I've got a Virginia Tech design solar kiln
operating along the PA/NY border. This
summer, at it's peak, I recorded temperatures
of 129 degrees and 19% RH. I have a temp and RH
rig with both sensors located near the middle
of the stack of lumber, and keep a daily eye
on them
We're now in the cloudiest time of year here
(Nov-Dec), and I'm wondering what might be
recommended as far as operational behavior goes
during these marginal months.
I'm drying fresh sawn 4/4 ash, which is entering
the kiln at 40-45% MC. I've noticed that even on
very cloudy days that I'll see MC losses early on,
but as the material nears 20% MC, the MC loss seems
to grind to a halt.
I can understand the lack of drying due to the
lack of sun and cold. The outside temp is trending
around 30 degrees typically, low 40's occasionally.
I tend to leave the fans off unless there's promise
of a little sun during the day.
With no sun, and a cloudy day, at best, the temp
inside the kiln is 10 degrees higher then outside,
and the RH in the kiln tends to run around 75-80%
this year (which has been an abnormally wet year,
and we've had snow cover for the last 3 weeks)
EMC charts show that at the 30-40 degree range,
a RH of 80% results in lumber at 16-18% MC ...
not far from the 20+% readings I'm seeing.
So ... my question:
If the current conditions outside translate to
no meaningful drop in MC in the kiln (from the
20% MC range), is there any "degrade" danger in
leaving the fans off, or should I occasionally
let them run for a day?
Also - is there a temperature and RH differential
"threshold" when it makes sense to let the fans
run? As an example, in the weather description
above, and if the sun does peak out occasionally
during the day, it's not unusual for me to see
a temperature in the kiln that's 10 degrees (or
a little more) higher then outside, and RH levels
in the 70-80% range. The EMC at those RH levels
would point towards some drying of the lumber,
but I'm just not sure what impact the lack of
temperature differential has on retarding the
drying process, and weather it makes sense to
run fans all day when they will barely squeeze
out any moisture from the lumber.
Thoughts and opinions appreciated.