Understanding kiln schedules

[JoshNZ]

If I were starting with say 27% MC lumber and reading this kiln schedule correctly, is it saying to set the dry bulb and wet bulb temp to 54.5C and 35C, and maintain this setting until the measured lumber MC reaches 25? Then change to the settings given in step 6?

And.. if I didn't have a dry bulb wet bulb setup but a temp and RH value, would I be using 54.5C and 28% RH?

And.. the equilibrium moisture content, is this saying what MC the lumber would reach if it were left in the environment with these settings indefinitely? (How is this a useful figure?)

This is the best sense I could make of it but please correct me where I'm wrong



 

[WDH]

What type of kiln is that schedule for?

[K-Guy]

What type of kiln is that schedule for?

That a conventional kiln schedule. In North America it is for 8/4 American Elm.

[JoshNZ]

Yep American elm. I didn't think you could control rh in a conventional kiln?

Are there dh kiln schedules around?
Edit: I did read in that same document how the schedules can be modified to suit low temp DH kilns. Still, is my thinking in following the schedule correct? And what is the equilibrium moisture content figure? How does it differ from the MC percentage figure?

[K-Guy]

@JoshNZ 
I believe you have an Ebac unit and they should be able to help you.

[rusticretreater]

A difficult subject.

The EMC and RH are for equalizing and conditioning.  Most of the US has 8%+- MC wood, gulf coast states like Ala and Miss are in the 11% MC range.  Most southwest are 3%-6%MC range.  You can find a map online for that.

First, you dry according to the left hand schedule until the desired MC is reached.  Then you test the stack for variances in MC.  Ends are usually drier.  The Virginia Forestry seminar teaches taking sample pieces out and cutting them to test for internal MC.  To equalize the MC it is recommended you dry to 2% more than your desired MC number so most wood goes to 6%.

So you remove moisture to a certain percentage and then put it back to equalize it.  The specific RH in the kiln meters how much moisture goes back into the wood for you to a certain extent.

Using the chart, you set the temp at the 8% mc reading(160 degrees dry) and from the center chart EMC 8.2% you set the RH in the kiln to 52%.  It sits at these settings until tests show uniform MC(which is now EMC) throughout the wood. Basically, if the wood surface is 6%MC and the inside is 8%, you only need to make the outside wood absorb 2% from the air. Then you move to the conditioning phase.  This makes the wood stable and it will hold its final dimensions when used in the proper section of the country.

Great doc on the subject with directions and every table you will ever need. Professional kilns, steam kilns, solar kilns are all covered. It is technical though.


https://www.fpl.fs.fed.us/documnts/usda/ah188/chapter07.pdf

[GeneWengert-WoodDoc]

The temperature is the maximum.  You can start cooler for six hours if you want or if your equipment starts working at 90 F.

Whatever you do, start'-up is really important.  During heating and the correct initial step for the first 12 to 24 hours, use a kiln EMC at or maybe 1% Drier than the EMC that the lumber has been dried at.  (Measure the surface MC to get an estimate of the average past EMC.). 

Never let the kiln EMC go below 5.0% EMC.  The drier values are no longer appropriate.

Your equipment may work at lower temperatures, which is ok.  However, getting the correct EMC is extremely important.

[YellowHammer]

The kiln schedules are a maximum setting, generally, you are safe for anything under that.  

Yes, the EMC is the moisture value the wood will eventually get, although it is asymptotic and will slow down considerably at the end.

The other thing is that you should know th maximum allowable moisture removal rates of each species and thickness you are drying and check it every day so you will get a feel for how the wood is behaving under that step of the schedule.  

The kiln schedules are really "suggestions" you can mix and match them, adjust dry and wet bulb settings, etc, as long as you stay as cool as possible, don't violate the max allowable drying rate, and don't get the depression too close together to cause sticker stain.  The goal is to stay just shy of the max allowable rate to minimize kiln time, most published kiln schedules are pretty "sloppy" and when I'm paying for the electricity, I don't like letting the wood coasting in the kiln.  

[JoshNZ]

Yup all making sense, thanks a bunch.

I don't want the wood coasting along either, the kiln is the only thing on the metre board at the moment, other than the odd power tool charging on the building site. Definitely racking up the units

[GeneWengert-WoodDoc]

I like your last comment about "coating."  The main purpose of running a kiln is to make money.  This implies that the operation is making the highest quality lumber (flat, check-free, correct color, correct MC) in the shortest time possible, using as little energy as possible.  In other words, some sort of overall evaluation program, measuring more than final MC, is needed for every load.  Stated another way, you need to be perfect and get better.

[wkf94025]

...you need to be perfect and get better.

Love it.  

I'm starting on software architecture for automation of my DH-assisted solar kilns, and look forward to much more fine-grained control of the climate and progression inside each kiln.  

For now, my wet finger in the wind is how many hours it takes for the DH drain line to fill up a 5gal bucket.  Similarly, water drops per second coming out of the kiln and dripping into the 5 gal bucket.

[GeneWengert-WoodDoc]

Did you watch the video that I did for NHLA and they are free?  One deals with schedules.

[JoshNZ]

If that was directed towards me, no I haven't watched any videos. Done plenty of reading through the various documents.

It's all making sense now that I'm actually doing it. I took 3 samples from the kiln across two species yesterday and oven dry tested, driest was 9% and the wettest 14%. I'm aiming for 12%. So I've set temp to 110F and the RH to 55%, I had to throw about 5L of water into the floor of the kiln to get it back up to 55%, and I'm sitting at that now with the compressor turned off. From a table I found that those settings produced an EMC of 9.2% which means the driest samples shouldn't get drier, while the wetter creep a bit closer to it, is that correct?

And once the MC range has tightened up a little and I want to condition, I set an EMC of 12% to rehydrate the outter layer of the boards for however long (didn't find any info about this, until it tests at 12% I guess?) And then shut everything off and allow to cool?

[GeneWengert-WoodDoc]

Your humidity discussion is indeed correct.  The surface of some pieces and the ends may come close to 9.2% MC, but the entire piece will likely be, at the lowest, under 10% MC, but not much.  If the 12% value is very important, 2% below the target, or 10.0% EMC is likely better.  Heat will help the wetter ones come down faster.

The term "conditioning" has two meanings worldwide.  In the eastern 2 3 of the contiguous states, condition means adding moisture quickly and at high temperatures 160 to 180 F.  The EMC is usually 4% EMC higher than the target, or 16% in your case.

In contrast, equalizing is setting the lowest MC that you want and the highest MC. In modern schedules with high performance equipment (versus the inferior equipment by today's standards used in the 1940s and 1950s).  Sometimes equalizing is called conditioning.

The schedule presentation of about 1-1/2 hours on NHLA.com, go to education and then webinars.  Free

There is also a concise description of schedule formation in DRYING HARDWOOD LUMBER.