36 BULLETIN 747, -IT. S. DEPARTMENT OF AGRICULTURE. 
The possible saving in fuel due to the " preheating " of the boiler- 
feed water may be readily found by the use of the following formula: 
(T— t) 
Per cent of saving=yg^rx X100 
where T=B. t. u. in water above 32° F. after passing through heater. 
t=B. t. u. in water above 32° F. before passing through heater. 
H=B. t. u. in steam above 32° F. at boiler pressure. 
As an illustration of the formula shown above, suppose the steam 
pressure in the boiler is 70 pounds' gauge and the initial temperature 
of the feed water is 60° F. If the water is heated to 200° F. by ex- 
haust steam before entering the boiler, the per cent of saving in fuel 
will be 
(167.94—28.08) _ 
(1,183.3-28.08) X100=12.11 per cent. 
The maximum gain that can be realized by using exhaust steam 
for heating feed water in an open heater is, with a gauge pressure 
of 70 pounds per square inch on the boiler, approximately 15.2 per 
cent, this being the case when taking water at an initial temperature 
of 32° F. and delivering it to the boiler at 212° F., the highest tem- 
perature that it is possible to heat water at sea level in an open ves- 
sel under atmospheric pressure. 
Table 5 gives the per cent of saving in fuel by " preheating " the 
boiler feed water from various initial temperatures to different final 
temperatures. 
For every 11° F. that the feed water is heated before entering the 
boiler approximately 1 per cent less fuel is required to generate the 
same amount of steam, and for each 11° F. increase in feed-water 
temperature, the boiler capacity is increased approximately 1 per 
cent. 
Besides the direct saving in fuel due to heating the feed water, 
the injurious effects of unequal expansion in the boiler, caused by 
having feed water at a low temperature, are diminished, and the 
life of the boiler is prolonged. It is easier also to keep a constant 
pressure on the boiler. There will be a further gain because of the 
smaller quantity of fuel consumed, due to the even firing, for when 
a fire is crowded to take care of a temporary overload a considerable 
amount of heat in the coal is lost by admitting an excess of air into 
the furnace and by a portion of the combustible matter being carried 
up the stack unconsumed. 
To reduce the per cent of saving in fuel, as shown in Table 5, to 
their equivalents in dollars and cents, let us assume that the boiler 
has a capacity of 40 horsepower and that it is operated 8 hours a day 
for 310 days in the 3^ear. With a combined boiler and furnace effi- 
ciency of 50 per cent, about 6^ pounds of coal per boiler horsepower 
hour will be consumed, or 2,080 pounds per day of 8 hours, when 
the feed water is admitted to the boiler at 40° F. If the feed water, 
