Dehumidification and Hardwood

[RSmith]

From an appearance quality standpoint, do you feel that dehumidification kiln drying is superior for all hardwood drying?



(Note from Kilndrying.org Administration: This question was originally mistakenly posted under forum expert Micheal Milota, Ph.D.  It was intended to be posted under forum participant RSmith's account.)

[Gene Wengert]

Drying quality depends on the temperature, humidity and velocity around and past the lumber.  The most critical quality time is at high moisture contents (above 50% MC) so that is when the temperature, RH and velocity need to be under close control.  Any drying hardware that provides the optimum drying conditions will achieve the optimum drying quality.  In truth, many different types of equipment (DH, steam, hot water, etc.) made by many different manufacturers will provide the proper drying conditions to achieve the highest quality.  (Of course, if the lumber is air dried prior to kiln drying, then virtually all quality is determined in air drying and the kiln equipment has almost no effect on quality.)

But, even with the proper drying hardware, quality is determined by how the equipment is operated.  (Analogy:  Is a Ford Fiesta safer or not as safe as a Buick Encore when driving from coast to coast?  The truth is that the driver is almost always the one that controls the safety, not the equipment.)  So, quality depends on the operator.

So, the only missing item for many operators is a definition of quality...what quality does the customer need?  So often, this definition is not very precisely given, so the operator does not really know what do do specifically.  Is an extra day need to equalize to achieve more uniform final MC or achieve a specific average final MC?  Is the stress (casehardening) in the lumber sufficiently removed?  And so on.

Now, most drying operations are trying to make a profit, so a better question about drying would be "Is a DH less expensive than steam, hot water, etc.?"  To answer this question, the annual volume, drying quality, energy costs, species and lumber thickness is needed. (For example, small scale is often cheaper with a DH; a DH cannot relieve stresses, so maybe a DH is not correct for some cases; DH is too slow for thick woods; etc.)