Steam and electrical consumption in a commercial scale lumber dry kiln

Kiln drying of lumber is the most energy intensive phase of the lumber manufacturing operation. It has been estimated that 60-70% of the energy used to manufacture lumber is used in the drying process (Comstock, 1975). During the 1970s and early 1980s the cost for that energy soared, and while energy costs have recently fallen, continued increases can be expected in the coming years. The trend in the lumber industry toward cogeneration of electric power further increases the potential value of energy the industry consumes. While energy is a major cost component of the drying operation, until recently little information on the actual energy consumption of commercial dry kilns has been available. The trend toward computer based kiln control systems should make this type of information more readily available in the future. Currently, some kiln instrumentation vendors are making energy quantification part of their control systems. Historically three methods have been used to quantify energy consumption. These include the use of theoretical models, the measurement of energy consumption on laboratory kilns and the measurement of energy consumption on full sized commercial kilns. Theoretical methods for calculating energy consumption are essential for engineering kilns and boilers. Their accuracy is limited by the accuracy with which vent losses and heat transfer coefficients for structure components can be estimated. While heat transfer coefficients for large homogeneous components can be accurately determined, losses near joints, doors and other openings are difficult to determine and can be significant (Laytner and Arganbright, 1984). Excess venting losses can range from 5 to 25%. Measurement of energy consumption using laboratory kilns has limited value due to the scale effect on transmission heat losses. Rosen (1980) has calculated that when all factors, including details of kiln design, were kept constant, and only kiln size was varied, energy consumption for drying southern pine varied from 1600 BTU/pound of water evaporated in a 100 thousand board feet (MBF) kiln to 18,000 BTU/pound of water evaporated in a 10 board foot kiln. The measurement of energy consumption on full sized commercial kilns, while most representative of real world