March 10, 1921 



HARDWOOD RECORD 



23 



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I 



6. 



Fig. 5 — Simplest Type of Recirculating Dry Kiln. BafOes "B" Assist in Pre- 

 venting the Heated Air from Passing UpwEird Along the Kiln Walls 

 Fig. 6 — Recirculating Type of Dry Kiln with Steam Jet to Increase Humidity 

 and Vents to Reduce It 



the heating efficiency has been increased by a rearrangement of the 

 heating coils, and the direction of the air movement assured 

 through the use of baffles (B). These baffles help to guide the 

 heated air up the central ehimnej^ and to prevent it from passing 

 upward along the side walls of the kiln. The circulation in a kiln 

 of this type depends largely upon the cooling effect upon the air 

 of the lumber and of the side walls of the kiln and must be feeble 

 at best. No provision for humidity control is made. With this 

 feature added, the kiln might appear like Figure VI. Here humid- 

 ity control is secured by means of the steam jet line (S) and the 

 intake and exhaust flues, the steam increasing the humidity when 

 necessary, and the flues providing for the escape of moisture-laden 

 air and its replacement by dry air. The jet line is so placed that 

 the circulation is increased by the escaping steam. Another means 

 for removal of moisture from the air in the kiln is illustrated in 

 Figure VII. The condensers (C) have cold water circulating 

 through them; moisture condenses on the cold pipes and is drained 

 off through suitable gutters. The resultant cooling of the air 

 assists the circulation considerably. However, even with this help, 

 the circulation is not always sufficient to maintain uniform condi- 

 tions. In these cases, recourse may be had to internal fans, as 

 shown in Figure VIII. Eighteen-inch or twenty-inch propeller 

 fans, spaced about four or five feet apart throughout the length of 

 the kiln will produce circulation ample for practically all drying 

 conditions. Unless specially designed for this work, the life of the 

 fans is liable to be very short when used under severe drying con- 

 ditions. 



The Internal Fan Kiln 



Just as it is possible to reduce the humidity by means of vents, 



as in Figure VI, so is it also feasible to do this in the case of the 



internal fan kiln. Figure IX illustrates how this may be done. 



It also indicates a housing around the steam pipes and a steam 





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jet line under the chimney. The purpose of the housing is simply 

 to assist in guiding the air. 



The regulation of humidity and the production of a high cir- 

 culation by a single means are accomplished in the water spray 

 kiln illustrated in Figure X. The water sprays (W) produce this 

 dual effect by cooling the air to the dewpoint and by creating a 

 circulation through impact on the air as well as through tempera- 

 ture difference. The temperature of the dewpoint, which controls 

 the humidity of the air, is in turn controlled by the water tempera- 

 ture, which is automatically regulated. The baffles (B) remove 

 entrained spray water from the air before it strikes the heating 

 coils. Condensers and steam spray pipes are usually added: the 

 former to use in place of the water sprays at the end of the run 

 and the latter for periodic steaming to relieve drying stresses. 

 This type of kiln is especially adapted to difficult drying conditions. 



The External Blower Kiln 



Quite different from the other tj'pes is the external blower kiln, 

 indicated in cross-section in Figure XI. There are many possible 

 arrangements of the ducts, the one illustrated being typical. These 

 ducts run the full length of the kiln, and are provided at intervals 

 with openings for the intake or discharge of the air. Heating may 

 be either b.y means of a single heating unit located at the dis- 



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Fig. 7 — Recirculating Condenser Dry Kiln. The Condensers Assist the Circu- 

 lation Besides Serving to Remove the Moisture from the Air 

 Fig. 8 — Recirculating Condenser Dry Kiln, with Internal Fans to Promote 



Circulation 



Fig. 9 — Internal Fan Type of Dry Kiln, with Steam Jet to Increase Humidity 



and Vents to Reduce It 

 Fig. 10 — Typical Water Spray Dry Kiln. The Water Sprays Produce Circula- 

 tion and Reduce Humidity 



charge side of the blower or by means of individual heating units 

 distributed throughout the intake (or pressure) duct in the kiln. 

 Humidity may be regulated by means of steam jets and condensers 

 or vents. Kilns of this type usually have ample circulation fairly 

 uniformly distributed, and are capable, when properly designed, of 

 operating under severe drj-ing conditions. The direction of forced 

 circulation, however, should always be with and not opposed to the 

 natural gravity effect, otherwise irregular conditions and stagna- 

 tion are almost certain to occur. 



Endwise flat piling is essential in all of the kilns except the type 

 indicated in Figure I, in which either endwise or crosswise flat 

 piling may be used. Vertical piling of either kind could also be 

 used in a kiln of this type, as shown in Figure XII. In fact, with 

 sufficient heating surface and large enough ducts, much improved 

 circulation and more uniform drying might be secured. This form 

 of piling permits the air to pass over both broad sides of each 

 board. 



All of the types so far mentioned use air as the medium of 

 heat and moisture transference. The kiln sketched in Figure XIII 

 differs from them in that it uses superheated steam for this pur- 

 pose. It also differs from them in that it is provided with means 

 for periodically reversing the circulation through the lumber. 

 Steam for circulation is supplied through the four spray lines (SL 

 and SR), only one pair working at a time, and the temperature 

 is maintained by means of the heating coils on the side walls. The 

 discharge of the steam from the jets produces a very rapid circula- 

 tion. Means for the escape of steam from the kiln must be pro- 



