Dealing with Variable Moisture Content in the Kiln Load

Mixed loads with varied moisture contents are an age-old problem with no easy solution. But here are some ways to approach the situation. June 13, 2014

Question (WOODWEB Member) :
I have high temp thermal oil kilns (250 mbf) with good air flow and moisture plates. I agree that the best way to dry balsam is to heat treat and then air dry but sometimes I have to dry straight non-moisture sorted balsam from start to finish in the kiln. How do I get the 15-20% of what I call soakers (40% plus) down to an acceptable percent without totally over-drying the other wood? I have tried low and slow, hard and fast and long equalization times (30 hours). Does anyone have any suggestions on air temps, wet bulb temps, venting times or fan speeds?

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
There are two issues. First, there can be variations in temperature (and also RH and air flow) in the kiln and we do know that a ten degree variation will have a large effect on drying rate and final MC. So, pay particular attention to variation related to location in the kiln.

Second, we do know that sinker (or whatever local name you use) has more moisture in the living tree than other wood which means longer drying time; plus, such wood also dries more slowly. If we mix two different types of green wood (higher MC and slower drying vs faster drying and less initial MC) in the kiln, there are three options. (Of course, heat treating and air drying, or pre-sorting are good ideas, but may not be practical in many situations.)

1. Ignore it and get a lot of wet wood after drying or a lot of over-dried wood.

2. Pull the load when the faster drying lumber is done, sort with a MC meter before the planer, and recycle the wet stuff back into a kiln load.

3. When the fast drying lumber is 2% (or 3%) MC below the correct final MC, then set the kiln at an EMC at 2% or 3% below their target MC. This will stop the dry lumber from drying further, but will allow the wetter lumber to continue to dry.

Do this equalizing at as high of a temperature as possible (under 200 F). This can take a long time (maybe doubling the normal drying time) so it is expensive; hence, number one or number two are often more attractive. Incidentally, I have seen some work trying to pre-steam the load before drying in the kiln, but the results were not very good. Patience (slow drying such as air drying) seems to work.

From the original questioner:

Thanks Gene. I will try option three the next time I have to dry it like this.

From Contributor R:
We are drying mixed balsam with ponderosa pine and have the same problem (either too many wets or over dries). Sorting before it enters the mill is not possible and with our high gas fired kilns (130' long) we are sometimes hand to mouth with the planer so longer hours are out. We dry by TCS and Accudry but rely on stack probing to verify end moisture results. Our HVP and 2 and better has dropped from 90+% to 70+%. I know mixing balsam in causes a headache for kiln operators but need to try and find a solution to raise the HVP.

From Gene Wengert, forum technical advisor:
First, this is an age old problem without an easy solution. In short, if you put two pieces of wood into a kiln and one of the pieces requires two days to dry and another four days (the reason why the difference can be species, thickness, bacteria, previous drying or initial MC, etc.), there is no way that we can run the kiln for two days without having wets, or run the kiln for four days without having overly dry pieces.

It would be effective if we could presort, but that costs money and sometimes cannot be done accurately. Green MC meters have been under development and have been used for years, but if they are used, then this means more sorting and stacking and handling, and more money. What has been used from time to time is an in-line MC meter after drying. The kiln is run with the intention of perhaps 10% to 15% to 20% wet lumber at the end. This wet lumber is pulled out, using the MC meter to identify it. When enough wet has been accumulated, then it is re-dried in short order. The cost of rewet pulling, redrying, etc. can be calculated to determine the optimum % wet in the first drying load. I have seen some studies trying to steam the lumber prior to drying to help, but such steaming is not highly beneficial.

Another alternative that works and is used in hardwood drying all the time is to increase the RH in the kiln when the direst lumber is as dry as desired. This stops over-drying, but lets the wets continue to dry. It extends drying time, obviously. In hardwood drying, the process is called equalization. In softwood drying, it is called equalization and also called conditioning. Note that variation in final MC can also be increased due to variations in kiln conditions - RH, temperature and air flow. It is for this reason that I developed with Larry Culpeper the zone kiln and TDAL kiln over 30 years ago; it worked well if properly maintained and if the temperature is correctly measured. Finally, if lumber is air-dried first, that will make the incoming MC more uniform and will eliminate the problem. Of course, AD costs money and takes time. So, "What is the best for you and your pocket book?" is tough to answer.