Secondary combustion thermocouple

[Nofossil]

I had a PM asking about my secondary combustion thermocouple. I decided to post a picture showing the installation as well as secondary combustion roaring along.

Sorry about the focus - the camera has a very hard time in this situation.

The thermocouple is an Omega stainless jacketed model with an ungrounded tip and a 1/8" diameter sheath. It's inserted through a 1/8" diameter hole drilled in the flange that the door seals against. It's bent back so that it's in the secondary chamber far enough to be useful, and clears the door refractory.

At the time this photo was taken, the thermocouple registered about 1400 F. As you can see, the actual combustion zone is just a bit hotter than that ;-)

 

[DaveBP]

How long has this probe held up to the heat and gases in this location?

And how are your refractory bricks holding up? Have you found a type of brick which you are satisfied with?

 

[Gooserider]

Nice, I figure it really doesn't matter just how hot the actual flame zone is for NCFS purposes - you could either use some kind of offset, or just go with saying "it's hot..." :coolsmile:

Is that fire brick structure in the burn area the "labrynth" setup you were working on a while back? How is that working out?

Gooserider

 

[jebatty]

I just ordered a probe similar to the one below but rated to 1292F for use to measure flue temp and display on my control panel. Do you think this one
1832F would work for what you are doing?

 

[Nofossil]

Answering several questions:

This is my current vastly simplified labyrinth. Still does a good job isolating secondary combustion from the cold water jacket, but the pieces are more readily replaceable. High-alumina firebrick rated to 2300 degrees still turns to gravel, but more slowly. The photo doesn't show it well, but the flame is almost all pure blue, with just a few streaks of yellow-orange.

The probe has been through at least two seasons. The thermocouple is an Omega TJ36-CAXL-14U-18 (not sure on the length portion of the part number - mine looks like a bit more than 12") which goes for $42 direct from Omega, and is rated to 2440 degrees ( I did destroy one putting it in the secondary combustion zone).

Omega has a wide range of thermocouples with excellent technical data. The KTSS-18U-12 looks similar to the eBay item listed above, except:

1) You know exactly what you're getting
2) It costs less

Omega even has helpful human beings who make up for their complicated and sometimes annoying web site. No, I don't work for them - I just appreciate a company that does a good job.

Goose is right - other than abstract curiosity about secondary combustion temp, all I need is an indication of how vigorous the secondary combustion is. In the current probe location, the range of good secondary combustion is around 1000 to 1400 degrees. Below 900, it's time to think about stirring it up and adding more wood if I want to get max output.

The graph below shows secondary combustion and flue temp for a recent short (about 4 hour) fire. This fire burned in total about a full firebox load of wood.

 

[twofer]

Thanks for the info and pic, nofossil.

 

[twofer]

Omega has a wide range of thermocouples with excellent technical data. The KTSS-18U-12 looks similar to the eBay item listed above, except:

With that being a 12" probe, and most flues being smaller than that, are you guys not worried about the exposed section of the probe sitting outside the flue affecting the temperature reading?

 

[stee6043]

Nofossil, did you add your refractory bricks in phases to see how each affected your performance? I've often thought about "blocking" the forward path toward the lower door like you've done. I wonder if you have any observations on how this brick affected temps/performance?

 

[Nofossil]

Omega has a wide range of thermocouples with excellent technical data. The KTSS-18U-12 looks similar to the eBay item listed above, except:

With that being a 12" probe, and most flues being smaller than that, are you guys not worried about the exposed section of the probe sitting outside the flue affecting the temperature reading?

The thermocouple junction is right at the tip - that's the only place that temp is measured.

 

[Nofossil]

Nofossil, did you add your refractory bricks in phases to see how each affected your performance? I've often thought about "blocking" the forward path toward the lower door like you've done. I wonder if you have any observations on how this brick affected temps/performance?

I haven't been completely scientific, but I'll say that the brick drastically reduces the temp of the door itself. One of the big effects of isolating the secondary combustion zone seems to be faster starts and more consistent secondary combustion. This design is pretty easy to clean as well.

 

[Hydronics]

Nofo, your brick setup is very similar to what Zenon (New Horizon) has available as a replacement (assuming you totally block the back & force the combustion gases to travel only from the front). I bought this brick setup from him but haven't tried it yet. His is specially casted double channel bricks & standard firebrick for the ends. Check out his website, click on the burn video, when it opens you'll see the new setup. I think he got the idea from his Biomass which is a copy of an Atmos lower chamber.

 

[Singed Eyebrows]

Nofossil; Before the untimely demise of the first thermocouple what was the highest temp you saw? It looks like your EKO is burning really well, Randy

 

[Nofossil]

Nofossil; Before the untimely demise of the first thermocouple what was the highest temp you saw? It looks like your EKO is burning really well, Randy

Ir was around 2500 °F , though I'm not sure I trust the thermocouple at that temp. For sure the secondary combustion zone reaches 'ash fusion' temp, which is around 2600 or 2700 degrees for most species of wood ash. This is the temp at which loose ashes start to bind together. My ashes are almost always bonded together a bit when I go to remove them.

 

[Singed Eyebrows]

Nofossil; Before the untimely demise of the first thermocouple what was the highest temp you saw? It looks like your EKO is burning really well, Randy

Ir was around 2500 °F , though I'm not sure I trust the thermocouple at that temp. For sure the secondary combustion zone reaches 'ash fusion' temp, which is around 2600 or 2700 degrees for most species of wood ash. This is the temp at which loose ashes start to bind together. My ashes are almost always bonded together a bit when I go to remove them.

Is there a secret to getting these high temps? From everything I've read 1800 to 2000 is normally tops. From your pic it looks like the flame is almost white hot, Thanks, Randy

 

[Nofossil]

Is there a secret to getting these high temps? From everything I've read 1800 to 2000 is normally tops. From your pic it looks like the flame is almost white hot, Thanks, Randy

I haven't studied other units. I don't know where you're supposed to measure combustion temp, so I can't really make comparisons.

The photo doesn't look like the flame. Cameras can 'see' in infrared, so the camera is washed out by the infrared in the flame area. When you look at it with the human eyeball, the refractory is glowing bright orange and is just as bright as the flame, which is mostly blue. You can see the refractory glow pretty well in the picture.

I am burning really dry wood and I'm sure that increases combustion temps.

 

[jebatty]

I believe CO combusts with a very light blue flame, and H combusts with a colorless flame. Also that H combusts at about 1400F. I commonly will have a nearly invisible "flame" combined with a roaring sound towards the end of the high burn, when all the fuel has been reduced to rich coals.

 

[Nofossil]

I believe CO combusts with a very light blue flame, and H combusts with a colorless flame. Also that H combusts at about 1400F. I commonly will have a nearly invisible "flame" combined with a roaring sound towards the end of the high burn, when all the fuel has been reduced to rich coals.

Early in the burn I think there are other more complex hydrocarbons in the mix such as methane. I see peak secondary combustion temps right after adding new fuel or 'stirring the pot'. I'll also see spikes when the fuel load shifts due to combustion. Later in the burn secondary temps seem to be a good deal lower, even though there's plenty of heat generated. Interestingly, I think that primary chamber temps are much higher later in the burn, so maybe the ratio of water jacket heat contributed by the primary chamber is higher then and compensates for lower temps in the secondary chamber.