is called the latent heat of steam at
atmospheric pressure, or the heat "from and at 212 deg. F." It is the
heat required to change a pound of water _from_ 212 deg. F. to steam
_at_ 212 deg. F., and is used by engineers as a standard by which to
compare the evaporation of different boilers.

In a boiler test the temperature of the feed water is usually
something less than 212 deg. F., and the steam pressure is commonly
higher than zero, gage. In the test outlined previously, the
feed-water temperature was 180 deg. F. and the pressure was 100 pounds
per square inch, gage. It must be clear, then, that the amount of
heat required to change a pound of water at 180 deg. to steam at 100
pounds gage pressure is not the same as to make a pound of steam
from and at 212 deg. F.

To make allowance for the differences in temperature and pressure,
the actual evaporation must be multiplied by a number called the
"factor of evaporation." The factor of evaporation has a certain
value corresponding to every feed-water temperature and boiler
pressure, and the values of this factor are given in the
accompanying table. Along the top of the table are given the gage
pressures of the steam. In the columns at the sides of the table
are given the feed-water temperatures. To find the factor of
evaporation for a given set of conditions, locate the gage pressure
at the top of the table and follow down that column to the
horizontal line on which the feed-water temperature is located. The
value in this column and on the horizontal line thus found is the
factor of evaporation required. If the feed water has a temperature
greater than 212 deg. F., obtain the proper factor of evaporation from
the Marks and Davis steam tables.

Take the data of the test, for example. The average steam pressure
is 100 pounds, gage. The average feed-water temperature is 180 deg. F.
So, in the table locate the column headed 100 and follow down this
column to the line having 180 at the ends, and the value where the
column and the line cross is 1.0727, which is the factor of
evaporation for a feed-water temperature of 180 deg. F. and a steam
pressure of 100 pounds, gage.

This factor, 1.0727, indicates that to change a pound of water at
180 deg. F. to steam at 100 pounds requires 1.0727 times as much heat
as to change a pound of water at 212 deg. F. to steam at atmospheric
pressure. In other words, the heat used in producing an actual
evaporation of 7 pounds under the