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Section I. U.S. and Canadian Species– Conventional and Elevated Temperatures Hardwoods Shown in this section are schedules for steam-heated kilns operated at temperatures between 100 and 180 °F4 (38 and 82 °C). Suggested schedules are essentially the same as those listed in U.S. Department of Agriculture Handbook 188, Dry Kiln Operator’s Manual. In general, these schedules call for changes in kiln conditions to be made on the basis of the average MC of the wood. The use of sample boards is required. (See appendix B, page 136 for a brief summary of a method for obtaining sample boards.) For some western hardwood species time schedules are also listed. Suggested schedules for products other than lumber, e.g. squares, handle blanks, etc., are listed under “other products.” For squares of species with no specific listing, use a wet-bulb depression number one unit higher than the one suggested for lumber of the same thickness. Thus, for 3- by 3-inch yellow birch squares, use T3-B3, rather than T3-B2 listed for 12/4 lumber. Assembled schedules can be found in Section VII. Schedules are intended for kiln drying from the green condition or when stock has been partly or well air-dried. These schedules are recommended starting points, and the prudent kiln operator will modify these schedules to accommodate regional species characteristics. For example, in the northeastern United States, kiln operators have modified schedules for hard maple, oak, and eastern white pine to accommodate the drying requirements and smaller diameters of trees being harvested. These schedules are listed in the Index to Schedules under the comments section for the selected species. Specific procedures for starting up the kiln and the first day or two of the run when drying air-dried or partly air-dried stock are given below. Specific Procedure for Air-Dried Stock (Average MC should be 25 pct or lower, with no material over 30 pct.) 4/4, 5/4, Most 6/4 (1) Bring dry-bulb temperature up to the value prescribed by schedule for the average MC of the controlling kiln samples. Maintain conditions during warming so that EMC does not exceed initial wet- and dry-bulb settings. (2) After prescribed dry-bulb temperature has been reached: (a) If the air-dried stock had not undergone surface wetting or been exposed for a considerable period to high RH just before it was placed in the kiln, set the wet-bulb controller at the prescribed wet-bulb temperature. Turn on the steam spray only if necessary to start equalizing. (b) If there has been surface moisture regain or if dried in a predryer above 75 percent RH, set the wet-bulb controller for a 10 °F wet-bulb depression and turn on the steam spray. Let the kiln run 12 to 18 hours at this wet-bulb setting, then change to the dry- and wet-bulb settings prescribed by the schedule. 8/4 (Plus 6/4 Oak) (1) Bring dry-bulb temperature up to the value prescribed by the schedule for the average MC of the controlling kiln samples, keeping the vents closed. Use steam spray only as needed to keep wet-bulb depression from exceeding 12 °F. (2) After prescribed dry-bulb temperature has been reached: (a) If there has been no surface moisture regain, set the wet- bulb controller at the prescribed wet-bulb temperature. Turn on the steam spray only if necessary. (b) If there has been surface moisture regain or if dried in a predryer above 75 percent RH, set the wet-bulb controller for an 8 °F wet-bulb depression and turn on the steam spray. Let the kiln run 18 to 24 hours at this setting. Then set for a 12 °F depression and run for 18 to 24 hours more before changing to the conditions prescribed by the schedule. 4A few schedules have maximum dry-bulb temperatures of 200 °F (93.5 °C). 5PDF Image | Dry Kiln Schedules for Commercial Woods Temperate and Tropical
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