22 
BULLETIN 894, U. S. DEPARTMENT OE AGRICULTURE. 
The following table is given for the convenience of the designer in 
figuring pipe areas : 
Table 4. — Square feet of radiating surface per linear foot of pipe. 
Diameter 
of pipe. 
Radiat- 
ing surface 
per linear 
foot. 
Inches. 
Sq.ft. 
0.275 
.346 
.434 
.494 
.622 
.753 
.916 
1 
li 
14 
2 
21 
3" 
One and one-quarter inch pipe is used most frequently for return- 
bend coils up to 30 feet in length and proves very efficient. On ac- 
count of friction 1-inch pipe should be used only where the coils are 
small. Large or extra-long coils should be of lj-inch pipe. 
As an illustration of the above method of calculating radiation, 
take a kiln which is to dry 2-inch oak and in which the maximum 
temperature should not exceed 145° F. The kiln is to be 15 feet 
wide and an average of 11 feet above the rails. There will be about 
7 pounds steam pressure available at the kilns. From the table we 
find that for 145° F. and 7 pounds steam pressure the constant K 
is 0.166. Therefore, the formula becomes: 
15X11X0.166=27.39 square feet radiation surface per linear foot of kiln. 
If lj-inch pipe is selected for the coil, the number of pipes used 
will be : 
27.39 
0.494 : 
=54 
The total amount of radiation surface in the heating coils will be 
the inside length of kiln multiplied by the radiation per foot. Assum- 
ing 39 feet for the length of kilu inside of walls, the total radiation in 
coils below rails becomes 39 X 27.39 = 1,068.21 square feet. The 
coils will be about 38 feet in this instance, and the return-bend 
fittings will make up the rest of the radiation. 
Pipes and Fittings. 
The best wrought-iron pipe should be used for work inside kilns. 
Galvanized pipe or fittings should not be used, as the fumes given off 
from many woods when drying decompose the coating. 
The pipe should be well painted with good high-temperature- 
melting paint. 
The return-bend fittings should be of the type known as "open 
pattern," as their larger radius tends to reduce resistance to the 
travel of the steam. 
