Principles of Dehumidification Lumber Drying

Basic heat pump (dehumidifier) theory is discussed along with the basic components and operation of low-temperature and high-temperature lumber dehumidification drying systems. The heat transfer and thermodynamic characteristics in the basic components of a typical heat pump, phase diagrams of different refrigerants, and the advantages and limitations of different compressors are discussed. The first generation of low-temperature dehumidifiers suffered from frequent compressor failures because they were unsuited to be operated above 120?F kiln temperature. Specifically, these compressor failures were usually caused by the oil in the refrigerant (usually R-22 or R-12) oxidizing at high temperature on the compressor valves and thereby causing the valves to leak. Second generation dehumidifiers now overcome this problem by using open or semi-hermetic compressors (when the electric motor is not cooled by the refrigerant), refrigerant R-114 which is more suited for high-temperature operation, or centrifugal compressors that do not require valves and have less oil in the refrigerant. These modifications allow a dehumidifier to be operated up to about 180?F so that drying times can be equivalent to conventional drying times below a wood moisture content of about 20 percent.

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