36 BULLETIN 147, U. S. DEPARTMENT OP 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= / q +•. X100 



where T=B. t. u. in water above 32° F. after passing throxigh 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) ^ rt <rt ■ 

 (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 1.5.2 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 year. With a combined boiler and furnace effi- 

 ciency of 50 per cent, about CI 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, 



