HARDWOOD RECORD 



Fchniary 10. l',i;;i 



By Donald R. Brewster* 



SpccialTst in the Kiln Drying of I^umber 



Every woodworking plant naturally desires to bring its kihi drying 

 practice up to a point where it will give maximum satisfaction and 

 service. But kiln drying which is satisfactory at one plant, may be 

 very unsatisfactory at another plant, which uses a different kind of 

 wood or makes a different class of product. In other words, kiln 

 drying requirenie"uts differ widely according to the species of wood 

 to be dried, the thickness of the lumber, the grade of the lumber, the 

 product to be manufactured, the number of different kinds of wood 

 and the amount of each to be dried, the moisture condition before 

 drying, the final moisture content desired, the permanency of the 

 plant and many other factors. In order that you may know what 

 constitutes good kiln drying at your plant, you must take all of these 

 various factors into consideration and draw your own conclusion in 

 the light of what I am going to tell you about good kiln drying in 

 general. 



The subject of kiln drying may be divideil into two main divisions 

 — first, the kiln, and second, its operation. 



Each type of kiln has its friends and its uses. Every kiln company 

 will give you along line of "talking points" calculated to show how 

 and why its particular kiln will give .you best results and be best 

 suited for your purpose. But when all of these points have been 

 viewed in the light of common sense and practical experience, it will 

 be found that there are only a few really necessary features in a 

 good dry kiln and no one has a monopoly on any of them. 



In the first place you need a building. This may be either brick, 

 concrete, tile, or wood, depending upon relative cost, permanency 

 desired, fire risk allowed and amount of initial investment possible. 

 For the permanent woodworking plant, probably a substantial fire 

 proof building of brick and hollow tile will prove most satisfactory 

 in the long run. Wooden kilns, on the other hand, Ijave many gooil 

 points in their favor, such as cheap and rapid construction, excellent 

 insulating properties, and portability. But no matter what material 

 is H.sed, any building will be satisfactory if it is tight and reasonably 

 free from the influence of outside weather conditions. The building 

 is, in fact, the least important factor in good kiln drying — any build 

 ing with four walls and a roof can be converted into a good kiln at 

 reasonable expense. 



Having the building, you must provide some sort of heat. A great 

 deal of lumber has been well dried in the j^ast with nothing but warm 

 air from a stove, or even an open fire. In the modern practice, how- 

 ever, this method does not recommend itself for drying expensive 

 lumber, because of fire risk and difSculty of control. In the small 

 shop good results are obtained bv keeping stock in a warm corner 

 near the ceiling, where it will gradually dry down to the proper 

 condition. For most purposes, however, steam is much the most sat- 

 isfactory form of heat, furnishing high temperatures with reliable 

 and accurate means of control. Some kilns have steam pipes within 

 the kiln while in other types the air is first heated by radiators out- 

 side the kiln and is then blown into the kiln and through the lumber 

 by forced draft. Each method has its advantages under certain cir- 

 cumstances. 



Introducing Moisture 



After lieat, the next requirement for gooil kiln drying is some 

 mean.s of introducing moisture into the air to prevent the outer sur 

 face of the wood from drying too rapidly so as to cause checking and 

 casehardening. This is sometimes done by evaporation from an open 

 water surface or by spraying the air with water. But for ordinary 

 requirements the most satisfactory method is to inject steam into the 

 air in the kiln through perforated steam spray pipes. 



The heated and moistened air must then be carried to the .surface 

 of the lumber to be dried so that the heat can do its work of evap- 

 oration. Evaporation of water consumes heat just as operating an 

 automobile consumes gasoline, and the amount of drying that takes 

 place is directly proportional to the amount of heat supplied to the 

 lumber. To provide the heat required for rapid drying the heated air 

 must be made to circulate freel,y between the boards. 



There are two ways of obtaining this circulation — the natural 

 draft and the forced draft. The natural draft takes advantage of 

 the fact that heated air is lighter than cold air, and consequently 

 rises, because of gravit.y. The lumber in natural draft kilns is there- 

 fore placed above the source of heat so as to be in the path of the 

 rising heated air. 



Forced draft kilns drive the heated air lietween the boards by some 

 mechanical means such as a fan or blower or a high pressure spray of 

 water. They usually also take advantage of the natural tendency of 

 heated air to rise. It is necessary, however, to limit the velocity of 

 circulation in forced draft kilns, as too high a velocit,y will dry the 

 outer surface too rapidly, causing checking and casehardening. 



*.-f talk gh'cn before the IVood-Workers' Club of Louisrillc, Ky., Dec lo. 



Example o( Poor Piling of Hardwood Stock 



