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 present state of the art, arising in the
conversion of the heat energy of the coal to the heat energy in the
steam. These losses may be classified as follows:
1st. Loss due to fuel dropped through the grate.
2nd. Loss due to unburned fuel which is carried by the draft, as small
particles, beyond the bridge wall into the setting or up the stack.
3rd. Loss due to the utilization of a portion of the heat in heating the
moisture contained in the fuel from the temperature of the atmosphere to
212 degrees; to evaporate it at that temperature and to superheat the
steam thus formed to the temperature of the flue gases. This steam, of
course, is first heated to the temperature of the furnace but as it
gives up a portion of this heat in passing through the boiler, the
superheating to the temperature of the exit gases is the correct degree
to be considered.
4th. Loss due to the water formed and by the burning of the hydrogen in
the fuel which must be evaporated and superheated as in item 3.
5th. Loss due to the superheating of the moisture in the air supplied
from the atmospheric temperature to the temperature of the flue gases.
6th. Loss due to the heating of the dry products of combustion to the
temperature of the flue gases.
7th. Loss due to the incomplete combustion of the fuel when the carbon
is not completely consumed but burns to CO instead of CO_{2}. The CO
passes out of the stack unburned as a volatile gas capable of further
combustion.
8th. Loss due to radiation of heat from the boiler and furnace settings.
Obviously a very elaborate test would have to be made were all of the
above items to be determined accurately. In ordinary practice it has
become customary to summarize these losses as follows, the methods of
computing the losses being given in each instance by a typical example:
(A) Loss due to the heating of moisture in the fuel from the atmospheric
temperature to 212 degrees, evaporate it at that temperature and
superheat it to the temperature of the flue gases. This in reality is
the total heat above the temperature of the air in the boiler room, in
one pound of superheated steam at atmospheric pressure at the
temperature of the flue gases, multiplied by the percentage of moisture
in the fuel. As the total heat above the temperature of the air would
have to be computed in each instance, this loss is best expressed by: