26 



HARDWOOD RECORD 



June 10. 1922 



possible maximum and the product in best condition. Uniform dry- 

 ness in a lumber pOe cannot be obtained unless the air circulation 

 through this pile is of sufficient volume to fill all of the passages sur- 

 rounding each course of boards and each board, and of sufficient 

 intensity to orercome the friction resistance to its movement through 

 these passages. Unless the circulation within the respective lumber 

 piles is uniform, the heat supply to the lumber cannot be uniform, 

 neither can the moisture removal from the lumber surface be uniform. 

 The drying within the piles wUl therefore be uneven, certain portions 

 drving faster, others slower, and-the drying time ^vill be lengthened. 



It is true that this condition may also obtain in some of the com- 

 partment kilns, and that it may be modified, or accelerated, by good or 

 poor piling. 



The length of time necessary to dry any lumber is controlled by 

 several conditions, which may be enumerated as follows : 



(a) The rate of transfusion of the moisture from the interior of 

 the wood to its surface when exposed to the maximum safe tempera- 

 ture. This rate varies with the species and to some extent with heart 

 and sapwood of the same species. 



(b) The proper schedule of permissible drying temperatures, since 

 the rate of transfusion increases with temperature increase. 



(c) The thickness of the lumber, or rather the distance through 

 which the moisture must pass. 



(d) The condition of the surface fibers and extent of any prerious 

 case hardening. Method of sawing, quarter or flat. 



(e) The initial moisture content of the wood and the desired final 

 moisture content — in short, the amount of moisture to be removed. 



(f) Structural peculiarities of the wood, and its ability to resist 

 rupture under unequal shrinkage stresses. 



(g) The continuity of the drying operation, without interruption, 

 \mder the schedule selected, with due consideration of the conditions 

 a to f . 



(h) And last, but by no means least, the previously mentioned uni- 

 formity of the heat supply to, and moisture removal from, all of the 

 boards in each pile. This can be accomplished only by an ample and 

 uniform air-circulation over all lumber surfaces in the kiln. This air- 

 circulation must be ample in volume and ample in velocity, and its 

 temperature and relative humidity must harmonize with the schedule 

 requirements. 



Concerning Relative Humidity 



The relative humidity of the air circulating over the lumber controls 

 the absorption by the air of the moisture which the heat causes to 

 transfuse from the interior to the surface of the wood, ami there to 

 become vaporized. The greater the relative humidity of this air, the 

 slower will it absorb additional moisture, and vice versa. It is through 

 the regulation of this relative humidity of the circulating air that the 

 kiln operator is able to control, and slow up, or speed up, as the case 

 may require, the rate of removal of the wood moisture from the sur- 

 face of the lumber, and thus protect it against case hardening and 

 consequent loss. 



This air-circulation, which has just been discussed, must not be 

 confounded with the so-called "ventilation" of the kUn. The air- 

 circulation does all of the heat and moisture carrying to and from the 

 lumber, whUe the ' ' ventilation ' ' may be concerned only in removing 

 from the kiln the proper portion of the moisture which the "circula- 

 tion" has absorbed from the lumber surfaces, by expulsion of a cer- 

 tain amount of this moisture laden air, with simultaneous substitution 

 of an equal amount of drier fresh air. Each of these performs a dis- 

 tinct duty, and it is by careful adjustment of the amount of ventilation 

 which the operator permits that he is enabled to maintain that relative 

 humidity of the kUn air which is necessary to restrict (or slow up) 

 the moisture absorption from the surface of the lumber and thus to 

 protect it against the evils of rapid surface drying. 



In any kUn, the Moist Air Fan Kiln and the Tiemann Water Spray 

 Kiln alone excepted, the air circulation through the lumber piles is 

 caused and controlled by the difEerence in the gravity of the air enter- 

 ing and leaving the lumber. 



In the Moist Air Fan Kiln the air circulation through the lumber 

 is forced in positive manner by the fan, and in the Tiemann Water 



Sprav Kiln it is induced and accelerated by the action of the water 

 sprays. 



Sine* the gentleman making the inquiry stated his unfamiliarity 

 with dry kiln problems, the writer deemed it necessary to present the 

 foregoing explanation in the simplest and most understandable form. 



In the opening paragraph of the inquiry it is stated that the drying 

 of the lumber would have to proceed at a " certain rate. ' ' This the 

 writer interprets to mean "large quantities to be dried at the fastest 

 practicable speed, compatible with good quality of product at minimum 



Nuw. quantity of output simply means kiln capacity and organiza- 

 tion of the drying operation, and this would remain the same in 

 either case, progressive or compartment system. We have already 

 discussed the drying speed and the conditions which control it. 

 These can hardly be altered, and must hold good in any kiln. 



Circulation Ef&clency Means Success 



The important variable, therefore is the "circulation through the 

 lumber piles in the kiln." All other things being eqtud, the Tciln teith 

 the best circulation will do tlie most satisfactory drying in the least 

 practicable time, because it will insure -uniform drying and wUl prevent 

 a condition frequently found where certain portions of the lumber in a 

 pile lag considerably l^ehind in drying, because these portions happen 

 to be less favored by the air circulation than the rest and thereby are 

 holding back the entire kiln charge. 



But in no kiln, no matter how good its circulation, can we dry the 

 lumber any faster than its moisture transfuses from its interior to 

 surface. 



The more direct from the saw the lumber can be brought to the kiln 

 without much exposure to sun and wind, the better will be the kiln 

 output, if proper attention is given to all other necessary and herein 

 stated conditions. 



Summing up on basis of the foregoing explanations, the writer 

 would advise against progressive kilns and in favor of compartment 

 type kilns for hardwood drying. Also, for endwise piling and against 

 cross-piling. Endwise piles should be about seven feet wide with cen- 

 ter chimney, thus giving the lateral travel through the sticker spaces 

 between adjoining courses of boards not more than three to three and 

 one-half feet. 



(Tlii^ Dimcux^on Will Be Continued June 25 — Editor) 



Why Lumber Is Steamed During Kiln Drying 



The Forest Products Laboratory is now enclosing in its corre- 

 spondence a slip of paper with the following information: 



"From the questions asked by numerous students taking the 

 short courses in kiln drying at the Forest Products Laboratory, 

 Madison, Wisconsin, it is evident that many who operate kilns 

 and handle lumber do not understand the object of steaming lum- 

 ber in a kiln. There seems to be a common impression that the pur- 

 pose of steaming lumber is to "remove the sap." This is far 

 from being the fact, for when lumber is steamed it takes on 

 moisture, as a rule, instead of giving off anything. 



"The reason for steaming lumber during drying depends on 

 when it is done, but nearly always the treatment is given for one 

 of the foUowing purposes: (1) to heat lumber through quickly 

 at the start; (2) to relieve stresses which otherwise would pro- 

 duce checking, casehardening and honeycombing; (3) to equalize 

 the moisture content and condition the lumber ready for use at 

 the end of the run; (4) to kill fungi and insects in the wood. 



"When lumber should be steamed, how long the treatment should 

 last, and what temperature should be maintained are points which 

 have been determined at the Forest Products Laboratory by tests 

 on many species of wood. A thorough understanding of the steam- 

 ing operation is essential, because the whole kiln charge can easily 

 be ruined by too severe a treatment. One of the chief needs of 

 many commercial kilns is proper steaming facilities, without which 

 a high degree of success in the artificial seasoning of wood is im- 

 possible." 



