Kiln Humidity Monitoring: In-Flow or Out-Flow?

Standard practice is to monitor the characteristics of air before it enters the pile, but monitoring exiting air could work under some circumstances. July 29, 2012

To Gene Wengert: I'm operating a small homemade DH kiln. When I set up, I placed my humidity sensor to read after the air has moved through the wood, not prior as you recommended in another thread.

What is correct? I theorized the air coming off the boards is going to be more accurate in RH/MC than the fresh air entering at opposite side. Wouldn't it be several points different until dry or stabilized? I had a drain line give me trouble and while I was in there I moved the sensor further away (still on after side) from D/H and that changed by 5 instantly.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
You are indeed correct that the relative humidity will increase as the air moves through a pile, especially through a pile of wetter lumber. With some conditions (including low air flow, thin stickers, high initial RH coming into the pile, and fast drying species), the humidity change can become large enough (the RH can get high enough) that the air is humid enough for staining to occur.

It is for this reason that the different dryers require slight modification from the published kiln schedule... That is, the temperature and RH in one dryer will be fine, but the thinner stickers, slower air flow, or whatever, will give a different drying rate in another dryer. So, again, that is why we measure the wood's response (drying rate every day) rather than rely totally on the schedule.

Also, because of this humidity change, the air flow in a kiln is reversed about every two hours. Longer cycles often will result in an abrupt RH change which the lumber may not like.

So, with that introduction, now to the question of which RH to measure... Incoming (which will be drier and more likely to check) or outgoing (which will be wetter and more likely to stain). Perhaps both would be good, but unfortunately, I have not seen a kiln schedule or other guidelines that talk about exit air conditions… what is good, ideal, proper, etc. for lumber drying. (Exception: About 30 years ago I developed the concept of temperature drop across the load and that incorporates both entering and exit temperatures. It is used for softwood drying today, especially southern pine.)

All the schedules and all the guidelines we have now, and have had for decades, are based on the entering air conditions, which are the most severe. The RH as given in a schedule is generally as low as it can be before checking occurs. With good air flow, 3/4" stickers, etc. and narrow loads (perhaps under 24'), the entering air conditions are okay for lumber drying, especially if the lumber is not soaking wet. If we had soaking wet lumber with a long air flow path (such as 24' commonly seen in package kilns), then it is probably better to air dry or pre-dry prior to entering the kiln. By air or pre-drying, we avoid the undesired RH changes as the air moves through the pile.

In your case specifically, the common DH kilns have somewhat narrower loads (and therefore smaller change in RH) than a large commercial dryer used to establish the kiln schedules 50 plus years ago. So, the change in RH is small enough that using the exit side is not fatal. On the other hand, it would be best to use the entering air conditions as that is what all the suggestions on drying are based.

Special note: The wet-bulb temperature on the entering air and the exit air side of the kiln are the same. Hence, we have always used only one wet-bulb in a kiln, except for long kilns where there might be one for one half and one for the other half lengthwise. What does change as the air moves through the load is the dry-bulb temperature. Hence, we use two dry-bulbs - one on each side of the load. (In a few cases, one DB is used, but the system is designed so that the incoming air is always blowing across the DB no matter which direction the fans are blowing.)

Although we measure incoming temperature and RH, we usually measure the exit air flow. This is okay because with air flow, what goes in must come out.

From the original questioner:
Thanks, Gene. I'm not running as large and complex system as you. I don't run as fast drying times as you, where the heat and RH is as critical. My concern is the final air RH coming out, which should be the RH/MC of the wood once it's stabilized. If I read your reply correctly, I'm okay reading on the out flow side, being a small kiln and not using aggressive heat to dry.