proper chimney

[matthew bower]

I have a few questions about the proper chimney to have when loading kiln carts.....does the gap in between the two bottom units need to be a specific length? does it very with species? does it change with what type of lumber cut? i have 24 seperate kiln tracks and i run incense cedar, panderosa pine sugar pine and white fir. thanks

[MichaelM]

Leave an inch or more between the packages.  Close at the top and bottom if possible.  That’s the short answer.

The “chimney” is probably a poor name because it implies that the air is supposed to go up or somehow vented.  The name comes from when wood was regularly air dried and wide packages (8 feet or more) were built by handstacking.  A chimney was built into the middle of a unit to allow the colder air (colder because it picked up moisture) to move downward and pull air in from the sides of the pile.  This created some natural convection so that airflow occurred when there was no wind.

In kiln drying, the chimney is there to allow the air to move from one unit to the next unit on a track.  If the sticker slots line up exactly, then you do not need any chimney as George Culp pointed out in a previous post.  I would agree until next week when new stickers are added or a saw gets off and the load height changes or when the units are coming from two different parts of the sawmill.  To compensate for all the imperfections we leave a space that somebody decided to call a chimney instead of calling it an air distribution gap simply a space.

In practice, the package width from the sawmill must be closely related to the cart width in the kiln.  Assuming two units per track, the cart width should be exactly equal to two times the unit width plus the chimney space.  This way the lift operators can set the unit at the outer edges of the cart and the chimney space will automatically happen.

The top can be closed by pushing on the side of the top unit so that the top few layers move in toward the center of the kiln cart an inch or two.  Some mills close the bottom by dropping a board, such as a 2x4, between the units.

[GeorgeCulp]

First of all if you really want to help the drying process then eliminate the chimney altogether. What I mean is instead of two 4' stacks per track you have only one 8' stack.

But.... you live with what you got. The biggest problem with establishing the "best" chimney size stems from the fact that column shrinkage will change "what you really want".

Again, what do I mean? The bottom 2 side by side stacks are only going to have their stick slots mismatched due to shrinkage differences vey little. However, the top two side by side stacks will be afftected by their shrinkage and all that down below them. I've seen the the top two side by side stacks to have their sticks completely mismatched.

If its the bottom side by side stacks you want them to be as close as possible, even touching. The top two side by side stacks to be as far apart as practical.

Why? When the stacks are spaced apart and the airflow exits the first stack a new set of "airflow entrance conditions" will be created for the second stack. This will cost you in total airflow. If they are touching such that the stack resembles an 8' wide stack then this will not happen and overall thru the slot flow will be maximized.

However the "catch 22" is that if you have variable shrinkage from one kiln column to the next then if the stacks are too close then air flow will be harmed dramatically or possibly even cut off completey.

It has always "baffled" me how there is anything positive to having two 4' stacks on each track.

[GeorgeCulp]

Assuming that you are going to have to have a chimney simply because you can't keep the sticks of each column in line through out the charge time the practical solution (not the ideal spacing) but the most practical one is a spacing that you can REPEAT for each and every charge.

I could get very deep into the aerodynamic and fluid mecahnical aspects of whats going on but it would be way to boring. Suffice it to say that the chimney itself imparts a change in aero characteristics of the charge. However, the fact remains that you have to have one. So, the best one will be the one you can repeat charge after charge after charge.  And the aero characteristics created by it will be the same every charge and you will learn how to deal with it cause it happens the same on evey chage.