104 
ON LIQUID FUEL. 
The extent to which the available heat could, in any case, become effective in pro¬ 
ducing steam by direct transmission to the boiler must, of course, be limited by the 
temperature corresponding to the pressure at which steam is to be raised. If that were 
50 pounds per square inch, the furnace gas could not be cooled down below 360° F. 
before being discharged from the heating surface of the boiler into the chimney. The 
quantities of heat which would in such a case pass away in the furnace gas, without 
being directly effective in producing steam in the boiler, would amount to 12 per cent, 
in the combustion of carbon, and 10"8 per cent, in the combustion of hydrogen, as 
follows:— 
Quantity 
burnt. 
Furnace 
gas. 
Quantity of heat requisite to 
produce increase of tem¬ 
perature = 300°. 
Equivalent evapora¬ 
tion of water at 
212° F. 
| 
Pound. 
Pounds. 
Heat units. 
Pounds. 
Carbon . 
1 
25 
300° x 5-750 zz 1725 
1-8 
Hydrogen . 
1 
70 
300° X 19-114 = 5734 
5-9 
These quantities of heat would therefore be wasted as regards production of steam, 
except in so far as they might be applied in heating the feed-water supplied to the 
boiler. 
But when, as in ordinary practice, the supply of air for supporting combustion is 
maintained by the draught of a chimney, the temperature of the furnace gas cannot in 
any way be reduced below about (560° F. without interfering with the draught of the 
chimney, and thus a waste of heat is occasioned considerably larger than that just men¬ 
tioned as being the minimum waste. 
In very many instances the furnace gas is discharged into the chimney at a tempera¬ 
ture very much more than 600° F. above the temperature of the external air, and then 
the waste of heat is of course even still greater in proportion as the temperature of the 
gas is higher. 
In the case of furnace gas, discharged at G00° F. above the temperature of the air 
supplied to the furnace, this waste amounts to 24 per cent, of the available heat result¬ 
ing from the combustion of carbon, and to 22 per cent, of that resulting from the 
combustion of hydrogen ; these amounts being equivalent to the evaporation of 36 
pounds of water at 212° F. per pound of carbon burnt, and to 11-9 pounds of water at 
212° F. per pound of hydrogen burnt. 
The amount of heat capable of becoming effective in producing steam cannot there¬ 
fore be greater than the difference between the total available heat and the heat thus 
wasted in the furnace gas. This amount is about 76 per cent, of the available heat 
generated by combustion of carbon, and about 78 pbr cent, of that generated by com¬ 
bustion of hydrogen. This comparison does not take into account those sources of waste 
which are due to imperfect combustion, but applies only to such portions of the carbon 
and hydrogen of fuel as are actually burnt in the furnace. In this case the comparative 
efficiency of these constituents of fuel in producing steam is as follows:— 
Combustion of Carbon 
Quantity burnt, 1 lb. 
Equivalent evaporation ; 
of water. 
at 212° 
at 60° 
Total heat of combustion. 
Available heat. 
Waste heat of furnace gas .... 
Heat units. 
14,500 
14,500 
3,480 
lbs. 
15- 
3-6 
lbs. 
\ 
• • • 
; 
! 
Effective heat. 
11,020 
11-4 
9-8 | 
t 
