HARDWOOD RECORD 



23 



Toward the end o£ the process abundant and 

 Vigorous circulation should be provided for, due 

 care being taken not to lower the temperature. 



The Dry Kiln. 



TYPES. 



As regards construction there are two general 

 types of dry kilns. In the radiator kiln the 

 timber is heated by coils of steam pipes under 

 the floor ; in the blower kiln heat Is supplied by 

 a current of air heated outside the kiln and 

 forced in by a fan. 



The common designations are "hot blast" and 

 "moist-air" kilns, but these refer to methods of 

 operation rather than to construction. Moist-air 

 kilns, whether of the blower or the radiator 

 type, are managed on the moist-air principle. 



PAETS. 



The essential parts of a dry kiln are the dry- 

 ing chamber, the steam coils and the ventilating 

 device. 



The drying chamber varies greatly in length, 

 according to the output desired and the process 

 used. It may be as short as 15 feet or as long 

 as 150 feet. In most cases its height is from ti 

 to 8 feet, with a width of from 10 to 30 feet. 

 Though it may be made of a number of mate- 

 rials, wood is widely used where the building 

 laws permit. The choice of material depends 

 otherwise upon the kind of lumber to be used 

 and the thoroughness of drying desired. Some 

 of the best drying is done in brick kilns with 

 concrete foundations. 



The steam coils which furnish the heat have 

 various forms in both types of kilns. In the 

 radiator type they extend under the floors from 

 the "dry" end of the kiln part way or all the 

 way to the opposite end. They may be of single, 

 double or treble thickness of inch pipe, accord- 

 ing to the heat required. To secure provision 

 for their contraction and expansion, and for 

 their drainage, special attention is given to 

 their installation. In the blower type of kiln 

 there are no steam pipes in the drying chamber 

 itself ; the heat is supplied by air heated outside 

 and forced into the kiln by a fan. 



To maintain in the kiln the circulation of air 

 needed in drying, there are a number of devices. 

 of which the simplest is possibly a system of 

 ventilator shafts for removing the lighter moist 

 air from the top of the kiln. In blower kilns a 

 fan is used for the same purpose, and in some 

 radiator kilns in which the humidity is kept 

 very high the only outlets are heavy dampers 

 through which the air must force its way. There 

 are certain makes of both radiator and blower 

 kilns operated on the moist-air principle, which 

 are equipped with condensing chambers for the 

 removal of a portion of the moisture from the 

 air. 



Doors are made of a number of materials, of 

 which canvas and wood are most widely used. 

 The doors are never hinged, but slide along a 

 track or in grooves, or are counterbalanced so 

 as to be thrown up like windows. 



The lumber is carried through the chamber on 

 trucks holding from one to three thousand feet, 

 or even more. These are usually of steel bars, 

 6 feet long, with a wheel at each end. Accord- 

 ing as there are two or three tracks in the kiln, 

 two or three of these trucks are used. In piling 

 the lumber upon them the pieces are not placed 

 in contact, but. as in piling for air-drying, are 

 "stuck" — that is, held separate above and below 

 by inch strips laid crosswise. 



Methods of Operating. 



There are two methods followed In kiln opera- 

 tion — the progressive and the charge systems. 

 The progressive system requires a kiln of con- 

 siderable length, say upwards of 50 feet. Tem- 

 perature and moisture are so distributed in the 

 kiln that in passing from the green to the dry- 

 end a load of lumber is first moistened, then 

 heated, and finally dried otit. In this way all 

 Btages of the process are under way in the dif- 

 ferent parts of the kiln at once. Circulation 

 may be carried on by a draft lengthwise of the 

 kiln which carries back the moisture from the 

 dry end of the kiln and deposits it on the wood 

 in the wet end, or, where the extreme of moist- 

 air drying is practiced, there may be no such 

 longitudinal circulation and very little circula- 

 tion nf any kind. In the latter case the hu- 

 midity is very high even at the dry end. The 



temperature may be fairly uniform throughout 

 the whole length of the kiln, in which case the 

 wood must be very hot on exit to insure the 

 removal of residual moisture, or a difference of 

 20° to 40° F. or more may exist between the 

 two ends. The progressive system of operating 

 is most widely employed in factories where the 

 wood to be dried is of fairly even thickness and 

 quality and where the drying is done in large 

 amounts and continuously. 



Kilns for operation in the charge system are 

 typically square. The kiln is charged and this 

 charge is dried before fresh material is admitted. 

 Wheeled carriages may be used to carry the ma- 

 terial, or it may be piled upon the floor or ar- 

 ranged along the walls. For novelty works and 

 other establishments with a highly varied prod- 

 uct of which smaller amounts are dried at once 

 tills method is widely employed, and wherever 

 particular attention must be paid to the thick- 

 ness and quality of the stock it probably yields 

 the best results. Conditions are controlled en- 

 tirely from outside the kiln and are varied to 

 suit the operator's ideas of the requirements of 

 the case. 



Drying may be continuous or intermittent. 

 From necessity or economy many kilns are op- 

 erated only by day. At night, in such cases, the 

 heat is shut off, the blower is stopped, and the 

 temperature is allowed to fall, while the humid- 

 ity may reach the dewpoint. Intermittent opera- 

 tion is not adapted to yield the best quality of 

 product. It would be highly expensive, owing 

 to the loss in heat in restoring the drying tem- 

 perature in the morning, were it not that in this 

 case only exhaust steam is used. 



Where the drying is continuous, the kiln Is 

 supplied with live steam when the engines are 

 shut down at uight. This means that drying 

 progresses evenly. The material is held at a 

 regular temperature and, provided the humidity 

 does not fall too low, checking and warping are 

 less apt to occur. 



Moist-Air Drying. 



It has been pointed out that moist-air drying 

 can be carried on in both blower and radiator 

 kilns, and that the moist-air system has the 

 wider use. Builders of radiator kilns, however, 

 generally advertise them as moist-air kilns and 

 refer to blower kilns as hot-blast kilns. They 

 lay stress on the damage done to the heavier 

 grades of stock by treatment in a fan kiln and 

 deprecate the use of any kiln with forced draft. 

 Careful study shows that as a matter of fact 

 most successful kilns drying lumber heavier than 

 pine and poplar are operated on the moist-air 

 system, whether or not they are equipped with a 

 fan. Moist air can easily be secured in blower 

 kilns by using the saturated air as it leaves the 

 kiln, or by admitting wet steam either into the 

 heater itself or into direct contact with the 

 lumber in the chamber, and in radiator kilns by 

 merely closing the drafts and gradually raising 

 the temperature. The humidity is generally se- 

 cured by steam, some operators, during winter, 

 secure it by piling snow upon the lumber as it 

 enters the green end of the kiln. In any system 

 of moist-air drying where no fan is used the 

 lumber, upon coming out of the kiln, retains 

 a residuum of moisture, which evaporates from 

 the heated surface in the open. For this rea- 

 son many operators, especially when the weather 

 favors, or shed room is available, keep the stock 

 exposed to the fresh air for a day or two after 

 it leaves the kiln. 



Most of the past and present improvement in 

 the kiln-drying of lumber follows the line of 

 moist-air operation. 



Preliminary Seasoning. 



Hardwood lumber is commonly air-dried at the 

 sawmill for a period of from two to six months. 

 Present demand for lumber is, however, so great 

 that there is strong inducement to market it as 

 air-dried when actually it is nearly or entirely 

 green from the saw. 



To supplement the drying at the mill, or to 

 make up for it when it has been omitted, kiln 

 operators who are particular about the quality 

 of their product very often pile the stock, as it 

 comes from the cars, in their own yards, and 

 allow if to air-dry there for a few months or 

 even for a year or two. This yard-drying un- 

 questionably improves the stock and indeed is 



indispensable for certain types of kilns. On the 

 other hand, for kilns which yield the best re- 

 sults when considerable moisture is present, 

 green material may be most suitable. It Is prob- 

 able that, for the sake of economy, yard-drying 

 will be eliminated In the kiln-drying of the 

 future without loss to the quality of the prod- 

 uct. 



The extreme form of preliminary seasoning is 

 found in the shed-drying of stock as practiced 

 by vehicle and implement makers. Shed-drying 

 for two or three years leaves so little moisture 

 in the wood that the kiln-drying which follows is 

 but a very brief process. 



From shed-drying it is only a little further 

 to the treatment given to such close-grained 

 woods as boxwood, which is dried for wood- 

 engraving blocks. Such blocks are wrapt in 

 pieces of coarse gunny sack and stored in small 

 barrels or boxes in a temperature of about sum- 

 mer heat, very frequently in a room directly 

 over a kiln. After about a year the blocks are 

 completely dried. So tedious a process can, of 

 course, be profitable only when the product 

 must be absolutely free from checks and other 

 imperfections. 



Preliminary Use of Steam. 

 In addition to supplying heat to the kiln, 

 steam may be used either to maintain the proper 

 humidity in the kiln, as in certain kinds of 

 moist-air operations, or to moisten and heat the 

 lumber before it enters the kiln. For prelimi- 

 nary steaming in the progressive system of dry- 

 iug, a steaming chamber must be provided at 

 the green end of the kiln ; with the charge sys- 

 tem the drying chamber may be used instead. 



WET STEAM. 



In preliminary treatment with wet steam, 

 when the drying is progressive, provision is made 

 at the green end of the kiln for a steam cham- 

 ber. This may be detached from the drying 

 chamber or may be a part of the drying chamber 

 cut off from the rest by a wooden or canvas par- 

 tition. It is large enough to hold a single truck 

 of lumber. Under the floor there is a perforated 

 steam pipe, usually running diagonally across 

 the bottom. If the heat of the unconfined steam 

 which enters through the perforated pipe is In- 

 sufflcient, as may be the case if the kiln is of 

 wood and much heat is lost, the steam radiators 

 used throughout the kiln in heating the charge 

 may also extend under this floor. The loaded 

 truck is run into the steam chamber, the outer 

 doors are closed as tightly as possible, and steam 

 is admitted. Although in one sense steam thus 

 used may be regarded as live steam because the 

 pipe conveying it runs directly from the boiler 

 to the steaming chamber, the pressure when it is 

 released in the chamber is reduced in effect to 

 zero, so that it is really exhaust steam. The 

 pressure of true live steam, though no greater 

 than a single pound per square inch, would de- 

 stroy a chamber built of ordinary masonry. 



Duration of treatment varies with different 

 operators ; it depends mainly upon the efficiency 

 of the steaming chamber and method of drying 

 which is to follow. The longest steaming noted 

 in this study, thirty hours, was given where a 

 completely detached wooden kiln was operated 

 on the charge system. The shortest, two hours, 

 sufficed with a concrete kiln in which a single 

 truck load was steamed at one time, preparatory 

 to treatment in a blower kiln equipped with 

 condensing coils. 



LIVE STEAM. 



A steel retort, or boiler, capable of withstand- 

 ing considerable pressure, is necessary for pre- 

 liminary treatment with live steam. Though 

 at present not in wide use, this method appears 

 to promise exceedingly well. It is true that 

 when operations are on a large scale an extra 

 handling of the lumber is necessary to move It 

 from the steaming retort to the kiln, but the 

 reduction of the time subsequently needed for 

 drying is so great as to offset this and result 

 profitably. A further saving is effected by the 

 fact that green timber, fresh from the saw, can 

 be treated as readily as older stock, sometimes 

 apparently more readily, so that preliminary 

 seasoning may be dispensed with. In respect 

 to the effect of live steam on the strength of 

 wood, experiments upon loblolly pine railroad 

 ties have shown that long treatment does pro- 

 duce weakness, 20 pounds pressure for four hours 

 having been shown to reduce the strength about 

 16 per cent, though a portion of this loss was 



