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drying to single digits p21/7/16
I've built my insulated chamber and have a dehumidifier coming in. My question is what variables to use (temp, time and RH) to get my 6/4 and 8/4 red oak from ~10% MC to ~7% MC.
I run my home built dehumidification kiln at 110F and follow a drying schedule like shown below. When the extraction rate falls off to 25% of the first day, I move to the next lower RH.
10% MC; 60% RH
So if you want 7% MC for your final value, make the last setting about 42% RH.
So I guess it wouldn't be advisable to set to 40% RH when I'm at 9-10%MC
It's impossible to say how long it will take to dry from one MC to another. It depends upon how much lumber is in the dryer, how much water the dehumidifier can process per day, and quite a few other factors. In my kiln with a full load of about 275 BF operating at 110F with a 40 pint/day dehumidifier, it takes about 12 - 15 days to go from 12 - 14% starting MC to 7% for 4/4 and 5/4 stock, so call it 2 days / % water removed.
No don't set the kiln at 40% when your lumber is at 9 - 10% MC, follow the chart and set it at 50% until it gets close to 8.4%, then set it to 45%, etc. Small changes, not big ones, especially when drying 8/4 stock and difficult to dry woods like white oak.
At 60% RH, the lumber's surface will dry to about 10% MC, but no drier. It will take many weeks to get that dry throughout the piece. So, the surface will be at 10% MC, but it is impossible to get the surface any drier. The core will therefore be wetter.
To achieve 7% MC lumber in a reasonably short time, you will need to use 25 to 30% RH or a bit lower at 110 F. That will give 6% MC at the surface.
As a rule of thumb, at 110 F and 30% RH, you might, on a good day, achieve with 4/4 oak about 1/2% MC loss per day. For 8/4 less than 1/4% MC loss per day. Why so slow? We are waiting for the water to move out of the wood and not for the air to scrub the water off the surface. So, the size of the compressor is not the issue at these low MCs, but rather how fast the water moves out of the wood. Temperature is a big factor at the end of drying. If the surface MC and the air's moisture are the same (such as 60% RH and 10% MC), then no drying will occur.
A complete listing of RH and MC values is given in many documents, including pages 16 & 17 in DRYING HARDWOOD LUMBER. (Download here at WoodWeb.)
Thanks Doc. I found your chart. I purchased an Ebac LD800 and the sparse info in their manual says not to increase temp more than 5deg Cent. per day. Not sure if damage to the wood should be expected when it is already down to 9-10%MC.