Heat absorbed per pound of combustible[56]
= -------------------------------------- (32)
Heat value per pound of combustible
The efficiency so determined is used in comparing the relative
performance of boilers, irrespective of the type of grates used under
them. If the loss of fuel through the grates could be entirely overcome,
the efficiencies obtained by (31) and (32) would obviously be the same.
Hence, in the case of liquid and gaseous fuels, where there is
practically no waste, these efficiencies are almost identical.
As a matter of fact, it is extremely difficult, if not impossible, to
determine the actual efficiency of a boiler alone, as distinguished from
the combined efficiency of boiler, grate and furnace. This is due to the
fact that the losses due to excess air cannot be correctly attributed to
either the boiler or the furnace, but only to a combination of the
complete apparatus. Attempts have been made to devise methods for
dividing the losses proportionately between the furnace and the boiler,
but such attempts are unsatisfactory and it is impossible to determine
the efficiency of a boiler apart from that of a furnace in such a way as
to make such determination of any practical value or in a way that might
not lead to endless dispute, were the question to arise in the case of a
guaranteed efficiency. From the boiler manufacturer's standpoint, the
only way of establishing an efficiency that has any value when
guarantees are to be met, is to require the grate or stoker manufacturer
to make certain guarantees as to minimum CO_{2}, maximum CO, and that
the amount of combustible in the ash and blown away with the flue gases
does not exceed a certain percentage. With such a guarantee, the
efficiency should be based on the combined furnace and boiler.
General practice, however, has established the use of the efficiency
based upon combustible as representing the efficiency of the boiler
alone. When such an efficiency is used, its exact meaning, as pointed
out on opposite page, should be realized.
The computation of the efficiencies described on opposite page is best
illustrated by example.
Assume the following data to be determined from an actual boiler trial.
Steam pressure by gauge, 200 pounds.
Feed temperature, 180 degrees.
Total weight of coal fired, 17,500 pounds.
Percentage of moisture in coal, 3 per cent.
Total ash and refuse, 2396 pounds.
Total water evaporated, 153,543 pounds.
Per cent of moisture in steam, 0.5 per cent.
Heat value per pound 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 × .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 × 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 × 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 212° per Pound of Dry Fuel
against B.T.U. per Pound of Dry Fuel