FOREST SERVICE WATER SPRAY DRY KIL^. 45 
To estimate the approximate amount of coil surface required, 1 
square foot of coil should be allowed for every 20 cubic feet of kiln 
space above the rails. If cold water is available throughout the year, 
less coil is needed. Where the temperature of the water is not over 
50° F. in the hottest weather of the season the coil area can be 
reduced 25 per cent. 
The condenser coil can be built up of 1-inch or 1^-inch pipe con- 
nected with cast-iron return bends and supported by hook plates. 
The supply is taken from the cold-water main through a gate valve 
to the bottom of the coil. The discharge is taken from the top of the 
coil and should be extended well down into the spray chamber, 
where it empties into the gutter. 
CONTROL INSTRUMENTS. 
REQUIRED DETERMINATIONS. 
The temperature and humidity in the water spray kiln are de- 
termined from two thermometer readings, one taken in the entering- 
air flue and the other in the baffle plates. The baffle plate ther- 
mometer gives the dew-point temperature. This thermometer should 
be placed in the lower opening of one of the boxes on the spray 
chamber side but in such a manner that it does not receive a direct 
spray of water nor rest on the floor of the kiln. 
The entering-air temperatures should be taken about halfway to 
the top of the pile and in the center of the flue. 
RECORDING THERMOMETERS. 
Four types of extension-tube recording thermometers are made, 
their principal differences depending upon whether the element 
contained in the bulb is (1) mercury, (2) nonvolatile liquid, (3) gas, 
or (4) a volatile liquid (called vapor-filled). 
For dry-kiln work the gas-filled and vapor-filled thermometers are 
found most desirable. 
The vapor-filled type is more rapid in action than the gas type and 
gives good satisfaction provided it can be installed so that no portion 
of the tube or case is at a higher temperature than the bulb itself. 
This is very difficult to do, particularly with the dew-point bulb, as 
some portion of the tube frequently has to pass near a steam pipe 
or through a space warmer than the baffles. Adjustment must be 
made for vertical distance between the bulb and the recording spring, 
due to the liquid head in the tube. A recent type of vapor-filled 
thermometer has been made with a large bulb, in which it is alleged 
that the temperature of the tube and case do not affect the reading. 
The vapor-filled type is more accurate than the other forms, because 
changes in the volume of the spring bulb or tube do not affect its 
operation, as it acts simply on the principle of a steam-pressure gauge. 
