﻿56 BULLETIN" 1136, U. S. DEPARTMENT OF AGRICULTURE. 



ferent automatic stacking and unstacking machines have been de- 

 veloped for this work. While they differ in operation, the resulting 

 piles are very much alike, except in the width and thickness of the 

 stickers. The layers of boards and the stickers are vertical, and 

 there is no space between the boards in each layer. As the lumber 

 dries and shrinks there is a tendency for the piles to become loose 

 and to lean. To avoid this trouble several take-ups have been de- 

 vised. These take-ups are intended to squeeze the load together 

 sidewise as the boards shrink and thus keep it always tight. A 

 serious objection to most of them is that they increase the weight of 

 the bunk very considerably; this is important where the bunks have 

 to be handled by hand. 



The principal direction of circulation with vertical piling must be 

 upward or downward through the lumber. In ventilated kilns it is 

 upward, in blower kilns either upward or downward, and in super- 

 heated steam kilns both upward and downward. 



Vertical stacking would seem, at first sight, to permit much better 

 circulation than flat piling. The unimpeded passage of the hot air 

 from the heating coils underneath up through all the straight, open, 

 vertical spaces between the laj^ers of boards seems very simple. How- 

 ever, it is not so simple as it appears. Trouble is experienced in 

 keeping the air in the lumber piles hot, it cools as soon as it strikes 

 the lumber, and then begins to descend and interferes with the air 

 trying to come up. Further, there is usually a space on either side 

 of the kiln and other sj^aces between the trucks through which the 

 air can rise more easily than through the piles. Another peculiarity 

 which may cause trouble, especially in superheated steam kilns, is 

 that the length of travel through the pile is comparatively great, 

 causing a large difference in drying conditions between the entering 

 air and leaving air sides of the pile. These points indicate why it 

 is necessary with vertical piling to use a specially designed kiln in- 

 stead of using a kiln designed for flat piling. 



DETAILS OF KILN OPERATION. 



The successful operation of dry kilns requires constant care and 

 attention. The results secured depend in a large degree upon the 

 operator, and he should be impressed with his responsibility to bend 

 every effort to turn out perfect stock. The operator must first 

 familiarize himself with the kilns under his supervision. Before 

 making the first run in a kiln, he should make a careful inspection to 

 assure himself that it is mechanically safe, that the heating coils, 

 traps, etc., are in proper working order, and that the instruments 

 have been checked, calibrated, and properly located. 



INSPECTION OF KILN. 



The kiln building should be kept tight and mechanically safe and 

 sound. Interior surfaces should be painted with a good kiln paint. 

 Doors should be kept in good repair and fitting tightly; poor doors 

 allow a great deal of heat to escape and upset the drying condi- 

 tions. Rails and rail supports should be inspected periodically and 

 the fastenings checked over. Pipes should be kept in proper re- 

 pair, and their pitch to the drain end maintained to allow free flow 



