Firewood Kilns How To Operate? HELP!!!!

[jadiehatcher]

Hey Everyone,

I'm in my first year of firewood production.  We mainly focus on retail ¾ Cubic Feet Bundles for retail stores.

I purchased a 36 Cord Kiln that is direct fired kiln with a 5,000,000 BTU burner (natural gas).  I figured incorrectly that the kiln would come with a magic button to run the perfect cycle for me.  Turns out there is a lot of trial n' error and testing to be done to find the most efficient kiln cycle for my desired outcome.  I'd love some feedback from experts and others who've dried firewood.  Below are the variables I have for my application.

• 1.   Mix of red oak / white oak and poplar.
• 2.   5' x 5' x 5' Kiln Baskets
• 3.   I can heat (Dry Bulb) my kiln up to 300 Degrees "safely".  Currently using a Set Point of 275 Degrees
• 4.   Desired moisture content of 18% or less (including core)
• 5.   Using a  Future Design Control Panel.  It's a pretty cool setup that allows you to build your own drying program with as many stages as you'd imagine.

I'd I'd love to get my drying time down to 30-36 Hours.  I'm at 54 Hours now....which is probably a little too long.
I've talked to several experts and there has not been too many cut and dry answers to my questions, so I thought I  might ask some of you that have more experience drying FIREWOOD (note not lumber!).

1.   What is the desired temperature of the core of the wood for the best drying condition?

2.    It is necessary to use a "rest cycle" to allow the moisture from the inside of the wood to equalize with the outside   temp/humidity?

3.    How often do you use a "dump cycle" to remove the moisture from inside the kiln?

4.    Is there a standard program that most operators use to dry firewood? 

5.    After the kiln cycle is completed do you continue to let the wood dry inside the kiln or do you open the kiln up to allow for a quicker cool down?  I learned the hard way, you MUST run the fans if you elect to leave the kiln closed up!


6.     How dry does the wood need to be to avoid mold issues for extended storage periods (3-4 months in a warehouse).

 Would love to hear / talk to anyone out there with some experience in this.

[Stephen1]

Welcome to forum! Please fill out your profile so we have an idea of where you are from. Different answers for different parts of country.

[K-Guy]

Welcome to the Forum

If you are in the US. The requirement is 140°F core temp 60 min.
For certification you must have a your kiln regularly checked to see that your documentation and kiln meet USDA standards.

I am also the pallet and firewood guy at Nyle guys.

[GeneWengert-WoodDoc]

The answer to your questions will vary depending on the market requirements for firewood...sterilization requirements and certification, final desired MC, bundle weight, and so on.

1.   What is the desired temperature of the core of the wood for the best drying condition?
We need to define what you mean by "best"

2.    It is necessary to use a "rest cycle" to allow the moisture from the inside of the wood to equalize with the outside   temp/humidity?
Only if this is part of the requirement for your particular market.

3.    How often do you use a "dump cycle" to remove the moisture from inside the kiln?
For green oak, each 100 pounds of green firewood will have about 35 pounds of water to evaporate.  Where will this water go if you do not have vents or leaks?  So, most people will have a vent slightly open in order to vent continuously.  Note that yellow poplar has about 25 pounds, so yp is quicker drying.  Aspen poplar has even less.  If you use hot flue gasses for your heat and not a plenum, you will automatically have plenty of fresh air.

5.    After the kiln cycle is completed do you continue to let the wood dry inside the kiln or do you open the kiln up to allow for a quicker cool down?  I learned the hard way, you MUST run the fans if you elect to leave the kiln closed up!
When drying is done and you are certain, you can shut it down and remove the wood when you are able.  Keeping it in the kiln while it cools does help equalize the moisture a little bit.  Emptying it promptly when dry means you can start on the next load promptly.


6.     How dry does the wood need to be to avoid mold issues for extended storage periods (3-4 months in a warehouse).
The gener al rule is no spots above 22% MC is safe.  As mold is an exterior event, not in the core, we are mostly concerned about the shell.  But a wet core can rewet the shell.

Special note.  Sweep out the dryer after every use.  Fine wood dust is easy to ignite

[jadiehatcher]

The answer to your questions will vary depending on the market requirements for firewood...sterilization requirements and certification, final desired MC, bundle weight, and so on.

1.   What is the desired temperature of the core of the wood for the best drying condition?
We need to define what you mean by "best"
I would define "best" as whatever would allow for the fastest drying condition and lowest moisture content.

2.    It is necessary to use a "rest cycle" to allow the moisture from the inside of the wood to equalize with the outside   temp/humidity?
Only if this is part of the requirement for your particular market.
From my understanding the "rest cycle" (fans circulating / vents closed / heat off) allows for the core heat/energy in the wood to force water from the inside out.  If the temp in the core is higher than the temp in the kiln it will try to equalize.....same with moisture content.  So I assume a dryer air condition inside the kiln allows for moisture from the core to work out of the wood?

3.    How often do you use a "dump cycle" to remove the moisture from inside the kiln?
For green oak, each 1000 pounds of green firewood will have about 35 pounds of water to evaporate.  Where will this water go if you do not have vents or leaks?  So, most people will have a vent slightly open in order to vent continuously.  Note that yellow poplar has about 25 pounds, so yp is quicker drying.  Aspen poplar has even less.  If you use hot flue gasses for your heat and not a plenum, you will automatically have plenty of fresh air.

We weigh ( 8 ) test baskets per kiln run.  On average we lose 25% to 30% of water weight (3000 Pounds In / 2100 Pounds Out).  My kiln has the ability to "auto vent" based on EMC / Relative Humidity.....just haven't figured out how to set the inputs for Dry Bulb / Wet Bulb.  The kiln is not sealed 100% (walls would blow out) so alot of moisture is forced out as more hot air is forced in.....just don't know if enough is being forced out.  My last test was 45' Heat Cycle / 10' Rest Cycle / 5' Dump Cycle.

5.    After the kiln cycle is completed do you continue to let the wood dry inside the kiln or do you open the kiln up to allow for a quicker cool down?  I learned the hard way, you MUST run the fans if you elect to leave the kiln closed up!
When drying is done and you are certain, you can shut it down and remove the wood when you are able.  Keeping it in the kiln while it cools does help equalize the moisture a little bit.  Emptying it promptly when dry means you can start on the next load promptly.
I like the idea of letting the wood equalize as it cools.....allows additional moisture to escape.  During my cool down stage the core temp was around 215 Degrees when the heat ended......four hours later with no heat in the kiln the core was still at 160 Degrees.......I assume that allows for drying to continue with residual heat from the core.


6.     How dry does the wood need to be to avoid mold issues for extended storage periods (3-4 months in a warehouse).
The gener al rule is no spots above 22% MC is safe.  As mold is an exterior event, not in the core, we are mostly concerned about the shell.  But a wet core can rewet the shell.
I use the Delmhorst J2000 Moisture Content Reader.  I use the 1.25" pins to measure each end of the wood.  I'm getting most reading at 10% or lower......HOWEVER......my core is still at 30% which I'm not too happy about. 

Mold is the primary issue I worry about as I will be storing the packaged wood for several months.

Special note.  Sweep out the dryer after every use.  Fine wood dust is easy to ignite
BEST ADVICE GIVEN!!!!

[GeneWengert-WoodDoc]

Correct me if I am wrong, but the product quality (unless sterilization by certain procedures is required) is not affected by drying speed.  Because the pieces are thick, compared to 1" thick lumber, much of the drying will depend on the heat and the humidity...hotter and drier is faster.  Some air flow to blow heat into the pile and to carry away the evaporated water is needed for fastest drying.

As water is evaporated when actual temperatures are above 230 F (temperature of air blown into a pile)3 the water evaporated will create pressure inside the kiln, just like a steam kettle, and will try to get out of the kiln through vents or leaks.  Leaks are bad as they will likely shorten building life.  EXCESSIVE VENTING IS BAD because it wastes heat and heat is not cheap.  Actually, at 230 F, the lumber will dry to under 22% MC at 100% RH (I know that air above boiling does not have an RH, so I should say saturated air at 230 F).  

But we do not have air that is that humid at 230 F in most cases.  So, you will find that drying time is mainly determined or dependent on the availability of heat energy...and the hotter, the more heat available.  So, we do not need a large vent at 230 F or hotter because the humidity (and EMC for those familiar with drying lumber) is always quite low at these high temperatures.

Again, depending on your market, you might find it better to say that your firewood has, on the average, a MC under 18% MC.  This way, one wet spot will still be ok, as well as one stick that is a tad above 18%...the average of your sticks is below 18% MC.  Test each load with a moisture meter, using long, insulated needles driven about halfway to the core and KEEP THE RECORDS.   Again, depending on your market requirements, you might state that you wood has been dried above 230 F which kills all insects and their eggs.

[jadiehatcher]

Thanks Gene.....good info.

I'm drying different methods to determine the most efficient manner.  I, like you, think the hotter/dryer then the better.....but I'm getting some different opinions on dump cycles.

My primary issue is moisture in the core and long storage periods for packaged bundles of wood.  I can't have mold issues in my bundles.

Thoughts?

[GeneWengert-WoodDoc]

Because of the temperatures over 212 F, the drying process is different (mass flow rather than diffusion) than a drying process at 130 F to 180 F.  Hence, you will be given different advice...some people are thinking of a lower temperature process.

An analogy:  consider a tea kettle that is heated to 150 F versus if it is heated to 250 F.  At 150, if you open the lid, you will get faster drying.  At 250 F, you can get very fast drying through a small hole and opening the lid does not make it boil faster.   What makes it boil faster is more heat...the process is controlled by the heat input and not the humidity.