﻿22 BULLETIN 1136, U. S. REPABTMEUT OF A ■. ; i t i t ILTURE. 



RATE OF CIRCULATION. 



Tlie Forest Products Laboratory recommends for all difficult dry- 

 ing work which demands uniform drying conditions a circulation of 

 :it 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 lie- 

 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 may 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 kiln 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 



