Before water fed into a boiler can be converted into steam, it must be
first heated to a temperature corresponding to the pressure within the
boiler. Steam at 160 pounds gauge pressure has a temperature of
approximately 371 degrees Fahrenheit. If water is fed to the boiler at
60 degrees Fahrenheit, each pound must have 311 B. t. u. added to it to
increase its temperature 371 degrees, which increase must take place
before the water can be converted into steam. As it requires 1167.8
B. t. u. to raise one pound of water from 60 to 371 degrees and to
convert it into steam at 160 pounds gauge pressure, the 311 degrees
required simply to raise the temperature of the water from 60 to 371
degrees will be approximately 27 per cent of the total. If, therefore,
the temperature of the water can be increased from 60 to 371 degrees
before it is introduced into a boiler by the utilization of heat from
some source that would otherwise be wasted, there will be a saving in
the fuel required of 311 ÷ 1167.8 = 27 per cent, and there will be a net
saving, provided the cost of maintaining and operating the apparatus for
securing this saving is less than the value of the heat thus saved.
The saving in the fuel due to the heating of feed water by means of heat
that would otherwise be wasted may be computed from the formula:
100 (t - t_{i})
Fuel saving per cent = --------------- (1)
H + 32 - t_{i}
where, t = temperature of feed water after heating, t_{i} = temperature
of feed water before heating, and H = total heat above 32 degrees per
pound of steam at the boiler pressure. Values of H may be found in Table
23. Table 17 has been computed from this formula to show the fuel saving
under the conditions assumed with the boiler operating at 180 pounds
gauge pressure.