Drying times on different length lumber (WF)

[fbushaw]

Does anyone have an idea on the differnece in drying times for Wf, 16' through 8'?? If you have a whole kiln charge of 8', would it dry faster or slower than a whole charge of 16' lumber?

[GeorgeCulp]

Hi, there's no doubt that "lengths" dry differently (assuming the same thickness and width). 8' definitely dries faster than 16'.

The only real way to know by how much is by tracking your lumber with an inline moisture meter. Its possible with a pin meter however the inline will give far better data (max mc/piece and av mc/piece).

Upon determing the difference in rate of drying of your various lengths you can then "build" your charges accordingly.

We're a southern yellow pine mill. After determing how our 2x4's, 2x6's, 2x8's, 2x10's, and 2x12's, 8-16 dried on an individual th x width x length basis we started experimenting with different charge building scenarios.

Ex - all 2x4's with 2x6x8 and 2x6x10
    - 2x4x8 and 2x4x10 with 4/4
    -  2x6x12, 14, 16 with 2x8x8, 2x8x10, 2x10x8, and 2x10x10

You can begin to see the possible combinations one can come up with.

Also think of building your charge based on "placement" on the charge. If you have more heat on the top of the kiln load than bottom, then obviously load the faster drying items on the bottom. Same with end to end temp variances.  Hopefully you don't have any temp variance within the kiln but if you do then here is a way to work around it.

If you have zone control you could adjust the DB in various zones t o account for the difference in drying rate based on length.

Hope this helps.  George Culp

[Gilman Blackshear,Ga]

George,
What do you think makes the shorter lengths dry faster? I have tested before drying only 18' and 20' SYP together which resulted in a lower drying time than if I had mixed them with shorter lengths.
What would be the explanation for this?
Thanks
Carri

[MichaelM]

Regarding which dries faster - short or long depends on several factors.  We always teach to sort as much as practical, but as George points out, folks who know their kiln and product characteristics can gain an edge without sorting.

I think we need to be careful when comparing high temperature drying to conventional temperature drying.  During high temperature drying a pressure is developed in the board and a lot of moisture is lost out the ends (research in NZ on radiata pine demonstrated this).  Therefore one might expect shorter lengths to dry faster at high temperature if everything else was equal but this might not be true at low temperature.

Another factor to consider might be sawing pattern in the mill and where the short boards come from in the log.  We know long boards rarely come from the outside of the log.  The short softwood lumber might be going in at a higher MC and take longer.

Some one suggested measuring package moisure contents at the planer.  This is a fairly easy way to determine for yourself how length affects final moisture content.

Thanks to everyone else for good comments.  Obviously this is a question with many answers.

[GeorgeCulp]

Carri, first off as Mike points out there is a big difference in high temp drying and conventional drying. My answer assumed you are high temp.

I have no explanation as to why the 18 and 20 footers dried faster.  I do know, in our case, it was blatantly obvious once we started tracking packages in each kiln charge that the shorts dried faster than the longs just as narrows dry faster than wides.

George