To Pre-sort or Post-sort? That is the Question
The objective in writing this series of drying articles isn’t to tell you how to dry your lumber. Instead it is to put ideas forward that are known to have some merit with regard to drying and let you decide what is relevant to your operation. The main thing is to be open to different ideas. In drying, more so than many other areas of manufacturing, the solutions tend to be site specific. This is because of the variability in wood from region to region and the impact that the logging, log storage, sawmilling, and green lumber storage practices have on drying. The topic this month is re-drying. Although we at FPInnovations have done some very specific work related to re-drying of SPF, this article will deal more generally with the concept of re-drying and how it can work to your advantage.
Re-drying is by no means a new concept. The idea has existed for many years and this technique is employed in a number of areas around the world. Where it is used, it has been adopted primarily to address problem species. Western hemlock and Eastern white pine are two good examples of species that exhibit problems due to wet pockets and where the concept of re-drying has been employed successfully. Any species or species mix where there exists a large variability in the drying rate between individual boards in the load is a possible candidate for re-drying. Other species that contain wet pockets include balsam fir, sub-alpine fir, trembling aspen, and balsam poplar.
The best way to illustrate the underlying justification for re-drying is to look at the differences in drying rate (and hence drying time) within a given species and product. Figure 1 (on page 20) shows how drying times vary within a mix of Eastern Spruce-Pine-Fir. As shown in these drying curves, the drying times for black spruce and jack pine are quite similar i.e. 50 hrs. versus 55 hrs. to a final moisture content (MC) of 15%. However, balsam fir shows considerable variation both within and between species. In fact the slowest balsam fir may take up to three times as long to dry as jack pine and black spruce. An obvious solution would be to sort balsam fir from the rest of the lumber and dry it separately. As shown in figure 1, however, this does not totally solve the problem as there is still considerable variation within balsam fir. Mills with a small percentage of balsam fir may find that pre-sorting it is the easiest way to deal with the problem.
For mills with a higher proportion of balsam fir or those dealing with any of the other wet pocket species, they need to identify in-kiln solutions to the problem.
The traditional way of dealing with wet pocket species is to modify the drying schedule to achieve the desired end point. The problem with wet pocket species is not that they dry slowly, but that there exists a wide range of drying rates between pieces. When drying most wood products, the MC tends to converge toward the end of the drying cycle. When drying a wet pocket species, however, there is a wide range in MC at the outset of drying and this tends to persist. If the drying schedule is simply extended, the wet pocket material will eventually reach the target MC but by that time the rest of the load will be well over-dried. This will inevitably result in extra drying degrade, in the form of warp, and potential problems from putting wood into service at too low a MC.
If the end use demands a narrow range of MC, then the drying cycle must be modified by adding an equalization treatment. This involves moderating the drying conditions to stop or slow the drying of the pieces that have already reached the target MC while allowing the wetter pieces to continue drying. Unfortunately this is a very slow process. As an example, when drying 8/4 eastern white pine for shop grade lumber, a majority of the material will have reached the target final MC by about 14 days into the drying cycle. Adding an equalization treatment to deal with the wet pieces typically extends the drying cycle for this material to about 28 days. Although the drying times are different, Figure 1 shows that a similar situation exists with balsam fir. Since balsam fir is generally being processed for a commodity product we cannot generally justify lengthy equalization treatments. As a result many mills end up having to develop a compromise between the proportion of “wets” and the amount of “over-dried” material that can be tolerated.
Re-drying as an Option
If someone told you that they could offer you an opportunity to reduce your drying time by 25% or more, most mill operators would jump at the prospect. This is the opportunity that re-drying presents. However, as with most things in life, it comes at a cost. The one big hurdle with re-drying is the extra handling that is requires to identify, sort, re-pile, and re-dry the “wets”. In order to make an informed decision on re-drying it is necessary to consider not only the factors that contribute to increased operating costs but also those that result in a net increase in profitability.
Figure 2 (on page 20) shows the distribution of MC after 40 hours of drying for a typical Eastern SPF mixture. At this point, 81% of the load is within the target final MC range but 19% of it is still “wet”. Table A (on page 20) shows that in order to achieve the target of no more than 5% “wets”, the drying time must be extended to 97 hours. Table A also shows the results at several other intermediate points. Therefore, in this example, the drying time is more than doubled in order to dry a further 14% (19% “wets” at 40 hours minus 5% “wets” at 97 hours) of the load. By that time, 89% of the load would be considered over-dry. In reality, this very seldom happens as mills end up stopping at some intermediate point, accepting a higher proportion of “wets” and sorting them out to sell as an “off grade” product. As shown in table A even a compromise like this results in a considerable increase in the proportion of over-dried material and the estimates of degrade, listed in the table, show the economic impact of this.
These data were generated by OASiS© and show the impact of extending drying time on percentage of over-dried material and drying degrade. This example is based on a mill drying Eastern SPF (15% balsam fir, 20% jack pine, and 65% white and black spruce) on a conventional drying schedule.
Consider the previous example with a mill drying for 40 hours and sorting out the 19% of material that is still “wet” after the initial kiln run. The idea behind re-drying is that this material would be accumulated and returned to the kiln to complete the drying process. In this case, it would take five charges of green material before enough re-dry material was accumulated to form a complete charge. This difficult-to-dry material would likely take about another 50 hours to finish drying on a conservative schedule. By doing this we end up with a much higher percentage of material being dried to the target MC range. This has benefits for everyone including better productivity at the planer mill and better performance of the final product in service.
The other benefit associated with re-drying is the freeing up of kiln time to dry more material. Using the previous example again, if the drying time without re-drying was 70 hours (a compromise between “wets” and “over-dried”) there is a potential to save almost 30 hours for every green load that goes into the kiln. Therefore, for every five charges of green material dried there is a total of 150 hours of drying time saved. Given that the one charge that needs re-drying will take 50 hours of kiln time, the net benefit is a saving of 100 hours. This is time that can be used to dry additional lumber.
• Decreased drying degrade
• Improved final MC uniformity
• Increased efficiency at the planer
• Decreased energy consumption
• Improved final product performance
• Process ensures that all material is dried to the specified MC and can virtually eliminate the potential for moisture claims
• Facilitates the process of kiln shutdown
The downsides of re-drying are:
• The cost of re-handling and re-stacking lumber
Re-drying is conceptually different than pre-sorting. In pre-sorting, we are trying to use a measurable property of the green material to predict fast versus slow drying boards. There are a number of systems that have shown good results for pre-sorting green lumber but they are all prone to some inaccuracy. In re-drying we are not concerned about what causes a board to dry quickly or slowly. By measuring the MC of each piece after an initial pass in the kiln, we have a sure-fire way of identifying the pieces that need extra drying time and dealing with them.
Ways to Re-dry
All of the above discussions have been based on the assumption that the material to be re-dried will be returned to the kiln in which the initial drying took place. This is the established way of handling re-drying and is currently practised in various mills in Eastern Canada and New England for the re-drying of white pine as well as in the Pacific Northwest for the re-drying of Hem-Fir. There are other options for handling the re-dry portion of the lumber mix.
One possibility is to install a small-capacity kiln to handle the re-dry material in smaller batches. This kiln could be designed to achieve the desired conditions for re-drying, which, as described earlier, will essentially be an equalization process. For proper equalization, humidity control to achieve higher equilibrium moisture content (EMC) conditions is important. The incorporation of a humidification system would therefore be desirable. In addition or instead of a humidification system, lower operating temperatures make it easier to achieve higher EMC conditions in the kiln. This is another way to achieve a more uniform final MC but at the cost of stretching out the re-drying time. Rather than using a kiln designed to operate at high temperatures, a lower-cost chamber such as a dehumidification kiln, low temperature kiln, pre-dryer, or heated warehouse with humidity control and minimal airflow could be set up exclusively for this material.
Another option that some mills have explored is to simply air dry the “wets”. Air drying (assuming it is done properly) is the ultimate equalization process as outdoor EMCs prevent any over-drying of the material. This option works well for species that have a final target MC of 12 to 14% or higher. Anything lower than this cannot be achieved in an air-drying yard, and would need to be handled in a dry kiln.
To address the issue of re-handling and re-stacking some companies have looked at using radio-frequency vacuum (RFV) kilns. These kilns can accept the lumber in a solid stack. There is a significantly sized operation in the U.S. Pacific Northwest that does exactly this for the re-drying of Hem-Fir. At FPInnovations we have done some pilot scale testing to evaluate this option for Eastern SPF. The results show that “wets” can be successfully re-dried in a RFV kiln, but this is an expensive solution being applied to a relatively low-valued commodity product. Careful economic analysis is required. FPInnovations can provide some performance data to anyone wishing to evaluate this option further.
Final Word on Re-drying
Re-drying is a drying option that addresses the problem of moisture variability at the end of the drying process, rather than at the start. As a result it is an alternative to pre-sorting and all of the extra costs associated with that (extra bins, material identification and differentiation in the green yard, and setting up homogeneous kiln charges). It works best when the proportion of material to be re-dried is relatively small. If, for example, the amount of material needing re-drying is 50%, it is not likely that a mill could justify re-handling such large amounts of material. Mill operators need to consider their own situations and pre-assess their potential to benefit from this drying strategy. FPInnovations has considerable information specific to different species to help mills make this assessment.
Peter Garrahan is a drying expert at FPInnovations Forintek Division.
December 1, 2011 By Peter Garrahan
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