Converting kiln schedules to lower temps?

[FeltzE]

I don't normally try to dry hardwoods straight off the sawmill, I normally air dry until at or below 30% MC.  But I have a requirement to dry cherry straight off the saw.

Is it appropriate to modify or convert the wet bulb depression to a lower temperature? 

Example: Table T8-B4* Cherry 4/4  MC above 35%   130DB, depression 7, 123WB. My calculation is that the Relative Humidity is 80.8% for that situation.

Can I simply use 120DB, and a 81% RH to achieve the same drying goal?  The goal being not to dry too quickly nor too slowly...

At 20% MC table T8-B4 calls for 160DB, depression 40, 120WB.  I calculate that to be a relative humidity of 30.6

I'm limited to 120 on the DH kiln. So shift the chart to 120DB with a 31%RH


Second question/note/statement ???, I have noted that the wet bulb depression itself probably shouldn't be directly applied. A 160 DB with 40 depression = 30.6RH... translate 120DB with a 40 depression = 16%RH, however 120 with 30 deg depression is closser to the 30%RH


Translation to RH seems safer to me... now that is said, I'm sure someone has a complete set of references that probably work better for this

* Table T8-B4 is refered to from http://www.fpl.fs.fed.us/documnts/usda/ah188/chapter07.pdf
DB - Dry bulb temperature
WB - Wet bulb temperature
MC- Moisture content of wood
RH - Relative humidity
Depression - what I get looking at what I write

[jimF]

You want the same driving force, which is EMC.  So you want to maintain the same EMC which is ~14.5%.  And at 120F a wetbulb of 110F will give you 14.5% EMC.  The difference between using EMC's and RH's is not much at room temperature but at higher temperatures the difference becomes larger.

[GeneWengert-WoodDoc]

Jim is correct about the difference being important at high temperatures and low RH values.  However, because the lumber is dry when you will have those conditions, it really will not make any important difference in quality or drying time.

However, at high MCs the settings using EMC and RH will be about the same.

I  Prefer using the same RH when adapting schedules for DH, as at high MCs, RH is the driving force...that is, the moisture near or at the surface for a wet piece is evaporated based on a linear function of (100 -RH).  We are evaporating free water mainly.

Now, because a cooler temperature will slow drying, you should probably drop the RH a little below the suggested values to get a more reasonable dying rate.  However, remember that shrinkage will increase when the RH is lower (or EMC).  So, dropping the RH or EMC is ok so long as it is to too big of a drop.

[FeltzE]

Thanks, I'll continue to convert the WB depression to RH then apply the RH to the lower kiln temps.  I am limited to 120 in the DH Kiln so I can't take advantage of the higher temp faster rates.

My biggest concdern is drying too fast causing cracking or honeycomb.