HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 13 
the center of the kiln and the air would rise in the center and 
descend on the sides. 
One of the greatest advantages of this natural circulation method 
is that the colder the lumber when placed in the kiln the greater is 
the movement produced, under the very conditions which call for 
the greatest circulation — just the opposite of the direct-circulation 
method. This is a feature of the greatest importance in winter, when 
the lumber is put into the kiln in a frozen condition. One truck 
load of lumber at 60 per cent moisture may easily contain over 
7,000 pounds of ice. 
In the matter of circulation the kiln is, in fact, self -regulatory — 
the colder the lumber the greater the circulation produced, with the 
effect increased toward the cooler and wetter portions of the pile. 
Preliminary steaming may be used in connection with this kiln, 
but experiments indicate that ordinarily it is not desirable, since 
the high humidity which can be secured gives as good results, and, 
being at as low a temperature as desired, much better results in the 
case of certain difficult woods like oak, eucalyptus, etc. 
This kiln has another advantage in that its operation is entirely 
independent of outdoor atmospheric conditions, except that baro- 
metric pressures will affect it slightly. 
THEORETICAL DISCUSSION OF EVAPORATION. 
In considering the drying effect of vapor alone (superheated 
steam) and of air mixed with the vapor, one very significant fact 
must be noticed. Saturate vapor alone in cooling and in order to 
remain saturate must absorb heat. Its specific heat is negative, so 
that the only way it can heat a body is by condensation. It is, 
therefore, incapable of producing evaporation. When air is present 
with the saturate vapor, however, the air can supply some of this 
heat, according to the pressure of the air present, so there will be less 
condensation. 
Still more important is the fact that when air is present with the 
vapor sufficient heat can be supplied to the body being dried by means 
of the air without greatly superheating the vapor, thus keeping a 
high relative humidity and at the same time supplying a sufficient 
amount of heat to carry on the evaporation. With vapor alone 
(superheated steam) a relatively high degree of superheating, which 
means a correspondingly low relative humidity, is required in prac- 
tice in order to supply the necessary heat for evaporation, after the 
material has become heated through to the temperature of the sat- 
urated vapor at the pressure used. Remember that the temperature 
of the wet wood corresponds to that of the wet bulb in the hygrom- 
eter when air is present, but very nearly to the dew point in the 
presence of superheated vapor alone. 
