HUMIDITY OF COMBUSTION PRODUCTS FROM WOOD AND GASEOUS FUELS

The capital and operating costs of steam kilns and the boilers necessary for their operation are so high-in Canada that they are not economically feasible for small operations drying softwoods. About thirty years ago, direct fired or hot-air kilns were intro-duced. In such kilns natural gas, propane, butane or oil is burned and the combustion products conducted directly into the drying chamber. This design eliminates the high capital and operating costs of the steam boiler, and permits the small operator to dry lumber economically. However, such kilns are not as versatile as steam kilns. While they permit good temperature control, their humidity control is limited: they can only reduce excess humidity by venting, they cannot increase it and therefore cannot relieve drying stresses at the end of drying. For this reason, they are usually limited to the drying of structural lumber. Due to the decreasing supplies and increasing costs of fossi