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Specific Procedure for Partly Air-Dried Stock (No material should be over 50 pct MC.) 4/4, 5/4, Most 6/4 (1) Bring dry-bulb temperature up to the value prescribed by the schedule for the average MC of the controlling kiln samples. Keep the vents closed. Use steam spray only as needed to keep wet-bulb depression from exceeding 10 °F. However, do not allow the depression to become less than 5 °F or moisture will condense on the lumber. (2) After the prescribed dry-bulb temperature has been reached, run a minimum of 12 hours on each of the first three wet-bulb depression steps of the whole schedule, but still observe the 5 °F minimum wet-bulb depression. Then change to the conditions prescribed for the MC of the controlling samples. 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. Keep the vents closed. Use steam spray only as needed to keep wet-bulb depression from exceeding 8 °F. However, do not allow depression to become less than 5 °F. (2) After the prescribed dry-bulb temperature has been reached, run a minimum of 18 hours on each of the first three wet-bulb depression steps of the schedule, but still observe the 5 °F minimum wet-bulb depression. When the kiln conditions coincide with those prescribed by the schedule for the average MC of the controlling samples, change to the MC basis of operation. Suggested Procedure for Frozen or Snow-Covered Hard wood Lumber During winter in the colder regions, a kiln operator may find it necessary to load a kiln with frozen lumber or lumber with an appreciable cover of snow and/or ice. While many operators go directly to the indicated step in the schedule, others use the following practice: Warm the lumber for 12 to 24 hours with dry-bulb temperatures of 80 to 90 °F and a wet-bulb depression of about 5 °F; operate vents as needed but turn off steam spray. After this warming period, refer to the preceding section on procedures for air-dried or partly air-dried stock with surface moisture regain. Softwoods(Conifers) This section lists schedules for drying softwood (coniferous) species in steam-heated kilns and direct-fired kilns operated at temperatures between 90 and 200 °F. The suggested schedules are divided into two groups: (A) those where changes in schedule are based on MC of the wood, and (B) those where changes in schedule are based on time. For products requiring very close control of final MC, such as furniture, millwork, and cabinet stock, better results may be obtained using the MC-controlled schedules. These are essentially the same as those listed in U.S. Department of Agriculture Handbook 188, Dry Kiln Operator’s Manual. Use of sample boards is required. As mentioned in the previous section on hardwood, some schedule modification may be desirable to meet product requirements or changing timber resources. Modified schedules frequently used in the northeastern United States are listed in the Index to Schedules under Comments. For the suggested schedules where the changes are made based on elapsed time in the kiln, considerable changes have been made from Agriculture Handbook 188. These time schedules are based on drying from the green condition unless otherwise noted. When drying partly air-dried or partly kiln-dried western softwood stock, choose the appropriate schedule from the Index, but use a 10 °F wet-bulb depression for about 10 hours prior to entering the proper step of the schedule. We feel these schedules are more appropriate for modern equipment and wood resource, should be more efficient, and require less kiln residence time. Suggested Procedure for Frozen or Snow-Covered Softwood Lumber Snow-covered or frozen softwood lumber requires a low-heat cycle prior to setting the drying schedule to prevent degrade in most species. General practice in the western United States is to turn off the spray line, with vents open and the heat on, for 8 to 18 hours depending on amount of snow or quantity of frozen lumber. The dry-bulb temperature should not exceed 100 to 105 °F. A frequently used practice in the northeastern United States is to run about 12 hours at 95 °F dry-bulb temperature with spray off and vents closed. 6PDF Image | Dry Kiln Schedules for Commercial Woods Temperate and Tropical
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