22 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
RATE OF CIRCULATION. 
The Forest Products Laboratory recommends for all difficult dry- 
ing work which demands uniform drying conditions a circulation of 
at least 25 feet per minute through the lumber piles. "Where require- 
ments are not so exacting much lower rates, may be used. If only 
the removal of the moisture from the kiln through ventilation is 
desired, a very low rate may be ample. In fact, certain types of 
kilns are being successfully operated without any visible means of 
moisture removal, leakage being sufficient to keep the humidity be- 
low the desired point. 
Generally high rates of circulation produce increased drying rates 
in wood as well as in many other substances, temperatures and hu- 
midities being the same; but actual data on the subject are meager 
and it is not possible at present to say how far it ma}' be commer- 
cially feasible to go in the matter of very high circulation, and to 
what extent similar effects may be produced by other means. 
TESTING CIRCULATION. 
Much trouble in drying is caused by poor or nonuniform circula- 
tion, and it is frequently necessary to determine the amount of cir- 
culation and its direction as a 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. About the only method which has proved satis- 
factory 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 shows clearly the 
direction of movement. It is, of course, necessary for the operator 
to be inside the kiln during the test. 
Tobacco, punk sticks, or rope may be used to provide the smoke, 
although it is difficult with these means to get a sufficient volume of 
smoke, and the fire risk is an objectionable feature. It is almost nec- 
essary, however, to use one of these methods in determining the cir- 
culation at an inaccessible point. A few punk sticks or a bit of rope 
can be tied to the end of a stick and poked into many places which 
could not otherwise be reached, Smoke from any burning substance, 
it must be remembered, tends to rise because of its higher tempera- 
ture ; hence the true circulation will not be indicated until the smoke 
has cooled to the temperature of the surrounding air, 
A special form of smoke machine for dry-kiln work has been de- 
veloped at the Forest Products Laboratory. This machine consists 
essentially of two small bottles and a few pieces of connecting tubing. 
One bottle is partly filled with hydrochloric acid and the other with 
ammonia. When air is blown through the bottles, fumes of the two 
chemicals are mixed, producing a dense fog or smoke which will drift 
readily with the air current. 
To secure proper results in smoke tests, it is essential that all the 
doors be closed and that the lain be operating in the normal manner. 
For higher velocities, such as those usually occurring in the flues 
of ventilated kilns and in the interior of some types of forced cir- 
culation kiln, the Biram type of anemometer is suitable. This 
anemometer is in essence a disk fan mounted upon pivot bearings 
