Kiln-dried lumber

Q: I need to increase the annual amount of kiln-dried lumber for our rough mill expansion. What good options do I have? More air drying, build a predryer, build a kiln or faster schedules? I am drying mostly 4/4 oak.

A: I must start the answer to your question with a question and comment. Is your rough mill operation operating at or near the highest yield possible? I have analyzed many operations where it is possible, without huge capital investments, to add 3 to 5 percent point of yield through better operation - equipment and employees. What this means is that the kilns have increased their production of dried parts by 3 to 5 percent without modifying anything in drying!

Another key item is, whenever you change the drying operation, make sure that the quality of the dried lumber is still acceptable to the customer, the rough mill in your case. So, the question you must ask yourself is, "What will be the quality of the lumber when I change drying options or change the present dryer operations?"

Here are a few drying options to consider:

1. It would be easy to increase the amount of air drying (leave the lumber on the yard longer), thereby lowering the MC of the incoming lumber into the kiln and shortening kiln drying time. The net effect is increased annual throughput of lumber. But air drying quality is often lower than other options, so rough mill yields will likely go down, even though drying production is up. This is especially true with oak. So, this is not a good option.

2. The highest MC lumber determines kiln drying time. A few wet pieces in a load when you start the kiln, even if 95 percent of the load is much drier, means drying times based on the wet pieces. If you eliminate loading a kiln with just a little bit of wet lumber, then you can shorten drying time. In addition, mixing MC in the kiln often leads to increased degrade and reduced yields. In short, better inventory management and loading of the kilns can result in increased production. Each day saved is worth around $30 per MBF in drying profit.

3. A good option is to shed dry, that is, air drying in a shed, with a roof to keep sun and rain off the lumber and movable mesh curtains to close in the shed when the lumber is drying too fast. Quality is superb and drying time is quite reasonable. Plus, incoming MC will be drier and more uniform than regular air drying, so kiln drying times are sometimes 25 percent shorter.

4. How much energy or fuel do you have available? If you have enough energy (probably double what you are now using with air drying), consider adding a warehouse predryer. This is a fast, high-quality, year-round drying option that will provide the lowest MC to the kiln. There are many advantages to these drying systems if operated properly.

5. Adding more kilns is always an option. However, with more kilns you may be tempted to bring lumber into the kilns at a higher MC then you are presently doing. With 4/4 red oak, each 2 to 2.5 percent MC addition to the incoming lumber MC costs about $30 per MBF. Overall, it is difficult to economically justify kiln drying 4/4 red oak green from the saw when there are other drying options available.

6. Unless your operation is very poor, there is little opportunity for increasing production by changing kiln schedules. Faster schedules can mean more over-drying, leading to cup, grain tear-out and shipping, and poor gluing.

In closing, I think that the assistance of an excellent drying/wood manufacturing consultant would be money well spent.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.