Calculating airflow in a kiln

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Quantifying airflow through lumber stacks. April 2, 2002

Can you predict the airflow through lumber stacks, if you know the kiln size, fan outputs, etc? There must be a formula for this. How does static pressure enter into the calculations?

Forum Responses
From contributor N:
The first step is to determine the fan performance. Since we are pushing air through a lumber stack, we encounter some resistance, which increases as the airflow increases. This is called static pressure. For my calculation, I normally use 1/2", but I know most people use a value between 1/4"-3/4". You can obtain these fan performance graphs from the manufacturer.

For the purpose of this example, we will use a kiln that is 27' wide and a stacking height of 12'. As a rule of thumb, half the "window" is lumber and half is free air on 4/4" lumber. We will be trying to get about 400 feet per minute airflow.

Window of open area: 27'x12'/2= 162 ft2

Air volume needed for 400 ft/min.: 400x162 = 64800 ft3

Our 42" fans will provide you with 20000 ft3/minute at 1/2" static pressure. Three fans should be sufficient for an airflow of nearly 400 feet per minute.

Total air volume from three 42" fans: 60000 ft3

Estimated airflow with three 42" fans: (60000/162)= 370 ft/min.

From contributor B:

This example only gets you to the theoretical air velocity through the sticker openings. It assumes your piles are perfectly piled, even ended, and the same width as the kiln, and that all the air goes through the sticker openings from one side of the load to the other.

In reality, you must allow for some losses, where air goes around or under the load. Even in a well-baffled kiln, there will be some losses around the ends of the boards. And if you have a pile of random length wood, or bundles or differing lengths, these can be even higher.

When we calculate estimated air velocity, we might assume as high as 30% losses, depending on the load configuration the customer is expecting. In a tiny kiln, your numbers will be better. In a large commercial kiln, these losses become a more important part of the equation.

From contributor D
Contributor B is right. I usually use 40-50% allowance depending on the product and stacking practices. One thing to remember - the velocity you need is related to the depth of the pile. For example, you don't need nearly as much velocity when you are blowing through 4' of 8' of lumber as when blowing through 20'. Also, species has a big effect.

From contributor N:
50% was a rule of thumb.

If one is to consider 4/4" lumber as 1 1/8" with 3/4" stickers, the actual opening is 40% of the face. (075 / (1.125+.75) = 40%.

This means that there is included 10% extra allowance for additional opening. Or in other words, 25% losses (25% of 40% is 10%). So I believe contributor B and I almost agree on the calculation.

I can only agree that the air velocity required will depend on the species, thickness, and depth of the kiln charge.

Although this may be controversial, I like to install a little extra fan capacity and then either install frequency drive (generally only affordable on larger kilns) or operate the fans in intervals. We have many customers who believe the interval-operated fans reduce energy consumption and improve drying quality.

Contrary to the first answer, you do not need to establish fan performance first. The first thing is to establish the airflow required or suggested for a particular species and thickness. This can range from 250 fpm for species like oak, 4/4 to 8/4, up to 600 fpm for hard maple 4/4-6/4, and even higher for many softwoods.

Next step is to figure out the area of the sticker openings that you have. Multiply the height of the sticker (inches) times the length of the lumber (feet). If you have several stacks, end to end, like both an 8' and 12' stack to give 20' overall, use 20'. Then divide the answer by 12 to get it into square feet. Then multiply this answer times the number of sticker openings. This gives the sticker area.

Next multiply the total area times the velocity to obtain cubic feet per minute GOING THROUGH THE LOAD.

As mentioned, there will also be some losses through the 4x4s, around the ends of the load, and so on. Therefore, you need to account for this loss as well. In a well-loaded kiln, you would have to add another 50% to the original cfm value.

Gene Wengert, forum technical advisor

From the original questioner:
My size is 18x12 area = 216-2 = 108. I not sure of my fan flow with my nile. Should there be an extra fan?

From contributor D:
18' is marginal with the standard two fans. It would be okay if you always dried air dried or thick stock. If you dry green lumber, especially stuff like maple or pine, a third fan would be a good idea, though when doing those species we recommend a smaller load anyway, so if you stack right, you can be fine with two fans. How's that for fuzzy?

Contributor D is correct when he points out that 18" is too wide. It is too wide for green maple and similar lumber no matter what fan speed, fan volume, etc. You will get too much change in EMC as the air moves through the load. That is, the EMC and temperature at the entering spot will be correct, but after 12' of air travel the EMC will be too high and staining is likely.

We address part of this problem by asking for 500 to 600 fpm through the load, but you also have to consider the load width. In fact, for white woods, we suggest loading the kiln half full for the first day or two in order to achieve the correct conditions throughout the load and also flash off the surface moisture. Then, add the remaining half of the lumber, blowing air initially through the dry part of the load first. More info on this is in DRYING HARDWOOD LUMBER.

Gene Wengert, forum technical advisor