A New Direct Heat Exchanger for Kiln Drying Lumber

A dry kiln was designed using a direct heat exchanger in lieu of a conventional steam boiler. The direct heat from a rotary, atomizing Johnson Oil Burner was used to heat the kiln. It is possible to heat a kiln with the direct products of combustion, but there is a great fire danger and a possibility of smoke damage to the lumber. The heat exchanger solves both these problems. The heat exchanger used consisted of a series of steel pipes 10 feet long, with wind boxes at either end, through which the heat makes four passes before being exhausted into the atmosphere. Initial combustion occurs in a fire-bricklined 16-inch tube, followed by passes through a series of smaller pipes. The temperature of the exhaust gases is under 200?F., resulting in an efficiency of 85 percent. The kiln building was constructed with tilt-up concrete panel construction in which panels were laid out and poured horizontally on the ground and when sufficiently hardened raised into place. A light-weight concrete mix made from exploded shale, and having superior co-efficient of thermo-conductivity was used. Two 60-inch fans circulate air through the heat exchanger, and air travels laterally through the lumber twice, a distance of approximately 40 feet. Humidity is added by spraying finely atomized water against the hot heat exchanger. Excess moisture is removed and outside air added by the use of two ventilating fans at the top of each chamber. Forced ventilation removes air quicker and more positively, which speeds the drying. Fire Eye conrols the operation of the oil burner. It prevents the operation of a burner motor or the flow of oil if the butane pilot light is not in operation. It stops the oil flow if the flame is smoky, or if it goes out. In case of a fire within the kiln, the excessive temperature will operate through the Fire Eye, stop the oil burner and activate the water sprays and close all vents. This extinguishes the fire with little damage to the lumber or the kiln. The greatest advantage of the direct heat exchanger over conventional steam kilns is its low operation cost of $2.83 per thousand board feet. This includes a depreciation charge of 88, charging and supervision labor 40, maintenance fuel oil 73, and electric power 48.

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