Heat Up Rate in Lumber Drying

Most wood-drying situations don't require careful control of the rate at which the load heats up. June 13, 2014

Question
In respect to softwood, is the quality of lumber affected by the heat-up rate? My understanding is if the heat-up is to slow the lumber might get soggy. If the heat-up is to fast the surface will dry down to quickly case hardening the lumber. If the heat-up is just right the water will flow at an optimum rate. What I mean by optimum rate is once the moisture gradient has been established the water will flow from the interior as fast as it leaves the surface. I also believe that in softwood it would be possible to establish a flow faster than we can evaporate water off the surface unless there is enough humidity.

Forum Responses
(Commercial Kiln Drying Forum)
From contributor P:
I think the heat-up rates should be different when comparing dry sort lumber to wet sort lumber. Since there is less water in dry sort lumber the heat conducts to the interior quicker. Therefore the kiln is allowed to heat-up quicker. The main reason for a slower heat-up with wet sort is to evaporate as little water as possible until the interior of the lumber gets warm. Wet sort lumber has more water so it takes longer for the heat to conduct to the interior. It is important the interior of the lumber gets warm before we evaporate lots of water from the surface to prevent case hardening. When I think about what I just said by controlling off the relative humidity in the kiln one could control the rate of water flow.

At lower temperatures below 130F the high relative humidity would allow less evaporation because the air is closer to saturation. Evaporating less water would allow more of the heat to be transferred to the interior of the lumber. Once the kiln gets up to 200F the lumber would be warm all the way to the interior and the excess humidity in the kiln will not only store heat for evaporating water but also control the flow rate of water to the surface. Another advantage to controlling off the RH in a kiln is you could dry lumber with all different moisture contents down to the same target moisture content in one charge.

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From contributor L:
You are confusing some properties - specific heat and thermal conductivity. You conclude that controlling on RH is a good idea and it is. Most higher temperature kilns (operating over 50C) use RH, EMC or depression as the control means. They are all essentially the same in the range that we deal with. RH is less common because until relatively recently, there have not been adequate sensors available.

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From contributor P:
You brought up a good point about the accuracy of measuring the RH in a kiln. I have never really seen any way better than a wet bulb. I have used the Honeywell RH sensor unit but they go out of calibration all the time. The wet bulb is not the best because of it could go dry. The only thing I can think of that would be accurate is a chilled mirror dew point measurement. Has anyone ever seen anything like this before?

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From contributor L:
Chilled mirror systems are expensive and accurate but not too good for a kiln. They generally won't survive in that atmosphere. There are better electronic sensors available now but calibration is still an issue.

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From contributor P:
I know of a company that has designed a chilled mirror dew point sensor for dry kilns of temperatures over 230(F) but is not yet out on the market.

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From contributor W:
There are plenty of chilled mirror products on the market but it is unnecessary and not practical in a kiln that dries the kind of lumber that can tolerate 230F.

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From Gene Wengert, forum technical advisor:
Knowing the relative humidity extremely accurately in a high temperature (over 212 F) kiln is not necessary. In fact, it is probably not necessary in any kilns over 130 F; that is, the present systems are just fine. As a great deal of lumber is air-dried or pre-dried before it enters the kiln, the market for extremely accurate devices is not necessary.

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From contributor P:
Gene do you not believe that an accurate heat-up is critical?

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From contributor P:
Just because I said the dew point meter can handle 230(F) does not mean that it will always be used at that temperature.

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From Gene Wengert, forum technical advisor:
Accurate heat-up would be critical for some hardwoods (such as oak) if they are dried green from the saw. Most oak is not dried green from the saw, however. Other species, such as white pine, hard maple, soft maple, ash, birch, and so on do not require an extremely special heat-up cycle, other than that very high humidity (that is, high RH that results with no venting) must be avoided. Species that are used for construction will benefit from a low initial RH during heat-up to establish tension set and thereby reduce warp. Extreme accuracy is not needed.

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From contributor P:
Why would you want to air dry wood if you can kiln dry it? Many defects come from air drying. A proper monitored kiln should have very little defects.

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From Gene Wengert, forum technical advisor:
If I air dry under a shed (for some woods) or if I air dry in the open (for other woods) I can achieve the lowest possible drying costs, all things considered. Please read Drying Hardwood Lumber for a discussion of costs. Certainly, if you have some quality losses, you may find that air drying is too costly, but in many cases, air drying is not costly and quality is not bad.

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From contributor P:
That makes sense. I was assuming that the wood is sold faster than it can be dried but this is obviously not always the case.