the 1956 Clean Air Act Memorandum
published by Her Majesty’s Stationery Office. The acts seek to ensure
adequate dispersal of sulphur dioxide and other pollutants produced in
normal combustion.
The acts do not apply to gas turbines which require special consider-
ation by the local authority, nor do they apply to plants with a gross
heat input of less than 150 kW. If the efficiency of the generating set is
taken to be, say 35 percent, then they will not apply to installations of
less than 52.5 kW rating. If it is not clear whether the acts apply or not,
the gross heat input at maximum rated load, including any overload,
should be obtained from the generating set manufacturer.
The Memorandum describes the method of calculating the mini-
mum height of the exhaust gas discharge. There is a recommended min-
imum efflux velocity of 6 m/s but this is unlikely to be a problem for diesel
engine exhausts; it is primarily applied to boiler flues and demands a ver-
tical discharge. In determining the minimum height, the Memorandum
takes into account three factors: the rate of emission of sulphur dioxide at
the maximum power output including any overload capacity, the type of
area in which the installation is situated, and the effect of adjacent build-
ings on flue gas dispersal.
The first two factors are used to determine, in the words of the Act,
the uncorrected chimney height, and the third factor is used to modify
this to arrive at the corrected chimney height.
The rate of emission of sulphur dioxide depends on the sulphur con-
tent of the fuel, and the maximum rate of fuel consumption. For BS 2869
Class A2 fuel (Class A1 fuel is no longer listed) the sulphur content
should not exceed 0.2 percent by mass, and the rate of fuel consumption
will depend on the rating and overall efficiency of the generating set and
should be ascertained from the manufacturer. If it is proposed to use
other fuels the sulphur content should be ascertained from the supplier.
The atomic weights of sulphur and of oxygen are 32 and 16, respectively;
it follows that for every kilogram of sulphur burnt, 2 kilograms of sulphur
dioxide will be produced.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.