DH Kiln Schedule 8/4 Walnut

[GBar]

Looking for input and suggestions on what folks use for DH style kiln schedules for 8/4 Walnut.
 
I have researched FPL-118 (dtd 2000) traditional steam schedules and the reference on page 95 stating that DH schedules use the same MC steps and RH values, but only the temperatures are lower for a DH kiln.  Nyle publishes DH schedule also, but are much more conservative.
 
I have just passed the 25% MC step.  In the L53 and L200 manuals, it has 25% MC to Final step as 120 DB/98 WB (or 45% RH).  This equates to an EMC of 7.5.  Another Nyle publication for the higher temp Kilns have a schedule that nearly matches what is in FPL-118.  That 25% step is 130 DB/80 WB (or 10.3 RH) which calculates to 2.0 EMC. very aggressive. 
 
After hitting 30 MC, I set DB to 115 degrees and the RH to 50% (DB/WB=115/96).
At 25 MC, I set 117 and 42 RH  (DB/WB=117/94). This is more aggressive than the Nyle DH schedule,  but I'm only seeing 1% to 1/2% MC loss per day.  Max drying rate on 8/4 walnut is 3.3% MC per day.
Therefore, today I set 120 and lowered RH to 28  (DB/WB=120/88).
 
I know this is more of an art than science. Very interested in folks personal derived schedules and experience with 8/4 walnut.  Also doing 5/4 & 8/4 upland white oak next...if you have schedules for that.
 

[scsmith42]

Just an FYI but 8/4 black walnut has a reputation for stalling out when it's around 25% - 30% MC. 

If your MC% readings are taken at the core of the boards, then the good news is that once you are below 25% MC it's all but impossible to damage the lumber by drying too quickly.

At this drying stage my L200 is set for 120F and 40% RH.  I keep track of both core and shell MC%.  

Towards the end of the kiln run if the RH% inside the kiln drops below 35%, I use a high pressure fogger to add humidity to keep it there, so as to minimize the delta between core and shell MC%.

[GBar]

Thanks for the tips.  I know my Brisket stalls at 160, but was not aware my black walnut would stall.
Attached is my kiln log showing the numbers.  it is below 25% MC currently.

I used Lignomat PK probes.  they are at the center of the 8/4.  There is some 6/4 in the load as well...those are the last 4 probe sets.  6/4 was sawn previously and air dried for almost a year.  so it started in kiln at 15 to 17 MC.

The log number are averages from the data logger.  The current temp range is 114 to 122 based on Dehumidifier cycling.  RH is ranging 25.4% to 39 percent based on temp swings.  The cycles of DH on/off are 16 min on and 22 min off, based on the high or low triggers.

[scsmith42]

Looks like you're losing around 1/2% MC per day in the core.  You can try to speed it up by increasing the temp but personally I tend to take a conservative approach and would probably keep things about where they are.

[K-Guy]

The DH schedules for the L200 and L53 are at a 120 max due to the refrigerant pressures, if they get to high you pop the high pressure switch trips it off to protect the compressor. Our bigger kilns are built to operate at temps up to 160.

[blackhawk]

I think that you are pretty good at 120F and 28% RH.  That is right at a 5% EMC.  The FPL-118 has the schedule for 8/4 walnut go down to 4% EMC once the lumber hits 25%.  That is a little aggressive for me though.  I usually never set my kiln EMC below 5%.  I usually run 8/4 walnut at 120F and 30% RH at this point.  You are doing the right thing by monitoring the moisture loss per day.

[GBar]

The DH schedules for the L200 and L53 are at a 120 max due to the refrigerant pressures, if they get to high you pop the high pressure switch trips it off to protect the compressor. Our bigger kilns are built to operate at temps up to 160.

K-Guy,
Thanks much for the response.  I had assumed the Nyle DH schedules temperature settings were based on some sort of limits, but was not aware it was refrigerant pressures.  I assumed it was based on the ability to remove humidity as the hotter it gets, DH units work harder to remove the moisture, otherwise less efficient.

What was interesting in your schedules was Group 4 final stage is more agressive than the final settings in Group 3.  Both at 120 degree F, Group 4 calcualted RH is 39.9% (6.8 EMC) whereas Group 3 calculated RH is 45% (7.5 EMC).  It did not seem to be intuitive.  I assume a typo, but the depression numbers of 98 and 95 are consistent across the Normal and the Alternate schedules.  It is clear Group 4 should be less agressive above FSP and is so published, but was a mystery why it was more agressive in final stage...and the final step starts at a higher MC 30 verses 25 in Group 3.

If we assume FPL-118, page 95 to be a reasonable thesis, it follows that the quoted citations are true:
    " A dehumidification (DH) dryer uses basically the same schedule as does a steam kiln. The difference between the two drying methods is in the hardware, not the way in which the wood is dried."
and note on bottom of table 8.3:
    "Note: that MC and RH values are identical in each schedule. The DH dry-bulb temperature is determined on the basis of experience with the equipment. The DH wet-bulb temperature is calculated using the dry-bulb temperature and RH. Because the dry-bulb temperature is lower in the DH schedule, it may be possible to lower RH slightly, taking advantage of the stronger wood and offsetting the slower drying that occurs at lower temperatures. Tc = [TF - 32]/1.8. "

Respectfully, It does seem there is a mistake in those Group 4 & 3 schedules. What Blackhawk cited of 28% RH (5% EMC) or 30% RH (5.4 EMC) for both Groups seems more reasonable.

I do realize that due to the natural variability of wood, published documents are guidelines and not Gospel...that goes for FPL-118 as well. 

Respectfully, interested in your insights to what is published in your manuals as compared to FPL-118.

[K-Guy]

The original owner set them up that way and I'm not sure but at that point it makes little difference. The wet bulb set point is high like that purely for efficiency, the compressor will run less but longer compared to a lower setting and at that point you are waiting on the wood.

[GBar]

The original owner set them up that way and I'm not sure but at that point it makes little difference. The wet bulb set point is high like that purely for efficiency, the compressor will run less but longer compared to a lower setting and at that point you are waiting on the wood.

Thanks for the insight.  much appreciated.

[K-Guy]

What is in  FPL-118 is correct for high temperature drying but our schedules are setup for the maximum temperature of the L53/L200 kilns of 120°F.