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Small DH kilns often have lower air velocities than conventional kilns, and this decreases the drying rate of green lumber. If this is the case for your kiln, you might cautiously decrease the RH in your kiln schedules to compensate for the reduced drying rate in the initial drying stages. This has the potential to create defects, so be careful. There are some species which are not listed in any publication. For example, what would you do if you want a schedule for mulberry? And what should you do if your kiln cannot match the temperature or RH requirements of a schedule that was originally formulated for a steam kiln? WOOD GROUPS FOR DH KILNS Different species have different densities and different green moisture contents, and they dry safely at different rates. Lower density species tend to dry without defects more readily than high density species, for example. This means that the species, the moisture content, the amount of lumber, and the desired moisture reduction rate all have to be considered when designing and constructing a dry kiln. In conventional kilns the rate of water removal from the kiln is largely restricted by the amount of steam heat available and the size of the vent openings, but in DH kilns the water removal rate is constrained by the auxiliary heater and the compressor capacity. Undersized compressors will not be able to remove water vapor quickly enough for the kiln relative humidity to decrease as water is evaporated, and mold and mildew can result if the relative humidity climbs above 70% or so.21 This means that the initial moisture content, the volume of lumber in the kiln and the ease of water removal from that species must all be considered in kiln design (particularly with regard to the size of the compressor) and subsequent use. Oversized compressors are less efficient than properly-sized compressors, so DH kiln manufacturers usually recommend compressors that will work for most kiln charges even if they are slightly undersized for drying full loads of some species at high moisture contents. Many hardwood lumber charges fall into one of three categories: 1) they have a middling green moisture content; 2) they are initially air-dried; or 3) they have to be dried fairly slowly to avoid defects. Water removal rates for loads such as these will be moderate and DH kilns will not require high-capacity compressors. On the other hand, some low-density hardwood species and some softwood species such as the pines can be readily dried with few defects, but these species have high green moisture contents and give up their water quickly. To dry green lumber from these species while keeping the RH below 70% you have only two options: you either have to buy a larger compressor than you’d buy for drying air-dried oak or you have to load the DH kiln with a smaller volume. If these aren’t typical loads it makes sense to downsize the kiln charge for fast-drying high moisture content hardwoods instead of increasing the compressor size. The ease with which water can be removed (and the potential for stain and/or mildew) is the basis for the idea of wood groups (as used by Nyle, a major manufacturer of DH kiln equipment). 21 Pearl, Virginia. 1996. How to prevent and remove mildew. University of Florida IFAS Extension. FCS3042. Found online at http://edis.ifas.ufl.edu/pdffiles/HE/HE63300.pdf. 89PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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