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Case Hardening?

Started by Paul_H, May 19, 2004, 10:02:36 PM

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Paul_H

I have a couple of questions regarding VG Fir and a DH kiln.

Does air drying the lumber for a month or so,help in relieving tension in the wood and prevent case hardening?

Is there a cheap,simple and effective way to add steam to the kiln at the end of the run to de-stress the lumber?

I have dried 2500 bf of 4/4 VG fir for a customer,down to 7%.It was fresh off of the mill and was stacked in tight piles(not stickered) I was under the impression that it should be air dried first.

When we planed up his boards today,there was chip out on the side cutting against the grain.He said that is normal but I don't remember having that problem when I ran my floor(VG)

The difference is,I stacked and air dried mine for at least a month before kiln drying.

Is there something to this?
Science isn't meant to be trusted it's to be tested

Kedwards

case hardening is the result of not equalizing and conditioning at the end of a load is typically a real problem with Oak. I equalize and condition all my wood. I dont use a DH kiln but I close down the venting and add water to the floor and let it steam for 24 hours . I equalize the MC% first otherwise you cant condition it later. I go to target MC - 2% for the driest board. On the conditioning I add water back to the kiln(5-10gallons)  to create steam and relieve the drying stresses. I target the final MC at target +1%(you will pick up some moisture in this process) For 4/4 lumber this can take up to 18 hours. It is also important to let it cool gradually to eliminate moisture gradients.
His thoughts tumbled in his head, making and breaking alliances like socks in a dryer without cling free

Don_Lewis

It isn't necessary to air dry first, but if you air dry, you are less apt to have to stress relieve . The benefits of drying from green outweigh the small hassle of having to do stress relief, but many, many people air dry to some extent. It sounds like you over dried some of your wood, hence the tearout..

Paul_H

Thanks Kedwards and Don for your help.

Don,
I am a little confused now.The lumber was between 7-8% when it came out of the kiln.I am often asked for an average of 7% for the D-Fir that I dry for people.

There were no pieces below 7% in this load.

             Is that still too dry?
Science isn't meant to be trusted it's to be tested

Paul_H

I phoned Nyle and talked to Don for a little while and I think I have a handle on the conditioning now.He went over the different methods of adding moisture into the chamber.

Thank you Don for your patience ;D
Science isn't meant to be trusted it's to be tested

Frank_Pender

Paul, for the methodology that I am using, I find that letting the lunmber airdry for at least 30 on the Fir and 60 plus on the hardwoods has produced great results for me.  I know it takes some time and all, but that seems to outwiegh the results. ;D
Frank Pender

Den Socling

Paul,

Did Don say that the shell of your lumber was probably well below 7%? What were his suggested methods for getting water back into the shell while using a Nyle?

Den

Paul_H

Frank,

If I had my druthers,I druther air dry too but the customer is on a tight time line,so I will have to rig up a system for conditioning.

Den,
Yes ,Don figured that the shell was well below 7%.I have been using a pinless Electrophysics moisture meter but I can see the need to start using a pin type,so that I can monitor the boards a little better.

Don told me that he knows of someone that adds water directly to the kiln floor,and another method that involves a couple of electric steamers.

The best sounding method was a small misting nozzle that nurseries use for spraying plants.They come with different size orifices that deliver a set amount of gal/hr.I believe that Nyle has these stainless steel nozzles available.

In the short term,I cranked the kiln temp up to 165 F and poured 5 gals of water out on the floor over a few hours(very hot,burns face)

The kiln temp is now 165 and the RH is just over 50%.
I don't know if that is correct or not.

Any input would be appreciated.
Science isn't meant to be trusted it's to be tested

Den Socling

Paul,

I don't know how you can condition in a DH kiln but I know a little about conventional kilns. You need to do it fast because you want to raise the MC of the over-dried surface, only. If you spend too much time trying to condition, you'll raise the MC.

To condition a conventional kiln charge, you heat with spray. You spray until the wood is 170, 180 or even 190'F. RH is at or near 100%. This goes on for 4, 8 or maybe 12 hours. The operator needs to 'learn' the kiln to know how long. I don't see how cold water misting could do much but Nyle must have seen some benefit.

Den

Paul_H

Den,

I don't know much about it either and I hope Al and Brian and a lot of others jump right in to the discussion

I didn't notice any significant drop in temperature when I added the water to the kiln.I'm guessing that with the amount of heat stored in the kiln and the boards,that 6 gallons or so added by a fine mist over a few hours would heat up very quickly.

I've been reading up on as much info on the subject as I can find but for me,nothing beats firsthand experience spelled out in laymens terms.None of the information I have read is specific to DH kilns :-/

Science isn't meant to be trusted it's to be tested

kilndry

Cold water conditioning (when done properly) allows you to rapidly raise the wet bulb temperature without raising the dry bulb temperature (because you are not adding any heat energy) so you can achieve the necessary depression without "chasing" the dry bulb. Conditioning is then possible at a lower db temperature and is a faster process. When drying temperature sensitive woods like eastern white pine, you can condition at a lower temp and thus reduce the chance of brown stain.

The key to cold water conditioning is getting a small enough water droplet to avoid condensing on kiln surfaces or the lumber, small enough to allow for rapid absorption into the air and/or wood. Proper nozzle size and adequate water pressure are important.

Doesn't matter what type of kiln it is, as long as the initial db temp is high enough. When the cold water enters the kiln airstream, it may lower the db and cause the heating system to come on, but it will not force the dry bulb over the setpoint, as live steam can do.

Throwing water onto the warm floor will work, you just have to wait longer for the water to evaporate and raise the rh in the kiln.


Paul_H

Welcome to the forum kilndry and thanks for the response!

I went over and checked the kiln a little while ago,and the temp was holding at 165 F,and 60% RH.From the index I have been using,it seems that the EMC would be just under 8% MC if left for a time.(I'm more asking, than telling ::))

I shut the heat off and will let the kiln coast down to around 110 F before I open up the doors to unload.

Sure hope it works out :)
Science isn't meant to be trusted it's to be tested

ElectricAl

Paul_H,

Kilndry brings good info about cold water mist.


Basically when converting the mist to a vapor ( to raise the RH% ) you loose temperature. It takes heat to do the vapor conversion.
The faster you can get to your target RH% or Depression and maintain your DB temp in the conditioning cycle the better off you are.  

Our kiln is a hot water conventional style using Wet/Dry bulbs.
We added 4 - 8' electric base board heaters from Northland Kiln to help maintain a constant DB temp.  Each heater is 2500 Watts or 10.9 amps @ 240v .  We also use hot water mist to minimize DB temp drop.  

It takes 2.5 gallons (20 lb) of water per 1000 feet of lumber for conditioning, but additional water will be consumed by stickers, blusters, and condensation.

It takes 1000 BTU's to evaporate one pound of water, so you'll need approximately 3000 BTU's per 1000' just to convert the water mist to vapor.

These are just rough numbers and every kiln requires some trial and perfection ;D
Linda and I custom saw NHLA Grade Lumber, do retail sales, and provide Kiln Services full time.

Den Socling

Now I get it. Its called cold water mist but you are misting only as fast as the mist can be evaporated without pulling down the DB. Wonder why this isn't used more often?

ElectricAl

Den,

It's hard to make mist small enough without a high pressure pump ::) and then there is the time factor to convert a droplet to vaper. You tend to get a wet floor even misting in front of the fans :-/

There are mini steam generators on the market that may fill the needs of the 5000' and smaller kilns.

Unsure of a price, maybe someone can help us out.

A mini steam generator really would not affect the DB temp much.
However a 2" live steam line @ 100psi might ;D

Linda and I custom saw NHLA Grade Lumber, do retail sales, and provide Kiln Services full time.

Den Socling

I may have mentioned this before but I use a little steam boiler to heat my vac kiln. At the moment, I'm drying 8/4 White Oak and 4/4 Red Beech. I use shots of steam to keep the RH up. This little boiler was used to heat a house. I picked it up from a demolition company that runs a salvage yard. It cost $300.  ;D

When I steam green wood to change the color of the sapwood, it works pretty hard. But normally, she sips fuel.

ElectricAl

Den,

I have read a couple of articles about White Oak and Vacuum Kilns not getting along :-/

But you seem to have a handle on it. Is it the type of machine that makes the difference?


Al
Linda and I custom saw NHLA Grade Lumber, do retail sales, and provide Kiln Services full time.

Den Socling

Al,

I've been telling people for years that you can't dry white oak in a vac kiln. I was wrong, again.  :D But it does take some special modifications and methods.

Den

jimF

The reason the temperature does not drop while using cold water mist is that when the vapor is absobed by the wood it is in essense condensing and giving off the heat the vapor took to vaporize.  In fact because it is forming chemical bonds to the wood it is giving off more heat than it takes to vaporize.  This is why when using steam the temperature keeps rising and you end up chasing the dry bulb.  Bob Little applied this idea  that Hart reported on a number of years ago.

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