Sanitizing Temps

[burdman_22]

Wanting opinions on how much longer to leave my wood in the kiln for sanitizing.

I'll start with, i know the international standard is 133 degrees internal temp for at least 30 minutes. and I've seen the suggestion on here saying to keep kiln at 150 (i think), for 24 hours. Also, i know it would be better to have an actual reading of the inside of the boards.

i assume this 150 degrees for 24 hours thing has some wiggle room, and I'm wondering if there is a formula or rate or something for how fast the inside of a 1 inch red oak (or any other wood) heats up for a given external air temp.

The temp in my kiln has been above 100 degrees since 0351 on the 21st. It reached 133 degrees 15 minutes ago (we had a few breaker tripping issues). At the rate my temperature has been increasing, it will take 11 more hours to get to 150 degrees. Surely I don't need to wait a full 24 hours longer for the temp to get to 133 inside the boards do i?

Just need a formula if there is one, not looking for advice on heating methods or any criticism, as I already plan to revise my methods after this load finishes. Just trying to get through this one first.

[beenthere]

What records do you need to supply to the customer or exporter to "certify" that you satisfy the wood is "sanitized" ??
I'd think a record of internal wood temperature would be needed.

Is it lumber?
Don't know of any formula but would likely need to have starting moisture content of the wood, grain direction, the species, maybe density, and maybe even growth rate. Also would depend on the kiln conditions such as air flow, etc. Tough one to answer.

[burdman_22]

https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapter_20_fpl_gtr282.pdf&ved=2ahUKEwi11KDa-KWJAxXbIDQIHd0VBW8QFnoECBkQAQ&usg=AOvVaw2KmTdyxfrP7JVptLFLs0jR

Found some info

[burdman_22]

Maybe someone can help me, cause I think I'm a bit confused. In chapter 20 of the wood handbook (link above), it looks like most 1 inch wood only requires like 20 minutes or less to reach an internal temperature of 133 (after the air reaches 160 degrees). This time appears to increase as the difference between wet bulb and dry bulb Temps increase (ill have to read up on what these mean and how to calculate for my kiln).

If I am sanitizing 1 inch red oak that is already air dried...is it really necessary to maintain a temperature of 150 or 160 for 24 hours? Where does this 24 hour time come from?

[YellowHammer]

Sanitizing temps and duration are a function of wet bulb more than dry bulb due to the heat transfer coefficient of water overcoming the independent thermal values of the wood, as well the enthalpy of the moisture coming off the wood. 160F exceeds the service life of most greases in kiln fan bearings unless they are packed with dedicated high temp grease.   Service life gets reduced from tens of thousands of hours to hundreds of hours.  I killed a LOT of fans until I started investigating the failure mode.   

The actual time the FDA requirements were originally based upon numerous thermodynamic equations to predict when the core temperature reaches the proper temperature, as well as published experimental data to confirm the calculations.  These old documents investigate quite a few variables including R value of the wood, the thickness, the delta T between the core wood temperature and the wet bulb temperature.  After I reviewed this at length several years ago, I decided that all of this could be greatly simplified by the simple use of a big Yellow Hammer, I.e. overpower and exceed.  Early on, I used 145F but when I ran some numbers, I didn't like the asymptotic tail of the wood core vs thickness, and didn't like the only semi cooked nature of the residual insects, (yes I dig them out to visually inspect them) especially when I was drying mantles and stuff.  Also, I found that the grease in the kiln fans degrade significantly at 160F to set limits on my upper temps, and so I and a few others settled on a kiln temp of 150F, and after taking TC measurements of the core wood, I was convinced that was a good number for all the species and thicknesses I dried.  In addition, WDH, and some others also independently followed suit, and through empirical evidence, determined that nothing survived a 24 hour cycle at 150WB.  Nothing.  I had all kinds of bug carcasses to prove it, and also, at least in my kilns, this temp and time produced 1% moisture drop in the wood, almost without fail and in fact does no harm to the wood.  So I have adopted 150F WB for 24 hours as it far exceeds the FDA published requirement, far exceeded the measurement error of the RTD's in my kiln, and served the purpose of allowing me to perfectly fine tune and predict the final moisture content of the lumber.  In addition, the subsequent 24 hour cool down period allows the wood to cool slowly, does a little equalization (based on some early prong tests I did several years ago) and also is convenient because its a very predictable time cycle, 24 hours to maintain, 24 hours to cool down, and so worked well into a routine workday.  So far, after selling a few million dollars of wood for a reasonably long period of time, I have had absolutely zero bug complaints from customers.  I also like this cycle because I reviewed it with my insurance and lawyers, and everyone agreed it would be a strong argument if I ever had a court claim, as almost any reasonable person would judge that the sterilization cycle far exceeds the letter of the FDA rules, and demonstrates, through excessive heat and time, a non shortcut attitude.  It's also easy to remember and explain to customers: "the FDA requires 133F from six minutes to a few hours, but I do a full 24 hours at 150F with no exceptions.  My bugs, mold, bacteria are and virus are dead." 

Anyway, that's how and why I do it.  I'm sure others have different reasons and cycles.

[Ianab]

I suspect the 24 hour guide is more a convenience thing? As in, if you crank up the kiln temp for the last day of the cycle, then you KNOW that any bug in there is well and truly cooked. Could you up the temp in the morning and move the wood out in the afternoon, to start a new load 12 hours sooner? Maybe, but you better be sure about your measurements. But if you combine the heat cycle with the last day of drying, like turn up the heat when the wood is at 9%, then you combine the last day of drying with the sterilizing part, so no time wasted. Wood at 9% is really hard to mess up, and if it comes out at 7%, sweet. 

[burdman_22]

Ok, Yellowhammer, thank you, you answered my last question (very thoroughly, and much appreciated). 

Now, I am sanitizing this wood for myself, to use as flooring. Currently it looks like my kiln is capping out at 134...though maybe it will climb a bit once it warms up outside (currently 47 out). It's been at 134 for 6 hours....if I find that it's not going to go above 134...should I just call this good? Or should I make some adjustments, then completely restart the cycle and try to get 150 for 24 hours?

[YellowHammer]

Ianab, that is exactly correct. 

Burdman, the absolute minimum I would run a sterilization cycle is 145F.  I can't tell you how many times I have opened up my kiln when I first started sterilizing to only 135F, and saw ants and beetles fly out when I opened the doors, all finding some little "cool spot" somewhere in the kiln, the pack, or somewhere.

I personally, as a business, have refused to sell wood before it had reached my 150 value, simply because I know, for certain, I am selling safe wood.  As I mentioned, this is overkill, but I like overkill in this instance.

If you can't hit temps, put a few (more) halogen lights in the kiln, or a chicken incubator heat bulb, put duct tape over all the vents, run the temps up and get the load hot.  Sterilization is not a gray area for me.  I get calls at least once a week from people who bought incorrectly sterilized wood from other kilns, all across the country, asking me what to do.  It's not something to play with.

[K-Guy]

I have posted this here before but am doing it again. I recommend following the attached instructions. It is over doing it but that way you know it's sterilized.

[burdman_22]

I guess we might pull out half of the load and retry with 500 bd foot less of lumber inside.

[blackhawk]

I embed a temperature probe into the center of a 3" thick piece of wood and put in every load.  This is thicker than anything that I will ever dry.  I put the probe in direct contact with the wood and seal the hole where the wire enters.  I sterilize at 140F for 24 hours.  I actually set my dry bulb to 142 just to be sure that I always keep that 140F minimum during the sterilization cycle, not as an average.  My kiln has been running at least at 120F for weeks before I start the sterilization cycle, so everything is heated up very well.  I don't start the clock on the 24 hours until my embedded temp probe hits the 140F mark

I use 140F because it gives a safety factor over the required 133F.  The 2nd reason is that most of your standard electrical cords and outlets are only rated to 140F.  Then you also have to remember that the cords and outlets generate heat from the current flow, so they will be at a higher temperature than the air.  If you are using 140F cords and outlets for your fans and heat lamps, then sterilize at 150F, that is a problem.  For extra insurance, I made sure that every electrical item inside my kiln was rated for at least 158F (70C).  I actually think everything was 75C rated.