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Drying 5/4 White Oak, TDAL won't equalize

2/16/16       
Derik Member

I've been drying with 2 new package kilns for the last 9 months, and as the supply of lumber on the yard gets fresher, the starting MC% is increasing. I've started my first charge of 5/4 white oak, to this point it's all been 4/4 and mostly red oak. I'm being very conservative, my wettest samples are currently around 37%. I'm at 100 DB and 94 WB. In the previous step of 100/96, the TDAL remained about 1-1.5 degrees different. After switching to the current step of 100/94 I've noticed the TDAL increase to around 3-3.5 degrees. The fan speed has remained the same, the DB is the same, why is my exiting air getting cooler and is it affecting the drying?

2/16/16       #2: Drying 5/4 White Oak, TDAL won't eq ...
GwnwWengert-WoodDoc

Measuring TDAL is extremely difficult Ina hardwood kiln. So, my first guess is that there is an issue with measurement.

But if the values are accurate, then the wood is drying twice as fast...TDAL represents energy loss from the air, so 1.5 degree TDAL for the same load of lumber is half the drying rate of 3.0 F. But, dropping the depression from 4 to 6 degrees should only increase the TDAL by 50%, not 100%.

So, the easiest explanation is measurement error. How close are the probes to the entering and exit sides of the lumber? They need to be very close...1/10" or even in the sticker opening.

2/17/16       #3: Drying 5/4 White Oak, TDAL won't eq ...
Derik Member

The front probe is above the fan deck in front of the fan and the back probe is about a foot from the wall in the plenum chamber about 6 feet up. It looks like the TDAL has settled in at about 2.2-2.3 which would be the 50% increase you said would be accurate. I'm seeing entering air around 100.2 and exiting air around 97.9.

The kiln adjacent is a step behind (100/96) drying 4/4 red oak and there is no difference between the entering and exiting air, which after reading your post has me more concerned than the kiln with a drop in temp. If there is no change in air temp across the lumber, does that mean it really isn't drying?

Like I said in my first post, I've been slightly conservative in my drying, it's been a back up operators role for 12 years with only track kilns and now head operator with the addition of the 2 package kilns within the last year. Maybe I need to move ahead a step on the red oak.

2/18/16       #4: Drying 5/4 White Oak, TDAL won't eq ...
GwnwWengert-WoodDoc

If the TDAL is zero, then the air is providing no energy so there is no drying. I suspect there is an error, as it is hard to believe that there is no drying unless the lumber is already quire dry. The EMC at 110 and 106 is around 18% so if the wood is wetter than 18% MC it has to be drying, so there has to be a TDAL. If the wood is 18% MC, then 110 106 is way off from a normal setting.

Note that every kiln schedule for oak is based on the wood's MC and not TDAL, so there are no guidelines for TDAL. From a drying standpoint, we try to keep the drying rate for most hardwoods constant, which means a constant TDAL. So, you should be given the desired TDAL and then maintain that until you get close to the end. Of course, we do have temperature limits and humidity limits that must also be observed.

In short, TDAL for hardwoods is so small that it is hard to measure, and has little or no guidelines or research, so it is not suggested. If it is used, then you must be very conservative, which means long times and inefficiency.

Let me know if I can help further.

2/18/16       #5: Drying 5/4 White Oak, TDAL won't eq ...
Derik Member

Thank you for your responses Gene. The red oak is at 100/96, not 110/106 which according to the book is basically the same EMC, but it seems just typing out questions and waiting for responses has helped a little. I've been looking at the history of the kiln, which allows me to look at the TDAL and it was only equal when the fans were in one direction. When the fans were in reverse the exiting air blows across the condensate return before the RTD which I think is giving a false reading. When that RTD is controlling the heat, as the entering air monitor it is working because the air is moving across the RTD and then the condensate return.

This load of oak is my first real mixed load as far as MC goes. Half the charge went in at 18-20% and the other half at 35-45%. Unavoidable because of the stock we had, but so far it looks like it's going well. The samples in the 18-20 range are slowly creeping down, about .2% a day and the higher samples are drying about 1% a day. I think the TDAL being 0 has to do with the drier lumber that is already around the EMC in the kiln and the condensate return line warming the air slightly before it reaches the rear RTD.

Again, I appreciate the responses, and I've learned a lot just in the past couple days, reading from the knowledge base and other forum posts.

2/18/16       #6: Drying 5/4 White Oak, TDAL won't eq ...
GwnwWengert-WoodDoc

Two comments...
When loading a kiln with mixed MCs, load the wetter lumber first and then run the kiln until this lumber is at the same MC as the drier lumber. Then add the drier lumber. This will not extend drying time.

For any lumber, always start the kiln at the EMC that is at or slightly drier than the lumber's surface MC. For air dried stock, this will be close to 12% EMC or a 10 depression. Starting more humid will add moisture back to the lumber and cause any surface checks to worsen.

It does appear that you have a problem with TDAL accuracy due to fan reversal differences. Further, you are drying based on the average of the whole load (TDAL has one overall measurement), yet with hardwoods, we dry based on the wetter wood. Next time the kiln is empty, turn on the system and fans and see if you have zero TDAL in both directions.

2/18/16       #7: Drying 5/4 White Oak, TDAL won't eq ...
Derik Member

Wouldn't loading half a charge in the kiln slow the drying process and cause uneven drying with all the voids let the air short circuit? This is a 75,000 BF kiln and I don't think I could baffle it well enough with all the empty space. I started the kiln at an EMC below the driest samples and so far all the samples have lost weight at each weighing, so hopefully there has been no damage to the drier wood.

The questions about TDAL were just based on observations of the kiln conditions and getting a better grasp on the drying process. I use weight samples to determine where I should be in the kiln schedule, not the TDAL, and am running a basic oak schedule from the Dry Kiln Operator's Manual. When my wettest samples tell me to move to the next step, and hopefully when the dry samples get to where I need to equalize, the wetter samples won't still be up any higher than 10 or 12%

2/18/16       #8: Drying 5/4 White Oak, TDAL won't eq ...
GwnwWengert-WoodDoc

Indeed the air flow will not be perfect, but it is better than checked lumber. Also, drying time for any load is based on how long it takes the wettest lumber to dry. That is why that it will not take longer for split loading. Ok?

2/18/16       #9: Drying 5/4 White Oak, TDAL won't eq ...
GwnwWengert-WoodDoc

Two comments...
When loading a kiln with mixed MCs, load the wetter lumber first and then run the kiln until this lumber is at the same MC as the drier lumber. Then add the drier lumber. This will not extend drying time.

For any lumber, always start the kiln at the EMC that is at or slightly drier than the lumber's surface MC. For air dried stock, this will be close to 12% EMC or a 10 depression. Starting more humid will add moisture back to the lumber and cause any surface checks to worsen.

It does appear that you have a problem with TDAL accuracy due to fan reversal differences. Further, you are drying based on the average of the whole load (TDAL has one overall measurement), yet with hardwoods, we dry based on the wetter wood. Next time the kiln is empty, turn on the system and fans and see if you have zero TDAL in both directions.


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