Horse Power]

Fig. 31 shows the relation of gas temperatures, heating surface passed
over and work done by such surface for use in cases where the
temperatures approach those found in direct fired practice and where the
volume of gas available is approximately that with which one horse power
may be developed on 10 square feet of heating surface. The curve assumes
what may be considered standard gas passage areas, and further, that
there is no heat absorbed by direct radiation from the fire.

Experiments have shown that this curve is very nearly correct for the
conditions assumed. Such being the case, its application in waste heat
work is clear. Decreasing or increasing the velocity of the gases over
the heating surfaces from what might be considered normal direct fired
practice, that is, decreasing or increasing the frictional loss through
the boiler will increase or decrease the amount of heating surface
necessary to develop one boiler horse power. The application of Fig. 31
to such use may best be seen by an example:

Assume the entering gas temperatures to be 1470 degrees and that the
gases are cooled to 570 degrees. From the curve, under what are assumed
to be standard conditions, the gases have passed over 19 per cent of
the heating surface by the time they have been cooled 1470 degrees.
When cooled to 570 degrees, 78 per cent of the heating surface has been
passed over. The work done in relation to the standard of the curve is
represented by (1470 - 570) / (2500 - 500) = 45 per cent. (These
figures may also be read from the curve in terms of the per cent of the
work done by different parts of the heating surfaces.) That is, 78 per
cent - 19 per cent = 59 per cent of the standard heating surface has
done 45 per cent of the standard amount of work. 59 / 45 = 1.31, which
is the ratio of surface of the assumed case to the standard case of the
curve. Expressed differently, there will be required 13.1 square feet
of heating surface in the assumed case to develop a horse power as
against 10 square feet in the standard case.

The gases available for this class of work are almost invariably very
dirty. It is essential for the successful operation of waste-heat
boilers that ample provision be made for cleaning by the installation of
access doors through which all parts of the setting may be reached. In
many instances, such as waste-heat boilers set in connection with cement
kilns, settling chambers are provided for the d