22 



HARDWOOD RECORD 



llarch 10. 1921 



Proper Drying Demands Good Circulation 



By Rolf Thelen^ 



In many ways the circulation of the air in tlie kiln is tlie deter- 

 mining factor in the success of the drying operation. Given a 

 uniform, ample circulation throughout the kiln, it is usually fairly 

 simple to provide and maintain the correct temperature and humid- 

 ity. If, however, the circulation is bad, no amount of expensive 

 equipment and apparatus will serve to yield satisfactory drying 

 conditions. The accuracy required in maintaining these drying 

 conditions varies, of course, with the wood to be dried and the 

 state of seasoning in which it enters the kiln. Any old hot box may 

 do to kiln dry previously air-dried inch stock of pine or fir, but 

 mighty few types of kiln are capable of drying thick southern oak 

 vehicle stock green from the saw! 



One of the big problems just now is the remodeling or adapting 

 of old kilns, formerly used only on previously air-dried stock, for 

 the drying of stock green from the saw, and much trouble is being 



Fig- 1 — Simplest Type of Ventilated Kiln 

 Fig. 2— Ventilated Kiln with Outlet Ducts Opening at Bottom o{ Pile. This 

 Arrangement Permits the Air to Drop Downward and Outward Through 



the Lumber 



experienced in the solution of it. A thorough knowledge of the 

 possibilities should be helpful to those who are confronted with this 

 problem, and it is hoped that this article may be of some assistance 

 in stimulating thought and investigation along these lines. 



Uniform and ample circulation is needed principally to distribute 

 the heat uniformly to all parts of the pile, and to permit the humid- 

 ity-regulating system to act uniformly and quickly throughout. 



The sketches are entirely disagreeable and intended merely to 

 illustrate principles, rather than details of design. 



While the circulation in those types having a central chimney 

 through the lumber is indicated, in all the sketches, as being 

 upward, it is quite possible to produce circulation in the reverse 

 direction, especially in kilns provided with forced circulation. The 

 type indicated is usually preferable on account of the greater uni- 

 formity of circulation produced in the chimney through the baffling 

 or mixing effect which the two converging streams of air have 

 upon each other. 



Circulation in compartment kilns is produced in a number of 

 ways, which depend principally upon differences in the tempera- 

 ture of the air at different points and upon mechanical means, such 

 as fans or blowers. To a lesser extent, dependence is placed upon 

 jets of steam or water. 



Compartment Kilns 



Compartment kilns may be divided roughly into "ventilated" 

 and "recirculating" types: in the former, the circulation is 

 intended to be produced by means of chimneys or flues, taking cold 



air from the outside and discharging hot air from the kiln, and in 

 the latter it is intended to be mainly internal, the same air being 

 used over and over again. Circulation in straiglit ventilated kilns 

 is usually very slow and lacking in uniformity; that in recirculat- 

 ing kilns may, with proper design, be made to reach almost any 

 desired speed. 



Figure I illustrates the simplest possible type of ventilated kiln. 

 Fresh air enters at the bottom, becomes heated as it passes over the 

 heating coils, and is supposed to move upward through the lumber 

 pile, and escape through the vent at the top. While this kiln 

 undoubtedly operates more or less as intended, the circulation is 

 usually very poor, and the air often descends through the main 

 portion of the pile instead of ascending, particularly if the lumber 

 is very moist. Further, two important principles are violated: 

 first, the moving air should come in contact with both broad sur- 

 faces of each piece of lumber to be dried; and second, the tendency 

 of the air which has come in contact with the lumber to sink 

 instead of rise should be assisted instead of opposed. The actual 

 amount of circulation, of course, depends largely upon the amount 

 of heating surface and the size of the duets. 



Figure II illustrates much the same type, embodying, however, 

 the two principles violated in Figure I. The chimney in the center 

 of the pile of lumber allows the heated air to rise before entering 

 between the boards, and assists in producing a natural downward 

 lateral circulation. The location of the openings in the outlet flues 

 increases this tendency. Although a distinct improvement over 

 Figure I, this kiln still leaves much to be desired in the way of 

 ample, uniform circulation. 



Even when the circulation through the ventilating ducts is com- 

 paratively small, it is still usually necessary to jjrovide additional 

 moisture to keep up the humidity in the kiln. Figure III illustrates 

 one way in which the circulation can be increased, and the humidity 

 raised at the same time by means of a steam Jet in the intake. 



•/« Charge Section of Timber Physics, 

 rot:, IVis. 



Forest Products Loboratorv, Modi- 



4. 

 Fig. 3— Ventilated Kiln with Outlet Ducts Opening at Bottom of Pile and 



Steam Jet for Maintaining Humidity and Increasing Circulation 

 Fig. 4 — Ventilated Kiln with Heating Coils Concentrated Near Center. In 

 Kilns with Central Flues It Is Not Always Necessary or Desirable to Dis- 

 tribute the Heating Coils Uniformly Under the Entire Pile 



This also serves to heat the air, and thus reduces the lead on the 

 steam coils. 



Figure IV illustrates the principle that the heating coils do not 

 need to be distributed uniformly from side to side of the kiln when 

 the heated air is taken up through a chimney in the pile. 



The first four sketches show straight ventilating kilns, recircula- 

 tion being prevented by means of solid horizontal partitions. The 

 simplest arrangement of a recirculating kiln is shown on the right 

 half of Figure V. The left half illustrates a modification in which 



