d of dry coal, 13,516.
Heat value per pound of combustible, 15,359.

The factor of evaporation for such a set of conditions is 1.0834. The
actual evaporation corrected for moisture in the steam is 152,775 and
the equivalent evaporation from and at 212 degrees is, therefore,
165,516 pounds.

The total dry fuel will be 17,500 x .97 = 16,975, and the evaporation
per pound of dry fuel from and at 212 degrees will be 165,516 / 16,975 =
9.75 pounds. The heat absorbed per pound of dry fuel will, therefore, be
9.75 x 970.4 = 9461 B. t. u. Hence, the efficiency by (31) will be 9461
/ 13,516 = 70.0 per cent. The total combustible burned will be 16,975
- 2396 = 14,579, and the evaporation from and at 212 degrees per pound
of combustible will be 165,516 / 14,579 = 11.35 pounds. Hence, the
efficiency based on combustible from (32) will be (11.35 x 97.04) /
15,359 = 71.79.[**should be 71.71]

For approximate results, a chart may be used to take the place of a
computation of efficiency. Fig. 39 shows such a chart based on the
evaporation per pound of dry fuel and the heat value per pound of dry
fuel, from which efficiencies may be read directly to within one-half of
one per cent. It is used as follows: From the intersection of the
horizontal line, representing the evaporation per pound of fuel, with
the vertical line, representing the heat value per pound, the efficiency
is read directly from the diagonal scale of efficiencies. This chart may
also be used for efficiency based upon combustible when the evaporation
from and at 212 degrees and the heat values are both given in terms of
combustible.

[Graph: Evaporation from and at 212deg. per Pound of Dry Fuel
against B.T.U. per Pound of Dry Fuel

Fig. 39. Efficiency Chart. Calculated from Marks and Davis Tables

Diagonal Lines Represent Per Cent Efficiency]

Boiler efficiencies will vary over a wide range, depending on a great
variety of factors and conditions. The highest efficiencies that have
been secured with coal are in the neighborhood of 82 per cent and from
that point efficiencies are found all the way down to below 50 per cent.
Table 59[57] of tests of Babcock & Wilcox boilers under varying
conditions of fuel and operation will give an idea of what may be
obtained with proper operating conditions.

The difference between the efficiency secured in any boiler trial and
the perfect efficiency, 100 per cent, includes the losses, some of which
are unavoidable in the presen