Sawing and Drying

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First solar kiln load results

7/15/18       
Carl

I just recently finished drying the first load of 4/4 ash
in the solar kiln I built a few months ago, and thought
I'd share the results and hopefully get some input on
the trial run behavior.

The load is just shy of 400 bd/ft, and all 8 foot lengths,
and was carefully stacked with very dry stickers.

The kiln is based on the Virginia Tech plans, with a
foot print of 14 X 7 feet. I decided to use a hinged
roof/collector setup up and a removable front/low wall.
I have a fairly decent tractor with forks, and wanted
to be able to easily load/unload the lumber, and the
forks allow me to lift the roof and prop it open.

Following the guidelines in the Virginia Co-op Extension
publication:

https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/420/420-030/42
0-030_pdf.pdf

I cut sample boards for monitoring MC rate of loss, and
also cut 1 inch samples from the ends of those boards
for oven drying and determining initial MC of the ash.

The oven dry samples showed the ash coming off the
sawmill at 38-40% MC. Based on off-the-stump MC chart
for hardwoods, this was a bit lower then what the chart
shows for ash (45%), but maybe not surprising - these
were ash trees that had been infected by the eastern
ash borer, and this year was the first year they did
not leaf out. The trees were dropped, the logs end
coated, and the logs were sawn within 4 days of
dropping, with the boards immediately put in the kiln
fresh off the saw.

As per the VT documentation, I cut two 30 inch sample
boards, used the formula to estimate the oven dried
weight of the boards, and put them in the pack so I
could pull them out and weigh them each morning, using
the VT doc formula for calculating current MC of the
sample boards, and applying each board's MC value
against the previous days MC value to come up with
the percentage MC loss.

The VT doc mentions quarter sawn lumber drying slower
then flat sawn - my samples were one of each, and
dried as expected - the quarter sawn board slower.

Below are the readings/measurements I took each
day - the percentage figures represent the percentage
of moisture loss compared to the previous day's reading.
The VT doc listed the max recommended drying rate
for ash at a little over 10% per day, which
became the "target" I was looking for. The MC-
values below are the actual daily MC values for the
sample boards determined by weighing each sample and
dividing the noted weight into the estimated oven
dry weight of that particular sample board
(scale resolution was hundredths of an ounce)

I averaged the readings of the two sample boards
to arrive at the single values below, and rounded
to nearest whole numbers. Days with no comments
indicate good sunny drying days.

Day 1 12% | MC-33 vents 1/4 open
Day 2 9% | MC-30
Day 3 20% | MC-24 unusually high rate of loss
Day 4 2% | MC-23
Day 5 7% | MC-21 closed vents a bit closer to 1/8 open
Day 6 4% | MC-19 cloudy/showers
Day 7 7% | MC-17
Day 8 6% | MC-16 vents now closed
Day 9 3% | MC-16 rain all day
Day 10 3% | MC-15 rainy day
Day 11 5% | MC-14 power out no fans
Day 12 9% | MC-12
Day 13 +3% | MC-13 rain/no fans - gained MC
Day 14 4% | MC-12
Day 15 9% | MC-11
Day 16 12% | MC-10 record heat starts
Day 17 5% | MC-9
Day 18 17% | MC-8
Day 19 +6% | MC-8.5 GAINED moisture

Some worthy-of-mentioning items ....

The last 4 days were as sunny and hot (touched
low 90's) as it gets here, and that weather
coming at the end of the drying cycle seems
to be the best case drying scenario I'm likely
to experience.

The first three days, moisture loss was very
fast (too fast), down to 24% - I find myself
wondering if this was due to the free water
being cooked off much more quickly then later
when the remaining moisture is mostly bound
water.

Things seemed to stall when the MC got to
the low 20's - up until the stall point, I
had the vents about a 1/4 open (aprox 100
sq/in of total open vent area). After reading
some of the Wood Doc's postings, I tried
closing the vents. Even fully closed, there's
likely 3-4 sq/in of opening around the vents
and door gap, and certainly some air leakage in
the structure, but I made a point of making the
kiln very tight (and very well insulated)
With the vents closed, moisture loss got closer
to the 10% target rate. Note to self: consider
closing the vents when MC reaches low 20's

The last couple of days, I took additional
MC readings using a pinless meter, and the
readings (after species correction and temp
correction at 1/4 inch depth) showed the
sample boards at 6% - even nudging high 5's
in some spots. I suspect that while the
weighing the sample boards is a necessity
at high MC's, as the lumber reaches final
MC, there may be some introduced error.
No matter - this kiln brought the load
down into (or very near) the 6 percent range.

I decided to leave the lumber in the kiln for
a bit to observe, and interestingly, the
MC rose the next day after the 6% reading.
I suspect there may be a point of "no returns
on investment" ... the lumber will hover
around 6%, and fluctuate based on outside
conditions - or perhaps any wetter pieces
are still drying a bit with the moisture
being picked up by the sample boards.

I'm going to post some responses that include
pictures and an interesting temp/humidity
measuring device.

Carl

7/15/18       #2: First solar kiln - structure pictur ...
Carl

Some pics of the kiln - the plan is to
stain it black when the board and batten
siding is dry.


View higher quality, full size image (1200 X 875)


View higher quality, full size image (1200 X 800)

7/15/18       #3: First solar kiln - kiln load pictur ...
Carl

I had a large piece of commercial rubber
roofing laying around, and use it as a
tarp to direct air flow through the pack,
and block air flow from wandering around
in other areas - it appears to be very
effective at getting good velocity through
the pack


View higher quality, full size image (1000 X 667)


View higher quality, full size image (1200 X 800)


View higher quality, full size image (1200 X 800)

7/15/18       #4: First solar kiln load - drying time ...
Carl

I found an inexpensive ($40) temperature/humidity
measuring device, and place the measurement
probes near the center of the pack.

The device has built in software that produces
timeline reports, and I've included the
report in PDF form. I marked it up to show
the MC of the sample pieces and the percentage MC loss for each day - the
report really gives insight to the daily
behavior of the kiln.

I bought a second one to monitor outside
conditions so I'd get some historical data
on performance at different times of year.

The recording device is:

Elitech GSP-6 Temperature and Humidity Data Logger Recorder

And can be purchased at Amazon:

https://www.amazon.com/gp/product/B01H3669T4/


View higher quality, full size image (900 X 801)


Click the link below to download the file included with this post.

20180703_kiln_run_report.pdf

7/15/18       #5: First solar kiln fans detail ...
Carl

I used three Lasko 20 inch fans, and made individual
fan mounts. With the roof lifted up to load,
the fan mounts tilt back against the back wall.

After the roof is lowered/closed, the fan mounts
are tilted against a stop strip on the bottom
of the rafters - I felt this tilted arrangement
might better direct the air move along the
collector rather then having them pointed
horizontally at the collector


View higher quality, full size image (600 X 900)


View higher quality, full size image (1200 X 800)


View higher quality, full size image (600 X 900)

7/15/18       #6: First solar kiln first load questio ...
Carl

After running the first load, I have some
questions I hope Gene can weigh in on...

1 - The fast rate of loss (too fast) when the
lumber was above 25-30% ... is this possibly
due to the kiln being more effective removing
free water moisture then bound water moisture?

2 - what type of degrade would one
expect when drying ash too fast early on
in the process (above, say, 20% MC)

3 - what would suggestions be for holding
cupping to a minimum? My first load had
almost no twist, bow, crook, and no end
checking, but there was quite a bit of cup.

It's worth noting that this first load was
"through sawn", and the worst cup
offenders were the pieces with juvenile
wood. The second load (in the kiln now)
was "grade sawn" with the associated
cants being left, and I'll be interested to
see how that effects overall cupping.

7/16/18       #7: First solar kiln load results ...
Gene Wengert-Wood Doc

1. I assume that you appreciate that the drying rate is based on the RH, velocity and temperature. With the vents open wide, plenty of air flow, and lots of sun, you will be able to dry the wood quite rapidly. Closing the vents a bit at the beginning, will slow down drying by raising the kiln humidity. The loss per day is the actual MC loss and not a percentage of that loss. So, 33% to 30% is a 3% loss and not 10% loss. So, you are not close to the maximum rate. The gain at the end might be because you are running the fans too much. Turn them On after the kiln has heated 10 F in the morning- -9AM ?- - and off around 7PM.

2. If you dry ash too fast, we might expect some minor cracking. You are not close to "too fast."

3. Cupping of wide pieces with the pith in or near is next to impossible. It is a natural event for wood. This cupping is a disadvantage of sawing "through and through" or live sawing. Slow drying encourages cupping.

7/16/18       #8: First solar kiln load results ...
Carl

Thanks Gene .... particularly the maximum
loss/percentage clarification ... sometimes
a little bit of knowledge can be a dangerous
thing.

It's obvious from your explanation that I
probably wont need to worry about drying
too fast with the size loads I'm putting in
the kiln, and your explanation:

"Slow drying encourages cupping."

Leads me to believe that it might be worth
experimenting with the size of the load,
reducing it until the drying rate approaches
the recommended maximum of 10+% for
ash.

All in all, it's been fun watching these first
two loads - me thinks one of the beauties
of your solar kiln design is that it's hard
for even the uninitiated to screw things up ;)

Carl

7/18/18       #9: First solar kiln load results ...
Matt Calnen

Gene, why does slow drying cause more cupping? I am a novice wood dryer. I have bought a couple of batches of lumber green from a farmer with a woodmizer and have not had trouble with what I would consider over cupping. Most of it was maple or ash, and 10” or less wide. I air dried the wood outside in a well stickered stack for two years that was covered on top, but the sides where open to the elements. I thought the wood came out good for what I paid. It had very little internal stress.

5/10/21       #10: First solar kiln load results ...
Jim Tomassino Member

Website: https://stuart.seacoastair.com/

Cupping of wide pieces with the pith in or near is next to impossible. It is a natural event for wood. This cupping is a disadvantage of sawing "through and through" or live sawing. Slow drying encourages cupping.


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