Abstract: The present invention provides a portable dry kiln for drying or treating lumber. The kiln can operate with an electrical generator and a gas, oil or waste burning heater to allow use in remote areas. The kiln is small enough to be readily transportable while improving thermal efficiency, humidity control, and eliminating fan motor maintenance problems. A cyclonic circulation of air extending through the kiln chamber is produced by the combined actions of an air supply system and a plurality of fans.