RMH CSA questions

Okay, I've done some searching but couldn't find my answer (maybe I'm asking the wrong questions).

I'm considering doing a compressor conversion as was explained by an earlier poster (ZeroFossileFuel's Post). Looks like a great idea and I have a dead compressor looking for a good project.

I understand that CSA needs to be maintained through the system from feed tube to exhaust for good burn. Should it be as consistent as possible or is it better if it increases just slightly as it progresses through the system (smaller at feed than at final exhaust)?

Also, does the CSA of the bell housing around the heat riser need to also be consistent or can it be larger? It makes sense to me that if it is larger it would slow the gases at that point and you'd just lose more heat to the primary heating and not get quite as much into the heat mass. If that's acceptable to me will it mess with flow at all for it to be larger than the rest of the system CSA?

I understand there is a relationship of the heat riser height to the burn tunnel length, but is that measuring the heat tunnel at the top (between feed and heat riser), at the bottom, or an average of the two?

Finally, is there any relationship to the CSA of the feed tube to the heat riser? I would think that a larger feed tube (hence more fuel at one time) might require a taller heat riser to ensure secondary fuel consumption.

Now I just have to figure out how to cut the tank and how to convince the wife I need a welder.

Thanks for any input or possibly search terms that might lead me to these answers if they've already been asked. If this information is out there, it might be a good subject for a sticky topic along with some basic calculations, critical dimensions, etc.

Brian in Oregon

Brian, you might want to go back to zero's post and follow the links he embedded as regards the basics
you could also read the sticky threads at Donkey's board, start by reading "start here", this will get you started
http://donkey32.proboards.com/index.cgi?board=discuss/

Holy cow. How did I miss those links? Thanks for pointing them out. I've seen that diagram from Ernie and Erica's site but it was always too small to read. I didn't even realize Ernie and Erica actually had a site and that it was the source of that image. That will help out a lot.

Only progblem I have with the diagram is I can't find "D" and it's mentioned for heat riser height.

inside to inside of the burn tunnel. in an 8 inch system this works things out to 24inches inside to inside of burn tunnel to 48 inches of heat riser from the base. with the 2 inch gap for a proper burn you will have a barrel height of 50 inches. you can do some things that will allow you to shorten this but for a basic stove this works.

Thanks, Ernie. That helps. I have Ianto's book on order so I'm hoping that answers a lot of my questions.

I'm not a student of fluid dynamics, but am I wrong that movement through a square chamber is less efficient than through a round chamber of the same CSA?

In the diagram on your site (the one from Rocket Mass Heaters) I can't locate measurement "D". Is that the mentioned inside to inside measurement of the burn chamber?

If the book answers this my apologies (I have a hard time waiting for shipments when my mind is stuck on something), but does measurement H matter so long as it is larger than the CSA of the burn chamber? If so there's a lot of extra space I have to take up between my heat riser's outside diameter and the inside of my barrel.

Thanks for all the help folks. I think I have most of this down, I'm just trying to round out the edges a bit.

Brian
(Near Portland, OR)

depends on how you mean efficient. for our purposes in an RMH round would be inefficient. We want the turbulence and mixing caused by the shape and texture of the burn tunnel and heat riser. in the exhaust ducting we want the opposite and use round smooth ducting to facilitate a clean flow.

now if you apply clarks to expansion of gasses then you get an idea of why the CSA is important that it remain constant all the way through the bench. you also need to assume that your expansion will remain constant due to the mass accumulating heat over time.

In our experience, the space between barrel and heat riser (H in the following diagram) can be substantially larger than the system CSA, but should not be smaller.

I suspect that the bigger it is, the more heat is radiated out from the barrel as opposed to going down into the bench. More stay time. If it's more than 4 times the system CSA, then the barrel is acting like a masonry-heater bell. We've seen this on some 6" heaters, and on Peter's batch boxes.

If it's more like 2-3 times CSA (a typical result using a brick heat riser), then it's acting more like the downdraft channels in a contraflow heater, and the exhaust gases are push-pulled downward by a combination of heat riser upward draft and exhaust chimney upward draft after the bench.

The diagram also shows the basic formula for heat riser height: at least 3x the height of the fuel feed, and at least 2x the length of the burn tunnel. So feed height : tunnel length : heat riser height would be 1 : 1.5 : 3+
For an 8" system, the brick channels that give the equivalent CSA are about 7" tall/deep by 7.5" wide, and the heights in inches are 16:24:48+. Depending on your bricks, the available space in the room, and your need for extra draft, a 50 or 52-inch heat riser is nice.
We do at least a 2" gap above the heat riser (keeping it within 1/4" of this dimension, half the radius, gives a very slight constriction that makes for a re-burn effect plus decent cooking temps in the top-center of the barrel).
And we try to do at least a 3" or 4" gap where the manifold enters the heat-exchange pipes, if it is coming straight out of the side of the barrel or a similar-sized manifold at a right angle.


This is an updated image of the same diagram from a different page on our website,
https://sites.google.com/a/ernieanderica.info/www/_/rsrc/1291441340872/rocketmassheaterpermitting/RocketHeater6in20000.jpg

Sorry about the missing "D" in the other one!

-Erica W

Ummmm, I can't find the "D" in this linked pic either. I'm beginning to relieve its kinda like big foot, or at least Waldo !

A current link to the diagram mentioned by Erica: http://www.ernieanderica.info/rocketstoves/rocketmassheaterpermitting/RocketHeater6in20000.jpg

And the missing "D" should be the length of the floor of the burn tunnel, from "B" running through "C" to "F".