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UPDATE: Got very delayed on the press because I've been setting up a new workshop. I'm building a micro rocket mass heater to keep things toasty in the winter, it doesn't have the heat core and exhaust welded on, still needs lots more work. I've never seen a mass heater this compact and the design is experimental so it will be interesting to see if it actually works.


My welder at the moment is 20 amp running on 15 so it's making the project drag on longer than expected. It makes it hard to sustain an arc. I have a more powerful 48 amp welder to use but I'm waiting on an adapter.




On 12/12/2020 at 6:10 PM, jason0012 said:

I am curious why you need 90 tons. A 12-16 ton press will move a lot of steel. Log splitters really are the budget way to go. 18-24 tons in a pre-built system for under $1000 is a pretty good start, and enough power to work some decently large stuff.

I have no need for a 90 ton press at the moment but there's no telling what I might want to use it for in the future. I'm getting into custom metal fabrication and automation. Was thinking of converting the press to run on 3 phase power in the spring, I want to try to automate sheet metal stamping.

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A:  Since your heater is experimental I strongly suggest that, if don't already have one, get a carbon monoxide detector for your shop.  If the design of your heater is not giving compete combustion to your fuel you have a risk of getting enough CO put out to be a risk.  Sometimes things don't scale up or down the way we plan. 

The design looks interesting.  Can you give us some details?  What is your fuel?

"By hammer and hand all arts do stand."  

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Good advice thanks, I'll certainly do that. I'll be using hardwood as fuel.

It's a bit hard to explain but I'll give it a shot, here are some pictures to help. This first pic shows how the central pipe is cut open to allow cooler air to drop down and pass into the secondary pipes towards the exhaust:


10/10 welds? :D





How it's supposed to work is that there's an extremely well insulated fire in the center of the tank. The heat travels up the center pipe and heats the top block which carries a decent amount of clay and steel mass - it radiates out heat. The cooler temperature exhaust travels down into the adjacent exhaust tubes welded onto the central one. The exhaust is shot back into the propane tank where it is pushed into the external exhaust, and finally out the chimney. All central pipes should be quite well insulated. I'm considering either using ceramic blanket and castable refractory on the pipes directly under the heater core or cob. I think I'll probably go with cob since there's less chance it gives off any nasty fumes or particles.

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This looks interesting but I have a couple thoughts and comments: 

1st, the fire box seems on the small side and you may have to keep feeding the fire fairly constantly to keep it going.  I see this as a drawback since you want a heat source to put out heat without having to attend to it very often.  With a wood stove maybe you throw in some more wood or stir up the fire about once an hour or so.

2d, since this seems to be driven by circulation based on gas temperature and density you want to make sure that there is enough temperature differential as the gas moves through the system to keep it circulating and drawing.  If the gas does not cool enough at the top of the central vertical vent it will not flow down the sides to exit out the bottom ports.  So, I'd be cautious about insulating the down draw sides.  You want those areas to cool off the gases.

3d, since you are using the solid structure on the top as a heat sink to radiate heat out into the room you want as high a specific heat in that mass as possible.  A block of aluminum might work well.  Clay has a low specific heat which means that heat does not travel through it well. This is why fire clay is used in cast iron forges to protect the brittle cast iron from thermal shock.  Clay does have decent mass which means it can store heat but you want the heat to radiate out.  Steel also has a lower specific heat than many metals which is why you can hold the end of a piece of steel in the forge and not burn your hand when you would with aluminum or copper. 

Where did you get the design?

I'll be interested to hear how this works out when you actually fire it up.  All my concerns may be theoretical and it may work a treat.  I'd be most concerned about how well it will draw.

Good luck.

"By hammer and hand all arts do stand."

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Thanks George for the helpful criticisms. You're right the fire area is very small and will require attention every few minutes. My hope is that the mass is small enough that it will heat up very quickly and radiate out lots of heat so I won't need to keep a fire going for long periods of time. If I had to remake it I'd definitely go with a gravity fed J-burner design instead of the L shaped burner I have now.

That's a good point about the exhaust potentially not being able to move through the system. I considered it as well and I'm just not sure. What I'm going to do is put lots of clay around that entire area temporarily to see if it works, if it doesn't I can pull it off and try only insulating the center pipe.

It's too expensive to buy a large aluminum brick and I think it might melt. The guys who build rocket heaters seem to use cob (clay / sand / hay mixture) as the heated mass. They have huge areas of mass to store heat while I only use a very small area by comparison, so mine needs to be as efficient as possible. If you have any other ideas about what would make a good heater block please let me know! Consider the temps get VERY high.


The actual combustion temperature inside the RMH is probably between 1500 and 2600 F cooling off slightly at the beginning and end of the burn cycle.

Basically I just researched rocket mass heaters for a few hours to understand how they work, then tried finding a really small one to copy but couldn't find anything like what I wanted so I just made up a custom design. 


Steve - yea those welds are terrible lol. Not like my skill level is great but my welding setup is very bad. My machine is cheap and under-powered, it can barely strike an arc and the welds barely penetrate. I welded some feet on the bottom of the L burner and those were some of my nicest welds yet, but I could easily snap the piece off. When I tested the 48 amp welder at a friends house it was like night and day difference. For the press I'm going to try constructing it with very minimal reliance on welds (I'm thinking about using 2x  1/2" steel brackets and industrial bolts on each side to secure the central plates).

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One thing I thought of after I posted last night is that you may be able to create adequate draft with chimneys from the lower exhaust ports.  If they draw well enough they may "suck" the gases through the system all the way from the firebox.  Are you thinking of some sort of inverted Y chimney or 2 separate ones?

The difference between materials in the "radiator" come down to whether you want something that will be warm for a long time (high mass/low specific heat) or something that will be hot for a shorter time (same mass/higher specific heat).  I'd suggest water or oil but you might have issues with boiling and steam pressure.  A block of copper would work too but might be hard to find and expensive.

These are all engineering and physics issues and if you hadn't already built your stove I would suggest a mockup or model to test some of the issues.

I'm still looking forward to hearing how it all works out.

"By hammer and hand all arts do stand." 

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Any chance you could do a very basic image to explain the exhaust port concept? I can't visualize what you mean unfortunately.

I don't have a block of copper but I do have enough 2-3" pieces of 1mm thick cut copper wire that could fill the square 10" steel vessel. Steel mesh could hold the copper pieces in place. Copper doesn't have any issues with giving off dangerous vapors at high heat right?

Ideally I'd like material that radiates heat for long periods of time so I don't have to make fires often. I work for 4-5 hours at a time, my work space is roughly 10 feet squared with no insulation and a crappy tin door that lets winter air in through the cracks :lol:. To make it even worse I need to mount the heater at should level because I have no space on the floor for it. I'm not sure I'll have enough mass to actually heat the room, it should be interesting to see if it actually works.

PS - You asked about how I thought up the design earlier and completely forgot to mention this guy:

It's basically a modified version of that. Just wanted to make sure he gets credit.

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