RMH CSA and The Heat Exchange Barrel

Maintain a constant CSA throuhgout, maintain a constant CSA throughout, maintain a constant CSA throughout... Correct?

So here are some introductory questions:

The flow moves up through the interior of the heat-riser, then the flow moves down between the exterior of the heat-riser and the interior of the barrel-- correct?

So, the CSA between the exterior of the heat-riser and the interior of the barrel must be the same as the CSA within the interior of the heat-riser-- yes?

Now here's where some mathematical absurdity comes into play:

Say you're using a system with a 28.3" CSA.

Also, you'll be using a 30 gallon drum ( http://www.amazon.com/exec/obidos/ASIN/B006P5RPOI/rmh12-20 ). The inside diameter of that barrel is 18.25".

So, if you want to maintain a constant CSA of 28.3" between the interior of the barrel and the exterior of the heater riser, then the exterior wall of the heat-riser must have a radius of...

sqrt ( [ (pi*9.125^2) - 28.3 ] / pi )

= 8.617

Meaning that the heat-riser would have an internal diameter of 6" and an exterior diameter of 17.2" !?

WHAT!?

I thought yall were using 10" duct.

Is anyone following this?

Here are some background-information quotes:

Len Ovens wrote:

Is 2 inches clearance between the top of the riser and the barrel covering it a good dimension?

Remember, constant CSA. The 6 inch system has a CSA of 28sqin.  So:

28/(3.14x6) = about 1.5 inch gap. In practice, just a bit more seems to work well. Paul's was 1.75 inch and that has worked well for me too. 2inch (or just over) would work for an 8 inch system with a 50sqin CSA.

Len Ovens wrote:What you are looking for is constant CSA (cross sectional area) through the system. Generally the squarer the better. If one part will be smaller than others, it should be the riser, but not by much. So size the riser first. 6inch seems to be the minimum size that works well. That is 6inch diameter for a round riser at 28sqin CSA. I do not know if 5.3 inchs square would be fine in a square configuration or if it should still be 6*6. Generally all of us have built a massless setup to test what we have outside before installing it inside. So figure out the CSA of your riser first then go through all of your runs and calculate them to make sure they are the same or slightly larger. Then look at each transition from one run to the next and make sure they are ok. From what I have seen, most people who have problems have had a bad transition (barrel to mass pipe mostly because it has to be hand formed of cob and is therefore hard to judge the CSA. In that case oversize is better. My core is steel and so was easier to make sure of.

Len Ovens wrote:

tomcampbell wrote:
It's tricky, though, to have read the post at the beginning of this thread and to have watched the video because one is tempted to think that they are consistent with each other, but I just noticed that the barrel in the video is probably a 16 gallon drum and not a 30 gallon (I've been wondering about what would be a 4 inch gap between the heat riser and the heat exchange barrel, given the combination of a 30 gallon barrel and a 10 inch outer duct for the heat riser!). woops.

It depends on which gap is meant. the gap at the top is determined by the inner diam of 6inches. with a CSA of 28 sqin. That would seem to make the clearance or head gap about 1.5 inches, but he (and others) have found 1.75 works a bit better. By the time the flue gas gets to the outer riser shell of 10 inches, even a one inch gap is over 30 sqin. This should be plenty so far as CSA goes. However, I have heard more than once that a 6inch system is the smallest that works well because of surface friction. So I am suspecting there is also a minimum gap for long runs even with a constant CSA. It seems to be less than 6inchs though

The other consideration is the barrel exit which is also 6inch and therefore would require at least a 1.5inch (or 1.75inch to be safe) gap all the way around assuming the flue gas has equal opportunity (this is sounding like a contract where I work ) to get to the exit from all directions. This is probably not the case because the exit is quite near the bottom of the space and so the bottom 1/3 is some what blocked. So to be safe I would add at least a third more and try to get at least 2.25inches gap or use an 8inch duct out and then reduce to 6inch.

Just something i have noticed on the forums as people have built a RMH, this barrel exit area has caused problems for a number of people. They have had to rework it to get it big enough. So this is one of those "pay attention" areas.

I don't know the barrel sizes off hand.... for two reasons... one they are not the same here. I am from Canada and our old 45gal drum is 55gal in the USA... not sure where that came from as the gallon was originally 10 pounds of water. So any size drum you are talking about may not make sense here. Second, I have not seen any floating around and so don't have access to any to measure. I have ended up using a water heater core with an 18inch diam. and 4feet high. This has allowed me to have the exit within the barrel. It has also meant the top is rounded and not flat. Harder to set a pot on, but doesn't seem to have effected operation.

I hope I have given enough to play with... all of these are prototypes and as always should be built and tested outside first.