5 Easy Pieces…to successful drying (part 2)
In the last issue, we looked at five areas where kiln operators can improve lumber quality and the kiln bottom line, all with little or no capital investment. These included everything from green lumber piling and storage, to air flow, baffling and schedules. Here we continue with five more areas you can target today to make a difference in your operation’s success.
November 28, 2011 By Peter Garrahan
No. 6 – Preventive Maintenance
The last place a maintenance person wants to go is the fan deck of a kiln that has just been shut down in mid-cycle because of a fan failure. Even fixing a seized bearing on a log infeed in -20° weather is more appealing. The message here is don’t wait until it’s broken to fix it. Get your kiln operator(s) and maintenance personnel together to develop lists of items that can be checked routinely. Identify what type of inspections or tests can be done, and how frequently they can be done to catch problems before they become “issues” that everyone in the mill knows about.
A good preventive maintenance program will include all of the mechanical components (i.e. motors, bearings, steam traps, valve actuators, etc.) as well as the building structure and kiln control system. Any time something fails in mid-cycle you should conduct an autopsy to see why, and whether you have missed something on your regular inspections that should be added to the list for the future.
No. 7 – Tighter Kilns
As mentioned in Point No. 4 in Part I (see CWP Jan/Feb 2008, or www.canadianwoodproducts.ca), achieving the conditions of a drying schedule is just as important as the choice of schedule. A common reason that schedule conditions are not met is leaky kiln structures. Damaged doors, poorly maintained door gaskets, vents not closing properly, and leaks around joints between kiln panels are all potential losses of kiln humidity. Since most of the kilns used for dimension lumber do not have the ability to add humidity in the form of steam or water sprays, it is crucial that moisture being evaporated from the wood be retained in the kiln environment when needed.
The easiest way to detect leaks is on a clear, cold winter day. To use a line from a video we produced years ago; “if you can’t see the kiln unless the wind is blowing, there’s a problem.” The only place any substantial amount of steam should be visible is from the vents, and then only when they have been opened by the control system.
Fix or replace doors as necessary. Gaskets should be inspected and replaced when they fail. Kiln joints should be sealed from the inside with a good quality sealant such as an industrial grade silicone, or the inner walls sprayed with a kiln coating. Vents should be adjusted to make sure they all close fully and tightly.
No. 8 – Stopping at the Right Time
Over-drying is a waste of kiln capacity and energy, and causes increased downgrade from warp. We finally have more tools to determine the end point in drying more accurately, and mill operators need to take advantage of them. Hot checks using a hand-held moisture meter are the traditional way of checking final MC. However, due to the inhospitable nature of a kiln, these tests are usually conducted very late in the process, and are usually only effective in identifying extreme situations.
In-kiln moisture detectors in the form of DC-resistance probes and dielectric probes inserted through sticker openings are two technologies available to supplement the hot check process. They offer the ability to monitor the MC on a continual basis. Over time, both of these technologies can be refined to achieve meaningful data for a particular operation. Don’t expect to plug either in and walk away. You will need to do some of your own testing to identify critical values for each product you produce. If used properly, these tools provide the operator with information to more accurately terminate drying from charge to charge.
The best measures of any system to terminate drying is how much the drying time varies and how consistent the final MC is at the planer mill. It is unlikely that every charge will have the same characteristics (i.e., species proportions, initial MC, log age, etc.), so we should not expect that every charge will dry in exactly the same amount of time.
The ultimate measure of a drying system is the consistency of the final MC of material arriving at the planer mill. Virtually all large, softwood dimension mills now have in-line moisture meters installed at their planer mills. If operated and maintained properly, these systems offer an irrefutable way of assessing the final MC of the product. Unfortunately, many of these systems are underutilized. If your in-line meter at the planer mill is used strictly to identify and kick out “wets”, you are missing some big opportunities.
Depending on what you are producing and the agreed to specifications with the customer, each product should have a clearly defined set of moisture objectives. Part of this should be agreeing on what the base line for acceptance is. In many situations this may be DC-resistance moisture meter readings, whereas in other situations it may be oven-dry MC. Regardless of which one it is, once the base line has been established it should become the standard against which the in-line meter is calibrated. Once the in-line meter has been calibrated, it then becomes the primary means to not only monitor final product quality (with respect to MC), but also a key tool for assessing kiln performance.
Following all of the equipment related points listed above will help prevent any induced variations in final MC, and an in-line moisture meter will verify how good a job you have done. Some amount of variability in final MC is inevitable due to natural differences between boards. You will know you have a well-tuned drying operation if every time you see a “wet” board you can also see a natural cause for it to have dried slowly, such as dense wood, wet pockets, etc.
No. 9 – Dry Yard Management
Few dimension and stud mills can justify facilities to protect dry lumber from the elements. As a result, dry lumber is vulnerable to some of the problems mentioned earlier, such as sticker breakage and board loss with handling. Strapping packages at the sawmill helps. Good spacing between packages not only helps prevent sticker breakage at this stage, but also promotes drying of any moisture that may accumulate in the material due to rain.
The best protection at this stage, however, is good inventory control. Ideally, dry lumber should not sit for any more than a few days before being processed at the planer mill. Some time in the yard is helpful as it helps equalize the surface to core differences in MC, and improves machinability.
No. 10 – Understanding Drying Behaviour
The last point listed in No. 8 deserves some attention on its own. Wood is a natural material and will inherently display variability in all of its properties. Even when drying a single species from the SPF grouping, we have to expect variations that will have an impact on the drying operation. For example, black spruce is known to have a wide variation in specific gravity. Since denser wood dries more slowly, this will be a cause for final MC variability.
Differences all the way down to the micro-structure of wood occur and contribute to variations in drying. Obviously, these differences between boards become even greater when we try to dry mixed species.
Over time, good kiln operators will become familiar with the properties of the material they are drying and will be able to develop drying routines specific to their situation. This needs to be all-encompassing, as it involves not only the characteristics of the material, but also the capabilities of the equipment, and, most importantly, the requirements of the customer. A future article in this series will deal more extensively with understanding wood properties and how they influence a drying operations performance.
In today’s rough markets especially, mill operators need to obtain every cent of value they can from the wood being processed. Although wood drying does not provide the ability to add value to wood that is not inherently there, it is often the source of needless reduction in value. The points listed above should provide some guidance on relatively low-cost ways and means for mills to avoid these losses.
Peter Garrahan is a lumber drying specialist with FPInnovations, Forintek Division, based in Ottawa, ON, and wrote this article specifically for Canadian Wood Products Magazine. He can be reached at 613-523-1232, firstname.lastname@example.org.
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