Dump Cycle Kiln Drying

A discussion of the value of periodic high-volume venting during the kiln-drying process. July 13, 2006

I am looking for any information on dump cycle drying using core probes, ambient
temps and chamber venting with an indirect gas fired kiln. I have up to 8 each heating and venting stages with my controller and the kiln capacity is approximately 20MBF.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
We had a grain dryer on the farm and we used "dump cycle drying" where we had different MCs and the dryer would dump a portion of the dry grain and continue drying the wetter grain. Is this what you mean by "dump cycle drying?" How would this work for lumber? Are you thinking that your kiln would have mixed MCs and that some of the driest lumber would be removed while the rest is left in? This is not workable, as we use different temperatures and RHs depending on the lumber's MC. Are you familiar with the basics of lumber drying?

From the original questioner:
Thanks for your reply. I am asking questions to see if anyone is using a dump cycle procedure. I have been informed that dump means high volume exhaust after a heating stage. Ambient and core probes are used during the heating stages. For an example, drying syp. run chamber at 130 deg. until wood core reaches 110 deg., then vent cycle starts and runs for 45 min. The next cycle the ambient and core temp are increased 10 deg., vent time 45 min. This will be done in a number of stage settings.

From contributor D:
Some companies in the Shenendoah Valley are using the method. If you don't know any of these companies, try to reach Dennis Clay. You might make contact through BoldDesign. We (PC Specialties) build the control systems for this method, but I don't follow the schedules/procedures.

From Professor Gene Wengert, forum technical advisor:
I did review a paper written about a technique that claimed great savings in energy by using a special venting system. Unfortunately, the theory was incorrect as some of the basic assumptions were wrong. It is really hard to save much money at all with improved venting.

I am always amazed at the new systems that are proposed for conventional hot air drying... What is wrong with the present systems (if operated correctly)? Hard to beat the cost and quality, although contributor D's vacuum system does seem to be an improvement for hard to dry items, such as thick lumber.

Regarding using core temperatures, I do not know of any research that has been done to indicate that such an approach offers an advantage or research that shows what temperatures should be used. It would be interesting to hear ans review technical documents about the process.

From contributor G:
The company in question is very pleased with the results that Dennis Clay's method has brought them. They currently have seven largish (I believe they are around 50K bf each) kilns running, each with the potential to utilize Dennis Clay's method. When he first asked me to design the control system, I was pretty skeptical but figured it was his problem... Seeing it in action on red oak makes you wonder what else you can get away with, regarding conventional kiln technology. (from PC Specialties)

From Professor Gene Wengert, forum technical advisor:
Remember that wood dries because of the temperature, RH and air flow of the air around the lumber. (This does not include vacuum systems, however.) So, if one system works better than another, it is because it is using different temperatures, RHs and/or air flow.

Although I am not familiar with the system that contributor G mentions, I do wonder what is wrong with the way lumber (red oak in this case) is dried in conventional kilns, assuming that the kilns use a modern system for operation. I do know that many kilns are run very conservatively by the operator and so oak drying is quite slow. But other operators do indeed use more rapid approaches with computer controls.

The basic question I have is if contributor D has developed a new set of temperatures, RHs and air flow values for oak, I wonder (#1) what they are and (#2) if one can achieve the same conditions with modern conventional kilns.

From contributor L:
We (Nyle) often use automatically rising dry bulb controls where the temperature is raised only enough to keep the lumber drying at a constant rate. This sounds more gee-whiz than it really is. The dump cycle is a variation on a conventional cycle that uses a time clock to run the vents and a wood probe to run the heat. It isn't all that amazing or different.