Q.
I am designing a small (1500 BF) DH kiln. I am now sizing up the fans and have a couple of questions.
I see messages that describe that the velocity needs to be on the order of 250 ft/min for most woods and up to 600 ft/min for some of the lighter stuff. Does anybody know what are the dominant factors that determine this velocity? I suspect one of two things are driving this design parameter (or perhaps both). One, the velocity could be required to give an even distribution of air throughout the lumber stack to eliminate "wet" spots. Or the velocity is necessary to achieve a surface evaporation rate.
My second question has to do with the sticker thickness. Is anybody aware of the driving parameter here? Given enough pressure, air can be forced through small passages. Has anybody ever experimented with thin stickers (say 1/4")?
I am trying to make this a compact kiln and any info will be a big help!
A.
1. The velocity has several major roles including the rate of drying at the surface when at higher MCs and also the uniformity through the load.
2. Sticker thickness is concerned about volume flow of air and not just velocity. The volume of air is important as well as the velocity--volume determines the moisture carrying capacity. Certainly very high velocity will carry a lot of moisture, but the drying rate will also increase. So, we find that 250 fpm (or up to 600 fpm) is best. Then to get uniformity through the pile, we use 3/4 inch thick stickers which gives good volume flow.
Velocity is discussed in DRYING OAK LUMBER.
Professor Gene Wengert, Forum Technical Advisor
velocity (ft per minute) x total sticker opening area (in square FEET) = required volume (cubic feet per minute)
So the first thing you have to do is calculate the TOTAL sticker opening area and make sure to convert it to FEET. For example, if you use 3/4" stickers, each sticker opening is 0.0625 FEET.
If you have 4' high packages with 4/4 lumber you will probably have about 25 sticker openings per pack x 3 packs high (for example) x 16' long (for example) = 0.0625 x 25 x 3 x 16 = 75 square feet (ft2).
If you want 300 ft/min air velocity, you will need 75 ft2 x 300 ft/min = 22,500 ft3/min (CFM) at 100% efficiency.
We usually figure, to be on the safe side, that about half the air volume will actually end up going through the sticker openings (the rest goes through bolster openings and through other openings which are not tightly closed up).
So in the example above, you should figure on enough fan capacity to produce 45,000 CFM.
There is a more detailed example under DRYING TOPICS at http://www.drykilns.com
