Small Charges in the Drying Kiln

Running the kiln partially loaded involves some tweaks and compromises. August 8, 2008

Question
When loading only partial kiln charges what is the best way to load the kiln? Example - 3/4 or 1/2 of a 25,000BF kiln. Does species and thickness make a difference?

Forum Responses
(Sawing and Drying Forum)
From contributor R:
I would load it in the way that gives you the most even and consistent airflow throughout the charge. You have to be able to baffle off the unused area.

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From contributor W:
In most cases you want to optimize air flow. That might be done in any number of ways such as leaving out a row, or a tier, or making the load narrower and blocking off around the ends. The problem with small loads is usually that there are holes with little air resistance and the air bypasses the lumber. If you avoid this in common sense ways, it should not really matter how you load the kiln. Naturally if you have a kiln designed to dry oak and you want to dry maple, you should reduce the face area to increase velocity. If you are drying thick material, make sure there is enough sticker space to handle to volume or air.

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From Gene Wengert, forum technical advisor:
When drying very wet lumber, partially loading the kiln is a compromise situation. Make sure that your boss or the person that owns the lumber understands that you will do the best you can.

Remember that narrow loads dry faster than normal wide loads, so when omitting a row (which is the best compromise) can mean that overall the wood will dry faster and that may result in checking. The RH and temperature are also a bit more severe throughout the entire load with a narrow load. So, perhaps you want to go just a little bit more conservative than normal with the schedule, but not too conservative.

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From contributor M:
I have a 5000 foot kiln that I often run smaller charges in. One way is to simply make the charge narrower in the air flow direction. Another way that I use in my smaller kiln is to take a nail gun and just built some boxes out of plywood to take up the space at the top of the load.

A couple of considerations in running small charges; it is nice if you can get about the same airflow through the load as you do with the larger charge. I have the luxury of a variable speed fan and an air flow meter, so I do a quick check with a new setup. The other thing is that the kiln will respond to changes quicker and might not look like it controls as well. So the general idea for me is to be even more careful with the small loads in regards to checking as I am with the large loads. You might not need the vents or as much venting with a small charge.