Steaming question for kiln dryers...?

[Big J]

Quick question regarding kilning.

 

I'm currently reading about how to reduce drying stresses in kiln dried timber and a short period of steaming at the end of the cycle seems to be the best way to address case hardening and achieve a consistent moisture content (no moisture gradient). The problem is that the descriptions and models for steaming apply to commercial kilns with operating temperatures of 100 celcius or more.

 

How could I apply a steaming cycle to a dehumidification kiln. I cannot far exceed 35 degrees without damaging the fans and dehumidifier. Apparently the steaming need only last a short time, raising the relative humidity to the point where equilibrium moisture content of the timber will naturally settle at 10%.

 

Any ideas are hugely appreciated!

[gensetsteve]

earlex wall paper stripper or a unit from a sauna

[Silver Back]

Is it necessary for dehumidifers? They tend to work slower than kilns, part of the problem with a kiln is that they alomost cook the timber to get rid of the moisture, dehum's will draw the moisture out slower. You could also control to some degree the process by the sticks you use - size and spacing

[Big J]

As far as I best understand, the dehumidification process is a gentle one, but drying stresses can still occur, predominantly from case hardening if there is a moisture gradient (say the surface of the board is 7% MC and the core is 13%). I would really like to try to get rid of that gradient but the only way that I can see to continue to dry the centre of the board without overdrying the surface is to increase the humidity. If I can get the humidity to the point where equilibrium moisture content is about 10%, I might have a good chance of getting a consistent 10% MC throughout the board.

 

gensetsteve - good thinking on the sauna unit - I'll have a look on the bay.

 

Jonathan

[tommer9]

Drying too fast is usually the cause of case hardening.

With the steam injection method you will require higher temperatures, and to keep the MC of the kiln at 120% for quite a while.

Most large kilns are steam heated, and use a combo of dry and wet heat. For hardwoods temperatures are between 10 and 180 farenheit, with fans blowing the air around at about 2-400 feet per minute. This in itself can cause problems with hardwoods though, as too much circulation can increase end splitting, even if the ends are sealed. For softwods the temp used is as much as 230 farenheit, but the problems you are having occur in hardwoods with this treatment.

Conditioning will help to counteract honeycombing and case hardening, the moisture being increased significantly for 4-72 hours after the required m.c is achieved.

If you are getting surface checking then more moisture at early stages is required in general.

This is a very useful PDF:http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr57.pdf

[WADKIN]

A slight tangent to the question really. But having seen a few guys attempt to run small kilns over the years most have dropped the idea following problems or just not being bothered with it all after the initial tempting idea of getting money in sooner than air drying.

For myself i never really had the luxury of room and when ever i need things drying i just pay to have a commercial kiln to dry it, there need not be a huge volume, i have had as little as 50 cube done in this way in the past. they are experts at it and dependant on the timber and volume being dried i have also found that the extra few pounds on a cube well spent, and it is something which i pass on to the buyer in the long run. Something which is badly kilned is pretty much worthless or at the very least will mean that the buyer faced with timber pinching bowing and cupping and all the other resulting issues that come with poorly dried timber will not be coming back so all the work in building the client base is lost.

Just my thoughts on it. And having said it i am sure there are people out there here include who do a good job of it, but there are also a whole lot who really bugger up a lot of wood.

[tommer9]

Excelent post wadkin. This is the reality TBH.

I do know of a fully computer controlled kiln that is for sale. It was never set up, and requires a container (such as a 60' shippping container) to build into (it comprises fans, computer etc etc) and will kiln hardewoods beautifully in 21 days turnaround (the guy bought 2 identical and only ran one). It is up for about 6k IIRC. PM me if you are interested.

[WADKIN]

It could be a wise investment for someone who plans on going into drying in a slightly bigger way, and can get his head round it. It really is a science in itself and different timbers need different handling and schedules,

I pay Wests in sussex to dry anything i need done, which in truth is not a lot because of who i sell to now. It also means that as they have different kiln options open to people, so that things such as sycamore can be done in one of their vacuum kilns, with excellent results when dealt with quickly enough. Just a case of finding someone who offers the service close enough to your location to save transporting too far.

[Mike Dempsey]

Hi

 

I have been using an alskan mill for about 8 years or so. I used to air dry all my timber for about 2 years or so outside until I moved workshops and lost my free outdoor storage area.

I came across the Sauno kiln about 18 months ago and bought the smaller one.

Building it out of the foam sheets was a bit of a nightmare because it is so fragile and crumbly and we ended up encasing it with fibreglass to a depth of about 5mm or so and strengthening the roof because this is where the door is chucked when the kiln is open.

 

Drying is a fine art in itself, but with a bit of practice, and careful monitoring I can dry up to 60mm beech in about 5 weeks or so down to 10%mc which isnt bad at all. Most of my timber is now cut to 100/1500mm square edged planks as you can get more in a kiln this way and it is easier to stack. The kiln has been designed to allow up to 8' planks to be put in and about 1m wide boards at the most. It is a bit of a bugger to fill especially on my own and can be a bit back breaking at the thicker sizes of timber.

The only steaming done is at the start when I add extra water and turn the temp up to 70 deg centegrade for five days and the turn it down to 40 deg and open up both vents to allow the moisture to escape. I have thought about putting a vent in the outside wall and running a pipe directly to that from the kiln vent so any moisture will vent outside. At the moment I run a dehumidifier for a few hours a day.

As a cabinet-maker with about 15 years experience I know when I am using crap timber and I can testify to the quality of my timber and so can the other cabinet makers who I have sold some to. So far I have kiln dried yew, oak, beech, silver birch, ash, sycamore, douglas fir, plum, and probably a few others.

I purchased mine from Classic Hand Tools and have good service from them. They were going to start a forum for users of the Sauno but havent started it. This would have been handy for all the questions that needed answered!

 

So, in my opinion, you only have to steam if you are on a tight schedule and trying to dry your timber as quick as possible as you would be in a commercial yard.

Ps. I know the big softwood timber yards dry their timber in 4 days. Thats probably why there is so much stress and warping in it when you buy it as well as the fact they only dry it down to 15%

[John Hughes]

I bought a smallish Ebac kiln on Ebay, used it once and it worked well .I dried a load of 1inch oak boards.

I don't have any space at the moment to set it up, so I now air dry and then move the timber into the workshop or much to my wifes annoyance under the sofa or any other space our house.:001_rolleyes:

As soon as I can get some more shed space I will set it up permanently and have another go.

So , yes , I am like one of the guys Wadkin mentions. I like the idea of being able to use the timber sooner but I am yet to get my money back from the purchase of the kiln.

John