Principles and Devices for Controlling the Drying of Lumber

It seems like almost everyone either has a new controller for their lumberdrying operations or is thinking about getting one. In the July 1988 Forest Industries, in "A shoppers' guide to electronic kiln controls," I described some of the benefits likely to accrue to users of these machines. Those benefits include improved grade recovery, greater kiln productivity, better use of limited steam supplies, and improved management and control of all aspects of lumber production. All of these lead to a better competitive position in the industry. In the companion article "Kiln drying advances in Western Canada," Graham Mackay talked about similar advances in electronic kiln controllers in Canada. In those articles we wrote in general about the principles, and about the characteristics of the controllers. We didn't describe them in detail. In this article I'll describe those principles in detail and I'll describe how the controllers work that use those principles. I think this is important because only by understanding those principles and controllers will you be able to optimize your lumber-drying operations. While this article was written with steam-heated kilns in mind, many of the principles still hold for direct-fired and dehumidification kilns. Before starting I want to make clear an important distinction between controlling the drying of lumber in a dry kiln, and controlling conditions in a dry kiln. Until recently, when we talked about kiln control what we meant was control of kiln conditions, i.e., the temperature, humidity, and rate and direction of air flow. We monitored AND controlled the temperature and moisture content (MC) of the air in the kiln. We did NOT measure and control the temperature and MC of the lumber in the kiln. We might have guessed at those conditions but we had no way to measure them and to base control of drying o them. That's changed recently, however, with the development of devices that allow you either to measure directly or to infer the MC of lumber during drying. That is in distinction to devices that only allow you to measure the condition of the kiln atmosphere. I'll reiterate that distinction in the discussion. I'll describe the more common principles of control in use today as well as some others that appear promising. For each principle I'll describe what's involved, the advantages and disadvantages, and where the principle is or might be used.