24 



HARDWOOD RECORD 



June 10. 1922 



YARD AND KILN 



<lA Section Intended to Promote EfSciency^ 

 and Economy in Lumber Dicing' ^ 



Advice on Selection of Dry Kilns Best Suited to Hardwood Lumber 



By C. J. Maural 



The following is in reply to an inrjuiry which reads as follows: 



' ' We are contemplating construction of dry kilns to dry hardwood 

 lumber, gi'een from the saw, providing we can successfully dry it at 

 a rate that would justify the investment and operation cost. Our 

 cut is 60 per cent gum, and the other 40 per cent mostly oak. The 

 thickness is largely one inch, with some stock up to two inches, and 

 also more or less five-eighths and three-quarter inch. We would dry 

 the stock to a 15 per cent moisture content, as a rule. It is our idea 

 to build the kilns 20' x 120', crosspile the lumber and dry it progres- 

 sively. We liave had very little experience with dry kilns and it is 

 our information that in progressive kilns the air circulation is longi- 

 tudinal. We would like to ask why it is not better to cross-pile 

 the lumber, as we note from your article in the issue of April 10th, 

 of the Hakdwood Eecord, that you advise against it. We would 

 appreciate it very much if you would outline the type of kiln, which 

 in your opinion would be best suited to the work we are trying 

 to do." 



The progressive principle of kOn drying is not properly adapted 

 to the drying of hardwood lumber green from the saw, such as men- 

 tioned in the inquiry. No doubt it is more convenient from the stand- 

 point of operation, and that of necessary attention, and it is also 

 perhaps less costly in construction, but experience compels the writer 

 to advise against its use for any drying where exactness in humidity 

 of the kiln air is of as much importance as in the case of green 

 oak lumber. As to the green gum, will say that, with proper manage- 

 ment, it may \>e possible to obtain fair results in progressive kilns, 

 but any steaming, prior to the kiln drying (which is rather beneficial 

 to gum lumber ) , is not practicable in a progressive kiln and would 

 have to be carried on in a separate steaming room at additional han- 

 dling cost. 



Advantages and Disadvantages of the Progressive Kiln 



The progressive kiln has some real and some fancied advantages, 

 and from the hardwood standpoint, many disadvantages. Its only 

 jjrojjer place is in the drying of soft woods. It does not tit into the 

 yard of the hardwood producer. 



The principal advantage and argument in favor of the progressive 

 kiln is in its smaller cost of construction for a given output capacity, 

 when compared with compartment kilns. In most operations this sav- 

 ing in first cost is more than offset by the increased labor cost of 

 handling the lumber through the kiln, which is considerable in the 

 ease of long kilns without mechanical provision for the handling of 

 the cars. 



Another advantage is in the direct progressive movement of the 

 lumber from the saw, through the kilns, into dry storage, or to shipping 

 track. It is quite possible to so locate compartment kilns, and connect 

 them to trackage, as to obtain the same result with less effort. 



A fancied and very erroneously conceived advantage is in the belief 

 that most anyone can operate progressive dry kilns, since all that is 

 necessary is to push the lumber in at one end and take it out at the 

 other, the air temperature and humidity conditions necessary for 

 the proper seasoning being immutably ' ' built into the kiln ' ' by the 

 man who designed and constructed the same. 



The i^roper, successful drying of hardwood lumber requires a series 

 of graduated heat and humidity conditions which must be established 

 and held uniformly, suitable at each ear position to the gradually 



diminishing moisture content of the lumber, so as to automatically 

 furnish the necessary moisture balance between the air and the wood, 

 to prevent too rapid a surface drying and its consequent damage, 

 or in reverse, to maintain a sufficient drying speed to avoid unnecessary 

 time loss. 



Such a carefully and distinctly graduated condition of increasing 

 temperature with decreasing humidity, however, is not possible in any 

 progressive kiln, especially not in a kiln which depends upon natural, 

 or gravity ventilation for the lengthwise movement of air, from end 

 to end. The writer would not consider it safe to dry oak lumber in 

 such a kiln, unless this material had been previously yard dried well 

 below the fibre saturation point. 



Moisture Content of Wood Must Control Temperature and Humidity 



It is generally conceded that in the drying of liardwood lumber 

 the temperature and humidity of the kiln air surrounding the lumber 

 must be controlled by the "known" moisture content of the wood, 

 and that the changes in the heat and humidity of the kiln air as 

 required from time to time, shall be made only on the basis of the 

 actual moisture content of the wood, and not merely by guess, in 

 which is only considered the length of time which has elapsed since 

 the lumber was placed into the kOn. This is the sole safe and sane 

 method in treatment of refractory hardwoods, and it cannot l>e prac- 

 tically applied in a progressive kiln, for when one car moves, all cars 

 must move, or a lot of valuable kiln space will be sacrificed. More- 

 over, there is nothing in the design or construction of the average 

 progressive kUn which wiU prevent serious variations in the air j 

 condition from that intended (or even temporarily established) for 

 any given car position. Clianges in atmospheric humidity, wind di- 

 rection or wind intensity have a frequent and very disturbing effect 

 on the graduated air condition desired in the progressive kiln. 



To obtain freedom from these disturbing outside influences, the 

 writer (also misled by the apparent advantages of the progressive 

 principle), several years ago constructed four progressive kUns of 

 the condensing type for the drying of hardwood. These were designed 

 with the greatest care and engineering effort, to make possible a full 

 and effective control of the graduated temperature and humidity con- 

 ditions as required by each of the twenty-two car positions, and to 

 assure a reasonably positive circulation of tlie kiln air through the 

 lumber on each kiln car. 



A heater coU, capable of the most minute regulation, was placed 

 transversely under each car position, and the condenser coils, located 

 at both side walls, were subdivided into easily controllable sections. 

 The building construction was fire-proof and air-tight. No effort 

 or expense was spared to obtain the desired end: to-wit, the jierfectly 

 and permanently graduated condition of temjieratnre and humidity 

 over the twenty-two car positions from end to end of the kiln. 



Yet, all of these precautionary measures to the contrary notwith- 

 standing, strong longitudinal air currents were set up by the tem- 

 perature extremes of the charging and discharge ends of the kiln, I 

 which seriously interfered with the drying conditions, made necessary 

 the installation of transverse dividing curtains at various points 

 throughout the kilns (which were found cumbersome in operation), 

 and while fairly good results were obtained in the end, these kilns 

 could not be compared in effectiveness, or operating ease and cost, 

 with any of the several blocks of compartment kilns s\il>sequently 



