30 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
woods, usually do not require a circulation greater than 25 feet per 
minute for a 5-foot air travel, and previously air-dried hardwoods 
require still less circulation. ’ 
TESTING THE CIRCULATION ~ 
Much trouble in drying is caused by insufficient or nonuniform 
circulation, and frequently determining the amount of circulation 
and its direction is a necessary preliminary to prescribing a remedy. 
The rate of circulation inside the average kiln is so low that most of 
the methods usually employed in the measurement of air velocities 
are not suitable. Although the velocity of the air occasionally is 
great enough to permit the successful use of strips of tissue paper to 
show its direction and intensity, about the only method that has 
proved universally satisfactory is to watch the drift of smoke and, if 
desired, to time its movement over a known distance by means of a 
stop watch. One of the special advantages of this method is that it 
Hydrochloric | 
4 . a 
Figure 5.—A smoke machine for testing the air circulation in dry kilns. The bottles 
are common ink bottles—aimost any kind of a bottie will do, but in order to avoid 
bulkiness a comparatively narrow one is desirable. The box and its handle can be 
constructed in any way desired, or in case of need could even be dispensed with 
entirely. Two pieces of bent-glass tubing, a cork, and a rubber tube @ complete the 
machine; short lengths of glass and of rubber tubing may be used instead of the 
bent glass, although they are not so good. The tube a should be long enough to 
allow the operator to extend the apparatus at arm’s length while biowing into the 
tube. Some operators prefer to fit the end of the rubber tube with a syringe bulb $ 
this is practically necessary when a mask is worn 
shows clearly the direction of movement. It has also some disad- 
vantages. Smoke from any burning substance, for instance, tends to 
rise because of its higher temperature; hence the true circulation will 
not be indicated until the smoke has cooled to the temperature of the 
surrounding air. The operator, of course, must be inside the kiln 
during the test, and it is essential that all the doors be closed and that 
the kiln be operating in the normal manner. 
Tobacco, punk sticks, or rope may be used to provide the smoke, 
although it is difficult with these means to get a suifcient volume, and 
the fire risk is an objectionable feature. A special form of fireless 
smoke machine for dry-kiln work has been developed at the forest 
products laboratory. It consists essentially of two small bottles and 
a few pieces of connecting tubing. One bottle is partly filled with 
concentrated hydrochloric acid and the other with strong ammonia 
water. When air is blown through the bottles, fumes of the two 
chemicals are mixed, producing a dense fog or smoke that will drift 
readily with the air current. (Fig. 5.) F 
