Sawing and Drying

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Solar kiln - questions regarding first drying run

5/26/18       
Carl

Am going to be starting up a solar kiln for the first
time in a week or two, and have some general questions ...

The kiln is based on the Virginia Tech plans, and
has a 6 X 14 footprint.

From my reading of Gene's many articles/responses
on drying, the goal is to dry the lumber as
quickly as practical without exceeding the recommended
MC loss per day for the given species.

Typical pile I'll be working with will be 4 feet
wide and 8 or 10 long, with the height determined
by using an optimum board footage value to collector
size/ratio that will hit the target drying rate
for the given species.

There are three fans, with high/med/low settings.
My reading indicates that air movement has the
greatest effect during the early/high MC stage
of drying, which I read as meaning that the air
velocity is most effective when removing the
free water, and as the lumber approaches 30% to 20%
MC, the air velocity is far less important.

Assuming the above is true:

Question 1 - fan speed:
Is there any merit to running the fans at high until
the MC is below 30%, and then switching to low(er)
speeds, or should they just run on high all the time?

Question(s) 2 - setting vent openings:
How does the vent opening size come in to play?
Should the amount the vents are opened be determined
by monitoring MC loss? In other words, if the rate
of drying is happening too quickly, should the vents
be choked down to reduce the drying rate? Or ... is
the vent opening determined by outdoor temperature?
Wide open in hot months, choked down in cooler weather?
I have found very few references to how the vent
openings should be manipulated during the drying
process - any insights would be appreciated.
I have six vents - 3 upper, 3 lower, with each vent
providing a 1 sq/ft opening.

Question 3 - weighting the pile:
Is weighting the top of the pile as beneficial
when kiln drying as it is when air drying?

First load in the kiln will be 4/4 white ash.
The plan is to get the logs sawn and in the kiln
as quickly as practical after dropping the
tree(s) and end coating the logs.

The VT kiln documentation lists the max MC loss
per day for white ash at 10.4%. The collector area is
120 sq/ft, and my plan is to load the kiln with about
350-400 bd/ft and monitor. From the VTech literature,
and given that we're approaching the high temperature
and maximum sunlight period, it appears the this
load could be dried in somewhere between a
week or two.

All responses appreciated

5/28/18       #2: Solar kiln - questions regarding fi ...
Gene Wengert

1. High speed all the time

2. Vents are opened to exhaust moisture and so they are not opened very fat for most hardwoods. Wide open vents will minimize heating, as the warm air will be exhausted after one pass through the load. With only 4– bf of a fast drying species, open the vents perhaps 1/4. Near the end, close them a bit as heat will help achieve the final. Low MC.

3. A thick concrete slab will absorb heat and release it later in the evening. It will be hard to get good stress relief with 100% RH. I never used weights in all the runs I did.

5/29/18       #3: Solar kiln - questions regarding fi ...
Carl

Thanks, Doc ...

Is there a typical target for the temperature
differential between inside kiln temp and
outside ambient temp? I seem to recall you
mentioning a differential of 20 degrees ...
maybe as the minimal differential that will
keep the drying process going at an appropriate
rate?

If I'm interpreting your response correctly,
the idea is to adjust the vent openings so
the air temp in the kiln doesn't drop too low.

I suppose in an ideal setup, outside air would
be introduced to the kiln, the vents closed,
and the air circulated until the RH goes way up,
and then the vent opened to exhaust the high RH
air, and pull in new/drier air, vents closed,
and so on.


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5/29/18       #4: Solar kiln - questions regarding fi ...
Gene Wengert-WoodDoc

Your idea about vent operation is how many non-solar kilns operate. Adjusting the vents to a small fixed opening is how to avoid sophisticated controls.

The kiln fans can be turned on when the kiln has roughly heated up 20 F above the morning low temperature, not current temperature. This will occur at about the same time every day for a month span or so. This means we can use a timer to turn on fans

2/22/22       #5: Solar kiln - questions regarding fi ...
John Pleasant Member

Wide open vents will minimize heating, as the warm air will be exhausted after one pass through the load.


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