September 25, lOlB. 



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Government Dry Kiln Improved 



BY HAIIIIY DONALD TIKMANN 

 Id Charge, Section of Timber riiy'lco, FV>rc8t Productit Lnborntory, Madlion, W'Ik. 



I'ut.:i;; till' ja.-t vr:ir coiicentratod study lioB been mndo of tlio 

 manner la nliich the nir moves n-iUiin the dry kiln and imrticularly 

 within Uie pile of lumber. Temporntures of Oic lumber lit various 

 points haw also l>een measured in this connection. The observations 

 h»TO develoi>ed some valuable conclusions. The importiuice of this 

 circulation within Uie pile has been Kf^ntJy tniphnsired. In fact, it 

 is evident upon consideration Uiat the humidities and temperatures 

 inside tJio piles of lumber nro largely dependent thereon. That is to 

 say, the temperature and humidity within the kiln taken alone are 

 no criterion of the conditions of drying within the pile of lumber 

 if the circulation in 

 any portion be defi- 

 cient. It is possible to 

 have an extremely 

 rapid circulation of air 

 within the dry kiln it- 

 self and yet have stag- 

 nation witliin the pile, 

 the air passing chiefly 

 through open spaces 

 and channels. Wher- 

 ever stagnation exists 

 or the movement of air 

 is too sluggish, there 

 the temperature will 

 drop and humidity in- 

 crease, perhaps to the 

 point of saturation. As 

 evaporation is a cooling *"'*^- ^- IMPROVED TIESIANN DKY KILN, SIX -THUCK BATTEKY FORM WITHOUT PARTI- 



Note. — In the Progressive type the condensers G arc placed at the dry end only and sprays F at 



moist end only. 



process, it is evidently 

 necessary to have a 

 continual supply of hot 

 air to keep up the temperature. For example, suppose the air 

 around a pile of lumber is at a temperature of 140° F. and a 

 humidity condition of 51 per cent. Then if the temperature at 

 any point inside the pile be 115 degrees (which may easily hap- 

 pen), at that point saturated conditions will exist. No evaporation 

 will take place there, and the timber may even gain in weight, while 

 the outer portions of the boards are drying rapidly. In this connec- 

 tion it should be observed that so long as evaporation is taking place 

 the temperature of the lumber itself (which may be measured by a 

 thermometer inserted in a hole bored in the wood) is always less than 



that of the air. When the lumber is very wet, its temperature is oluioBt 

 identical with the wet-bulb of a hygrometer but, as it becomes drier, 

 its temperature will lie somewhere between that of the wet bulb and 

 tliat of the actual air temperature. 



Tho extraction of heat from the air which comes in contact witli 

 the lumber, through evaporation of moisture causes it to cool and 

 to tend to descend. This tendency is considerable, and may bo suf- 

 ficient to counteract the tendency of tho air, heated by steam pipes 

 beneath, to rise through the piles. It has been found sufficient to 

 produce a reversed circulation causing it to pass downward instead 



of upward from steam 

 pipes placed beneath 

 the piles. This condi- 

 tion is particularly 

 manifest when wet lum- 

 ber is placed in the 

 kiln, and especially so 

 when it is cold or 

 frozen. In the former 

 humidity regulated kiln, 

 in which inclined piling 

 was used, the arrange- 

 ment was such as to 

 cause the circulation to 

 be forced upward 

 through tho pile of 

 lumber and downward 

 in the spray chambers 

 on either side. When 

 flat piling is used it 

 has been found that 

 even with a good cir- 

 culation in the kUn itself stagnation sometimes may exist within 

 tho center portion of the piles to so great an extent that mold may 

 form between tho layers of lumber. With the inclined piling as 

 originally designed for use with this kiln, the effect of the cooling 

 is not nearly so great as with the flat piling, but it may be sufficient 

 to cause unnecessarily slow drying in portions of the pile. In a large 

 sized operation wlicn the forced circulation is in the opposite direc- 

 tion from that induced by the cooling of tho air by the lumber there 

 is always more or less uncertainty as to the movement of the air 



FIG. 1. DIAGRAM.VTIC SECTION OF IMPROVED HUMIDITY REGU- FIG. 2. DIAGRAMATIC SECTION OF IMPROVED DRY KILN WITH 

 LATED DRY KILN WITH SPRAY FLUE IN CENTER. SPRAY CHAMBERS ON SIDES. DOUBLE TRUCK FORN. 



