1867, C.F.Allen and L.W.Campbell of the United States obtained the patent of wood superheated steam drying device. During the first world war, H.D.Tiemann of the United States made a pioneering study on superheated steam drying of wood. From 1950, F.F.P.Kollmann and others in the Federal Republic of Germany, and from 1972, Nanjing Forestry College in China has done a lot of work in the research, popularization and application of superheated steam drying.
Superheated steam, as a medium in drying production, is produced by the steam evaporated from wood and the saturated steam sprayed from steam injection pipe flowing through the heater and being heated to superheated state. Compared with wet air, superheated steam has high heat release coefficient, high heat transfer efficiency, strong permeability to wood, unstable superheated state, and tends to release heat quickly and convert it into saturated steam, so wood heating and drying speed is fast. In addition, at the same temperature, the relative humidity of superheated steam is higher than that of wet air, so it has higher equilibrium moisture content of wood and mild drying conditions. However, the air tightness of the kiln shell is very high to prevent air infiltration and abnormal leakage of the medium in the kiln.
The superheated steam drying process is simple and easy to operate, and there is no need to adjust the wet bulb temperature. Dry bulb temperature is usually divided into two stages: the moisture content of wood is above and below the fiber saturation point.
The influence of superheated steam drying on wood strength and its application scope are similar to those of high temperature wet air drying.