Equalizing

[admin]

a user sent in this question. admin


Is there an advantage to using two steps of equalizing in the drying process?  Dry, equalize then dry again, then finish with one last step of equalization.  We are trying to bring our standard deviation closer on White Fir.

[MichaelM]

Equalizing can mean different things.  If one is drying hardwood lumber to 7%, equalizing is done at a 5% EMC or about a 35F wet-bulb depression.  In this way, no board dries below 5% MC.  For alder dried to a 9% mc, the equalizing would be done at a 7% emc or a 25F wet-bulb depression.  This is described in the Dry Kiln Operators Manual Chapter 9 downloadable at http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=101&header_id=p.  The procedure regularly results in lumber that is within a 1% moisture content range as required for furniture, flooring, and cabinetry.

For lumber dried to 15%, this doesn’t work so well.  One would have to set the EMC at 13% or approximately a 6F depression.  The drying would slow unacceptably and there would probably be lots of corrosion in the kiln.  Therefore, it is difficult to equalize at all in the conventional sense.

We often recommend about a 20F depression when the average moisture content of the wood in the kiln is in the 20s, say 25%.  This is an EMC of 7-8% and reduces extreme overdrying without dramatically reducing the rate of drying on the wetter pieces.  Pieces that ate down in the 10-12% range only have a difference of 4 to 5% (MC-EMC) while pieces at 25% have a 17 to 18% difference (again board MC minus EMC).  The drying rate is nearly proportional to the MC-EMC so one would expect the wetter boards to be losing moisture 3-5x faster than the drier boards.  Do this at the highest temperature in the schedule because water moves faster inside the wood when the wood temperature is higher.  This will not get you +/-3% on white fir, but it is a reasonable trade off.

It’s not clear to me why one would do a two-step equalization or equalize, then drying more.  I have heard of people trying a steaming cycle at various points during the drying cycle, but I don’t recall any conclusive results.  We do have a lab kiln and can try things like this for you, unfortunately it takes money.  But these are things you can also try at the mill for several charges.  Making a valid comparison is the hard part (in a lab or a mill) so that it is the kiln schedule causing differences, not some other factor.

[BrandonL]

I am a quality assurance manager for a large hardwood flooring manufacturer. I am happy to be a member of this forum and am looking forward to interacting with the community. Our final dry mc% tolerance is 3%. We are striving to increase our process capability among all kilns discharges to be equal to or better than 3% (Pp and Ppk indices). A longer equalization achieves those results, but the cost is about 2 to 3 extra days in the kilns. My question is, is this the best approach to reducing the standard deviation of a kiln charge as measured by bake-out?

[MichaelM]

 Sorting before drying as much as the mill equipment, number of kilns, and sensing technology allow will help.  This might be for heart-sap, grain direction, initial moisture content, or whatever impacts drying the most. 

After it is in the kiln, some people say prestreaming helps with variability but I don't know that it does so without adding time to the cycle.  I'd also worry about the effect presteaming might have on color in hardwoods.