Keeping the Customer Happy
Like many things, the moisture content (MC) of wood can seem a simple and straightforward subject until you start asking questions
- How dry does the wood need to be?
- How much variability can be tolerated?
- How do I measure the MC of the wood?
- How do I get the wood to the desired final MC condition?
- How do I make sure my customer receives the wood at the MC condition it was produced in?
Regardless of whether we are producing wood for fine furniture or 2x4s for house framing, these questions need to be considered and addressed if we want to minimize the potential for in-service problems that can occur when wood is supplied and installed at an inappropriate final MC. Problems detected after a product goes into service will be costly to repair and will damage the reputation of the supplier and the wood industry in general. It is therefore in a company’s best interests to work with end users to identify their needs and assure they are receiving wood that will perform well in the intended application.
The general rule on how dry is to base it on what MC the wood will achieve in service. The moisture content that wood attains in service is determined by the temperature and relative humidity that it is exposed to. For every combination of temperature and relative humidity there is an associated property that is referred to as the equilibrium moisture content or EMC. There are tables in most kiln manuals listing EMC conditions for most ambient and kiln operating conditions. At normal exterior ambient temperatures the EMC will be about 6%, 11%, or 20% when the relative humidity is at 30%, 60%, and 90% respectively.
Since temperature and humidity vary considerably with weather patterns over the year, we usually concern ourselves with the average EMC. Exterior EMC conditions in Canada will range from about 11% to 15% depending on the time of year and location. Interior EMC conditions will generally be lower and can range from 2% or 3% during the winter to 10% or 12% during the summer. The interior EMC conditions explain why we typically specify a final MC of 6% to 8% for wood to be used in furniture or flooring. At that level, the wood is in the mid-range of annual EMC conditions and may pick up a little in the summer and lose a little in the winter.
Another factor that impacts on final MC specification is the complexity of the assembly being made. For example, a wood molding is relatively narrow, is not part of a bigger structure and can therefore undergo larger MC changes with little impact on its performance. On the other hand, wood glued into panels for use in a table top will experience problems with splits and/or warp if the moisture changes too drastically. The same rules apply for structural wood products. Single studs or joists can undergo relatively significant changes in MC without a negative impact on their performance. Glued-up assemblies such as I-joists, glulam, or cross-laminated timber will be more sensitive to final MC changes. This means that material to be used in these products must be dried closer to the final MC it will achieve in service and have less variability.
Final MC specifications will therefore vary depending on the specific product, its application, and possibly even geographic location. The onus is on the wood supplier to work with the end user to identify the specific requirements. Since the range of end uses for wood is expanding we can expect that final MC requirements will not be the same for all products in the future. This makes the job of the kiln operator more challenging.
Targeting a Specific Final MC
Achieving the required final MC is a matter of having good information at hand. I met a kiln operator once that could know when his wood was dry by tapping a rock on it. There is actually some scientific basis to this method, but the manner in which he applied it did not allow him to be too accurate. He could generally tell when it was dry enough but not necessarily differentiate, for example, between material at 16% versus material at 14%. The more specific the final MC demands are for a product, the more sophisticated we need to be with regard to measuring techniques.
Hot checks with a hand-held moisture meter, temperature drop across the load, and drying time may all be good indicators when trying to achieve the 19% and less requirement for grade lumber. However, if you are trying to optimize your drying operation or to achieve specific final MC targets, more sophisticated tools are needed.
There are now a number of in-kiln devices for measuring final MC. There are two general categories. One is based on pins embedded into individual boards to measure DC-resistance. The other is based on readings from metallic plates or strips placed through a bundle. Both will require some effort to set up and calibrate, but once this is done, they can be used reliably to zero in on the required final MC more effectively.
Weighing individual samples or a portion of a load is another way of obtaining intermediate and final MC information. The most accurate method of monitoring individual board MC is by means of the sample board technique commonly used in the hardwood industry. This technique can be done through manually weighing the sample boards on a regular basis or automatically by having the sample boards placed on load cells. Load cells can also be used to weigh an entire bundle or portion of a kiln load. This latter method can provide good information on the average MC of the load but does not provide any board-by-board information.
Regardless of which technique(s) you use, there is no substitute for experience. All of these techniques work better once they have been refined for a specific application.
Reducing Final MC Variability
If you have determined that you need to reduce final MC variability in your product, there are a number of ways to achieve it. Final MC variations can be the result of inherent differences in drying properties from board to board, or can be the result of differences in drying conditions. The easier one of these two to deal with is differences in drying conditions.
Uniformity of Drying Conditions
In an ideal setting, every board in a batch kiln would be exposed to exactly the same drying conditions. One of the objectives of the kiln operator should be to maintain and operate the kiln in a manner to achieve that. Here is a list of items that should be considered:
- Minimize dry-bulb temperature variability around the kiln (maximum deviations of +/- 5 F from setpoint for a kiln drying commodity grade material or +/- 2 F for a hardwood/value-added operation).
- If dry-bulb temperature is uniform, wet-bulb temperature uniformity will usually follow.
- Avoid wide swings in wet-bulb temperature by venting uniformly from all regions of the kiln. Proportional venting will also help reduce wide swings in WB temperature.
- Steam or water spray nozzles need to be uniformly spaced, kept clear and functioning properly to get good coverage and uptake of the spray into the kiln air.
- Ensure that you have enough airflow. High-speed softwood dry kilns should be able to achieve at least 800 to 1000 fpm. Faster drying hardwoods and white pine need about 500 fpm. Slower drying hardwoods (especially in smaller kilns) still need at least 300 fpm.
- Maintain uniform airflow through the load (no more than +/- 100 fpm from the average airflow).
- Stickers, for most kilns, should be at least 3/4-inch thick. Older kilns with lower airflow will benefit from a slightly thicker sticker.
- Lumber should be piled neatly so that there are no gaps through or between loads to allow short-circuiting of air. Random length lumber should be box-piled.
- Gaps above, below, and at the ends of the kiln load should be baffled.
- Maintain the design width of the plenum.
Uniformity of Material
In an ideal setting, every board in a kiln load would start at the same MC and have the same drying properties. Short of achieving that, any measures to improve the uniformity of material going into the kiln will help achieve a more uniform final product. Here is a list of items that can help achieve a more uniform initial product:
- Logging and log yard inventory management to ensure even-aged logs are being processed.
- Green lumber inventory management to ensure even-aged packages are being placed in the kiln.
- Reduce sawing variation.
- Dry one dimension at a time.
- Pre-sort green lumber to reduce variability.
- Pre-air drying (if done properly) can help reduce natural variations in initial MC.
Other Measures to Achieve Final MC Uniformity
There are other measures that can be taken either in the kiln or after drying to achieve and/or ensure a more uniform MC to the end user:
- Running longer and slower drying schedules will tend to reduce variability in final MC.
- Incorporate an equalization step in your drying schedules.
- Depending on the required final MC, let material sit on stickers in the yard, a warehouse or a humidity-controlled storage facility for a period of time prior to the next processing step.
- Use an in-line meter to identify material that falls within the MC specifications for each product.
- Incorporate an in-line meter with some sort of re-drying system to handle pieces identified as too wet.
Mills processing softwoods for commodity products will need to become more flexible and responsive to a wider range of drying quality demands. Final MC uniformity is just one aspect of drying quality that will be affected. The good news is that this has been achieved in other wood processing sectors and there are many ways of achieving the new objectives. The key is to identify what the new objectives are and then identify the right solutions for your material and equipment. In this respect, opening and maintaining good lines of communication with the end user is essential.
Peter Garrahan is a drying expert with FPInnovations.
November 8, 2011 By Peter Garrahan FPInnovations
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