Process Controls Versus Schedule Controls in Commercial Kiln-Drying Operations
Here's a technically informative, if somewhat contentious, debate about different approaches to computerized kiln control, and the role of human operators in the kiln-drying process. June 13, 2014
Question
I was just wondering if anyone really understands the difference between Schedule and Process Controls. I believe that schedule controls are outdated by 20 years or more. I can't understand why Schedule Controls are even used anymore. Why not leave lumber drying up to a computer rather than a kiln operator with a calculator?
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
The problem with full computer control for hardwoods is that the connection between what the lumber is doing and drying quality is not clear. Drying rate alone and drying gradient within the wood are not enough information. Of course, what is wrong with the old, outdated controls and a calculator? They have dried billions of board feet of lumber successfully. They dry with the same quality and same speed as more sophisticated controls.
Incidentally, kiln operators can see warp, stain, checking, and other quality items that we cannot see with a computer. The kiln operator is still essential for hardwood drying. Having said that, I suspect that every new commercial hardwood kiln sold in the U.S. is computer controlled with many control options. This has been true for many years.
For softwood drying, probably every kiln sold for the last 20 years uses some sort of logic based , computer control basis. In the old days we used process controllers, but now we use even more advanced controls. Softwood kilns still do not run without an algorithm (set of instructions). It is the set of instructions that is the key.
From the original questioner:
The problem with old outdated controls and a calculator is the kiln operator to do as good as a job as a computer has to stand there forever watching the charge of lumber with a calculator adjusting the schedule. Don't get me wrong though we still need kiln operators to adjust the computers to optimize the computers performance. There are ways of calculating MC in lumber and shutting down kilns on MC and not by schedule time. Changing setpoints during a charge depending on what is happening in the kiln must be better than guessing what is going to happen because of what happened on the last charge.
From Gene Wengert, forum technical advisor:
A kiln operator does not stand forever running a kiln. For hardwoods, time involved is less than an hour a day. Oftentimes a computer kiln runs faster because the kiln is hotter or drier. For softwood dimension, even less time is required of an operator. A well run kiln by an operator will dry as fast as a computer kiln. This is because it is the lumber in most cases that determines or controls the safe drying rate. Certainly a good operator is not guessing, but uses knowledge and kiln data to make good decisions. I do favor computer kilns, especially for softwoods. Remember who developed the first TDAL zone kiln for softwoods (at Union Camp)? Answer: myself and Larry Culpepper.
From the original questioner:
Did your TDAL zone control run by time or shut down by calculated moisture content? I haven't seen many fully automatic kiln controls around. Please let me know of any on the market today.
From contributor F:
The reason we are still drying by schedule control is that kiln manufacturers refuse not to progress to manufacture a system to dry by process control. "What is wrong with the old, outdated controls and a calculator?" The process can easily be performed quicker and resulting in higher quality if the lumber is monitored properly. Just because we have dried billions of board feet does not mean it cannot be performed better. The problem with current computer controls, as well as schedule controls, is that they do not monitor the lumber in regard to quality items. There is a method that can see warp, checking and other quality items. By monitoring these quality items the method can advance the kiln set points much quicker.
From Gene Wengert, forum technical advisor:
The TDAL kiln provided more heat where the lumber was wetter (on the average) by using zone controls and also estimating MC during drying. It also estimated final MC quite accurately so that shut down could be controlled. They are widely used in the South USA. There are many automatic controls today - almost all softwood kilns and many hardwood kilns. Manufacturers are too numerous to mention. No kilns I have seen measure warp or checking during drying. How would one do that directly without spending huge sums of money?
From Gene Wengert, forum technical advisor:
Incidentally, there are some hardwood kilns that require the operator only to put a wood sample board on a scale and the computer weighs the sample board, measures the MC and calculates the best kiln conditions, predicts future settings and so on. This system allows for visual inspection of the wood but yet lets a computer control the process.
From contributor F:
It is very easy to monitor warp and such for the same expense that is currently incurred by using the scale/computer method you mentioned. No kilns presently monitor such quality items because kiln manufacturers refuse to provide such systems. Such system can reduce the drying time by more than 35% while at the same time improve the quality output.
From Gene Wengert, forum technical advisor:
A completely automatic kiln is sold by SII. It measures MC, calculates drying rates, controls kiln conditions, avoids step changes, etc. This system was initially patented by Bob Little.
From Gene Wengert, forum technical advisor:
Contributor F - you state "The reason we are still drying by schedule control is that kiln manufacturers refuse not to progress to manufacture a system to dry by process control." Are you in the USA or familiar with our kilns? In North America, very many new commercial-sized hardwood kilns today use process control, and probably all softwood kilns do.
Did you not read my comments about warp? Warp is basically not a kiln drying defect, except for cup. It is related to stacking and wood characteristics. It takes about eight days to dry a load of red oak from 25% MC to 7% MC. Are you stating that a computer kiln will dry in 35% less time or in 5-1/4 days without starting out at higher temperatures or lower RH’s? Southern pine is dried in 18 hours at 240F. Without going to hotter temperatures or higher air flow (which certainly make the wood dry faster), are you saying that you can dry this wood 35% faster or in less than 12 hours? In doing this faster drying will improve quality, even though there is little quality loss?
From contributor W:
The reason there are not more sophisticated control systems is that we don't have standard trees yet. Also, the idea that kiln manufacturers refuse to make more sophisticated controls is not correct. Kiln manufacturers will make anything that people will buy. The control systems that measure moisture content with oins are almost always slower than a decent operator. The ones that measure weight are better but many people don't want to pay for them. It is really an economic question and it is hard to justify really sophisticated controls.
From contributor F:
Gene, you both know who I am and where I live. For others, yes, I live in the USA. I am very familiar with the current kilns in the USA. Warp is very much dependent on how wood is dried, cup, twist, crook and bow and don't forget surface checks are very much determined by how the wood is dried.
SII has a completely automated control based on moisture content (weight) but it does not monitor quality factors such as warp and such. I know Bob Little and have talked to him extensively concerning his patent and my patented system. Yes USA manufacturers do refuse to manufacture this control system. I have approached if not every manufacturer almost all and they refuse to look at it. If manufacturers don't advertise the product, buyers don't know what is available to them, so they don't ask for this system. It is not surprising buyers don't ask when manufacturers refuse to consider it.
There is significant amount of loss from poor quality resulting from warp. Elimination of any loss is profit in the pocket. This system also improves brightness of the lumber. I don't call lumber with closed surface checks as being quality lumber. The checks are still there. I have performed the fundamental research, ran pilot studies and used this system in a commercial setting. In both the pilot study and commercial setting the method reduced the drying time and improved the quality.
From the original questioner:
The key to effective lumber drying is to remove the water from the surface of the wood only as quickly as it will flow from the inside of the wood. Water can only be evaporated and transported by the air from the surface of the wood. If the water is removed from this surface at the correct rate we would avoid warp, checking and other bad things that happen to the lumber. Each lumber species hardwood or softwood dry a unique rate. There are inexpensive Process Controls out there that dry lumber at the perfect rate. The FCC controls the RH and the speed of heat up so to optimize this rate for all species. There is no schedule but a set of parameters for each species to adjust the rate.
From Gene Wengert, forum technical advisor:
How can you dry wood if there is no gradient of moisture from shell to core? In conventional drying, water moves as vapor (diffusion), and that requires a gradient. As the temperature gradient is close to zero, the gradient is established by MC gradients. (If water moves from the surface as fast as it is coming from the core, then there is no gradient). Of course, if the shell dries faster than the core, this sets up a gradient and the moisture will move from the core. I suggest that you review the work of J.M. McMillen on gradients in wood and then we discuss (privately) what he has shown. Once the gradient is set up, moisture moves from the core as fast as it leaves the surface. Can you show me one peer reviewed document that shows that your information about rates is related to drying quality? How does one determine the perfect rate?
From Gene Wengert, forum technical advisor:
I have yet to see a peer reviewed document that relates hardwood warp (except cup) to drying schedule, except to a small amount that is not important on a practical level. I assume that you are talking about hardwoods because you are mentioning checking. In any case, since most hardwoods are not kiln dried green from the saw (except for white woods and they seldom check), controlling checking or warp would not be possible. What about softwoods? Is there such a relationship given in Drying Southern Pine by Peter Koch?
As I stated earlier, if you dry faster, then you have a hotter or drier or higher air flow situation. If such hotter, etc. conditions work, then why not use them in a standard kiln? Likewise, if certain temperatures or humidities work to develop flatter lumber, why not use them in a standard kiln? I have not heard of your patented drying system and I have no idea why a kiln company would not want it, unless it is too expensive.
From contributor G:
To chime in from SII Dry Kilns, I'll disagree with Gene a little bit and say that we still do sell a fair number of fairly basic round chart kiln controllers for commercial hardwood kilns. As Contributor W alluded to, the type of control system a customer purchases often comes down to money, and on smaller installations the cost per BF for the controls can be quite different when comparing the various systems available. It can be hard to justify $30,000 or more for a computer system when there's only one kiln involved. As suppliers it's better for us to offer a few options so the customer can choose what's best for them.
From contributor F:
It is definitely not any more expensive, in fact it is less expensive. It is curious that so many kiln manufacturers have refused to look further into it when it is economically competitive. Yes, I am talking about appearance grade lumber, usually hardwood. If you look at the international journal "Drying Technology" you will find peer reviewed articles on wood drying as well as the articles I have written for that journal. It has been well known for a long time that warp is controlled by how the wood is dried. J. McMillen published articles supporting the idea. He stated that the faster you dry lumber, without other degrade occurring, less warp will result in the final product. While much of the surface checks are initiated while air drying, many are also initiated when first put in the kiln after air drying. Why air-dry when it becomes economically competitive to put straight into the kiln from the saws?
Up to now I have used the same settings as is published and commercially used. The system was actually called for back in the 1950’s by Ray Reitz at the US Forest Products Laboratory because he knew the limitations of the present schedules was that by using moisture content based control there was no way to determine the drying stress level which was the object that limited the drying of lumber. By knowing the stress level you can use the same settings but sooner and achieved higher quality lumber. Stress is the only possible degrade that limits the drying rate. All other possible degrade dictates to dry fast. So it makes sense to monitor stress levels and not moisture content.
From the original questioner:
Gene - I did not say that there is no gradient of moisture from the shell to core. You kind of have me wondering what you mean you say "If water moves from the surface as fast as it is coming from the core, then there is no gradient." Then you say "Once the gradient is set up, moisture moves from the core as fast as it leaves the surface."
From Gene Wengert, forum technical advisor:
I remember about 25 years ago when one instructor at a drying short course said that you could surface check oak lumber after air drying. John McMillen was in the room and he then stated that the person had put drying technology back 50 years. He was fuming mad. He continued that it cannot happen with normal air dried lumber and normal kiln operation. I see the instructor now and then and he still recalls that learning experience.
Although faster drying does give less warp, the difference between drying in ten days versus eight days will not do that. Also, it is when the lumber is green to 30% MC (roughly) that warp is potentially controlled slightly by drying speed, not when it is under 25% MC. Certainly, if air dried lumber is put in to a kiln and then is allowed to gain moisture, you can get a lot of cup. Side bend is not controlled by drying conditions, neither is bow. Again, I am assuming that there is no rewetting.
As almost all oak is air-dried or pre-dried prior to the kiln, I am hard pressed to see how any special drying system from 25% MC to 7% MC would affect quality, compared to normal. Can you tell me more on how this would happen? Of course, much of the discussion above assumes that every piece of lumber behaves similarly. One key for a good kiln operator is to select the proper pieces for controlling the process. Because this is not an exact science, the kiln will be operated somewhat in a conservative manner. Although data may exist that lets one know that it is safe to change kiln settings based on a number of selected pieces in the kiln (maybe ten or so), there can easily be another 10,000 pieces that are not being measured. Therefore, to protect the unmeasured lumber, the kiln is not operated at the optimum for the few pieces being measured but a conservative factor is thrown in. This can account for longer drying times in practice compared to a research kiln study; small research kilns do not need this conservative factor.
From Gene Wengert, forum technical advisor:
You stated that moisture moves from the core as fast as it leaves the surface. This means, if it starts from the beginning of drying, that there is no gradient. No gradient means no drying. In practice, however, we do dry the surface fast enough to establish a gradient as that is what will move moisture from the core. Once we have a gradient, then moisture movement is relatively constant. To assure that it is, we keep raising the temperature and lowering the RH. Determining the magnitude of this gradient and associated stresses is a key for check prone species. But with 10,000 or more pieces of lumber in the kiln, how do we know which pieces to monitor? If we do not know, then we must include a safety factor in kiln operation. Contact the US Forest Products Lab for the work of McMillen on stresses in drying.
From the original questioner:
That is not exactly what I meant. I realize there is a moisture gradient and as well a temperature gradient. The goal of a slow heatup is to keep the moisture gradient to a minimum as the maximum flow rate has not yet been achieved. Do you know that both the temperature gradient and moisture gradient is what dries lumber. I tried a test once. I had and pot of almost boiling water and a piece of green wood. I measured the weight of the wood and then put it in the boiling water for a while then took the wood out of the water and then measured the weight which was less. I tried this test more than once and always the weight was less. This is why high RH dries lumber the humid air brings more heat to the lumber than dry air. If you understand this I will explain to you why high RH is better for lumber quality.
From contributor F:
I was not at that kiln course you mentioned, so I don't know what was exactly said but I know air-dried lumber can contain checks from the yard and also obtain checks from the initial step of the kiln process. Not everyone runs the kiln properly, use proper schedules nor are all kilns in proper running condition. Also, wet wood can surprise someone who usually does not encounter it. By drying slowly after air-drying a great deal of warp can occur. Drying oak as well as other species are often dried slowly at first causing much warp. Also, improper air flow causing reduced drying rate induces warp.
I stated that I did fundamental research, pilot studies and ran commercial studies not just work in small research kilns. The kilns I used were large and I had good success. While wood is a natural material and is inherently variable, irregularities in the drying load are caused more by inconsistencies in the kiln conditions from one end to the other than variability of the wood. If the kiln is in good running order, the wood will progress uniformly in the kiln.
Going to mills one can predict where the de-grade will occur in the load and which mill will have high amounts of de-grade by looking at their operational procedures. If you say you have to have a conservative schedule to overcome the unmeasured pieces you may as well throw out the kiln samples and dry by some selected time method for each of your kilns. The true reason that moisture content based schedules were developed initially by the Forest Products Laboratory was that there was no way to measure stress level at that time. With controlling by stress level instead of moisture content one can experience significant time savings, decrease in degrade and increase profits.
From Gene Wengert, forum technical advisor:
To the original questioner: The reason that wood in boiling water loses moisture and shrinks is that the fsp at 212 F is considerably lower than the fsp at room temperature. Hence, there is a lot of water that moves out of the cell wall. Further, the air in the lumens expands and pushes water out of the piece. The effect you observed is well known. Your connection of that event to an explanation of heating at high RH is incorrect.
From Gene Wengert, forum technical advisor:
Checks originate at high MC’s. They can be closed at 25% MC and the re-open when put in the kiln. New checks are not created in a kiln (unless there is moisture regain) at 25% MC initial MC. If there is any tension set, it would be even harder to create new surface checks. I do believe that McMillen is correct.
Kiln drying air-dried lumber too slowly will not create warp unless there is moisture regain. I have never heard that air flow causes warp. Do you mean at high MCs or at 20% MC? Or are you thinking that the slow drying uses a high EMC that results in moisture regain? As your latest post implies, a properly operated kiln not have quality loss for air-dried lumber. Rather, it is an improperly operated kiln.
Regarding conservative kiln schedules, please do not throw out the conservative US FPL schedules and procedures that have worked so well for over 60 years. I can give someone who is drying oak and advanced kiln schedule that will dry air-dried oak faster than the conservative FPL schedules. If your system dries faster, that is all you are doing is drying hotter and at lower RH. But how do you know which of the 10,000 pieces of lumber to monitor. Will there be another piece, not measured, that will have higher stress? There will be - both wood variability and kiln variability. Therefore, you need to be somewhat conservative when you base the kiln operation on 10 pieces out of 10,000.
Did you run your new system on green oak or air-dried oak? I have two mills in MO that will run air-dried oak at 180 F dry-bulb and 140 F wet-bulb initially. They are really fast and claim no quality loss in kiln drying. (Note that the Dry Kiln Operator's Manual and Drying Eastern Hardwood Lumber both talk about accelerating schedules). Anyone who has a new drying approach could use an accelerated schedule and claim time savings. They are reducing the safety factor that is built into the FPL schedules.
This acceleration is not done with standard operating procedures because even though the 10 kiln samples indicate 25% MC maximum, the operator knows there are some 28% and maybe even 30% MC pieces among the 10,000 pieces not measured.
I still maintain that any improvements in kiln operation under 25% MC will not change the wood quality (provided that moisture is not added or the lumber is not over-dried). If there is no quality change, then any time savings will not be sufficient to pay for a complex system.
From the original questioner:
You still have not explained why my theory of heating at high humidity is incorrect. Obviously from your last response you understand that water flows from just heating the lumber and not from just moisture gradient. Can you tell me the disadvantages of drying with high humidity when compared to low humidity?
From contributor R:
I thought I'd drop our attitude here at PC Specialties, being a manufacturer of process controllers and vacuum dry kilns. Our attitude is that a kiln operator is a vital component of a dry kiln. An individual who has experience dealing with the wood, understands it from hands-on operation, and who is there each day to get a gauge of how the wood is drying is vital, in our opinion, to a quality hardwood drying operation (no experience personally with the factory-style softwoods operations).
That said, some manufacturers produce very complicated and successful (and very complicated and unsuccessful) systems to remove the need for some or all of the hands-on that a kiln operator normally provides. I won't comment on the durability, utility or any type of ROI of these systems. My personal opinion is the more complicated the system becomes, the less likely it is to work long term, although that is highly dependent upon the engineering choices made during the design of the system. There are people who want to remove the kiln operator from the system, and there are people who want the kiln operator to manage the operation of their dry kilns.
From contributor W:
Heat transfer rate is a function of temperature difference. It is true that if you pass air that has a dew point higher than the wood temperature, heat transfer will be faster because you will be condensing water on the wood. Who wants that during the drying phase? The energy level of air (enthalpy) is usually expressed in the USA as BTU/#dry air. It is essentially constant at the wet bulb temperature. In other words, air at 120F dry bulb and 120F wet bulb has almost the identical amount of heat in it as air at 160F DB and 120F WB. Yet disregarding condensation, the rate of heat transfer is dependent on dry bulb.
From contributor F:
Gene, using your argument that billions of board feet have been dried so there is no need to change things, well, we should still be using oxcarts to travel across the country, not the Ford Model T and definitely no need for an F150 pickup truck. And we should be using draft horses not skidders to pull logs out of the woods. While most of the variability in the load comes from the kiln condition and maintenance and some from the inherent variability in the wood, the uncertainty of operating the kiln comes from not knowing the stress level within the wood. In the publications from FPL in the 1950's, they stated that the conservative nature of the schedules was due to lack of knowledge of stress level. They knew back then that one could advance the kiln settings after about 1/8 of the original MC had been removed.
But the exact point at which this could be done was not able to be measured at that time. Therefore they produced conservative schedules. And now we can know the exact point! So in the larger commercial operations we don’t need to stay with the oxcarts. The so called safety factors are not really safety factors but cover my backside factors because we don’t have enough knowledge. We now have that knowledge. Contributor R mentions his opinion is “the more complicated the system becomes, the less likely it is to work long term”. I agree if all that was done was to complicate the situation without solving a problem. The computer control systems now used is nothing more than throwing a computer at a problem without solving the question of what is the stress level.
Think about it, what does MC tell you at any point during drying except when to turn the kiln off? Nothing, it does not tell you anything about avoiding stain, warp, checks or such. So using a computerized load cell to weigh a sample does not address the problem. This new system tells you what the stress level is, if warping is occurring, if checks are starting, if some equipment is not operating to the point that it is effects the conditions and more. All at the same equipment costs of current systems. By being able to monitor the limiting factor - stress - you effectively increase the capacity of your kilns and reduce costs. No one will eliminate the need for the kiln operator. It is essential that he is in tune with the kilns he oversees. However, the better the tools he has, the better the job he can do. This new system is not complicated and gives understanding of what is going on in the kiln and provides Eureka moments of “Oh, is that why we do such and such” . Yes, the owners that have only one or two kilns cannot justify spending $20,000 to $30,000 on controls, but there are many owners that have more than two kilns.
From Gene Wengert, forum technical advisor:
To contributor F: You did not read the entire statement I made about the weighing and schedule system. You did not quote the next sentence. The statement was "What is wrong with the old, outdated controls and a calculator? They have dried billions of board feet of lumber successfully. They dry with the same quality and same speed as more sophisticated controls." So, by your analogy, what is wrong with ox carts if they travel at the same speed and give the quality ride as an F-150?
From the original questioner:
To contributor R: I believe we need kiln operators but we still need engineer's designing process controls that work obviously. A kiln system that is engineered properly will work and will always work. An operator's job should be to make sure the kiln is kept up. There is a huge list of quality control things the kiln operator can be doing. Leave the high tech lumber drying to the engineer's that make the process controls. There are companies that offer support for the kiln operators like modem connection to the PC in control. Some of the process control software comes with anything you can think of because the engineers are continually updating it to meet the customer’s needs, including custom reporting, graphs, activity logs and specialized debug screens mainly of the use of the engineer's when the kiln operator calls them.
Again with a proper engineered process control and the right service the kiln operator can do many other things to improve lumber quality that the control has nothing to do with. Why not have a process control that does the most when it is going to keep working forever (nothing lasts forever but you know what I mean).
From Gene Wengert, forum technical advisor:
I still do not see how a system kiln drying air-dried oak can improve quality. There is no present quality problem for the kiln drying of air-dried oak, unless the lumber is rewetted or over-dried (granted that air-drying itself may create defects). If most commercial operations were to dry oak green from the saw to control defects, they would have to triple, at least, the number of kilns. That is a big investment for them, including an energy plant.)
You talk about measuring the amount of stress, warp and checking. I assume that this is in ten samples (or more) in a load. You continue to ignore that even if your system measures stresses, checking and warp in ten or more samples, there are still 10,000 pieces that are not measured and the stress levels and decision points in those pieces would certainly be different. So, you need to go somewhat slowly to protect all the lumber in a load.
You talk about the stress limit in a piece of lumber. Please explain how knowing the actual drying stress levels in a piece of wood tell you when to increase the drying rate, etc. Don't you also have to know the strength in order to establish the stress limit? The maximum stress for one piece may be ok for that piece, but if the next piece has less strength, that stress level may be too high. How do you measure the strength? Do you use an average strength or a minimum strength? Do you measure the strength right at the surface fibers or internally or on a gross basis? If you are correct that a major problem with kilns is their variability, how does your system address this? Would you have to build new kilns to go with your control system, or modify the heating, venting and air systems?
From the original questioner:
To contributor W: The energy level (enthalpy) of the air equals the enthalpy of water vapor plus the enthalpy of the dry air. The higher the RH, the more enthalpy in the air. The point to this is energy is what dries lumber the temperature and RH is just a way of measuring this energy. In most cases high RH is better for the lumber in terms of stress also the more energy flowing to the lumber per second the faster it dries. As far as who wants to condense water on lumber well think of what the dew point and wet bulb temperatures are. The dew point is the temperature at which vapor turns to liquid. The wet bulb is the temperature liquid turns to vapor.
Let me give you a few numbers to visualize this. A dry bulb of 160 and a wet bulb of 120 the dew point is 115 the RH is 31.54 the enthalpy is 118(btu/lb). You do not ever want a dry bulb of 120 and a wet bulb of 120 because that is 100% RH it might start raining in the kiln. A dry bulb of 160 and a wet bulb of 145 is what you want because the dew point 144.188 is still lower than the wet bulb(lumber temperature) and the enthalpy(energy drying the lumber) is doubled at 235.9(btu/lB). I still have had no one tell me why drying at a lower RH is better than high RH. When I talk about high RH I mean as high as it can go meaning the wet bulb temperature still has to be above the dew point.
Energy dries lumber bot temperature. I have seen kilns that heat up fast and slow (building RH up) and if you look at the heat source output once the kiln gets up to temperature you will be using more heat in a high RH kiln than a low RH kiln. Most all stresses are cause before and during the initial drying phase (heat up). Most stresses are caused by heating the dry bulb up to fast lowering the RH.
From the original questioner:
Looking at the stresses in wood while drying works but you would have to look at a lot of samples (too many). Drying with a slower heatup and high RH stops most all stresses unless the stress started before the lumber gets to the kiln so why stress. A slow heatup does not mean longer drying times. The humid air carries more energy, drying the lumber faster even with a slow heatup.
From Gene Wengert, forum technical advisor:
I went over the fact that humid air does not transfer more energy to the lumber unless there is condensation. Contributor W indicated the same.
From Gene Wengert, forum technical advisor:
When there is hot air surround the lumber, humid or not, the heat is transferred at a rate proportional to the temperature difference between the surface and the air. The moisture in the air is not a factor - never has been and never will be unless there is mass flow to the lumber (condensation). Please check any heat transfer book and you will see that what I stated above is true. You are confusing the increased energy in humid air as being increased heat.
Because the drying process is typically controlled by mass transfer and not heat transfer, this means that a high humidity will slow down the drying process. Heat will speed it up but will also weaken the wood fibers and can make shecking result, such as when oak is boiled in water. You also had confused the lowering of the fsp as the temperature increases as some other effect.
From the original questioner:
If the wet bulb and the dew point temperatures are almost equal water wood evaporate on the surface of lumber cooling it down to dew point. Dew would form from the humid air and heat the lumber. Would this cycle not keep water flowing aside from the drybulb temperature?
From the original questioner:
The heat from the heat source heats both the dry air and the water vapor in the air. The energy in the dry air evaporates the water off the surface of the lumber in turn cooling the surface to dew point. The water from the air condenses on the lumber surface giving off heat. Most of all the energy heating the lumber beyond the surface layer is from the condensing water. The energy in the dry air mainly evaporates the water off the surface and warms the surface to above the dew point.
From Gene Wengert, forum technical advisor:
To the original questioner: For a very wet surface, in air warmer than the wood and not at 100% RH, the surface will evaporate water and will cool to the wet-bulb temperature, not the dew point. You are incorrect to suppose that moisture will condense from the air on this cool surface. Even if the temperature of the wood is the dew point and the air has the same dew point, there will be no condensation. You are also totally incorrect to suppose that "Most of all the energy heating the lumber beyond the surface layer is from the condensing water." In your supposition, how can we have water being evaporated and also condensing?
Introductory heat transfer will show that heat is flowing from the air to the wood due to the temperature difference. For a very wet surface, the wood's surface temperature is the wet-bulb and the air temperature is the dry-bulb. So, heat flow is proportional to this temperature difference, DB-WB. Incidentally, with your boiling water experiment, you will note that there is an initial loss of moisture, but then drying stops as equilibrium at 212 F is achieved.
From contributor O:
We don't believe that computerized systems should have total control based on information gathered simply because the computer isn't gathering as much information as an operator might see. I don't think the systems that are measuring and calculating the MC of a few samples can be trusted and therefore the drying schedule has to have an inherent safety factor. I don't believe that you can measure the stress in a few samples and say that conditions are optimum. I do believe that, from experience, you can set an EMC that suits the species and that only computerized controls can change the drying conditions as smoothly as should be done.
I'm running a vac kiln so things are a little different but consider this. I just pulled a load of holly. It was 4/4, 8/4 and 12/4 mixed. I checked MC with a digital Wagner recorder. The 4/4 ranged 6.8-8.0%. The 8/4 ranged 6.4-7.6%. The 12/4 ranged 7.0-7.7%. I never looked at the wood and I never worried about MC during the seven days that it was drying. The computer can show me what the calculated MC was and the drying rate and the temperature and the humidity and the pressure at any minute of those seven days but, at any minute, no one parameter was a concern because I set the conditions and the rate of change when I started the drying. It's a good rate for holly, which can be very difficult to dry. My process controllers do the work. I think that this is how conventional kilns should be run, also.
From contributor F:
Gene, I am glad to see questions directed to the process of the new system. But I would like to clear one thing up first. I did not misrepresent you. You said "They have dried billions of board feet of lumber successfully. They dry with the same quality and same speed as more sophisticated controls."
This new system is more efficient and faster with as high or higher quality material produced, therefore the moisture content based methods are outdated. My analogy is accurate. There is a difference, the speed is faster and the ride is better. Not the same. This new system provides the kiln operator with a better tool he can use. The highly respected researchers in the mid 1900's knew what limitations they had when they put together the schedules but it was the best at the time. They called for what this new system provides and they would welcome it today. I believe that providing results from peer reviewed research and discussing the merits and benefits of such research is not unprofessional. As I pointed out above, I did not misrepresent nor misquote you.
Now to the exciting stuff - questions. All these questions and more are answered in the papers and patent description. But to answer the ones you asked; Using the new system requires keeping the kiln in good maintenance to avoid the variability due the kiln condition, I assume you agree that kilns should be properly maintained. No modification of any of the kiln mechanics is required. There is no need to obtain the stress that causes failure which some may call failure stress nor the elasticity of all or any of the pieces. The stress level is related to strain and strain rate. The device that measures this is mounted on the surface but it measures more than the surface fibers and not exactly the gross strain. The system does require knowledge of the initial safe drying rate of each species. You have published safe drying rates for some species. I don’t know if you consider them safe at the start of drying only or throughout drying. But they would be good enough to start off with. This addresses all your concerns about the strength of the wood.
By knowing this you monitor the strain rate and when it changes it indicates stress level changes. Stress reversal is clearly presented but a more important point in drying is peak stress as I coined. Through careful analysis, peak stress can be observed and it is at this point the settings can first be changed. The patent goes into this more deeply. You keep bring up 10 samples and 10,000 pieces. There are two points to this concern. The first I addressed already - the variability of the load is composed of the inherent variability of the wood and the variability of the kiln conditions. The variability of the kiln conditions overwhelms the inherent variability of the wood. Maintain the kiln and most of this problem disappears. The second point is the progression of drying through the kiln. Lumber progressively dries from the sides of the stack to the center. At first no drying occurs in the center of the stack because the air becomes saturated, which is the reason we have fan reversal.
All this can be recorded by this system, as negative strain in the center and on the exiting side. Therefore all that needs to be monitored are the sides of the stack. When the settings are advanced, portions further in the stack experience the settings the sides of the stack previously experienced.. Therefore you don’t need any safety factors, just knowledge and data which this system provides. This system displays the wood’s response to fan reversal by swelling, non-uniform venting through the kiln changes in settings, if checking is occurring and more. This system shows the operator how the wood is responding to the air it is exposed to. All this is reported and supported with research data in the papers and patent.
From the original questioner:
Gene, I talked to my teacher and yes my condensing theory on the wood was totally false. The wood temperature would be at the wet bulb not at the dew point. We never want water to condense on wood. Just so you have a little background on me. I am 24 and have been in the lumber drying industry for four years. I work under my teacher. He has been drying with high RH for many years because he has done many experiments drying both ways. I have witnessed enough practical examples to 100% believe him but yet I still have an open mind. This is why I like having these conversations with you because you are the only person I know that both are opposed to my proposition and can really understand things to the detail I am looking for. I really would like to try to prove that since high RH air holds more energy it can transfer more energy. I think you think that the energy in the water vapor is not used in any shape or form to transfer energy to the surface of a solid. Is that correct?
From Gene Wengert, forum technical advisor:
If there is no condensation and no movement of the moisture air into the wood, then the energy in the vapor in the air is not a factor. It is interesting to note that this energy in the vapor, potential energy, goes out the vents in a vent-type kiln and is lost from the drying operation. Some people will use a vent heat recovery system to capture some of this energy by condensing the vapor in the vent heat recovery system. In a DH kiln, this energy is recaptured by condensation and is re-used within the dryer.
From Gene Wengert, forum technical advisor:
To contributor F: I am still perplexed. Let's say that I load a kiln with 50,000 BF of red oak 4/4 lumber. Some is at 28% MC (the wettest) and some is well air-dried and at 16% MC. The rest is in between. Some is a little thicker than normal. Some is quartersawn (less strain); a lot has mixed grain in the same piece. Some is bacterially infected and some is fast grown (southern oak) and requires slower drying than the other. Some is circular sawn (weaker surface; internal checking more likely) and some is bandsawn. In the old fashioned drying, we try to accommodate many of these factors, but we have a built-in safety factor just in case.
How would you be able to accommodate all this wood variability in a single load? I can see how you can measure strain or stress in one piece, but not throughout the load without a multitude of sensors. Doesn't each sensor cost money? Wouldn't it be likely that one sensor would indicate that drying should be slow to protect that piece, even though the rest of the sensors indicate faster drying?
From contributor F:
Yes, the sensors cost money but no more than the load cells used in a weight based system and they are reusable, not disposable. So the sensors are not a problem. Just like with the weight based system, you would sample mostly from the most sensitive material, like the high moisture content and bacterially infected (which is likely to be the high MC anyhow) and any other scenario you can conceive of. We would hope the operator has kept records of his air yard and knows which is the wettest. By using an aggressive but safe setting, the most sensitive lumber would be brought to the others at the fastest possible rate and when they are ready for a settings change, the others would be also. This change would be sooner than a weight based system that could not detect the best time to advance the settings. This is the heart of the system - knowledge.
The tree ring orientation in the board is of no concern. The system records the strain but analyzes the relative strain rate. Again, you do not need to measure stress throughout the whole load because of the progressive nature of the drying process, from side of stack to center. There is no need for a safety factor, just knowledge of your material and system. The system is able to relay to the operator how the wood is responding to the settings. This is a better tool for the operator to use.
By the way I do not say the old fashion way is obsolete. It is the most economical in some situations, like an operation that has only one kiln. It is just that it will be slower with more possibility of degrade. This is not a new tool just to have another tool. It is one that provides more and better ability to the operator.
From contributor W:
To the original questioner: What takes place as air passes over the wood is adiabatic saturation. The air remains at the same enthalpy which in this case means the same wet bulb temperature. Some of the sensible heat is converted to latent heat. That is, the temperature drops and the moisture content (or RH) increases. However, since no energy is being added the enthalpy can't change and since enthalpy can't change, wet bulb can't change. The RH in the narrow range of temperature change that we are talking about, is directly related to EMC and the RH and EMC of the air has to be lower that the surface of the wood or water will not evaporate.
On the other hand, if the EMC is too low, it will over-dry the surface and cause checks. Therefore, with most species, we have to pay close attention to the RH (EMC) and cannot simply adjust it higher to be more efficient. The lower the EMC of the air, the more total energy is used per pound of water removed, but that is unavoidable in a conventional or DH kiln.
From contributor L:
Don't you need (as with climate/drying rates) different data for much type of wood to know how they react to stress?
From contributor F:
For optimum performance yes. Some species could use research but we know what settings are at least safe and they can be used to start with. As far as how they respond, no. All material responds to drying the same way whether it is wood, ceramic, textiles or food products.
From the original questioner:
The control I use starts with a safe set of parameters and the parameters are adjusted for each species being dried. It finds the best rate for removing water from the wood to minimize stress.
From the original questioner:
What kind of things would a kiln control need to be superior over existing kiln controls on the market today?
From contributor F:
How do you define optimum (factoring in some moisture loss rate that is not related to the limiting factor (stress level))? All species can be dried faster than currently dried with improved quality. Why use trial and error when you can directly monitor strain without guessing. If you minimize stress, drying will take three years and a day. The idea behind all schedules is not to minimize stress but to maintain it below the failure level. Would you not consider a system superior to what is offered today that performs the job faster with resulting higher quality material and is able to inform you of what is occurring in the kiln and how the wood is responding exactly.
From contributor F:
I was not clear in my last post. I wanted to say that if you try to minimize stress during the drying it will take three years and a day. In the final product you want to end up with no stresses. To get no stresses in the final product you do not minimize stresses during drying.
From the original questioner:
Contributor F - I know that wood could be dried faster and also better quality. In an engineering prospective what makes a kiln control superior? How would a control monitor exact stress levels and what sensors would be used?
From contributor F:
The patent does not stipulate what type of sensor, many are able to achieve such (LVDT's, optical, magnetic field effect and more).
