FOREST SERVICE WATER SPRAY DRY KILN. 11 
should be hard-&urned. The walls should be laid up in tempered or 
cement mortar. The difference in expansion between the inner and 
outer faces of the exterior walls caused by the difference in tempera- 
ture will create numerous small cracks, particularly in solid brick 
walls in cold weather, which should be painted up with an elastic 
cement rather than mortar. Where tile is used, selection should be 
made of a type in which the openings run horizontally rather than 
vertically in the wall. The tile should be properly scored for plas- 
tering. Both sides of tile walls should be plastered with cement 
mortar. 
Where kilns over 50 feet long are built of brick, tile, or concrete, 
it is advisable to build the interior and exterior walls 12 inches thick, 
particularly where fireproof roofs are used. 
Walls of monolithic concrete or concrete blocks are highly ab- 
sorbent of moisture unless thoroughly waterproofed. It is very 
difficult to hold a high humidity within the kiln where the walls will 
absorb moisture readily. The heat loss through such walls is also 
very great. 
Figure 2 shows a kiln with brick walls and wood roof; figure 3, a 
kiln with tile wall and tile and concrete fireproof roof. 
DETAILS OF CONSTRUCTION. 
SPRAY CHAMBER. 
Whether side or center spray chambers should be selected depends 
upon the temperatures to be used in the kiln. The side-spray cham- 
bers produce a greater and more even circulation than the center- 
spray type. The side-spray type is recommended for all low and 
medium temperature runs and for high temperature runs on very 
green stock. The center-spray chamber is somewhat less expensive 
to install than the side-spray chamber, which requires extra piping 
and fittings, and is suitable for kilns where high temperatures are to 
be used. 
Where side-spray chambers are used the inside width should be 
12 inches. For center-spray chambers the width should be 15 
inches. The height from the bottom of the kiln should not be less 
than 5 feet 6 inches, and the top of the chamber wall should be about 
18 inches above the top of the rail when kiln trucks are used, or 6 
inches above the loading floor when trucks are not used. 
'The spray-chamber walls may be wood, brick, or concrete. Where 
wood is used, narrow cypress shiplap 1J inches thick laid vertically 
(see fig. 2) will probably be found most satisfactory, and should be 
well nailed with cut nails to 2-inch and 4-inch cypress cleats. This 
wood should be set green. Hard-burned brick walls 4 inches thick 
laid in cement mortar and plastered with cement plaster prove satis- 
factory, as do also integral waterproofed concrete walls 3 or 4 inches 
thick. (See fig. 5.) 
