Walnut End Checking in the Steamer

Troubleshooting a checking problem with Walnut related to conditions in a large steaming chamber. October 3, 2009

We have recently developed a problem in our walnut lumber. There are end checks that run about 4 inches deep on each end and have a honeycomb appearance. It is worse in the thicker stock (6/4) but still noticeable in some of the 4/4. The first thing we did was adjust the drying schedule, but that didn't help. We had recently put a new steamer into operation and assume that is responsible for the defect as we have had no trouble before this. Our old steaming process used low pressure steam blowing directly into a poorly sealed chamber (a tarp over a concrete pad). We were satisfied with the results but were forced to change our process by the DNR due to local sewer regulations.

Now we have built a more traditional steamer building that contains a pit of boiling water in the back of the chamber with two small vents towards the bottom front to release pressure and create circulation of steam. We have experimented with different temperatures (currently at 210F for 90 hours). The color is good but not exceptional by any means. My opinion is that the lumber is drying out during the beginning of the charge (this would explain the color issues also) even though there could be numerous explanations.

It's very cold here in the winter. It takes about 3-4 hours for the steamer to get up to temperature. We experimented with cooling the lumber slower after the charge was over with no luck either. We thought the moisture might have escaped the ends too quickly with the cold dry air. We are currently looking for an accurate humidity sensor that will withstand the environment of the steamer. We probably were wrong to assume by heating with boiling water we would automatically be at 100% RH. Do you think that running a misting system during startup would help or something along this route?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
End checks (which is what you are seeing even though it looked like honeycomb as they went deeper) in walnut are a common problem. If your steamer is not at 100% RH, then you are creating them there. Boiling water will normally keep it at 100% RH, but if there is a cold spot in the steamer, that spot will condense moisture and prevent the humidity from getting really high. (Cold wall or floor or roof.) Are you monitoring WB or DB temperature?

If, after steaming, the ends dry too fast (no end coating and lots of air flow), then this is where they are developing. End coating must be applied after steaming and after the wood has cooled. Do not change the kiln schedule (go back to the original one). End checks develop very early in drying, not middle or late stages. That is why I suspect some end drying in the steamer. Low RH may also be why your color is not outstanding. I assume that you do not have any heating source in the steamer other than the boiling water.

From contributor A:
Just wondering, but have you tried to mark the lumber to see if the ends near the vents are checking worst? Most steamers I have seen, the water trough runs the length of the floor under the piles.

From the original questioner:
Thank you for your responses. Our sensor measures the dry bulb temperature. With our current problems it would seem to make more sense if we could have both readings or at least a RH sensor. We have a portable temp probe/wireless transmitter that we have moved around in the steamer yielding uniform temperatures. However we haven't checked by the ceiling.

We don't end coat our walnut (we know we probably should) as we have never experienced any trouble until the last couple of months. We have been running the kilns at 300 cfm. Our lumber isn't end trimmed. Our lumber is manually trimmed (upcut chopsaw) after drying.

Just to give everyone a better picture of the building, it basically resembles a small dry kiln (steaming capacity 18,000ft). The interior is lined with stainless steel with all joints filled with silicone. In the rear is a 4'x4'x26' pit (this goes completely across the room). In the pit is a 2,000,000 btu heat exchanger submersed in the water. This is the only source of heat in the building. The manufacturer uses the same ceiling structure as a dry kiln so there is a fan deck above the lumber. This is open so there is approximately 7 ft between the top of the packs and the actual ceiling. The manufacturer offers an optional fan to aid in circulation. We thought we wouldn't need a fan in a steamer so we didn't put it in. It could be added any time.

We marked some packs a couple of weeks ago that were located by the vents to see if they are any better or worse, but we won't know until next week when we get them in the kiln.

Well, I guess our next step is to map the steamer out into locations and paint these numbers on every pack following them through the system (although we have noticed no packs with zero defect). Also we need to put in some sort of RH tester.

From Professor Gene Wengert, forum technical advisor:
The only way to measure RH is to use a wet bulb at these conditions. Can you cut a sample, weigh it, put it in the steamer and then weigh it after steaming? Hopefully it has lost no weight. For a building this large and with the poor steaming capacity, you need the fan. What powers the heat exchanger? Do you have live steam available?

From the original questioner:
Our power to the exchanger is low pressure steam. Yes, we do have live steam available. Today we are trying another sample batch with samples cut and we ran live steam into the building to try to get it wetter during startup. Still looking for sensor - any suggestions?

From Professor Gene Wengert, forum technical advisor:
Run the live steam into the water bath so that it is wet steam. For a sensor, use a standard wet-bulb.