^.\y.v«^>'Ay.\:;aiBo:oa:;^;:>itystiiiOT;j^^ 



Some l>Jew Kiln-Drying Thoughts 



Editor's Note 



The following papur was road heforo the annual wfot 

 ciatiou. In-ld at the Auditorium hotel, Chicago, Decemher 

 panj-, Indianapolis, lud. It contains some suggestions th 



It is not my intention to take up the time of this meeting with a 

 long discussion of any theoretical drying system. You gentlemen are in- 

 terested in what can be done and has been done rather than what might 

 be done. With this idea in view, what I sliall have to say will be based 

 upon observations made under ordinary factory conditions and actual 

 tests made under these same conditions. I shall take up briefly the 

 troubles resulting from improper ilrying, with their causes and the 

 conditions which produce them, following this with a brief description 

 of the conditions under which successful drying can be accomplished 

 and these troubles eliminated. I have incorporated in this paper the 

 results of a series of tests made under the conditions which I shall 

 describe and which will illustrate better than any argument what is 

 possible under correct drying conditions. 



The troubles resulting from improper drying, which affect the panel 

 manufacturers and a part of which are common to all manufacturers 

 of wood products, are blistering of veneer, veneer shrinkage, opening 

 of joints, warping, case-hardening, honeycombing and cheeking. As 

 you all know, case-hardening, honeycombing and checking result from 

 excessive heat or lack of moisture or from both, the surface drying 

 first and preventing the escape of moisture from the center of the 

 stock. The other troubles are caused by under drying. I do not in- 

 tend to convey the idea here that improper drying is always the cause 

 of all these difficulties. A part of them may, at times, be caused 

 by other things regardless of how the product is dried. But I do say 

 that where the stock has not been properly dried some of these 

 troubles will result. 



Unsuccessful drying is the result of improper conditions as to heat, 

 humidity and circulation in the drying chamber. These conditions 

 may be all or some one or more of the following: insufficient humidity, 

 insufficient circulation, excessive heat, lack of uniformity as to heat, 

 humidity and circulation in all parts of the kiln, and the variation of 

 these conditions from time to time during the drying period. While 

 it is perhaps not possible to dry each individual piece of wood in 

 the same degree, it is certainly true that approximate uniformity can 

 be attained only where the conditions are uniform throughout the 

 drying chamber. The temperature cannot be permitted to vary. Like- 

 wise the amount of moisture must be uniform throughout the room. 

 There must be a continuous and rapid supply of fresh air to all 

 parts of the kiln and the prompt removal of the air after it has been 

 used. The problem, therefore, is first to produce these conditions, 

 and second, to control them once they are produced. To produce the 

 conditions as to heat and humidity, it has been found best to heat 

 the air to the desired temperature and hiunidify it to the proper 

 degree before it is permitted to enter the drying chamber. This, 

 of course, cannot be done where the radiation is inside the drying 

 room proper. There are additional objections to having the radiation 

 in close proximity to the drying product. The best results in rapid 

 and uniform circulation can be obtained, I believe, by having both 

 the intake for fresh air and the outtake for impure air on the floor 

 line. In this way the warm air rises rapidly and passes to all parts 

 of the room, while the cooler impure air sinks to the floor line and is 

 removed. This insures a constant and ever changing supply of fresh 

 air, heated and humidified to the proper degree, and brings it con- 

 tinuously into contact with the work to be dried. The control of 

 circulation as well as that of heat and luimidity should bo taken care 

 of automatically. 



I desire at this point to emphasize the importance of humidity in 

 any drying system and that there must be some provision made for 

 producing and controlling it. The amount of moisture naturally in 

 the air is uncertain, varying with the outside conditions, and cannot 

 be depended upon. There must be some method for producing it, 

 but any arrangement which provides for it without accurate control 

 and regulation will not produce results. Moisture in the drying air 



inK I))' the Xalional \'enoer & Panel Manufacturers' .\sso- 

 s and 1), by Uenr.v Hunter of the Hunter Dr.v Kiln Cona- 

 at will 1)0 valuable to Hakdwood Recuhd readers. 



is responsible for the open pore and no wood product can be success- 

 fully dried without it. Without an open pore, the center of the stock 

 cannot be properly dried while the outside ease-hardens and honey- 

 combing sometimes results. On the other hand, stock dried with open 

 pore can be more easily worked and presents a better surface for the 

 adhesion of glue. Veneer and cut-stock will be more pliable, flatter, 

 and will have good color. The stock can also be worked in less time, 

 with less waste and less expense. 



With the drying conditions regulated and controlled as already in- 

 dicated, lumber can be best dried at a temperature of 140 degrees 

 F, with the humidity held at 57 to 63 degrees. In drying panels, 

 the temperature should not>be so high, 120 degrees being the limit. 



The following results were obtained from the tests made in a factory 

 which was having most of the troubles above, enumerated. They were 

 obtained under the conditions described as being essential to proper 

 drying. The first test was upon a number of panels taken from the 

 glue room. The first moisture test on the core stock showed a varia- 

 tion in moisture content of from 3 to 14 per cent. The first moisture 

 test on the veneer and cross banding showed from 6.7 to 8 per cent 

 of moisture. After being re-dried under the above described con- 

 ditions for fifteen hours, the core stock showed a variation of from 

 3.5 to 4.5 per cent in moisture and the cross banding and veneer 

 showed from 2.9 to 5 per cent. After the panels were laid, they 

 weighed 65 pounds 4 ounces. After being dried for 22 hours, at 

 120 degrees, the weight was 63 pounds 8% ounces, a loss of 1 pound 

 11% ounces or an average loss to the panel of 3.4 ounces. After the 

 panels were dried, they showed a moisture content of from 4.7 to 6.7 

 per cent, an average of 5.75 per cent. 



A test of twenty oak boards taken from the tempering shed to 

 be used as core stock, showed a moisture content ranging from 2.9 

 to 19.4 per cent. Eighteen maple boards taken at the same time, 

 showed a variation of from 3 per cent to 6.7 per cent moisture con- 

 tent. These boards were taken from the same piles at the same time 

 and had all been kiln-dried. After being re-dried for thirty-sis 

 hours at 140 degrees, the oak showed a variation in moisture con- 

 tent of from 2 to 3.8 per cent, an average of 2.7 per cent. The 

 maple boards showed a variation of from 1 per cent to 3.6 per cent, 

 an average of 1.6 per cent. This stock was worked up into six 5-plj' 

 oak panels, size 18x33, and six 5-ply mahogany panels of the same 

 size. After being laid, the oak weighed 57 pounds 4^2 ounces and 

 the mahogany 52 pounds 15^4 ounces. After drying, the oak weighed 

 55 pounds 4 ounces and the mahogany 50 pounds 13 1^ oimces. The 

 loss of moisture on the oak was 2 pounds % ounce, an average of 

 5.4 ounces to the piece. On the mahogany, the loss was 2 pounds 2 

 ounces or an average of 5.6 ounces to the piece. The moisture con- 

 tent in the oak after drying was 7 per cent, in the mahogany, 7.1 per 

 ceat. 



These results speak for themselves. The variation of the moisture 

 content of stock dried under improper conditions, and the uniformity 

 of the same when dried under correct conditions is apparent. The 

 danger of having part of the work come through in good condition, 

 and a part of it go wrong, is eliminated. It makes your drying sys- 

 tem certain and dependable. There is no more reason for guess work 

 in your drying than in any other department of your factory. Create 

 the proper drying conditions and control them. Pile your stock 

 properly, so as to prevent the forming of pockets, and the obstruc- 

 tion of circulation. Test your stock for moisture content before 

 placing it in the kiln. Treat it accordingly and test it before re- 

 moving. Learn your factory conditions. Do not expect your stock 

 to remain dry after leaving the kiln when your shop conditions are 

 wrong. With correct factory conditions and the application of the 

 above described principles in your drying department, your drying 

 troubles will be greatly lessened, if not entirely eliminated. 



—31— 



