NC State Extension Publications

 

My involvement in computer dry kiln controls dates back to 1984 when I attended an International Woodworking Fair in Atlanta (the secondary wood manufacturing machine show). One of the nominations for the Challenger Award was a computerized kiln control. I remember one of the icons of the secondary wood manufacturing industry running up to one of my friends and asking if he should purchase one of these machines so he could replace his kiln operator. After the show, North Carolina State University was given the computerized control to use with their laboratory kilns. The generosity of the vendor was driven by the idea that the experimental kiln could serve as a test site. The manufacturer’s computer technician could never figure out drying logic, or maybe we could not figure out his control logic, but needless to say the manufacturer and the early computerized kiln control passed into oblivion.

We have become a much more mature industry. The major goal in drying should be maintaining an excellent level of quality and then push production cost lower. We know that a knowledgeable kiln operator is needed to insure that we get quality and a reasonable kiln through put. With that in mind, many kiln operators view computerized kiln controls as a tool to help them do a better job.

The following is a list of benefits that can be derived by using computer controls. Not all controls provide all of the listed benefits, but the list will serve to get your thought process going if you are considering a new control system and/or kilns.

  1. Better monitoring and control of temperature – The electronic sensors used in computerized controls tend to respond to temperature changes within a kiln faster than the old gas tube sensors. When coupled with modulating valves (valves that can partially open versus on/off valves) electronic sensors allow a more precise temperature control in most situations. Other vendors have coupled the on/off valves with control algorithms (PID) for very accurate and precise control of temperature and humidity. Computerized controls are better suited for track kilns that have multiple temperature zones where each zone is controlled by its own valve versus a series of manual controls. Multiple heating zones can provide more uniform drying by channeling heat energy where it is needed. For example, a track kiln when first started may be loaded with lumber that has been air dried for several weeks on one end and freshly sawn material on the other end. With only one zone the entire kiln charge will be exposed to one set of conditions, largely dependent on where the dry bulb is located. With a multiple zone system, the zone with the air dried material may reach temperature quickly, closing down the heat for that zone, while the cooler zone loaded with the green lumber may have its heating valve open 100 percent because it is not up to temperature.
  2. Schedule changes without an operator present – Computerized kiln controls can advance stored kiln schedules based on time, predicted drying rates or input from remote in kiln probes such as resistance pins or load cells.
  3. Monitor kiln sample moisture content – Depending on the type of system, moisture data from the lumber load can be monitored either semi-manually or by remote sensors. Three basic systems are:
    1. Manual – This system can range from the operator manually weighing samples using the oven- dry sample method and then basing their decision to advance the kiln schedule on this data, to an electronic scale system tied directly to the kiln control which eliminates the operator having to do the math and which can advance the kiln schedule automatically. Positive attributes of this system are: you can get good results in terms of moisture data, it forces the operator to look inside the kiln, and some automatic advancement of the schedules can be used. Since this system most nearly duplicates what most kiln operators are currently practicing, it may be the path of least resistance in terms of technological acceptance by plant personnel. The negative side of this system is that it does take an operator’s time, thus moisture cannot be continuously monitored.
    2. Pin type resistance measurements – Pin type resistance measurements are similar to those that a pin type electric resistance moisture meter would give. Positive attributes of these systems are: the moisture can be continuously monitored and the schedule updated. Also the probes can be stuck in the middle of the lumber stack. Some systems also give shell and core moisture readings which may be very helpful in drying thick stock. However, above 30 percent and below 6 percent moisture it is very difficult to get accurate moisture readings from pin type electrical resistance readings. In some matched sample data I have worked with, the readings from the control were lower than the actual moisture content at the beginning of drying (about 28% average moisture content 4/4 white oak) and towards the end, the moisture content displayed by the control was lower than the actual moisture data. Some vendors have incorporated the use of oven dry samples in conjunction with their pin type resistance probes to correct the accuracy of the readings at higher moisture contents.
    3. In kiln weight systems – These are load cell systems that continuously weigh the moisture content samples inside the kiln to give a moisture content reading. The operator still prepares the oven dry moisture content sections for each sample. In new systems the reliability of the load cells has greatly increased. Positive attributes of this system are: it continuously tracks the moisture content of the load, and can continuously advance the schedule. Reported accuracy results by kiln operators have been good. However, the system does not track the moisture of the inside of the load.
  4. Control fan speed during drying – Variable speed fans can be tied in directly with computer controls. Fan speed can thus be automatically changed as the schedule is advanced.
  5. More information is available to the operator – Graphic displays have made many advances. Some operators have found many useful ways to utilize this data. Uses have been from evaluating the progress of conditioning by comparing the dry bulb temperature on each side of the load versus the wet bulb, to being able to find traps that are stuck open by looking at the percentage a steam valve is open versus the temperature in a heat zone and comparing this to other zones.
  6. Manage steam demand – Steam use can be controlled or prioritized by controlling kiln start up and conditioning. This may be a very important factor for companies that are boiler constrained and cannot consider additional boiler capacity.
  7. Monitor kiln conditions and shut the kiln down automatically – Parameters can be set up in many computerized controls to automatically shut down the kiln if the parameter is exceeded. For example, if the dry bulb goes above a certain temperature, or the wet bulb depression gets too large, the kiln will automatically shut down. Some controls will automatically call the operator (even at midnight) and alert them if there is a problem.
  8. Troubleshoot kiln problems – See Item #5.
  9. Ramp schedule changes – Instead of large step changes as in the traditional Forest Products Laboratory (FPL) schedules, computerized controls can ramp schedules. Instead of exposing lumber to rapid equilibrium moisture content (EMC) changes and creating or deepening checks, the changes in EMC can be slowed.

Some Additional Thoughts Concerning Computerized Kiln Controls

  1. You still need an operator – To use computer controls to their full potential you will need an operator who has a good understanding of drying. In addition to their kiln drying knowledge, they have to be willing to learn to use a computer and understand computer logic.
  2. You still need to look inside the kiln daily – Continuous moisture readings are only part of the equation. You need to see if the lumber is mildewing or checking. The concern is not limited to moisture loss, quality is the number one issue.
  3. Sample selection is critical – If you are using some type of continuous moisture monitoring system, sample selection is just as critical, if not more so, than if you are doing manual samples. If you just pick dry samples, the computer will not have good data to work from.
  4. You must understand how the computer collects and uses data – That includes knowing which moisture content samples the computer is using to advance the schedule and which is your controlling dry bulb temperature. Some systems allow the operator to designate on which samples to advance the kiln schedule. Others can use the average of the wettest half, or wettest three, etc. Also, some European controls use an average dry bulb temperature while most of our North American schedules are designed to use the hot side (incoming air side) as the controlling dry bulb temperature.

    Many kiln operators use different methods to buffer their kiln schedule advancement. For example, in a package kiln with low airflow, a kiln operator may stay a step behind in the schedule or use their wettest sample to avoid raising the temperature before the center of the charge of green red oak is below thirty percent. The question may be raised, are there any ways to buffer the kiln schedule advancement with a particular computerized kiln control? Some of these problems can be avoided if a good pre-drying system is in place.
  5. Maintenance is critical – In a manually controlled system, the operator can easily compensate for a maintenance problem. For example, a steam valve that leaks can be manually shut off. With a computerized system, the computer cannot shut off the manual valve. Thus, steam bypasses the control valve and gets into the coils and heat is added to the kiln even though the valves are shown to be shut off. A situation like this could cause a temperature over-ride during conditioning, resulting in lumber that is not stress free even though the computer was trying to get the correct dry bulb and wet bulb temperatures for conditioning.
  6. Computer controls cannot compensate for poor drying practices – Most sawmills that are known for their drying practices have a drying system in place. That is, they emphasize good sticking and stacking practices, use shed, fan shed or pre-dryers to avoid drying defects that occur prior to kiln drying, and have knowledgeable and motivated kiln operators in place.
  7. Look at the service and support record of the potential vendor – Do they have a good support record for their products.
  8. The computerized controller should be user friendly – An operator should be able to easily navigate their way through the system. Kiln data should be presented in an easy to understand manner. The operator should be able to easily enter or customize drying schedules.

Author:

Department Extension Leader and Extension Specialist (Sawmills)
Wood & Paper Science

Publication date: May 1, 1997

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