Air Velocity

[Kierman Dimitri]

Hello all,

Does anyone know of a good resource for information on Air Velocity? The DKOM is somewhat veage when it comes to this subject. I would like to try and establish some general guidelins in this area like FT/MIN at various stages of drying ETC...............


Thank you

[Joe D]

What are you drying?

[Kierman Dimitri]

We are drying anything from 2x3 to 12x12 DF approx. 100,000 BF charges in a package loaded DH kiln. Our most common size is 4x5.

[Joe D]

In drying green softwoods high air velocity and limiting the distance the air travels across the load is extremly important to achieve rapid and uniform drying. Conversly when drying air dried hardwoods high airflow is not as critical.

Ball park figures:
High temperature drying of softtwood dimension lumber, 1400 FPM
Conventional temperature drying of softwood 4/4 and 5/4 lumber, 700 to 900 FPM
High temperature drying of softwood timbers, 1200 FPM
Conventional temperature drying of softwood timbers, 700 to 900 FPM

Also on large timbers you might consider moving to larger sticker sizes (from 3/4"-7/8" to 1-1/2" - 2". And leave a space between the edges of the lumber.

The above has to be modified for species limitations and kiln limitations. When you say you have a large DH kiln I imagine it to be a package kiln with a relatively long air travel. You are probably limited by the amount of moisture the DH unit can remove. So you run a fairly mild schedule, with longer run times. The mild schedule probably mitigates the lack of high airflow.

It is still impotant to maximize airflow with the tools you have.

Merry Christmas.

[MichaelM]

Joe gave a good answer.  You might also look at the wet-bulb depression in and out of the load.  If you have a large wet-bulb depression going in and a small one coming out (say like 30-40F in and 5F out) then maybe you could use more airflow.  Wet lumber in the middle of the kiln is sometimes an indicator of poor airflow.  Mike M.

[Kierman Dimitri]

Thank you for the information both Mike and Joe.

Mike, am I understanding correctly when I say that if there is a large TDAL then I could increase the airflow and if there is a small TDAL then the airflow is sufficient? and if this is the case then should I measure this temperature drop across the entire chamber in a package loaded kiln, or accross only one load within the chamber as there is a significant difference between the two measurements?.

[MichaelM]

If there is a big depression going in, then expect a bigger temperature drop.  If there's a small depression going in, then expect less of a temperature drop (because the drying rate is less).  What you are tring to avoid is having the air approach saturation (a narrow wet-bulb depression) before it get reheated.  There may be times when you want a narrow depression going in, such as with the 2x12 DF.  At these times the drying is slow everywhere.

Measure the drop after the air passes through the load and before it is reheated.  In a package kiln or side loader this means from plenum to plenum (unless it has swing out reheat coils).

While we are on this, imagine the worse case - say the air becomes saturated halfway through the load moving from left to right.  In this case there is no drying at the center or the right.  After the fans reverse the right dries but the center still does not.  So the sides of the load dry but the center does not during the first part of the schedule .  The center never catches up unless there is a lot of time at the end for equalizing.  If you do not have this wet pattern and your DH unit is working at maximim, you probably have enough airflow for your kiln.  Assuming the wood can take it and you can work the DH unit more, this pattern would be another thing to look for as you dry faster.  From a package kiln, keep track of the middle units and side units as they are run across the in-line moisture meter (or use a hand-held meter).

[Kierman Dimitri]

Mike,

In our package loaded kiln without compromising the plenum area at both ends, would it be better to have a space in-between each column of lumber or would it be ok to have them right up tight to one another?. I  am making reference to where the air passes through the stacks of lumber from front to back not end to end and we can load the chamber with 5 rows deep using 48inch wide packages and have approx a 12 inch space or chimney between each row or we can load 6 rows of 48inch wide packages right up tight to one another and have no space at all.

[MichaelM]

I would keep the packages together, but leave a small space, 2", between each.  You don't want to significantly reduce the plenum.  If the packs are butted tight and the sticker slots don't line up then the airflow might get blocked.  Also, use the side and top baffles.

[GeorgeCulp]

Concerning your question of keeping the distance between the columns far apart or tightedged here are some things to think about.

1. Do you have forklift crossouts in your columns? If so, and via these crossouts you can "tie" the columns together such that they are all "lowered" equally when they shrink then tight edged is best. (but you gotta be on the money here.

2. the downside of having a large distance between the kiln columns is that as that after the air exits the first column, in order to enter the second column it must re-enter via the "layer entrance conditions".

The "entrance" to each layer is at a point where the top and bottom pieces of lumber form 90 degree angles. This is a less than optimal entrance.  Without getting overly technical the gist is that it will cause your airflow to go down.