PROGRESSIVE DRYERS 



The actual treatment undergone by the timber must in general be 

 the same whatever the type of dryer, if a satisfactory result is to be 

 obtained. The terms " progressive " and " compartment " apply to the 

 manner of moving the wood or controlling the thermal and hygroscopic 

 conditions. 



In all progressive kilns the timber is piled on trucks, and moved at 

 regular intervals through zones of varying temperature and humidity. 

 The heating apparatus and means of circulation are therefore arranged 

 so that one end of the dryer is relatively cool and moist and the other 

 end hot and dry. The intermediate space gives a gradual change from 

 the cool, moist conditions to the hot, dry state. The length of time 

 necessary for drying corresponds to the time taken to move the truck load 

 through the dryer. A progressive dryer is essentially suited for dealing 

 with large and regular suppHes of one quality and thickness of timber. 

 If two different thicknesses are put through the process for the same time, 

 it is obvious that either the thicker wiU not be dry, or that the thinner 

 will be over-dry. It is possible that several units could be installed, each 

 working on one kind and thickness ; but each unit is necessarily large, as a 

 considerable length is necessary to get good circulation, and as the width 

 of the tunnel is fixed by the length of the longest plank. Generally 

 speaking, one unit ^^dll deal with 60,000 super feet of i inch hardwood 

 per month. The air. circulation may be obtained either by fans or by 

 thermal means. If fans are used, an electric motor or steam engine is 

 required to supply the motive power. 



With thermal circulation there is neither moving machinery, motors, 

 engines, nor pumps. If the dryer is of proper proportions, the circulation 

 and re-circulation of return air are found to be sufficiently active, as there 

 is a constant source of heat at the heating coils, and a regularly renewe4 

 cooling effect from the fresh loads of cold, wet wood that are put in at 

 regular intervals. Even if there were no outlet for the air, an internal 

 circulation would be maintained. The air would rise naturally at the 

 hot end, pass horizontally along through the stacked loads of timber, 

 become depressed because of the cooHng effect of the fresh material, and 

 would finally return underneath through the space under the floor level. 

 As saturation would be soon reached and drying cease if all outlets 

 were closed, a wet-air outlet chimney is provided, while an inlet is allowed 

 for fresh air to enter to replace that which is drawn off by the chimney. 



For very heavy evaporations a fan-operated progressive dryer might 



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