Drying Yellow-Poplar At Temperatures Above 100 Degrees C.

A single, simple drying procedure was examined to formulate a general description of the process of high-temperature drying of yellow-poplar lumber. One-inch boards from a single yellow-poplar log were dried in a steel frame inserted between the platens of a press which was electrically heated to temperatures from 121?C. to 232?C. During the first, constant-rate stage of drying, the temperature of the specimen remained at 100?C. as the wood dried by vaporization of water at the surface; the drying rate was a linear function of time, controlled by the rate of heat transfer from the platens to the surface. The second stage began when free water could no longer move to the surface fast enough to prevent surface heating; drying took place inside the specimen, moving from the surface outward as indicated by temperature gradients with the temperature remaining near 100?C. in the center and outward to the point where water was being vaporized. The drying rate was approximately linear with the square root of time, indicating that it was controlled by the rate of heat transfer through the wood, rather than the rate of moisture movement out of the wood. When the temperature in the center exceeded 100?C., steam pressure inside the wood probably increased the rate. When the drying rate no longer varied linearly with the square root of time, and moisture content ranged from about 6 percent at 232?C., to about 13 percent at 121?C., the third stage, conditioning, began. As the inerior of the wood and the drying conditions came into equilibrium, the center slowly dried from the fiber saturation point, which appears to be about 20 percent at 100?C. for yellow-poplar, down to equilibrium moisture content, heat transfer remained the controlling factor, because there was much less water to be moved. Shrinkage was greater in thickness than in width for both radially and tangentially oriented specimens dried at high temperatures. Specimen shape, not grain orientation, controlled the amount of shrinkage. Volumetric shrinkage for wood dried at 27?C. is greater than shrinkage for wood dried at higher temperatures, possibly because growth stresses are relieved at higher temperatures. Temperature had little effect on volumetric shrinkage from 100?C. to 232?C. Applying this data to a practical system would mean three-stage drying. The first stage could be speeded up by the use of some efficient heat transfer medium, such as hot oil or super-heated steam. The third stage would take place at lower temperatures and high relative humidities, and radio frequency heating might be used. Based on this data, it would require 1 hour to dry ‘4 by 4 yellow-poplar lumber heartwood or sapwood from a moisture content of 80-100 percent to a moisture content of 8-10 percent. These drying methods have not yet been evaluated to determine practicality.

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