The First Load is Complete PDF Print E-mail
Written by Randy   
Monday, 30 June 2008 00:00
The first test load of lumber is now complete and some good lessons were learned. Total time to dry the 500 bdft of oak and cherry was 59 days.

The first interesting point to address is that "complete" is an arbitrary term.  The end of a kiln charge depends on what final moisture content is desired.  For my load, 7% was the target.  How the moisture content is determined will also impact when "complete" is complete.  The weight of a sample board was the final judge for this load.  When the sample board weighed out to 7%, the moisture meter supplied a range of readings from just under 6% to 10%.  Different locations on each board, some flat sawn and some quartersawn, location in the stack, different densities in each board all impact the moisture meter.  It's impossible to get the same value at all locations and it can drive you crazy trying to determine when the load is done.  The sample board or boards give an accurate reading and dispense with the variations in readings at every inch of a board.  So once the sample board reached 7%, I considered the charge complete.  Ironically, the load will stay in the kiln until my next load is ready to go in so the drying continues. 

Fifty nine days seems to be about 29 days too long.  But I think a large number of the days were due to the changes made to vent positions.  My original logic was to keep the vents nearly closed during the beginning to ensure some moisture stayed in the kiln and the oak wasn't dried too quickly.  Once the wood reached about 30% and most of the danger of damage was passed, I thought opening the vents all the way would allow for quicker removal of moisture and faster drying.  Turns out that was an incorrect assumption.

The biggest lesson learned is that high temperatures take a priority over moisture venting.  With the vents wide open, the maximum temperatures were still reaching 110°F but it was clearly slowing the overall rate of drying.  With the vents closed or nearly closed the maximum temperatures went up and there was still enough venting to increase the drying rate.  In the future, the vents will get closed and stay closed.

First load chart of sample board moisture content vs. days of drying

Above is a chart of the sample board moisture content vs. the number of drying days.  Two important points to check out.  Day 17 was when I made the decision (a wrong one) to reposition the vents to full open.  It is easy to see the drying rate immediately slows down some. The next major change took place on day 31.  This is when I changed the vents to a position only slightly open.  The chart clearly shows an increase in drying rate at day 31.  The only other vent change was made on day 56 to completely close all vents.  The reduction in the drying rate after day 42 (or when it reached about 10%) was not due to any vent changes or long term weather changes.

The other major lesson that was learned is that I need to improve the baffling at the ends of the lumber stack.  "Improve" is probably a generous word since there was no end baffling at all.  I could easily tell a large amount of heated air was bypassing the stack around the ends.

For the next load, the vents will stay mostly closed and the ends will get some baffling.  I think these two changes will definitely decrease the drying time.

I hope to have the datalogger information for internal temps and humidity up some time soon.  I do know that with the vents mostly closed, my highest temperature was 148°F, and that's on the floor just before entering the stack.

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