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Tweaking the kiln

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Forum topic by RussellAP posted 03-28-2013 05:21 AM 669 views 0 times favorited 9 replies Add to Favorites Watch
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RussellAP

2959 posts in 975 days


03-28-2013 05:21 AM

I just finished my home made kiln and I’ve been taking some measurements. It isn’t pulling the air at the right rate out of the heater box. It is pulling air and cooling it, or it’s pulling it so slowly it cools off by the time it reaches the fans.

I have some temp probes and humidity meters coming along with two more fans to increase the volume of air it moves. I can kick up the heat with higher wattage bulbs but I want to try to pull more air first. I think I can pull that heated dry air a bit better. I did a smoke test today and it does pull it down, but it takes too long to clear the smoke out of the heater box.

I’ll let you know when I get the parts.

I might mention that I used to do refrigeration, so I do know a thing or two about heat and airflow. It’s how it’s applied to kiln technology that is difficult.

-- A positive attitude will take you much further than positive thinking ever will.


9 replies so far

View Rick M.'s profile

Rick M.

4138 posts in 1068 days


#1 posted 03-28-2013 07:22 AM

Heat rises, what if the bulbs were at the bottom?

You might have better luck with a dehumidifier in the bottom, blowing out. Those are commonly used in screen drying boxes which are basically the same thing.

-- |Statistics show that 100% of people bitten by a snake were close to it.|

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kizerpea

746 posts in 1056 days


#2 posted 03-28-2013 12:08 PM

+ 1 heat rises

-- IF YOUR NOT MAKING DUST...YOU ARE COLLECTING IT! SOUTH CAROLINA.

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JoeinGa

3335 posts in 695 days


#3 posted 03-28-2013 12:44 PM

I asked the same questions (about heat rising) Go look at his original post about the kiln and you’ll see why he did it this way
It’s titled “My new toy” http://lumberjocks.com/topics/48268

-- Perform A Random Act Of Kindness Today ... Pay It Forward

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RussellAP

2959 posts in 975 days


#4 posted 03-28-2013 01:03 PM

Thank you Joe, I never realized just how much people retained from 7th grade science class. lol.
Yes, the heat would run away in the summer if I used the heater on the bottom, this way I’m hoping to heat a small chamber with radiant and convective energy to lower humidity and raise the temp just slightly. In fact in the summer I may not even need the lights if its hot enough.

The problem I seem to have is that the structure is large and those two fans are too small to move the air at the rate it needs to move.

Not looking for wind, just need to move the volume of air from one end to the other in less time.

-- A positive attitude will take you much further than positive thinking ever will.

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WoodyG

33 posts in 1861 days


#5 posted 03-28-2013 02:01 PM

Russ, I have a kiln I made for drying wood and it works seemingly very well? I am not under any time constraints to get it dry but it is a dehumidifier type of kiln. I start it off with two heat bulbs for 24 hrs then
shut them off and just run the machine which generates enough heat to keep it plenty warm. I run a hose out the bottom into a pan and keep track of how much water is coming out. This kiln is air tight and sealed all
the way around….....very important step….....so putting in a few small pieces of wood…...well this whole
thing would have to be sized to the use you are making. My dehumidifier will pull two gallons a day…....no
problem but doing this to fast is not good either? I would say you have some research to do for the best
results?

-- WoodyG

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Wildwood

1100 posts in 823 days


#6 posted 03-28-2013 02:20 PM

Drying wood in a homemade kiln or woodshed easy to understand and if know wood looses MC through evaporation. Therefore, air circulation more than or just as important as heat.

Advantage of a homemade kiln is better control of drying environment. Controlling combination of air circulation and some warmth required.
Looking at your shelving tells me you should improve air circulation first. One reason old appliances with open metal or plastic covered racks very popular with kiln builders is air circulation. Have seen many different open slat wood racks used in plywood box kilns. Also cooling racks from someone’s kitchen used in a beer cooler.

Have seen lights & fans mounted on top, sides and bottom of homemade kilns. Some kilns vented and some not.

Russell, bottom line there is a learning curve to achieve your goal. Cannot lose if improve air circulation first!

-- Bill

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RussellAP

2959 posts in 975 days


#7 posted 03-28-2013 02:20 PM

Woody, one of the challenges I face is the diversity of opinion about how to specifically dry bowls. There are many fundamental philosophies which include how thick to turn the bowl for which method you use to dry it. There are endless varieties and methods used, so after reading as many of them as I could find, I used all that info to design my kiln. Most of the other bowl kiln folk choose the light bulb method over the dehumidifier method because drying boards is different than drying bowls. Drying wood boards requires a certain amount of heat, or I should say that heat is more important in the board drying process than it is in drying bowls. Heat is actually the enemy of kiln drying bowls. The two most important things for drying bowls successfully is air movement and lower humidity. Ambient air enters the light box where there is heat in many forms, radiant, convective, conductive, and that chamber basically expands the air and obliterates the moisture by volume. Then it’s pulled into the kiln where it mixes with the moist air from the evaporating bowls and gathers that moisture as it cools and is blown out via the fans.

Now all this science has to be applied to each individual type of wood you dry.
Not all woods dry the same. This is where having a three tier system works to my advantage, I can take bowls that need to dry a bit slower and use the bottom shelf, whereas maple and box elder can do fine on the top shelf.

I just need to tweak the air flow a bit, those two fans do a good job, but two more would do an excellent one.

-- A positive attitude will take you much further than positive thinking ever will.

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RussellAP

2959 posts in 975 days


#8 posted 03-28-2013 02:30 PM

Bill, you are so right about the leaning curve. Good thing I have fat tires, lol. Air flow is THE most important thing in bowl drying, and lower humidity, (usually understood as heat), is secondary but still necessary.
After the air flow is established I plan to increase the volume of the heat box. If I double the fans I need to increase the volume of air being dehumidified, so when my meters get here I will be able to establish how much I’m dropping the humidity and I can go from there.
Keeping the box air tight is a challenge so I’m thinking I might recaulk all the joints. It’s impossible to tell if I have a leak though. All I can do is blow smoke into the heat box and watch it clear.

-- A positive attitude will take you much further than positive thinking ever will.

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Rick M.

4138 posts in 1068 days


#9 posted 03-29-2013 01:35 AM

The bulbs will produce the same amount of heat whether they are at the top or the bottom, the difference is the temperature differential from top to bottom. Basically the kiln is more efficient with heat at the bottom. While air throughput reduces the humidity, raising the temperature allows the air to hold more moisture and speeds evaporation. One thing that should help airflow and is to use wire mesh shelves.

-- |Statistics show that 100% of people bitten by a snake were close to it.|

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