High Temperature Drying

[Craig Jensen]

I am interested in any input people may have on High Temperature drying and it's effects on Hemlock and White Fir.
By high temperature I am speaking of schedules above 220 degrees F.
Does anyone do this?  Better?  Worse?  Tips?

[MichaelM]

Craig,
230 or 240 seems to halve the drying time compared to 180 if the kiln can get to the higher temp easily.  If you push these species too hard you will start to see collapse.  I am not aware of people that have gone beyond about 220.  If I was going to try it, I would start the schedule in the 190 range, then go up in temperature after 12 hours or so.  We have dried hemlock up to 270 in the lab kiln.  Shrinkage increases a little as the temperature goes up.  The wood gets a lot darker too.
Mike M.

[Craig Jensen]

Do you notice a difference if you separate the White Fir from the Hemlock?

[Joe D]

I have no experience running hemlock or white fir but have a lot of experience running southern yellow pine at high temperature.

Whenever doing something new address with caution and know its risks.

If you accept the risks I would think about something like the following using dimension lumber as a first run at it. I assume you have a modern track kiln with adequate heating capacity and airflow.  I would control the dry bulb off of the exiting air side. I would use a conservative dry bulb temperature of around 200F. Remember this is exiting air so your entering air dry bulb would be around 230-240 after the kiln heats up. The wet bulb initially at 180F. As the lumber dries there will be less demand for heat and the dry bulb temperature on the entering side will drop. 

You need to ramp up to these temperatures over let say five or six hours depending upon your kiln heating capability. Don't vent until you get to dry bulb temperature. To dry at high temperatures you need a minimum of 1200 FPM airflow through the lumber, with 1400 FPM being much better.

Good stacking is critical in high temp drying because the atmosphere inside the kiln (the EMC is around 2 to 3%) is extremely dry. In the SE many operations use load weights (concrete & steel) to minimize warpage.

A big trick in high temperature drying is when to pull the charge. Because the process is so fast there is a great deal of MC variability, but if you leave it in too long you risk severely over drying (why we use weights).

Also remember different woods have different characteristics. If I remember correctly from a couple times working with mills in the west, hemlock has wet pockets. So you will probably have to redry some.

So those are my thoughts. Please think about them and then come back with your concerns.

Good luck.

[Craig Jensen]

Thanks Joe and Michael:
A little background -
We run 4 84' double track kilns at 170psi.  We are a northwest studmill (8' and 9' 2x3's 2x4's and 2x6's) and have 3 basic species mixes that we run.
We run DF and Larch (dried separately)  DF on an elevated schedule that reaches 210 at about 14 hours - larch on a high temperature schedule that reaches 240 in about 8 hours.  Both take 12 to 14 hours to be dry.
When we used to dry them together, we could not reach high temperature (above boiling temp), and drying time was about 20 hours.
We run Spruce, Lodgepole, and Alpine fir, (Spruce and Lodgepole together - AF separately)  The Spruce / lodgepole schedule closely mirrors the DF schedule, and the Alpine closely mirrors the Larch schedule.  These all take 16 to 18 hours to dry.  When we ran these species together, we could not reach high temperature, and drying times were 20 to 28 hours.
We run Hemlock and White fir together.  This schedule runs 24 to 30 hours, and tops out at about 220.
All of our schedules are based on exiting air.

I have a theory that species with lots of bound moisture dry better at high temperature, but we don't have the heating capacity to reach high temperature with species that have higher percentages of free moisture like Spruce, Lodgepole, and Douglas Fir.

Am considering lobbying management to find a way to separate Hemlock from White Fir as well, and see if we see teh same reduction in drying times as well as reduction in drying defects if we can follow the same pattern we found with the other two mixes.

Any thoughts?

[Kierman Dimitri]

Mike, In reading your response above I was wondering if during your testing where you halved the drying time going from 180 degrees to 230 or 240 degrees if you may have noted the nearest significant increase in the drying rate say starting at 100 degrees?

[fbushaw]

Craig,
Do you sort your WF in the mill?? Drying your WF with High temp schedules, do you get alot of under dry lumber?

[Craig Jensen]

Hey Francis!  How are ya?

We do not sort at the mill right now.  Currently we get whatever mix comes from the woods Hemlock and White Fir.
This percentage seems to fluctuate from 10% Hemlock up to 35% Hemlock.
The result seems to be lots of wet hemlock and lots of overdry white fir.
Am thinking if we could sort them out and just dry white fir, or just hemlock, we could do different schedules that are specie specific and reduce the overdry white fir.

But there is no room to sort at the mill, would have to be at the logyard in our case...

Craig

[fbushaw]

Craig,
Do you leave your vents closed untill your kiln reaches max temp, or do you let them open earlier?

[Craig Jensen]

Currently we keep the vents closed except for fan reversals through the entire schedule.

[fbushaw]

How do you keep your wetbulb down, and have the right depression? Do have issues with stain?

[Craig Jensen]

Stain is not an issue with studs, does not down grade our product.  However, the stain we do get planes off.

Keeping the moisture in the kiln converts it to vapor in the high temperature environment, which acutally is more effective in transfering the heat to the wood.  So the "humidity" in the kiln ceases to function as humidity and starts being a heat tranfer medium.  We experience a substantial drop in steam load once we exceed about 215 degrees, as we are able to keep the heat we put in the kiln and re-use it.  As we vaporize the moisture in the wood, that steam then continues to heat the rest of the wood, is my theory.  Not sure how that would affect dimensional lumber, but it has not hurt studs at all.  We actually experienced a reduction in drying defect when we raised our temperatures from 190 to 220. 

Craig

[GeorgeCulp]

I'd like to follow up and reemphasize something Joe Denig mentioned above.  Its the role of airflow.

When you increase your airflow it "allows" you to use a higher temperature. What do I mean?

The greater the airflow the tighter the difference will be between entering and exiting air of the layers. At first glance you may think that isn't good because very little heat is being transferred as compared to the much greater temp drop across the layer of lower air flow systems. But in actuality it is far more better - simply because all the lumber on the layer sees a more uniform temperature and thusly dries more at the same rate and therefore gives you a tighter standard deviation and....higher quality lumber.  (If you monitor the driness of your lumber adequately and correctly.)

When airflow is increased (and assuming the max heat output of your heat source is fixed) it will take longer to get to temperature but you will still dry faster.

What do I mean here?  EX - take airflow from 700 ft/min to 1400 ft/min.  As I said above, as the speed of the air increases less heat will be transferred to the wood in a layer. However, since the airflow has increased by 2x then twice as much heated air is circulating through the lumber per unit of time....

...and you will gain far more heat transfer to the wood.  How do you benefit? Primarily two ways - 1. Due to a tighter standard deviation of lumber you can now dry less because you can raise your average MC 2. and because the amount of heat that is being transferred has increased to the much greater volume of being circulated you dry faster.

High air flow in softwood is a win - win propostion.

One thing you will notice and may not like. If your max heat output of your heat plant is normally reached with the lower airflow then with the higher airflow you will take longer (and maybe a lot longer) to heat your dry bulb set point.

And you may not like that. But think about it here - what is the goal, to create hot air or hot lumber?

Be well.  I hope this has helped.

George Culp

[Craig Jensen]

George,
I have an off the wall question.
Is is possible to get to the higher temperatures quicker with a lower air flow, and then increase the airflow once you are up there?
Has anyone done any testing on this they would like to talk about?  (I'm sure we have all done testing we don't want to remember . . )
 

Craig

[GeorgeCulp]

Craig, first off sorry to take so long to reply.

Yes, you can lower the airflow and you will get to temperature in the kiln quicker. However, and depending on how long you wait ti raise the airflow, your dry temp will go down.

I really wish everyone could embrace the difference between getting the air in the kiln up to temperature versus getting the lumber up to temperature.

In essence though you will eventually get the lumber to temperature with lower air flow however it will take a longer total drying time and the quality will be diminished.

[Craig Jensen]

George,
Thanks for your reply. One advantage to time between posts is the opportunity to do a little testing. Your reply agrees with our informal test results.
Thanks again though. Funny how a faceless reply will convince management when I can't.