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kiln schedules

Started by JustinW_NZ, August 01, 2016, 08:08:54 AM

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JustinW_NZ

Hi all

Hey does anyone have a sample list of kiln drying schedules for different species in a DH kiln?

I'm running a Ebac unit in a 20foot refer container and seem to be getting good results, but perhaps I could be cranking it through faster?
So any guides for hardwood in these arrangements would be great  :P

Cheers
Justin
Gear I run;
Woodmizer LT40 Super, Treefarmer C4D, 10ton wheel loader.

longtime lurker

What's your current schedules look like? Are you giving it some air drying first or does it go into the kiln before the leaves have turned brown?
What species are we talking anyway?

Aside from that, here's what I do:
I air dry first, in an ideal world I'd never kiln anything over 20% MC, but sometimes I have to play real kiln operators and get it out the door sooner. Personally the drier it is before entering the kiln the happier I am.

If I'm playing real kilns, I tend to use the  "Dry Kiln Schedules For Commercial woods"  as published by the US forestry service. There's enough schedules there for local stuff that I can find something close enough in terms of drying characteristics to base a schedule off. By something close I look at density, shrinkage, propensity to collapse, general bastardry of wood etc etc.
I take good notes of what schedule I use and any tweaks I make to it for future reference, though mostly I don't mess much if something is working. A full kiln charge of timber is worth more then a day shaved off the schedule IMHO.

If I'm air drying I worry less about kiln schedule and more about degrade as it air dries: some species I put them in the breeze, some get put under fans to accelerate the process, some get wrapped in wet hessian to try and slow dying down. (I've got three species that 6 months to get  them to 20% is verging on " too fast" in terms of limiting collapse)

Once below 20 % MC, the only limit is how hot the kiln gets, you can run it pretty hard. Because most of my timber is at that point before entering the kiln I fitted an auxiliary (wood fired) heat exchanger loop to our DH unit: the only way I could speed up kiln throughput was by bringing the charge up to working temperature faster.
The quickest way to make a million dollars with a sawmill is to start with two million.

JustinW_NZ

Thanks for that input LL

I too am trying to air dry as we have some very nasty wood here for all the defects.
It usually takes 10 months to get into the 20% range.

My biggest thing was I think once its air dry can I easy just open up the temp and let it rip?
Which I guess the answer is yes!

Our main wood species we deal with are;
NZ Native beech - Hard, red and silver. (silver beech easy) - Den posted some pictures of samples he first tried to dry of NZ red beech and they say a lot  :D
Eucalptus - mostly always prone to collapse and horrible problems.
We use the breeze carefully in summer otherwise we can cause a lot of problems.

Our softwoods here by and large are very easy and simple to deal with.

Cheers
Justin
Gear I run;
Woodmizer LT40 Super, Treefarmer C4D, 10ton wheel loader.

longtime lurker

Theres basicly three ways to deal with collapse right?

(I used to think there was only two, but Den's bringing me around :D)

Steam recondition the timber, or dont let it collapse in the first place.

Yanno... I've got no real issue with steam reconditioning. It takes a lot of the guesswork out of it: just strip it out, stack it in the yard and let it do what it wants then bring it back in a steam chamber. 95% of it will come good, though theres still some internal honeycomb to contend with but thats a non issue in terms of market expectations. Theres a reason the bigger mills go that way and its all about how long you want your money tied up in timber that cant go to market because its not on spec for moisture content.

Or dry it that slow with that much care that little or no collapse occurs. You still loose some to degrade though done right its surface issue that planes out rather then the full internal honeycomb stuff. Mostly. I got some Caledonian Oak thats making my life a misery right now that even a steam chamber would struggle to pop back out, I couldnt saw timber that out of square if I tried to, maybe I need to find a market for one piece washboards or something:



 

 


We go the latter route though it means having $ tied up for a year in air drying timber, plus the headaches of careful storage. I'd like one of Den's kilns just for the cashflow aspects of it: be nice not to buy logs this week to sell in 12 months time, and not be having to guess the market that far ahead in terms of sizing etc etc.

Anyway, if you can get it under 20% in reasonable order you should be able to run the kiln wide open. With regard to drying degrade at that point the only other thing that can happen that hasnt yet is if you over dry it. Just be a bit careful on the ramp up in thicker timber... you want the charge to be heated through before she starts pulling moisture out otherwise you can get some case hardening resistance. Thats why I retrofitted the aux heating system, I was drying the shell before the cores had got hot on the thicker cross section pieces.
The quickest way to make a million dollars with a sawmill is to start with two million.

JustinW_NZ

Thanks for sharing your experience with this.

I'm going to add kind of a third way to deal with collapse...
Cut the wood to stop it from happening.

Now the reason I say this is because I have found if I quartersaw the logs (our main market is flooring anyway, so I want it to be 100% stable) then I get MUCH less problem.
It is probably around the fact the wood gives up moisture slower so it self regulates to a degree?

Now I know this is a pain to do BUT for the troublesome woods I do not hesitate any more - I quatersaw them and leave them in whatever width, so whatever crook happens I deal with on the edger once dried.

The only other guys cutting volume of one of the NZ native timbers have 7 vacuum kilns so use technology to get around the problem in the wood.
We just use the above method and no one can fault the wood coming out thus far.
Its only the stacks I leave out in direct sunlight have a few boards on top that go a little wayward...

That's my experience anyway.

steam reconditioning otherwise is a great simple and tested way to go otherwise in my view.

Cheers
Justin
Gear I run;
Woodmizer LT40 Super, Treefarmer C4D, 10ton wheel loader.

longtime lurker

Interestingly enough I was looking for another article in my "library" (ie collection of old apple boxes full of miscellaneous industry related papers I've downloaded to read over the years because screens give me headaches) and pulled a related one out:

Improving seasoned hardwood timber quality : with particular reference to collapse       Innes T.C. 1996
Link here:
http://eprints.utas.edu.au/20796/

but I warn you its a big download and really heavy reading most of which went waaaay over my head. But I got the high points.

From what I gather theres a correlation between collapse and ambient temperature, with lower temperatures being good and higher temps (during initial drying be it kiln or air) increasing the severity of collapse. 28° Celcius seemed to be a maximum temperature to avoid collapse in E. regnans.
(Not much good to me - thats a cold winters day here, but maybe something to play with there: severity of collapse winter vs summer should be something you'd notice if you knew to look for it I'd reckon, based on the "real world sawmillers" scale of "if I cant see the difference by eye then theres not enough difference to matter" )

Anyway, the low temp thing is probably part of why the vac kilns are doing so well.

Another interesting tidbit from that paper was this:

"The high AE following the power failures highlighted the care which is needed in
restarting a kiln under conventional (setpoint) control after a power failure. Oliver
(1995c) gives a qualitative description of what is likely to occur during a power
failure and fan restart, and makes some recommendations for a restart procedure. This
is summarised below.

When the fan cuts out, it takes a few minutes for the air to become still. After this occurs,
the wet wood and still air in the stack move toward equilibrium, with the result that the
wood surface moisture concentration rises. As soon as the fans are started, the air in
contact with the wood rapidly increases in moisture concentration. There is thus an
increase in moisture concentration gradient across the boundary layer, and hence more
rapid drying of the surface of the wood. This results in an increase in moisture
concentration gradient within the wood near the surface, which can easily lead to surface
checking.
If the dry bulb temperature of the air in the plenum reduces below that of the air in the
stack, then this effect does not occur. However, if the dry bulb temperature in the plenum
becomes significantly lower than the wet bulb temperature of the air in the stack (which is
approximately equal to the wood temperature), then restarting the fans will result in a "sea
breeze" effect which is likely to result in checking. The restarting technique
recommended by Oliver (1995c) is to first switch on the water sprays (assuming that kiln
humidification is by water spray in the plenum at entry to the stack). The fans should then
be started slowly, either by using the speed controller if it provides sufficiently low fan
speed, or by running the fans for short bursts only every few minutes until the moisture
concentration and temperature gradients have been restored to their values before the
shutdown. "

It's something to bear in mind when the power goes out... I just wish I didnt have to trawl through 200 pages of hard reading sometimes to find this stuff.
(And I wonder how that relates to the on/off cycling of passive solar kilns? Thats a Gene question if ever there was)


The quickest way to make a million dollars with a sawmill is to start with two million.

JustinW_NZ

very interesting.

Any funnily enough I have less problems in winter than the peak of summer.
No numbers to back it, just the NZ calibrated eyeball  ;D

The kiln restart part does make sense but I do wonder how much it applies to a DH kiln over old much higer temp kilns? (yes its all about the RH I guess)

I would love to visit and look at your library one day, sounds perfect  :P

Cheers
Justin
Gear I run;
Woodmizer LT40 Super, Treefarmer C4D, 10ton wheel loader.

longtime lurker

Actually my "library"sucks. Other then actual books on shelves it is literally a heap of articles downloaded and printed over the years that are stored in a couple of boxes. I download to read because I find it easier if I can scribble notes in margins etc, but its a nightmare to find anything in there, and half of its probably just junk that should be thrown out.

A great place to start with articles of interest is the Forest and Wood Products Australia website: they do a fair bit of research into local species some of which obviously cross over to you guys in terms of species and general industry practice. Best thing about their research is its funded by the industry, so its mostly written to be understood by industry participants.
As usual with those kinda things, its aimed at the big end of town and its a matter of taking what you can from it.

This is a link to research and development page for processing - they've got a couple of other R&D pages accessible through the links at the top.

http://www.fwpa.com.au/rd-and-e/processing.html

The quickest way to make a million dollars with a sawmill is to start with two million.

GeneWengert-WoodDoc

When we wrote Kiln Schedules for Commercial Wood: Temperate and Tropical, we did include schedules for all Australian and NZ woods we could find. The book is on line
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr57.pdf

Remember that the DB temperatures are maximum.  The moisture contents are for the wetter pieces.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

longtime lurker

The only problem I have with the published schedules - and I'd say I'm not alone in this - is that they were published in 1988. Kiln technology has changed somewhat since then - big kilns are still mostly conventional (though the control mechanisms have become more precise) and the information is as good as ever for them, but medium and small installations have swung away from steam to embrace DH, solar (either passive or hybrid), vacuum, RF, and probably a few other methods as well.

Which is good, innovation is a wonderful thing. What I see happening in Australia is that aside from the major softwood players with enough scale to justify waste fired boiler heat for large scale conventional's, theres a swing away from low temp conventional to solar hybrids in the 20-40000 BF size for the medium hardwood guys, and to get throughput you just install more of them.
At the small end since we installed our DH kiln here I've had every operator of a conventional kiln in 300 miles front to inspect: they might chuckle at my haphazard chamber etc etc... but no-one laughs at the quality of the product, the low levels of degrade in species regarded as difficult to impossible, and they get real quiet when they see the numbers in $/m3 to operate it.

While not insurmountable, there is a learning curve for operating in the low/ medium temperature range with repeatable and cost effective results, and (here at least) kiln schedules are as closely guarded as the recipe for Grandma's Pumpkin Scones.

I guess what I'm saying is that 30 years after publication of Dry Kiln Schedules, it would be nice if they maybe expanded the one mostly blank page that discussed changing schedules to suit various other now common kiln types.
The quickest way to make a million dollars with a sawmill is to start with two million.

GeneWengert-WoodDoc

I wrote the section in the book on how to convert the schedules to DH.  The simple technique is still used today.  So, DH schedules are covered 100%.

I also put in low and medium temperature schedules, and not just higher temperature steam kiln schedules.  So, the information in the schedule book is still useful today for almost every kiln operation and equipment...just make sure to use the appropriate sections or chapters for your equipment.

Note that all schedules are a bit conservative in order to assure high quality in all types of equipment and operations.  With excellent operations, slight acceleration is possible without losing quality.  We do not claim that these are the only schedules for lumber.  Rather, they were the schedules used commercially and they were both high quality and economically efficient, so one can use them even today for safe dryng at a competitive cost.  The theory of dryng lumber has not changed in 75 plus years.

Incidentally, I personally traveled to QLD and elsewhere, including NZ, to get schedules and talk to kiln operators.

I also developed the daily drying rate concept to fine tune kiln conditions.

RF and vacuum do not use a schedule of this sort.  The book is for hot air drying systems.

In Drying Hardwood Lumber, written in 2000, the only change in the schedules was to suggest use of 160 F as the top temperature for 4/4 through 6/4.  It had been 180 F, although 8/4 and thicker had used 160 F already since 1950. This change reflected the fact that we were drying more lower grade than in the past.  I also suggested that the low relative humidity at the end be raised slightly to avoid over-drying, which is more likely with the modern, powerful kilns.

The schedules in the Kiln Schedules for Commercial Wood book have been used in the US to dry over 30 billion BF of lumber every year, even today.  The schedules have been used throughout the world for many species.  The only big difference today is for the pine progressive kilns.  Otherwise, they are used for high quality drying in close to 100% of all hot air drying systems.  Certainly, a few operations have tweaked the schedules to more closely fit their equipment.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

GeneWengert-WoodDoc

If you have this kiln schedule book mentioned, there is one "made up" species on page 146.  Can you find it?  I guess we were younger and a bit crazy perhaps.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

JustinW_NZ

Gene, love the idea of throwing in a made up species in there to see whos 'awake'  :D

And LL -


A shot of silver beech all quatersawn going into the kiln avg moisture about 26% - will see how it comes out.

good discussion, I will try and work the DH calc across when I have time, I really need to print that manual out.

Cheers
Justin
Gear I run;
Woodmizer LT40 Super, Treefarmer C4D, 10ton wheel loader.

longtime lurker

Quote from: GeneWengert-WoodDoc on August 06, 2016, 06:45:56 AM
I wrote the section in the book on how to convert the schedules to DH.  The simple technique is still used today.  So, DH schedules are covered 100%.

I also put in low and medium temperature schedules, and not just higher temperature steam kiln schedules.  So, the information in the schedule book is still useful today for almost every kiln operation and equipment...just make sure to use the appropriate sections or chapters for your equipment.

Note that all schedules are a bit conservative in order to assure high quality in all types of equipment and operations.  With excellent operations, slight acceleration is possible without losing quality.  We do not claim that these are the only schedules for lumber.  Rather, they were the schedules used commercially and they were both high quality and economically efficient, so one can use them even today for safe dryng at a competitive cost.  The theory of dryng lumber has not changed in 75 plus years.

Incidentally, I personally traveled to QLD and elsewhere, including NZ, to get schedules and talk to kiln operators.

I also developed the daily drying rate concept to fine tune kiln conditions.

RF and vacuum do not use a schedule of this sort.  The book is for hot air drying systems.

In Drying Hardwood Lumber, written in 2000, the only change in the schedules was to suggest use of 160 F as the top temperature for 4/4 through 6/4.  It had been 180 F, although 8/4 and thicker had used 160 F already since 1950. This change reflected the fact that we were drying more lower grade than in the past.  I also suggested that the low relative humidity at the end be raised slightly to avoid over-drying, which is more likely with the modern, powerful kilns.

The schedules in the Kiln Schedules for Commercial Wood book have been used in the US to dry over 30 billion BF of lumber every year, even today.  The schedules have been used throughout the world for many species.  The only big difference today is for the pine progressive kilns.  Otherwise, they are used for high quality drying in close to 100% of all hot air drying systems.  Certainly, a few operations have tweaked the schedules to more closely fit their equipment.

Its obvious that I'm reading the book wrong Gene, or havent looked outside the little bit of it I use to understand the bigger picture. To be honest I've never downloaded and printed the whole thing out either, just the bits that I needed to use, and those are  consulted regularly.

So I open the book to the correct page for Asian and Oceanian species, pick a species thats listed thats close in drying characteristcs to what I'm trying to dry, and it directs me to a schedule. Most of what we do is covered under T3-C2, but on some of the softwoods we go hotter, Kauri points to T10-D5S.

I open the page for the correct schedule and it gives it to me. Mostly I follow the schedule pretty closely until we hit 150°F dry bulb at which point  I'm maxed out for temperature so I hold on 150 and keep increasing my wet bulb depression to final. It works, I'm happy

I've never seen anything on low temperature schedules much beyond start colder and follow the steps. Which bit am I not reading that I should be?

And thanks BTW - I consult that manual pretty much once a week or more. I'd hate to be trying to run a kiln without it, you guys did a truly great thing in writing it.

The quickest way to make a million dollars with a sawmill is to start with two million.

GeneWengert-WoodDoc

Run as hot as you can up to the maximum in the schedule.  Use the same RH, or EMC, as in the "normal" schedule.  Very simple, I do believe.

Using 150 or 160 maximum is very good for hardwoods and many softwoods.  All air DB temperatures are maximum suggested.

Some low temperature schedules include 1,2,3, 117.  We could invent lots of them, but we printed only ones that had been used commercially.

Quiz
1.  What letter does almost every schedule start with?
2.  What does the "s" means at the end?
3.  What does the first number mean?  A bigger number means...?
4.  What does the second letter mean or designate...hint is something to do with MC?
5.  The last number if small means the wood is prone to developing _______?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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