30 



HARDWOOD RECORD 



March 25. 1922 



YARD AND tCI LN 



cA Section Intended to Promote Efficiency^ 

 and Economy in Lumber Dicing ^ 



Why Steam Lumber and How? 



By C. J. M. 



Much has been written about the steaming of hardwood lumber 

 and oak in particular, but the subject still seems somewhat unde- 

 fined in the minds of some of our friends among the kiln owners 

 and operators, as evidenced b_v several recent requests for informa- 

 tion as to why, how and when they should steam. 



Moderate steaming is never harmful, except in case of "bent" 

 stocks, such as for instance bent rims for wagon or auto truck 

 wheels, which cannot be steamed with absolute safety and require 

 special precautions. Otherwise moderate steaming is generally 

 beneficial to the hardwood. 



Now by "moderate" steaming is meant quick steaming to a 

 moderate temperature with the largest possible volume of steam 

 which can iic thrown into the kiln in the shortest practical time. 

 Slow steaming is permissible only when warming the cold lumber 

 after the kiln has been charged. In any steaming thereafter, the 

 operator should always aim to saturate the kiln air quickly, with 

 a large volume of steam, so as to cause a prompt and heavy con- 

 densation on the surface of the lumber before heating the lumber 

 to the maximum permissible temperature. 

 Initial Steaming 



The most convenient, economic and efiScient method of warming 

 the lumber after it has been brought to the kiln cold, or possibly 

 frozen, is with low pressure or exhaust steam, blown direct into 

 the kiln. I mention exhaust steam at this point because of its 

 comparative cheapness and complete sufficiency for this purpose of 

 warming the lumber to the first drj'ing temperature. It will do no 

 harm to go 15 degrees or 20 degrees above that starting tempera- 

 ture; provided, that the lumber is permitted to cool b:iik to sf.nrting 

 temperature of the respective schedule, in fully saturated air, 

 before any drying whatever is allowed to take place. This means 

 that no ventilators or condenser coils must be permitted to func- 

 tion until the starting temperature of the selected schedule has 

 been reached. This is most important. 



This initial steaming will, if properly done, accomplish a double 

 result. First, it will heat the lumber through very evenly and 

 quicker than in any other way, excluding all damage. Second, such 

 steaming deposits a very heavy film of moisture upon all surfaces 

 of the lumber by condensation of the steam, which moisture is 

 quickly absorbed by the surface fibers, and which penetrates the 

 more readily since the interior of the lumber is much cooler than 

 the kiln air, moisture always tending from the hotter to the cooler 

 matter. 



This moisture absorption will soften the outer fibers and will 

 relieve any stresses which may already exist, due to casehardening 

 from weather exposure in yard or transit, and the wood is thus the 

 better prepared for the necessary transfusion of its moisture from 

 center to surface, and for the more serious business of uniform 

 shrinkage. 



Lumber is never steamed to bring about any changes in the 

 "sap," chemical or otherwise. Such an assumption is totally 

 unfounded and erroneous. 



Steaming Must Be Reliably Controlled 

 The steaming process is very simple, but it must bo careftiUy and 



positively controlled as to the highest permissible temperature, 



and this is not so simple. 



Assuming a kiln full of oak lumber which is to start drying at 



115 degrees. This lumber may be steamed from its cold condition 



to 135 degrees, but no higher. The ordinary thermometer is 

 utterly insufficient here, because the kiln cannot be entered, since 

 at that temperature and 100 per cent humidity it will be entirely 

 too hot for the operator to make reliable temperature readings 

 within the kiln far enough from the door to be truthful, and with 

 the door closed. Such a proceeding would be useless, foolish and 

 dangerous. Moreover, the 135 degrees must not be exceeded in 

 any part of the lumber in the kiln. 



Consequently an extension thermometer becomes essential and 

 imperative in such a case. The tlexible extension should be at 

 lea.st 25 feet long to permit the placing of the bulb right into the 

 center of the second pile of lumber (from the door). 



An ordinary recording thermometer of the gas type, with a 

 25-foot flexible extension, will serve the purpose very nicely. Only 

 one such thermometer is necessary in a kiln plant, if the same is 

 properly mounted on a solid stand, so that it can be moved from 

 kiln to kiln as needed. 



,\ one inch hole drilled through the kiln door, or the door .iamb, 

 will admit the bulb and flexible extension tube, and this hole can 

 be closed with a cork or screw plug when not in use. 



By the use of such a thermometer, arranged as explained, steam- 

 ing becomes an exact operation, free from guess, error or danger. 

 Without such definite control it had better not be attempted. The 

 valves controlling the admission of steam to the kiln for purpose 

 of steaming must be accessible from the outside, or must be 

 equipped with extension handles which will make control from the 

 outside possible and positive. 



In the foregoing we have explained the advantages for steaming 

 at the start of the drying operation, recommending that such first 

 steaming be done with exhaust steam, and we have likewise 

 explained the necessity for an absolute control, from the outside 

 of the kiln, of the steaming operation, through the temperature 

 as it is built up in the center of the lumber piles, and how this 

 may best be accomplished in a sure and safe manner. 



The reasons for this first steaming were stated as convenience, 

 thoroughness, uniformity and economy in warming the lumber to 

 drying temperature and, the softening of the outer fibers, thus 

 relieving anj' existing strains due to casehardening and bringing 

 the outer layers of the wood to the be-st possible condition for 

 moisture transfusion from center to surface. In elaboration of this 

 latter benefit it should be stated that a thorough initial steaming 

 (within the proper temper.iture limit) will also tend to establish 

 a uniformity of transfusion, and consequently drying of the lum- 

 ber, which otherwise might not take place, because of the differ- 

 ence in the resistance to transfusion in the various boards which 

 may have been previously exposed to more or less rapid surface 

 drying. 



Further Steaming 



If, after this initial steaming, the greatest care were exercised 

 in the gradual cooling of the lumber (in the tightly closed kiln), 

 back to the starting temperature, witliout permitting any moisture 

 to escape, and then in starting and continuing of the drying proper, 

 under the temperature and humidity conditions prescribed by the 

 right schedule, no occasion for further steaming would arise. 



Unfortunately most kilns arc not sufficiently air tight, nor suffi- 

 ciently controllable, so as to produce and maintain in all parts the 

 exact temperature and, more important, the exact humidity condi- 



