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Kiln dried lumber moisture regain to ambient EMC

6/12/19       
Carl

I've been operating a Virginia Tech design
solar kiln for a year now, and am curious
about the need to dry lumber below the
ambient EMC if the lumber will be stored
for an appreciable length of time.

In other words, are there any advantages
to drying lumber down to, say, 8% MC when
it will be stored in an unheated building
and acclimate in the 12-14% MC range?

When the lumber is eventually pulled from
the building for use, and handled in a
way that brings the MC down to proper
range - 6-8% - are their any advantages
to initially drying the lumber to that
range, or is drying to the ambient range
of 12-13% adequate?

Gene has mentioned in the past that
properly wrapping 6-8% MC lumber with
4-6 mill plastic will prevent moisture
regain ... but I can see issues with that
approach if pieces are pulled from the
wrapped pile and re-covered numerous times.

The lumber in question is predominantly
4/4 ash

One of the reasons I'm asking is that I've
observed that the solar kiln quickly pulls
the free water from a load, and fairly
quickly works at the bound water from the
30% down to 20% range. At that point,
drying slows, and it seems that getting
the last 6% to 7% out (from the ambient
12-14% range) can take quite a while.

Also - in the winter, reaching the ambient
12-14 range was possible, but it appears
there's not enough heat available to get
down to the 6-8% range(northern PA).

At any rate, the kiln is performing
really well, and I'm just curious if MC
regain to ambient "nullifies" drying the
lumber to low MC's initially, or if there
are warp reducing benefits of initially
drying all the way to 6-8%.


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6/13/19       #2: Kiln dried lumber moisture regain t ...
Mark B Member

So are you considering pre-drying your material down to say 10% for material you wont need/sell right away and then flash drying it on demand to 7% in smaller loads as needed?

6/13/19       #3: Kiln dried lumber moisture regain t ...
Carl

> ... down to say 10% for material you wont
>need/sell right away and then flash drying
>it on demand to 7% in smaller loads as needed?

That's pretty much it ... but more like down
to 12-14% (ambient EMC around here), and if
in the winter, bringing it inside to acclimate
in a 30-35% RH area, or, if in the summer,
possibly "flashing" it as you mention.

I guess my main question is whether there
are any benefits (warp reduction, ease of
drying to lower MC later) to drying as low
as I can go in the kiln if it's going to
acclimate later at the 12-14% MC range

Carl

6/14/19       #4: Kiln dried lumber moisture regain t ...
GeneWengert-WoodDoc

I cannot think of any great reasons to dry to 7% MC, give or take, and storing the wood at 14% EMC, compared to drying the wood to 14% MC and then storing it.

However, in practice, if you have a load that you will stop at 14% MC, you will actually have as much as 5% MC variation on each side of 14% MC, so the wood will be 9% to 19% MC. If, on the other hand, you dry to 7% MC, you will likely have 1 to 2%MC variation and as it goes up to 12%MC (a little under the ambient EMC due to hysteresis), the MC will be very uniform. So, then when you are ready to dry it and sell it, it will dry quickly back to 7% MC, versus the 9 to 19% MC stuff that will have a wider MC range even after storage, requiring a bit longer drying.

One other small consideration is that any insects will be happy with much of the 9 to 19% MC wood, while drying to 7% MC will eliminate any insects...the lyctid powderpost beetle will not get into drying wood in a kiln and the other insects will find the wood too dry to survive, although the ambient storage can result infestations.


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