Fiber saturation point: not an issue

Why the fiber saturation point is not a critical element in proper kiln drying practice. September 26, 2000

I have five Irvington Moore brick steam kilns and my problem is this: In my smaller-capacity kilns (40,000 board feet), once I get below the fiber saturation point (FSP) or actually below 25 percent moisture content (MC), sometimes I get to a "freezing" point where the wood stops drying.

I have variable-speed fans and I usually run them about 75 to 80 percent until I get to this stage, but I still worry that some of my bunks in the middle have not reached FSP. So, for a safety margin, should I increase my fan speed, add more heat, or just drop my wet bulb and crack the vents? All would increase drying, but I was curious if one is a safer bet than the other to reduce drying defects like checks and honeycomb.

First, let's correct one small point. You never will be at the FSP. The FSP is approximately 30 percent MC, but that refers to the MC of a cell and not to the piece of lumber. When a piece of lumber is at 30 percent average MC, the shell is way below and the core is still above.

Sorry, but this is a pet peeve of mine. I hear knowledgeable professors teaching students about drying and talking about the FSP, but the FSP is not a critical point in drying lumber because a piece of lumber is never all at the FSP at once. The surface cells are there perhaps in a few hours; the core may take weeks to reach FSP.

Now, to your question about very slow drying. If you do not raise the temperature, as called for in the schedule -- which assumes that some of the lumber in the center might be a little wetter -- then you will have the very problem you're worried about. Since about 1945 we have dried in excess of 250 billion board feet of oak lumber using the schedule in the book without any problems, provided the wood wasn't bad and the schedule was followed. So, use the schedule. Also, use the wettest half of the samples for testing, not just the wettest one.

Regarding fan speed, start with approximately 275 to 300 feet per minute (fpm) through the load. Do not worry about the percent level to achieve this velocity. Below 20 percent MC, you can drop the speed to 200 fpm in most kilns.

Checks and honeycomb are initiated above 45 percent MC (average MC), so your concern about avoiding defects is not critical at 30 percent MC or lower in most cases (with 4/4 lumber).

I am concerned about your statement "cracking the vents," since most kilns have a control system that automatically controls the humidity in the kiln and you do not need to manipulate the vents manually. Certainly, use the automatic control system 100 percent.

I think that a one- or two-day visit by a practical "expert" in drying might be very helpful. He/she could review your procedures, discuss some of the advanced subjects on drying, and so on. An on-site visit is much more beneficial than attending an advanced seminar. (I do offer both on-site visits and seminars and have been doing this for years.)
Gene Wengert, forum moderator

I'm glad someone else has a pet peeve about the use of FSP. It is thought of as some magical point; really, it has no meaning in the drying process.

I think it got its magical reputation from the numerous studies in the past which tested wood for its mechanical properties and shrinkage. The wood was first dried before being tested and THEN each sample was increased in MC. At that point, testing showed that many mechanical properties, as well as shrinkage, stopped changing at about 30 percent.

However, if you test wood that has never been dried, i.e., as it comes down to each particular MC, shrinkage and mechanical properties start to increase well above 30 percent, and at as high as 75 percent in some species. So, FSP has no meaning when wood is initially being dried, such as when we stick it in the kiln; only after it has been dried.

From the original questioner:
Thanks for the tips.

I was just always told to watch putting the heat on above FSP, to let my lumber get to around 25 to 30 percent before I initiate heat, and to increase my fan speed after I reach FSP.

"Cracking my vents" just meant that I lower my wet bulb until my vents start to open, to increase drying time if I hit a snag in drying. I usually follow the schedule pretty closely. If I see checking start to occur I will tend to back off the schedule a little. I am a "greenhorn" kiln operator, however, and I guess I let oak intimidate me a little.

I normally dry partially air-dried material, but sometimes they stick me in a situation of drying green and I practically use my kiln as a predryer for the first 25 days.