What can we expect from a 6 inch Thermal Mass Rocket stove..

[nutrvd]

I have recently built a Thermal mass rocket heater. ... it is almost complete except for the cob bench. There is a photo below.
Before cobbing the bench I would like to check that the performance I am getting is what can be expected.
It based on a 6 inch size. The exhaust pipe is 6 inch diameter and the heat riser combustion chamber and feed tube are 5 1/3 inch square.... These parts are all constructed in firebrick (except the exhaust pipe. The riser is insulated with perlite / fireclay mix. The gap between the riser and the barrel is 2 inches on the top and 1 1/2 inches on the sides. 
The combustion chamber is 1/2 length of the riser and the feed tube 1/4.

It starts well and draws well... some smoke comes from the feed tube on occasion, but rarely. This is usually remedied by fiddling with the wood. Overtime the bricks get very hot. The barrel is warm at the bottom and gets hotter higher up. The top of the barrel is quite warm but it is not very hot.... gives a steady heat but it does not belch out heat. The exhaust pipe is warm where it exits the stove but mostly I can manage to keep my hand on it without burning. Further along the pipe it is much cooler and where it exits into the chimney / fireplace it is barely warm at all.

The question is whether this level of described performance would seem correct. I guess I was expecting the barrel and top of barrel to be much hotter. I was also expecting for there to be more heat in the exhaust pipe. This adds doubt as to whether it is performing below potential. If I had a pot of water on top of the barrel, I would not be able to bring it to the boil (though it would get hot). Any comments would very much appreciated.

[satamax]

How's your flue transition?

[nutrvd]

Thats a good question.... I kind of extended the side of the barrel to the ground with mesh and covered the mesh in clay / sand mix.... now a few layers of clay have gone on. The pipe leaves from this. How big the internal dimensions are ? I am not sure .... it is the size of the barrel extended to the ground on one side of the stove.

I am thinking of taking the barrel off and looking inside, perhaps smooth the inside out a bit....but it is not easy to get the barrel off... so I am thinking of cutting the barrel all the way around and then replace it with cob covering the joint. Is it correct to think the the barrel (especially the top) should be hotter?

[Robert]

Usually the top of the barrel should get red hot... i am boiling the water with no prolems... the exit to the chimney usually is also quite warm...

[satamax]

What you need.

[nutrvd]

Yes

[colinsaengdara]

Nutrvd,
How many times have you burned? If you built your stove with standard fire brick, that is a dense material, and it does take about 20 to 25 minutes to get fully up to heat. That will be a fact of life for this stove. But immediately after the build there is a burn in period needed to drive all the moisture out of the clay materials. It takes a tremendous amount of heat to pull ALL of the moisture out. Until this is done, the cob is stealing all the heat from your combustion. If your stove is rocketing well, keep burning. The moisture in your heat riser clay/perlite mix may not be completely driven out yet. After that, I would expect your barrel to get red hot half an hour into a burn.

Your system will dry out. I would not pull it apart. And I would hold off cobbing your bench until your exhaust temps from your barrel transition are very hot.

I suggest patience.

- Colin

[docbb]

Hello,

The dragon seems a little wet on the picture, isn' it ?
The bare pipes of the not-yet-built bench are extracting much more heat than with a built bench, so the gases are cooled too much and you encounter a stack-stall.
Is the tunnel isolated ?

[nutrvd]

Thanks for the feedback... Indeed as I am using it is improving. By now we will have burnt it maybe 10 times and the temperatures have been rising. It draws really well. Does not smoke.... but still the top of the barrel is far from red hot. The exhaust pipe where it leaves the manifold is now too hot to keep my hand on but further along where it exits into the chimney it is still barely warm. I understand that these temperatures in the exhaust will be higher when the cob is done.
Is the tunnel isolated ? I guess what is being asked is how much insulation is placed around the burn chamber? If that is the question then I have placed very little insulation around it.... it is made out of 4 1/2 inch thick firebricks. The fire bricks get too hot to touch after about 2 hours of burning.

I have 3 doubts. Firstly that the gap between the top of the riser and the barrel might be too big. It might for instance be 2 1/2 inches rather than 1 3/4. Secondly that the manifold might be too small internally or that the internal profile too rough and lastly that the exhaust pipe is too long. There are also thoughts that there should be a vertical pipe inside the chimney for some distance. At the moment the exhaust goes into the fireplace which is then bricked up.

I am also starting to realise the benefit of not using such a dense material as firebrick .... but that is something I do not intend to change.

I was thinking to remove the barrel, have a look inside, cob the inside passages nice and smooth, maybe enlarge the manifold and maybe decrease the height of the barrel by 1/2 an inch or so?

[nutrvd]

So I opened it up.... and found that the gap between the top of the riser and barrel was only 2.5 cm..... so I have raised the barrel and made the gap 5 1/2 cm. Now it is all back together again and this evening will try it again.
The manifold seems to be adequately large... also got me wondering if a gap of 3 inches between the riser and the sides of the barrel of 3 inches is too much....

[peterberg]

The top gap too small will lead to problems, quaranteed. When the top gap way large, say 3 ft, the rocket will run like nothing else. There's a minimum, but the maximum isn't defined yet.

[Donkey]

Dec 18, 2013 22:56:11 GMT -8 nutrvd said:

So I opened it up.... and found that the gap between the top of the riser and barrel was only 2.5 cm..... so I have raised the barrel and made the gap 5 1/2 cm. Now it is all back together again and this evening will try it again.
The manifold seems to be adequately large... also got me wondering if a gap of 3 inches between the riser and the sides of the barrel of 3 inches is too much....

As Peter said, the top gap can NOT be too large (not if it's a practical height), it seems that it WAS too tight.
If your stove doesn't improve immediately, check the manifold again, it may be just marginal and in need of a subtle widening..

[nutrvd]

Thanks Peterberg and Donkey .... it is helpful having experienced people to bounce ones own experience off