4 BULLETIN 300, U. S. DEPARTMENT OF AGRICULTURE. 
must be cleared, it is frequently economical to use as fuel the wood 
cut in clearing. 
It is estimated that 1 pound of coal will convert from 7 to 10 
pounds of water into steam, and that there are about 13,000 British 
thermal units in 1 pound of coal. The heat loss from a bare boiler 
containing steam at 125 pounds pressure on an ordinary summer day 
is about 1,200 British thermal units per square foot per hour. This, 
allowing for fire-box losses, is equivalent to about 14 pounds of coal 
per square foot of bare boiler surface per shift of 10 hours, or on a 
boiler having 226 square feet a heat waste of 339 pounds of coal. 
The economy of covering boilers with insulating material is seen 
from the following calculation: 
Boiler, 54 inches diameter by 16 feet long, contains 226 square feet surface. 
125 pounds gage pressure represents 352° F. temperature. 
Air temperature assumed to be 80° F. 
Coal cost at dredge assumed to be $11 per ton. 
Loss per square foot of bare surface per degree of temperature difference is 
3 British thermal units per hour. (Authorities give this as from 2.7 to 3.) 
Assume that 1 pound of coal produces 7 pounds of steam. 
Latent heat of steam at 125 pounds gage=865 British thermal units. 
The total loss from the bare boiler per hour will then be 
3.0X (352-80) X 226 $11.00 _ 
865 X7 X2000 =$0.167 
Thus the loss per shift of 11 hours would be $1.84 and the loss per 
month of 52 shifts would be $95.68. 
For a working pressure of 125 pounds per square inch, a boiler 
covering of about 2 inches should be used. In tests the efficiency of 
a 2-inch heat insulator has been found to be as high as 90 per cent. 
That means that by insulation 90 per cent of $95.68 can be saved 
or $86.11 per month. To cover a boiler, as described, costs about 
60 cents per square foot. Thus, the boiler covering would be paid 
for in a little more than a month and a half of operation. The 
above calculations are based on an air temperature of 80° F. For 
lower air temperatures the saving would be correspondingly greater. 
For convenience in reckoning the temperature corresponding to 
the pressure in the boiler registered by the gage. Table 1 is given: 
TABLE 1.—Steam temperatures at various pressures. 
Gage Gage 
S 
pressure | ; eae pressure | ¢ fellate 
per square Pea per square = 
inch. ‘ inch. : 
= 
Pounds. oe Pounds. ie ihe 
0 212 100 338 
10 240 150 366 
25 267 200 388 
0 298 || 250 406 
eel 
