Kiln Control Technology and Drying Parameters

      This mostly hypothetical discussion develops into a back-and-forth about whether wood stress should be used as a determining factor in kiln control systems. June 13, 2014

Question
Are kiln controls in today’s market as good as they can get? What would a superior kiln control need to be superior over existing controls? Gene I am most interested in your thoughts. I guess I am looking for answers to the above questions that are practical to our available technology. Justifying cost I don't care about though.

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
In order to answer this question, I need to know the species or group of species, thickness and the starting MC.



From contributor W:
Nothing is ever as good as it can get. I hope the controls available today are better than ten years ago and the ones in ten years are better than today. It doesn't mean everyone uses the latest thing. There are plenty of people drying with manual controls and old control systems. In some cases, they do a better job than any sophisticated control could do. I compare wood drying to cooking. Some people can make great bread in a camp stove and some need the latest oven with steam injection and all the bells and whistles.


From contributor L:
Contributor W said: “Nothing is ever as good as it can get. I hope the controls available today are better than 10 years ago and the ones in 10 years are better than today. Doesn't mean everyone uses the latest thing. There are pleanty of people drying with manual controls and old control systems. In some cases, they do a better job than any sophisticated control could do. I compare wood drying to cooking. Some people can make great bread in a camp stove and some need the latest oven with steam injection and all the bells and whistles.”

I totally agree. Although I love the way my Lignomats run, my hot-rods are totally manual. They will go from ambient temp to 140F in no time. The typical computerized system usually has way too many safe guards and limitations built in - not to say that they can't be modified under the hood.



From contributor F:
Computers are not the necessary ingredient for a superior control. Neither are bells and whistles. All present controls are based on moisture content which is the wrong parameter to monitor. Stress is the limiting parameter and is what should be monitored. If the wrong parameter is monitored you cannot expect to have the superior control. A better tool is what is needed. Hope will never achieve better results when better available technology is ignored.


From the original questioner:
Ok an example if you need one is SPF. Starting 80% MC drying down to 15%MC. 2x4, 2x8, 2x10, 2x12. Surely we have technology to improve the very best kiln controls on the market.


From the original questioner:
Contributor F - is your way of measuring the stress out on the market? I am interested with this stress monitoring.


From the original questioner:
I agree with Contributor W but if the computer has all the bells and whistles then even the best kiln dryer has an advantage. Your point doesn't really make sense if you think about it.


From contributor F:
Those that have manual controls can do a good job. Just because a good job is better than a poor job does not mean an even better job cannot be performed with a superior tool in the hands of one that understands the principles. The problem with manual systems is not that they are not automated. The problem is that moisture content does not provide you with the truly desired information. Throwing a computer at a moisture based system does not provide better information. One can monitor stress, getting better information, and progress the drying manually and get superior results. As mentioned on the other thread, indirectly, the stress based system is not on the market, but the technology is available. Have you looked up the patents yet? Those provide most of the information.


From the original questioner:
I cannot find the patents without the patent number or a full name.


From contributor O:
After operating vapor tension controllers I think the accuracy of the sensors used today give tighter kiln control and coupled with a PLC/PC combinations with moisture probes or load cells. Today’s kiln operator has a lot more tools and information to do a better job drying.


From contributor M:
"Throwing a computer at a moisture based system does not provide better information." I'll join the fray by disagreeing with that statement from Contributor F. There seems to be a couple of competing theories here on the best way to monitor drying being debated in the background, while the discussion also tries to relate drying ability with the level of control. Here's what I think, and I don't mean to rant but the underlying tone here seems to be that kiln manufacturers are short changing the industry with our current level of control.

A well trained experienced kiln operator can dry wood properly with almost any type of control given kiln equipment that is suitable for the purpose and in good operating condition. The more advanced the control system is, in regards to the information it can present to the operator about the kiln conditions and the wood's status, the better prepared the kiln operator will be to advance the schedule, and the more confident he will be in his drying operation. Therefore, a kiln operator that has access to current conditions, including MC, as well as good histories for the current load, will be better informed and more likely to make the proper schedule changes in the most timely manner. This would apply whether he/she was making the changes manually, or relying on a computer to make the changes based on parameters the operator has set in the schedule.

To say that using a computer in a moisture based control system will not provide better information is false, I believe. Regardless of how you measure the MC, which is all operators can measure today using commercially available systems, the more information the operator has, and the more current that information is (continuously monitoring mc), the better they can do with their kilns. If you have a vapor tension controller that works well, and you take MC readings on a daily basis, you can make schedule changes after you have taken your readings and feel confident in the schedule. Then you wait until you take readings again, and make another change. It works fine and is a very common drying method.

On the other hand, if you have a system that measures MC continually and you have confidence in its abilities based on your own experiences and you have written a schedule based on MC there is no reason that the computer system will not do a good job adjusting the kiln conditions automatically based on these readings without further kiln operator input.

As kiln manufacturers we put a lot of time and effort into making control systems that work, that are easy to use, and that can be relied on to help improve kiln drying quality as well as shorten drying times. Different operations desire or require different levels of sophistication, and the time-tested methods of MC measurement, whether probes or weights or TDAL or whatever should not be written off because there is another method being developed that may or may not prove itself in the commercial marketplace.

Computers and moisture monitoring systems in use today can be a very useful tool in kiln drying, and to state otherwise does a disservice to all the folks who continually work to improve kiln drying. If and when a stress monitoring system is on the market I am sure it will be tried and tested and the results will show whether it is viable or not. Let's see it in use and we can all decide for ourselves.



From contributor F:
I still say "Throwing a computer at a moisture based system does not provide better information." I will also say again that "Those that have manual controls can do a good job. But just because a good job is better than a poor job does not mean an even better job cannot be performed with a superior tool in the hands of one that understands the principles. The problem with manual systems is not that they are not automated."

But first, I do not agree with Contributor M that “there seem to be a couple of competing theories here on the best way to monitor drying being debated in the background,” I see this being the main discussion here, not whether one uses a computer. To put aside the computer discussion, my point about the computer is that, it of itself does not improve the control system. A person manually controlling the kiln can take daily measurements and use a standard drying curve to predict what hour the samples will reach a particular moisture content. So how does a computer providing the current MC actually provide an improvement in the quality of data that it generates? You may gain convenience but not better data? You obtain the same information either way. Computer generated MC is not an improved information over manually generated MC. Contributor M states that “The more advanced the control system is, in regards to the information it can present to the operator about the kiln conditions and the wood's status,” That means current MC and bells and whistles. More frequent information does not mean better information.

Talking about computer versus manual control of the kiln throws off the discussion which is directed towards why a stress based system is superior. That is the core of what I’m saying. As stated earlier, the respected researchers in the 1950's called for a system to monitor stresses instead of MC based systems they themselves published. They adjusted the schedules trying to use additional knowledge they gained when they realized that when approximately 1/8 of the original MC was lost, peak stress should have occurred. It is truly fascinating to read the old research reports to see how close they got but not close enough to accomplish their goal - a stress based system. The limiting factor in appearance grade lumber drying is stress so the goal in achieving the optimum system is to monitor stress. Stress data is better data. Now we are able to have a stress based control system. So far all I have seen are people trying to defend an old comfortable method. I have not seen anyone point out some inherent flaw or weakness in the stress based method. I welcome any comments.

Contributor M said “I am sure it will be tried and tested and the results will show whether it is viable or not. Let's see it in use and we can all decide for ourselves.” It has been tested commercially and it proved to match the claims of 35% reduced drying time. I am always ready to talk to any company that desires to provide their customers with a superior system.



From contributor B:
No way or no how are kiln controls as good as they can be and will get. The huge weakness in kiln controls today is the inability to sense moisture content and stress development in the lumber. Pin sensors are fairly blind above the FSP and have some drift due to contact relaxation. Weight based systems have some drift and have some inaccuracy in the original estimation of MC. Plus they usually use a sample that might or might not represent the load. Using conventional samples is slow, time consuming, and has sampling issues and other problems. Coming in the near future are a variety of microwave based sensors that accurately measure MC from about 100% MC down to 7%MC. There is a system in Australia that shoots the microwaves through a dummy sticker and collects MC readings at a number of points. There is a system that will measure a pack of lumber (for softwood drying). There is a system under development that has small sensors that clip over the side of a board in a lumber pack and thus can make readings on whole boards and not samples.

Coming are wireless sensors that will transmit the data without being connected (for anyone who has tried to load and unload a kiln with the weight based wires in the way). Measuring stress is an interesting concept. It can be done. I personally think it will be a long time before stress measurement replaces MC as the primary control function for a kiln. For one thing, there is not a knowledge base of what stress level causes damage in wood over the temperature, moisture content, and time range that lumber is dried at. In other words, if we had the sensors now, we still would not know how to operate them. I see stress measurements as initially, at least, being used to augment MC data. My understanding is that there are companies with these sensors well under development. Maybe on the market and I missed it?

There is work underway at some laboratories to bring the principles of CAT scanning to the dry kiln - probably a long way off. Acoustic emission was an idea tried about 15 years ago. We might see that idea arise as a supplemental way to detect checking in a kiln. Some sort of computer, either a full blown computer or a chip is more or less essential to provide the menu or schedule for control. There is a way to make the computer a more integral part of the system. Modeling might allow the prediction of drying while the load is running and allow the computer to do a better job of fine-tuning the drying process based on the data it is getting and the predictions for checking, etc. it is making. In short, I think that the superior systems we have now could become primitive in 10 or 15 years. Yet I am still using a kiln with capillary bulb controls and a chart recorder, so I know the change won't be sudden, but incremental.



From contributor G:
I believe that superior kiln controls can only exist with experienced kiln operators. Characteristics of wood vary from region to region, town-to-town, season-to-season, and day-to-day. There will never be an out of the box kiln control that will produce perfectly dry lumber 100% accurately. The operator will be key to making the superior kiln control as long as he is able to utilize the information provided and with his experience he will mold his controller to dry his wood. With an experienced operator and the more tools he has to work with the closer to the superior kiln control we will become. I find it hard to say that one way of controlling a kiln is better than another but put them all together and you’ve got something to write home about. I can’t imagine that the superior kiln control doesn’t have a computer to collect data, display conditions, and perform critical changes at just the right time. The controller that can be controlled by more than one factor and display as much information about the conditions in the kiln is the superior kiln controller. There are a few controllers out there that combine different factors to advance though schedules and are close. Combine in some third party measuring systems and a good operator who can intrepid that data you’ll have it.


Contributor B - yes, there are a few different MC monitoring systems available and a few being worked on. I agree on your evaluation of each method. However, why is this work being performed? MC does not tell the operator the information he needs. It is not the right direction for research to be pursued. (My perspective in this discussion is for appearance grade lumber. The drying of dimension lumber should be controlled by MC monitoring of some form, directly or indirectly).

As for stress based systems Contributor B said “For one thing, there is not a knowledge base of what stress level causes damage in wood over the temperature, moisture content, and time range that lumber is dried at.” This is true if you are talking about a predictive modeling system. You can throw in the statistic problems also; along with the model is not directly hooked up to the kiln environment. All the material properties need to be known for each species and the variability for each species. This would take a few life times.

Contributor B said “if we had the sensors now, we still would not know how to operate them.” Technically, it is impossible to directly measure stress. There are a few sensors that indirectly infer stress by measuring strain or change in length. There are also films that measure strain but it is said to measure pressure and is confined to measuring only compressive strain (not productive to avoid surface checks). However, monitoring stress level by strain rate, it can be performed now and we do know how to operate them mow. The absolute stress value is not needed for this method. The material properties are not needed, only safe optimum drying rates for each drying stage. Which is easy to obtain and Gene has some good initial values to start with. The only reason to have MC to be monitored with strain rate would be for the operator’s confidence during transition. The only reason this method is not on the market now is the resistance of the kiln manufacturers. Work with CAT scans is a productive tool for research but I don’t see it as a method for control. Work with acoustic emissions has been performed at least since the 70’s. The problem with this work is that it only detects surface check development after they already exist. So they’re not avoiding them – the information is too late. Most of my work has been performed in kilns using capillary bulb controls and a chart recorder, but to optimize based on what is happening to the wood you need to measure how the wood is responding to the drying process which capillary controls do not do.

I agree with Contributor G in that the experienced operator is essential. However, they are limited by the tools they have available to them. If their tools do not provide them with the information they need they are blind. Also, more is not better. There is an old engineer principle called KISS “keep it simply simple”. The more stuff you have the more stuff that can fail. More extraneous information the more confusion you will experience. The question is not whether to use a computer or not. The question is does the fact that a computer sits on the control desk going to help you or not. If the computer does not give you the information you need, it will hurt you in decision making. Depending on bad or extraneous information is worse than having little information. An experienced operator can make good decision on little information but can be lead astray with bad information.



From contributor M:
To contributor F: Please explain to us why kiln manufacturers are resisting this type of control system?


From Gene Wengert, forum technical advisor:
For how many species would knowing the stress level help drying quality? Is knowing the stress after stress reversal occurs helpful? In other words, would stress knowledge (and then changing the stress level, assuming that stress is in the normal or typical range) at low MC’s affect quality? If so, with which species and thicknesses?

Many species of hardwoods are dried in five days with excellent quality – yellow poplar, soft maple, ash, etc. Even if drying could be shortened by 1/4 day, often the operation would not have someone there to unload the kiln immediately, so shorter time is not helpful. Is stress level related to warp (such as cup), discoloration, staining, etc.? (Although stress is the cause for warp, I do not think that knowing the stress level in a piece will help predict warp such a crook or cup.) Has anyone published research that has related stress development at different MC’s to drying quality? Has anyone determined the safe stress level at different MC levels, so that if stresses are below the safe level, then drying can be accelerated?

In a typical kiln with 10,000 pieces of red oak lumber being dried green from the saw (which is seldom done), perhaps 100 pieces (or 1%) will have objectionable surface checking. So, would you agree that the drying rate for the kiln was already very close to the optimum and faster drying is not possible without increasing degrade? Maybe a little slower might have reduced the checking in these 100 pieces. If the slower drying took an extra day that would cost about $1250. Is it worth this amount of money to reduce checking in 50 pieces (250 BF worth about $250)?

In my frequent visits and consultations with hardwood lumber drying operations, I see more problems from management issues and not from kiln operational issues. I see losses due to incorrect MC measure and incorrect MC specification. I see casehardening stresses poorly relieved. I see poor sampling to determine kiln samples.



From contributor F:
Contribtor M - I don’t know why manufacturers are resisting this type of control system. It would be best for them to answer that question. Gene asks “Is stress level related to warp (such as cup), discoloration, staining, etc.?” Yes it is. All knowledgeable persons in drying know that the faster one dries lumber the greater the tension set and the less warp experienced. This has been well published. All directly related to stress level and drying rate. Also, it is well known that the less time the lumber stands in a kiln at the higher MC’s the less discoloration; directly related to drying rate which is related to stress level.

Whether the lumber has been air-dried or not all lumber experiences stress reversal and peak stress in the kiln. Many operations still experience degrade because of lack of brightness. Quicker drying at lower temperatures increases brightness. A reduction of at least 35% on a normally five day run is more than 1.5 days not 1/4. The operators would be able to turn around a load and gain production with 1.5 days. With difficult to dry species much of the costs are in inventory, land costs, delay in turn around, lack of just in time concept. By reducing the kiln time by 35% or more one can start to justify reducing or eliminating air-drying. By drying by stress level the final level of casehardening is reduced and the time needed for conditioning is reduced. This has been published also.

Gene, you have published safe drying rates; those are good initial starting rates. Further more precise values can follow. We already addressed the variability of lumber verses variability of kiln conditions within one kiln and conclude that the kiln variability is much greater than the lumber variability. Kiln maintenance is a management issue. As you mentioned you see more management problems than kiln operational problems. Even so, just because they get by with less than optimum production rates they would benefit greatly by improving their production rates. Gene says, “I see casehardening stresses poorly relieved.” As I mentioned casehardening is greatly reduced by this method.



From contributor B:
“However, why is this work being performed? MC does not tell the operator the information he needs. It is not the right direction for research to be pursued.” This research is being pursued because almost all hardwood lumber is presently being dried using MC based schedules. And there is a great amount of room to improve drying of some species like oak with the development of sensors that work well above and below the FSP, and because there are currently no MC sensors that will do that job. Also, almost every kiln sold in the last ten years has some sort of PLC or computer based control system that can be interfaced to better MC sensors. So I think there is a wonderful opportunity to improve drying and get the technology in service with the development of better MC sensor.

In regards to Dr. Wengerts "safe drying rates" - very good idea. In fact, that is generally my main control strategy when drying oak. But notice that the safe drying rates are expressed in MC loss per day, not stress or strain, unless you are referring to data or a publication something I have not seen. I am not against measuring strain or stress and support the development of that line of thinking, but I don't see an abandonment of MC as a primary control factor for convection heat based wood drying in the near future. I do see a possibility of measuring not only MC but also MC gradient during drying allowing the computer to estimate or calculate stress or strain as an additional parameter in control strategy.



From contributor F:
Measuring MC gradient during drying has been tried and does not work. There is no direct relationship between MC gradient and stress level. As mentioned earlier to use any predictive method you need material property values for each species that would take a few life times to collect. And then you would need to include material variability. As far as work on MC measurement system, why continue down a line of less than optimum when a much better method is available? "But I don't see an abandonment of MC as a primary control factor for convection heat based wood drying in the near future" But why is there the resistance to go that direction? No good technical reason.


From the original questioner:
There sure is an indirect relationship between MC gradient and lumber stress. You cannot have stress in lumber if the moisture is removed from wood at the correct rate. This rate is dependent on species and can be controlled by the RH in a kiln. Using a chilled mirror dewpoint sensor one can accurately control the RH in a kiln.


From contributor F:
Sure, MC gradient is the cause of stresses but there is no equation that can be developed that one can predict the stresses observed. People have tried and not succeeded. Now they are trying to use fuzzy logic to sort of predict. There is also no MC loss rate other than zero that will develop no stresses during drying. If you are talking about no final stresses after conditioning then the best stress relieve occurs when the stresses developed are the greatest during drying.


From Gene Wengert, forum technical advisor:
The best control system would be one that measures color, measures defects (such as warp), and so on. When measuring stress to control color or to control warp, you are doing no better than measuring MC. You are measuring a secondary variable. If someone claims that lumber can be dried faster, then why not accelerate the standard MC based schedules a bit? I have seen that the smooth schedules already are able to accelerate drying, as they begin elevating the temperature earlier than the step schedules. Note that almost all step schedules in the book are conservative to account for poor controls, etc. We suggest accelerating them.


From contributor F:
"If someone claims that lumber can be dried faster, then why not accelerate the standard MC based schedules a bit?" Because in doing so you do not know what you are doing to the stress level without measuring stress and are asking for failure. Which is one reason why I done suggest to do so by smoothing the step method.


From contributor M:
To contributor F: It sounds like for you it is either control according to stress, or stick to step schedules when you read your last post. This would indicate you believe that all the advances in drying technology do not really work and that the results kiln operators have seen when using new technologies do not really occur.

Two questions for you: Do you believe that any modern control system that is commercially available is superior to step schedules run manually on chart recorders? Second, since you keep implying this and did not answer earlier, why do you blame kiln manufacturers for not developing a stress based system? If the designer of such a system believes in it, why wouldn't he commercialize it himself and sell it directly to the mills? Why would he need to involve a kiln manufacturer?



From contributor F:
It's not that the new MC base systems do not work in telling you what the MC is or that the operator cannot speed up the process based on these devises but that an operator does not know how he is effecting the stress level without measuring it, and this is risky. The current systems can measure and do the math for the operator. It can keep records, but the current systems do not give the operator any better information than manual methods. I see an advancement as something that provides the operator with better information.

As for your last question, how many inventors are rich? Banks don't give out loans based on patents. Venture capitalists only look at big money items like biotech stuff. My idea is too small to them. I am not independently wealthy. I have spent my money on the legal fees. Kiln manufactures have the capital and facilities to hand such a project and can realize returns. I can give you some reasons that the manufactures gave me but I think it would be more appropriate if they decided to give their reasons themselves.



From Gene Wengert, forum technical advisor:
What stresses are you worried about at 25% MC and lower? As most hardwoods are air-dried or pre-dried first, then stress is not an issue. (The reason we AD or PD firs, is that a kiln costs about $25 per day per MBF, so one cannot afford to KD green in most cases when drying time exceeds five-seven days.) Regarding acceleration, I recall that smooth schedules provided the same quality as step schedules and drying was about 18% faster. So, what can your system measuring stresses do compared to this type of system?


From contributor F:
In short, the patent provides better information than current systems. I respect kiln operators who work with less than the best possible information and still produce a good product.



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