Wet Pockets in Kiln-Dried Wood
Bacterial infections can cause streaks of 30% moisture content in wood that is mostly at 7% MC. March 4, 2009
Question
We sawed and dried some California nut walnut crotches about a year ago. We checked the flitches in the kiln and had good moisture content at about 7%. I did not check every piece in the load though. I pulled a flitch out of storage and checked the moisture content. This flitch had a grey streak in the wood and every sequential piece had the same grey streak. This grey streak was over 30% moisture. All of the adjacent area was still 7%. What could cause this? All of the moisture is concentrated in an area 2" wide and 10" long. I am using a Wagner L606 to check the MC%. I thought of a roof leak or something as a source of the wet spot but it is in every piece of the boule. Weird or what?
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
Wet pockets (or water pockets) are not that uncommon in some species and are usually associated with bacterial infections. Of course, the moisture meter could be responding to a high mineral content in the streak and giving you a false reading as well.
From contributor G:
Were you using a DH kiln?
From the original questioner:
Nyles L200.
From Gene Wengert, forum technical advisor:
The grey streak is the clue that tells us it is either bacterial or mineral and not related to drying equipment.
From contributor G:
I have 3 Nyle L200's. I really like them, but when you get down near 8%, it can be tuff to get them to 6%. You will get wet pockets; but, you can fix that. First, the kilns need to be really well insulated. I set my starting temp at 90 degrees. I bought the automatic overtemp vent system from Nyle for each kiln and I love them. After the kiln reaches 90, the stored compressor heat will take it to whatever temp I want without raising the temp setting of 90 on the controller. The problem with DH drying is that when you reach, lets say 10% MC in your samples, what little moisture is left re-evaporates before it can trickle down the coil into the drain. I shut down the compressor and vent it out through the overtemp fan. It works great and saves energy. I don't go over 125 degrees, so as not to damage the compressor. You can set the L200 unit outside the kiln and use ducts for the intake and exhaust, and run 130 degrees or more. I would consult Nyle before doing this.
DH drying is different from steam and other methods of drying. For little guys like me, it's great. Like any tool, you have to learn how to use it. I will say that it is hard to damage lumber in a Nyle L200 - you would have to do something really stupid.
Sometimes you have to shut down the unit and run the fans for a few hours, then start it back up. The "wet pockets" will soon start to fall in line with the rest of the charge. Wet pockets are common with DH drying, but they will come down. I figure my cost of drying at about 5 cents per bd ft. That's not bad at all.
From Gene Wengert, forum technical advisor:
I have run a DH kiln and never saw wet pockets related to this drying method. I have clients that run DH and they have never mentioned any wet pockets except what we can relate to bacterially infected wood (and as stated earlier, the colored streak is another clue).
Remember that DH uses hot air, controlled RH and air flow; so does steam. The only difference compared to steam is that the DH temperature is lower (and in some DH kilns the air flow is lower). Above 30% MC, drying in DH and steam are at about the same rate of drying (if operated properly and the equipment is correct).
We must make sure that we are all talking about the same event. Dry lumber at 7% MC with a small spot or pocket of wetness that can exceed 20% and sometimes 30% MC. The color change tells us that the problem is in the wood, not in the equipment. Steam kilns have the same phenomena with bacterially infected wood. It is not just DH.
From contributor G:
Well, like I said, you have to learn how to use the tool. All I can say is that I have operated 3 L200 kilns for at least four years. I know the tool quite well. The commercial steam kiln operations I have visited start at 160 degrees. The DH at 90. Steam drying and DH drying are two completely different techniques. I don't see how you can even compare the two. I will add, that as you increase the temp closer to 130 in your kiln, the wet pockets will start to come down quickly and equalize. It's all about temp and air flow.
From Gene Wengert, forum technical advisor:
I have not seen a hardwood kiln being started at 160 F; 160 F is the hottest that most hardwood kilns will achieve. Almost all hardwood kilns would start between 100 and 130 F.
Regarding bacteria, we do know that they are common in older logs and are found near the butt. They increase the green MC, may discolor the wood, and the wood dries more slowly. In this specific case, the person is working with butt walnut. Because of the high MC, sometimes the logs will not float in water and are called "sinkers."
The two types of kilns use different hardware, but the technique is identical - hot air, controlled temperature, RH and air flow. They are more alike than different, especially with hardwoods.
From the original questioner:
I assume it would dry out if left in the kiln for a few runs?
From Gene Wengert, forum technical advisor:
Yes, but never allow dry wood to regain moisture. It is likely that this would happen if sent through a second time. So, better to just wait for the wet spots to dry out, sorting such lumber. If it is indeed bacterial, it is wise not to use such wood as it is weaker, maybe be discolored and smells if the HR is high at times.
