should be put in air-tight
tins or jars for the determination of moisture; the balance of the
sample (the total weight of which should be from 10 to 50 pounds,
depending on the total weight of coal used in the test) may be
packed in a wooden box lined with paper to prevent splinters from
mingling with the sample. A duplicate coal sample should be kept at
the plant to be used in case of loss of the sample sent to the
chemist.

The Bureau of Mines has published a bulletin or pamphlet giving the
analyses and heating values of the various kinds and grades of coal
from all parts of the United States. (Bureau of Mines Bulletin No.
22.) This bulletin can be used to learn the approximate heating
value of the coal. Simply find out what district the coal used in
the test came from, and its grade, and then refer to the bulletin
to obtain the heating value of the coal. If a chemist can be
obtained to make a heat test, however, it is better to use the
heating value he determines.

Suppose that during the test the coal used was run-of-mine
bituminous having a heating value of 13,500 B. t. u. Every pound of
coal fired, then, carried into the furnace 13,500 heat units, and
this value therefore is the _input_ to be used in calculating the
boiler efficiency.

During the test 5,000 pounds of coal was fired and 35,000 pounds of
water was fed and evaporated. This means that 35,000 / 5,000 = 7
pounds of water was evaporated per pound of coal burned. This is
the "actual evaporation," and the heat required to evaporate this 7
pounds of water is the output to be used in calculating the
efficiency.

Every fireman knows that it takes more coal, and therefore more
heat, to make steam with cold feed water than with hot feed water;
also, that it is somewhat easier to make steam at a low pressure
than at a high pressure. So it is plain that the heat required to
evaporate 7 pounds of water into steam depends on two things,
namely, (1) the temperature of the feed water and (2) the pressure
of the steam in the boiler. From the data of the test, both the
average feed-water temperature and the average steam pressure are
known, and so it is a simple matter to find out the amount of heat
needed to evaporate 7 pounds of water from the average temperature
to steam at the average pressure.

A pound of water at 212 deg. F. must have 970.4 B. t. u. added to it to
become a pound of steam at 212 deg. F., or zero gage pressure. This
value, 970.4 B. t. u.,