Brown Stain in Ponderosa Pine

[robshow]

Could anyone suggest a schedule that you are using for limiting brown stain?
Recently i have researched the issue and attempted to adjust some schedules but have not seen any difference.
We use double track LSI kilns with line shaft fans 100' and also double track Wellons with cross shaft fans 66'.
The Brown stain occurs in both types on kilns in shop 5/4, 6/4, 8/4 (worst in the 8/4)
Im getting ready to use a schedule that is published in the book KILN-DRYING WESTERN SOFTWOODS by Edwin Knight,  it states that it is a 6/4 schedule that will limit brown stain.
Any suggestions?

[HencoV]

Hi Robshow,

What is your current kiln schedule? Also what air speeds do you get on the exit side?

[robshow]

Thanks for the reply.
Not sure how to answer the first question, do you want me to describe BD WB and time at each step.
Air flow is average of 725.

[HencoV]

If you can, post a screenshot of the process graph for one batch..showing set point and actual values

[robshow]

Here is the schedule we use on 6/4 shop.

[HencoV]

Comments on you schedule and process graph regarding Brown stain. Not always 100% scientific or in line with literature, but my observations and opinion based on experience on kiln control none the less. Use it… or not

In short -kiln brown stain is formed when extractives like proteins and sugars accumulate at the evaporation front below the surface of a board. These extractives caramelise above certain temperatures (Somewhere around 50+oC/ 122+ oF) forming the brown stain…often highlighted when varnish is applied to the plained surface. Brown stain does not affect the strength of the timber.

Looking at your schedule, it is evident that the compiler of this schedule was attempting to create a  “soft” drying condition. Also seeing that the brown stain is an issue, I am assuming you are drying for appearance grade.

Looking at your process graph and schedule, the following observations:

Heat up and spray: You are not using a spray for humidification. This is possibly neutralized by the very slow heat up phase. Though not necessarily bad, I would use a wet steam spray to humidify the air inside the kiln, at least up to the first time I start venting. You can probably reduce you drying time considerably if you use a spray/humidification during the heat up phase, instead of relying on the woods moisture to supply sufficient humidification

Venting: During the first 15 hours of your drying cycle, your vents are 100% open for ± 40% of the time.  Even though this is at a low temperature and high EMC (±15.9%), this could already force the evaporation front deeper into the timber, setting you up for a barrier where the extractives gets stuck later, and creating an insulating “dryer” outer layer which will reduce energy transfer to the centre of the board and reduce your drying rate…requiring more energy to dry the timber.

I am not a fan of proportionate / modulating vents. Even though it makes more sense from a control point of view to control the WB more accurately, it comes at a cost of constantly loosing pressure, air flow and energy transfer through the stack. I compare it to trying to inflate a tyre with a huge gash in the sidewall…pressure never really increases enough to keep the tyre inflated. Same applies for the pressure differential over the fans and between the constantly open vents. (but this is a title for a complete study on its own)

Air flow, Wet bulb and Main steam:  Your process graph shows an avg WB reading that fluctuates every time the fans changes direction. Your WB avg line should follow the vent set point arc. It looks like it varies by ± 5 Deg F from around 50 hours into the process in each direction. I don’t trust the WB value. It indicates that there is possibly an air flow issue between forward and reverse, or one of the probes making up the average giving a very low or high reading .

The fact that the vent position does not change much/ enough to get the WB avg onto a less fluctuating arc, tells me that the logic controlling the vent is not reactive enough. (Control not sensitive enough to WB temperature changes, dead band too wide, hysteresis error or PID loop not tuned properly.)  I don’t believe in using average values for this most critical part of drying. At each wet bulb, measure and record the air flow over it, with all doors and vents closed and fans running, in both fans forward and reverse directions. Identify the probe with the highest and most constant air flow in both directions and use only that probe for WB control and venting.

Your main steam valve on the other hand, opens more in the one direction than the other. This indicates better air flow (more energy uptake/demand) in one direction than the other. It could also indicate a baffle problem or loose panel or faulty probe in the direction where main steam closes more ..you need to investigate. You will probably find that one or all the stacks on one track have a slightly higher MC% or even patchy drying.  If your proportionate vent control was good, the vent position would be “more” open in the direction that the main steam is “more” open to compensate for more moisture coming off the timber..again indicating a problem with the venting control.
Ok?

With that out the way, is there a fix for brown stain?

 I believe that by understanding the dynamics of your kiln, and applying some common sense, one can reduce, if not overcome the problem of brown stain.  Everybody I have come across who had a problem with brown stain, opted for the “softer” drying schedule route as you have, very few have achieved what they set out to do, and if they succeeded , they were not 100% sure how. No published schedule is a perfect fit for the next kiln. Even two identical kilns don’t react the same to the same schedule due mainly to variables like different air flow, heat transfer and venting capacity &strategy. These should be balanced and within the safe schedule limits that the specie you try to dry can handle. I regard published schedules  as the “safe” starting point. 

The key is to attempt to keep the evaporation front as close to the surface as possible. In other words, match the vent set point with the evaporation rate. Try the following with your kiln:

Set the proportionate venting to be as sensitive as possible.

Phase:   DB at end of Phase:   WB/Vent  Set point:   Spray SP:   Hours
1.          165 Deg F   :                       145:                 142:      8:
No venting until WB SP is reached, increase DB until 165 at 8 hours
When the vent close to more then 40%, go to Phase 2 WB/Vent SP
2.          165                             141                 No Spray
When the vent close to more than 40%, go to Phase 3 WB/Vent SP
3.          165                             138                          No Spray
When the vent close to more than 40%, go to Phase 4 WB/Vent SP
4.          165                             135                          No Spray
When the vent close to more than 40%, go to Phase 5 WB/Vent SP
5.          165                             131                         No Spray
When the vent close to more than 40%, go to Phase 6 WB/Vent SP. This should be around 25% MC
6.          165                             127                         123

Stay on this SP until you reach desired MC%

Note: The 40% position limit is just a guess. If you study the way your kiln react to these setting, you may find that you may need a higher or lower value as indication for when you need to increase the WB depression.

Good luck.

[robshow]

Thanks for the info HencoV !!!
Thanks for putting so much thought and effort into your reply, im still trying to digest it.

We try not to use the steam spray for heating up although we have a 4/4 schedule that uses it. We tend to have an issue with kiln stain if we do too much live steam (we already use quite a bit  conditioning on the tail end).
I like what you said about the venting and pressure drop. This definitely cant help with descent air flow through the units.
We are going to begin an air velocity test in our kilns and i will check the WB locations and try that which you suggested.

I didnt quite understand the schedule that you created. Some of the terms and the format of it. As you see my schedule format is a little bit different.
At this point with some of the changes we have made our 5/4 shop is looking a lot better. I wont know about the 6/4 until they run that next week.   

Again, Thank you for the help. Im going to share this with others (electricians, maint supervisor) at my mill and see what kind of changes we can make.

[LarryL]

Has this problem always existed, or is a new experience? One thing to consider, are you sawing fresh logs or decked logs? If decked how old are these logs, and have they been stored under water(sprinklers etc). While in California years ago I dried millions of feet of Ponderosa and Sugar Pine. Fresh logs are pretty forgiving, most conventional schedules can be used. Old decked or sour deck logs posed the biggest challenge. Low initial temperatures and low humidity were required, Conditioning could create issues. Lumber that was dried to 12% or less wasn't a problem. Boards that were above FSP showed brown stain issues. The main to consider early in the schedule is not to pull the moisture so fast that it pools on the surface. If your airflow is not sufficient to remove the water from the surface it is a tough battle. We had different schedules for fresh logs versus decked logs. We also had summer and winter schedules. Use a hygrometer to confirm that your recording temps and sensors are correct. Especially your wet bulb readings. Your humidity may be higher than you think. You might have to remount your wet bulbs to insure there is proper airflow over them to give you accurate readings. Too high humidity at the wrong time of the schedule is a recipe for brown stain.

[robshow]

Thank you LarryL.
This is the first time in my 6 year career that i have had to deal with the brown stain issue.
These are decked logs and only about 6-8 months old (at our site) we do a good job watering with sprinklers.
I will look into the WB locations and check air flow and calibration.

The charges of 6/4 that we are running now at the planer still dont show any improvement from last schedule adjustment.