in the steam during a boiler trial, where superheated steam tables are
available, the formula for determining the factor of evaporation is that
already given, (2),[22] namely,

H - h
Factor of evaporation = -----
L

Here H = total heat in one pound of superheated steam from the table,
h and L having the same values as in (2).

Where no such tables are available but the specific heat of superheat is
known, the formula becomes:

H - h + Sp. Ht.(T - t)
Factor of evaporation = ----------------------
L

Where H = total heat in one pound of saturated steam at pressure
existing in trial,
h = sensible heat above 32 degrees in one pound of water at the
temperature entering the boiler,
T = temperature of superheated steam as determined in the trial,
t = temperature of saturated steam corresponding to the boiler
pressure,
Sp. Ht. = mean specific heat of superheated steam at the pressure and
temperature as found in the trial,
L = latent heat of one pound of saturated steam at atmospheric
pressure.

Advantages of the Use of Superheated Steam--In considering the saving
possible by the use of superheated steam, it is too often assumed that
there is only a saving in the prime movers, a saving which is at least
partially offset by an increase in the fuel consumption of the boilers
generating steam. This misconception is due to the fact that the fuel
consumption of the boiler is only considered in connection with a
definite weight of steam. It is true that where such a definite weight
is to be superheated, an added amount of fuel must be burned. With a
properly designed superheater where the combined efficiency of the
boiler and superheater will be at least as high as of a boiler alone,
the approximate increase in coal consumption for producing a given
weight of steam will be as follows:

_Superheat_ _Added Fuel_
_Degrees_ _Per Cent_
25 1.59
50 3.07
75 4.38
100 5.69
150 8.19
200 10.58

These figures represent the added fuel necessary for superheating a
definite weight of steam to the number of degrees as given. The standard
basis, however, of boiler evaporation is one of heat units and,
considered from such a standpoint, agai