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I have heard of kiln operators spilling buckets of water on the kiln floor or spraying packs of hardwood lumber with garden sprayers in attempts to “home brew” conditioning solutions when purpose-built mist or steam systems were not installed. Don’t follow their example! Water puddles will evaporate too slowly to provide the quick boost in RH needed for stress relief, and rewetting the lumber with large water droplets from low-pressure sprayers will increase the likelihood of defects in the finished lumber. I’ve even heard of people using rice cookers to add steam to small dry kilns, but they can’t begin to generate the amount of steam required. If for some reason a company running DH kilns didn’t want to use a water mist system, commercial electric steam generators are available. High pressure steam is not required, but an appropriately-sized steam generator might cost you almost what you paid for the DH compressor setup. It might also require either 240 or 480 3-phase electrical connections. As an example, a 10 kW steam generator will produce roughly 35 pounds of steam per hour. This might be a reasonable starting point for a kiln with 4000 BF capacity based on the five to ten pounds of water per hour per 1000 BF rule of thumb mentioned above, but the size of the steam generator might need to be increased depending on the fraction of lumber volume to kiln volume. CONDITIONING IN A DH KILN. DH kiln schedules for hardwoods usually terminate at an EMC of about 4%, but some DH kiln operators run their kiln with a final step-EMC of 5% so they can equalize while they’re drying to the target MC. The driest boards are already restricted to drying below the EMC in the kiln, so their conditioning procedure might be to increase the dry bulb by 10° when the slowest-drying sample board reaches the target MC and then and to add water back to the kiln as a mist to provide the added moisture needed for conditioning. Most of the Nyle-supplied schedules finish drying at 160°. If you use those schedules, you might think that you could just do the final drying and equalization at a lower temperature than the schedule calls for, slowing down the kiln a little bit and then boosting the kiln temperatures for conditioning at the higher RH required. This might not be practical because of compressor limitations. Table 4 reproduces a sample schedule that Nyle provides to its customers for 4/4 to 6/4 red oak: Table 4. Nyle-provided drying schedule for 4/4, 5/4, and 6/4 red oak lumber RED OAK 4/4, 5/4, 6/4 MC% DB °F DEPRESSION °F WB °F EMC% 106 17.5 105 16.2 102 13.3 96 9.9 90 5.4 90 3.8 90 2.6 110 2.1 Green to 50% 110 50 to 40% 110 40 to 35% 110 35 to 30% 110 30 to 25% 120 25 to 20% 130 20 to 15% 140 15% to Final 160 Equalize and Condition to Target MC% 4 5 8 14 30 40 50 60 53PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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