Adding Fans to a Dehumidifier Kiln

      Is it worth it to exceed the design air circulation in a dehumidifier kiln? By the numbers, probably not. May 15, 2012

I'm building my kiln now, and I'm at the point of framing up the fan housing. It's a Nyle 200 drying approximately 4000 bf. 4 stacks 8' long. Chamber is 20' x 13' in case I ever need to dry longer lumber. I plan on building baffles/partitions to block off the extra space. Fans are all 16 diameter; hp; 1,500 cfm. Plans show 2 fans. I have additional fans not being used from another kiln that I can easily put in now if a little more airflow would be better. Ideas?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
What species? What thickness? What incoming MC?

As the NYLE L200 is designed for 4 MBF of oak, if you have an easier drying species, then it is 2500 BF capacity. In either case, the size of the compressor is limiting the drying rate, not the air flow. If you increase the air flow, you will potentially dry the wood faster until the RH is high and the compressor can no longer remove the moisture fast enough. If you will have air dried lumber, then air flow is not a factor in the drying rate, so low flow is fine and means less money for electricity.

The fan formula is CFM =
250 fpm (a good air velocity)
x sticker thickness in inches / 12
x number of sticker openings (or layers of lumber + 1)
x length of lumber

Example: two 8' packs end to end mean 16' of lumber, with 3/4" stickers and 25 layers.
cfm = 250 x (.75 / 12) x 26 x16 = 6500 cfm
Then add 50% for leaks and you need about 10,000 cfm. In this example, your fans would be extremely undersized.

From the original questioner:
Species I saw is usually maple, red and white oak, cherry and walnut. 90% of the time it's all 4/4 and I like to air dry before going into the kiln if possible, usually down to 20%. All loads are of the same species. My stacks are usually 37 layers x 4 8' stacks running end to end so air is blowing across two 4' wide stacks. I wonder if putting in two more fans on separate knife switches would be good for turning half off for easier drying woods?

From Professor Gene Wengert, forum technical advisor:
With air dried, you do not need the extra fans. With oak, you do not need the extra fans. With green hard maple, you do not have a powerful compressor, so you need to shrink the load size, which will increase the fan velocity, so you do not need larger fans. Go with the size suggested by NYLE.

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