Fig. 31. Section through a typical Moist-air Dry Kiln.
These heating coils are usually installed in sections, which permit any degree of heat from the minimum to the maximum to be maintained by the elimination of, or the addition of, any number of heating sections. This gives a dry kiln for the drying of green softwoods, or by shutting off a portion of the radiating coils—thus reducing the temperature—a dry kiln for drying hardwoods, that will not stand the maximum degree of heat.
Fig. 32. Single Pipe Heating Apparatus for Dry Kilns, arranged for the Use of Live Steam. For Low Temperatures.
Fig. 33. Double Pipe Heating Apparatus for Dry Kilns, arranged for the Use of Live Steam. For Medium Temperatures.
In the Moist-air or Natural Draft type of dry kiln, any degree of humidity, from clear and dry to a dense fog may be obtained; this is in fact, the main and most important feature of this type of dry kiln, and the most essential one in the drying of hardwoods.
It is not generally understood that the length of a kiln has any effect upon the quantity of material that may be put through it, but it is a fact nevertheless that long kilns are much more effective, and produce a better quality of stock in less time than kilns of shorter length.