Drying for Value Added
Lumber producers are looking at ways to increase their average dry product value and to secure new markets for their wood. In most cases, these new markets have different demands on the dry lumber quality. Sometimes these demands are well known and defined by the end user. In other cases, they are less clearly defined but still need to be determined and addressed by the kiln operator.
July 5, 2012 By Peter Garrahan FP Innovations
Changes in drying requirements will inevitably put some operators in the position of having to use kilns that were developed to dry for one end use and now having to use them for another (more demanding) application. This article will deal with the equipment and operating considerations that need to be taken into account.
Over the years, the softwood dimension industry has had to deal with one drying specification for the vast majority of its production. The NLGA requirement to stamp material as “S-Dry” called for at least 95% of the material within a shipment to be at or below 19% MC. Cross-Laminated-Timber (CLT) is a new product that is getting a lot of attention in the marketplace. CLT is a composite product made up of edge-glued pieces of lumber to produce panels that are, in turn, laminated to achieve the required thickness. As with most composite products, uniformity in moisture content helps maintain the product’s dimensional stability and geometric shape. For CLT, the recommended target MC for the raw material is 12% +/- 3 (ANSI/APA PRG 320). Glulam is another composite product where final MC specifications differ from regular grade lumber. Feedstock for glulam should be at 15% or less at the time of manufacture (CSA 0122-06). These specifications are quite different from the NLGA requirement listed above and will place extra demands on the drying operation in order to achieve. Other products such as I-joist flanges and MSR may also have different, client-specific final MC requirements.
|Glulam beams also require a narrower range of final MC for the individual components.|
As far as a drying operation is concerned, there are two things that need to be addressed. The first is to achieve lower MCs with less variability and the second is to be able to switch from one drying spec to another on short notice.
Achieving a Lower Final MC with Less Variability
Many kilns that are used to dry traditional commodity-grade softwood products may not be able to dry to the news specs without some changes or additions. The following is a list of the performance-related aspects of a kiln that should be addressed.
- Temperature Uniformity – The kiln should be able to achieve and maintain the set-point dry-bulb temperature to within a tolerance of +/-5 F. This takes into account swings in temperature as the heating system cycles on and off as well as variations around the chamber. To see if you are achieving this temperature, you may need to install extra temperature monitoring, which can be a stand-alone system or part of your kiln control package. If you are not achieving this, you will need to take measures to address the situation, which could include installing proportional control of the heating system (to address wide swings in temperature) or to conduct a heat balance of the kiln (to address temperature variations).
- Relative Humidity Control – In general, if you are trying to attain better uniformity in final MC you will need to have the ability to achieve higher relative humidity at some points in the drying cycle. There are a number of things that can be done to assist with this, including:
- Making your kiln tighter by fixing holes, adding insulation and replacing worn door gaskets.
- Running at a lower dry-bulb temperature will make it easier to achieve the required relative humidity
- Adding a humidification system. This could be a steam spray or high-pressure water spray.
- Updating/Upgrading Kiln Controls – A single dry and wet bulb in a 120-ft. long kiln does not provide sufficient process control when trying to attain tighter tolerances in final MC. This relates closely to the notes on temperature uniformity listed above. In conjunction with a control system, zone control of temperature will also help achieve a more uniform final product. Zone control will usually require the installation of other control valves or dampers to regulate the addition of heat on a zone-by-zone basis.
- Improved Monitoring of MC During Drying – Better information on the wood MC during drying will help the kiln operator make better decisions on how to operate the kiln, including the most appropriate shut-down time. Information on in-process MC can be gathered by:
- Installing an in-kiln device to remotely monitor MC. This could be in the form of DC-resistance pins or dielectric plates that are inserted through the sticker openings. Either of these systems can be linked to the control system.
- Conduct more frequent hot checks and/or gather more data during each hot check.
- Improved Monitoring of MC After Drying – Final MC monitoring at the planer mill serves two purposes. The most immediate benefit is that it can be used to assure that every shipment or order meets the MC specifications set by the customer. If needed, pieces that don’t meet spec can be dropped out for other products or for re-drying. The second benefit to be gained is using the information to modify the manner in which future charges are dried. In-kiln determinations of final MC are limited with regard to the level of detail and proportion of the load that can be assessed whereas an in-line meter at the planer mill provides the opportunity to gather MC data on every piece that comes out of the kiln. A properly installed and calibrated in-line meter at the planer mill is still the best source of information for assessing the performance of a drying operation.
Achieving Greater Flexibility
Producing a wider range of products dried to different specifications also requires more flexibility in operating. Flexibility can be with regard to response time and the ability to switch from one product to another more rapidly. Flexibility can also refer to being prepared to handle different products at the kiln.
- Improving Response Time – When drying to lower final MCs there are several reasons a faster response time is desirable. First of all, with a wider range of products, producing for inventory will result in an increase in the amount of rough-dry material in inventory, which is not a popular choice with mill operators. Secondly, holding lower MC material in inventory for long periods of time can mean having to take special precautions to maintain the MC such as warehousing or other protection from the elements. There are several ways in which response time can be improved:
- Just-in-Time Production – Create better links between the sales and production groups to reduce inventories at all stages and reduce the overall “time in process” through improved material flow.
- Combining Kiln Charges – Identify products with similar drying properties and dry them together, even if it means having to run a gentler (but longer) drying schedule to minimize differences between them.
- Smaller Kiln Charges – If you have excess kiln capacity, look at running smaller kiln charges. With proper care (i.e., adding additional baffles) most kilns can be operated with partial loads. A track kiln can be run effectively with only one track filled. Similarly, package-loading kilns can be run with fewer rows of bundles. The ultimate way to dry smaller kiln charges is to buy smaller kilns when it comes time to replace or add to existing kilns.
- Operating Flexibility – Along with changes to equipment, the operating procedures at the kilns may need to be modified. In the past, many operators have been able to use one or two drying schedules to cover most of their products. Producing for a wider range of final MC and product quality specifications will necessitate having to develop a broader menu of drying schedules and shut-down procedures. This puts more pressure on the kiln operator to develop those schedules and more demands on the time of that individual to monitor things more closely at the kilns.
- Implementing equalization treatments at the end of the drying phase is another way to facilitate drying slightly different products together. Equalization usually requires the addition of humidity to the kiln environment, which may mean having to add equipment as described earlier.
If you are embarking on a project to produce a new product with more demanding drying specifications, make sure that all the members of the team involved in the project understand that there will be implications with regard to drying. The main one is time but there may also need to be some changes or additions made to equipment. Keep in mind that drying rate (% MC/hr) slows considerably as the wood MC gets lower. Therefore, reducing final MC by only a few per cent will have a significant impact on drying time.
In summary, new opportunities in wood products will only be successful if mills have the capability and ability to produce those products to the desired quality levels. A review of your current drying operations will reveal if you are ready.
Peter Garrahan is a scientist, wood drying, with FPInnovations’ Lumber Manufacturing Group. He can be reached at 613-523-1232, email@example.com.
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