Question
I need some information on kiln drying large timbers - 4 x 6's to 12 x 12's or so. It will be done in New Mexico with our low RH. I think a high temp steam kiln is better for these woods. Ponderosa pine, Doug fir, white fir and spruce are our native woods. Am I correct in saying there are two ways to steam dry? 1) Blow heat off of steam piping with fins like a radiator. 2) Blow steam directly into kiln to cause the moisture in the timbers to boil off or, in a sense, vaporize at 100 deg c? I am going to dry furniture grade pine with steam or high-temp direct fired heat. I have read lots and have not yet found out about large beams or timbers. I thought also to kiln dry the log after it is debarked. Maybe I would have a better value added byproduct. This is a new startup, not yet off the ground.
Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Whoever suggested that you dry large timbers is not too experienced. The largest we would dry is 6x6. I have not seen any info about larger sizes. Cracking is too severe. Low RH is not want you want, incidentally.
You think that steam is better, compared to what? Direct-fired? Yes indeed, but with such large sizes, I am not sure that a kiln is the right way... AD under cover might be best.
Incidentally, do not inject steam directly into the kiln atmosphere. Do not go above 130 F. Do not dry a large piece and then plan to resaw it. Cut the size you need first. Again, you apparently have been given bad advice in this as well.
I am not a kiln man yet, however, I am a sawmiller - 3rd generation. I am getting tired of selling green beams and watching the plaster crack in multi-million dollar homes. Been doing it for a long time. Got to be a better way to get the timbers stable. All flat roof, exposed ceiling, custom cut.
The final MC for timber framing (20% MC give or take) is quite a bit different than for red oak lumber intended for furniture, as indicated. I did not see that the rf-vac people were drying to 7% MC. However, in the initial posting the person was talking about a steam kiln and not rf-vac. Can you imagine trying to dry large timbers of red oak in a steam kiln? Of course, the initial posting was about pine, spruce, and DF. Please reread my posting in light of the original posting - West Coast species, furniture grade, resawing into lumber, etc.
If the original question is about how to dry timbers for timber framing, the answers would be different than if one were drying for resawing. Note that resawing is not taking a large piece and making it a bit smaller.
Note: Resawing a large timber into furniture grade lumber means resawing a large cant into several 4/4 or 5/4 sizes (or maybe 8/4) and not just sawing off an inch or two to resize the piece after drying. I think that you have misunderstood the use of this term.
Look at www.hb-almelo.nl. They sell superheated steam kilns.
What I am talking about is exposed beams in the house. Put up green, they twist and crack in place. So some checking is okay. Also, carvers spend lots of hours carving on green corbels just to see it crack and split to heck. Most homes here use all exposed ceilings with either beams or vigas (hand peeled with drawknife). Vigas so green you can't pick up a 10' 10'' post (almost 1 lb per bdft.). Also, what I meant by RH was that in my location in NM, high desert, the humidity is low.
To sterilize and set the pitch, you need to go to 133 F and 180 F respectively for brief periods of time. This can be done in direct fired or in steam kilns. The wood does not care about the source of heat.
In any case, the faster and hotter that you dry, the more and larger cracks you will have. Cracking can be controlled somewhat by using higher RH in the kiln (do not vent).
The site you list, www.hb-almelo.nl, is a company in Holland. I would think that you would do better to go with a North American company (spare parts, repairs, knowledge about drying your species, etc.). Although we do not use superheated steam drying in North America, we do use high temperature drying with much success. However, this is for dimension lumber. We would not use it for timbers. Also, there is no strength loss for Southern pine, but other species do have losses.
Again, you might consider AD followed by a one day heat treatment.
One thing to consider is that I can dry a load of AD oak and increase the price by say $300 per MBF. Assume drying time is 10 days in conventional or 3 days in vacuum. If I dry a large timber and take 20 days (or 6 days), will the timber have added $600 per MBF to its value? (In other words, a kiln has the potential to make a gross profit of $30 per day ($100 per day for vacuum). If not, then maybe I should use the kiln to dry oak and make a lot of money. I perhaps can AD the timbers or contract their drying with someone who will do it cheaper than $600 per MBF.
Let's hypothesize that we're working with 10" x 20" x 30' oak or pine timbers. I have not looked into actually drying rates, so I'm going to grab some figures out of the air that hopefully will be close enough for practical discussion. From an air drying perspective, we're going to be looking in the neighborhood of one year or more to air dry, right? Timbers this size would take, what, 3 months or more in a DH kiln? But in a vacuum kiln, perhaps 20 days or less.
Timbers that are air drying are not costing us anything in terms of utilities to dry, but they do represent an inventory cost in terms of purchased inventory (logs) that are tied up in a long drying process. Also, they are going to impact our operational cash flow, because we have capex tied up in inventory that cannot be used for monthly OpEx needs.
If a vacuum kiln is used, then we can turn our inventory 12X times a year, versus 1X. Thus, can't we generate more OCF with a vacuum kiln, producing more volume, than going the air drying or DH route?
On a separate note, would a solar kiln be a practical option? If it would bring the drying cycle down to a level similar to the DH kiln, we could perhaps build more solar kilns to handle the volume, and thus not have the electrical OpEx associated with a DH kiln, but still turn inventory 4X times per year. Thanks in advance for the feedback.
You have indeed indicated an area where vacuum kilns are really great... large pieces of wood that take too long (i.e., too expensive) to dry conventionally. I do think that you have to limit your thoughts about drying oak to just red oak; the occlusions in white oak will not let moisture move as easily as in red oak (or other porous species).
Drying time in a kiln for such a large red oak timber would be in excess of 6 months, assuming you want the entire piece to be at 30% MC or less and you do not want hundreds of checks. Even then, I am not sure you can do it. So, your only conventional choice is AD.
So, following your example, if we can AD in 18 months and can vacuum dry in 20 days, then you would have to have an inventory 27 times larger when AD than when vacuum drying. But, the point I was trying to make is that with a vacuum kiln, we might get 4 loads of 4/4 oak through in 20 days and we would make $300 per MBF on each load (gross profit). So, if you use a vacuum kiln for timbers, you are really losing money, as you will not be making 4x$300 or $1200 every 20 days. So, the question is, are you in business to dry wood or make money?
