Elbow translated to length

I'm pecking away at my planning and understanding of RMHs.  Part of understanding is to read as much as I can on things smokey.  Today I ran across something on an exhaust fan page that says 90° elbows = 10' of straight run.  Is that what y'all see in your construction?

What exactly is the claim?  That an elbow and it's associated mass will absorb the heat equivalent of 10 feet of run?

No, that's a tinner concept comparing how much velocity you lose from the friction of a 90 degree elbow with a 10 foot run of the same circumference. Every run of duct creates friction, eventually (if the run was long enough) giving the net effect of having no pressure at the end of the duct. Round duct has the least friction, a triangular, two piece elbow at 135 degrees (not ever made) would have the most. When designing a duct system one tries to create the "smoothest" transitions so the fan originating the air pressure has to work the least hard and still have air arrive at it's destinations.

This concept doesn't really apply to RMH's because you're not using a fan to blow heated air through the system; you're using the second law of thermodynamics to heat cob and, if the system is made right, to entrain the exhaust to leave the system in the direction you want it to go. Erika (sp?) might have a better explanation of the RMH.

Chris

It is my understanding anything that causes friction/impacts the flow rate impacts how a RMH system operates.  Too many elbows/too long of length and the system won't run.  Too small of pipe and the system won't run.  Excessive narrowing or expanding of the cross sectional area of the exhaust and the system won't run.  Muck up the math and get the gaps wrong and the system won't run.

So if there's turbulence/friction in an elbow, it's slowing the flow of the exhaust gasses.  I didn't make a connection between flow rate and heat absorption . . . that's a point.

ElfNori, you make some good points. I haven't built a RMH yet so I can't speak of my experience in relation to my trade. I would point out that heating systems that use ductwork depend on the delivery of heat to a specific opening in the system, where the RMH depends on heat radiation emanating from any and all parts of the duct.

I have looked at some of E & E's pics of builds they've done. They regularly use 2 90 degree elbows to make a u turn in the system, sometimes more than once. Seems to me that once a heater is made, if the math involved in figuring the delivery velocity works the same as the radiation coming from any particular couple of feet of duct in a system, that it should be a whole lot warmer where the elbows are, maybe even uncomfortably so since the space where two 90 degree elbows is located should be radiating 20x more than any regular foot of duct run.  Ask them.  Anyone who's sat on one of these babies found that to be so?

I can say that this 180 degree set of elbows would never be allowed in a traditional duct system.

http://picasaweb.google.com/eawisner/DanaAnnexRocketStove#5263850922207281922

And I totally envy the folks who have this heater, it's a work of art.

Chris

When I took my stuff to the workshop to build a mockup, I took some of the flexible stainless ducting with me.  Ernie said I couldn't use it for the complete run, but might be able to get by using it for elbows and U's.  He said something about "recovery time". He said if the flexible ducting was used for an elbow it would take a certain amount of straight smooth ducting for the flow rate to recover.

I'm pretty good at leaps of logic (aka "connecting the dots") but I think we need to ask Ernie to weigh in on this, don't you?

I'd like to compare the effect on air flow of a commercial elbow versus the same radius/length of the stainless flexible ducting.  Maybe when I get to the point of doing testing I'll be able to do that, but don't hold me to it.  I'm building a 7" system and the flexible ducting I have is 6", so I won't be using it for this system.

I wait with bated breath.

Chris

ElfNori wrote:
When I took my stuff to the workshop to build a mockup, I took some of the flexible stainless ducting with me.  Ernie said I couldn't use it for the complete run, but might be able to get by using it for elbows and U's.  He said something about "recovery time". He said if the flexible ducting was used for an elbow it would take a certain amount of straight smooth ducting for the flow rate to recover.

I'm pretty good at leaps of logic (aka "connecting the dots") but I think we need to ask Ernie to weigh in on this, don't you?

I'd like to compare the effect on air flow of a commercial elbow versus the same radius/length of the stainless flexible ducting.  Maybe when I get to the point of doing testing I'll be able to do that, but don't hold me to it.  I'm building a 7" system and the flexible ducting I have is 6", so I won't be using it for this system.

Well, Ernie sent me, so let's see if I can help...

- Yes, elbows of any kind will offer more resistance than a straight duct.  But the figure of 10x seems a little extreme.  It might be for a rectangular elbow, or a corrugated one, instead of a smooth elbow like the adjustable ones we prefer.  I'd love to see Chris's documents on theoretical flow through various shaped objects - that would be a really nice complement to our 'gut and check' research!
The 10x factor might not be an engineering reality -
- it might be intended as a worst-case if the exhaust fan is pushing air against its natural direction - a ninety in the wrong direction could easily create a heat-trap or cold-air plug that would give more resistance than a horizontal ninety.
- or it might be intended as a stern warning to HVAC apprentices not to put too bloody many elbows in there, as it makes the system twice as expensive and half as effective.

- No, this resistance does not mean that there is more heat delivered at the elbow, at least as far as we can tell from RMH's we've built.  But we could be confused because we also tend to put cleanout traps at the 180 degree bends, so there is some variation in the surface heat anyway. 

Tinknal wrote:
What exactly is the claim?  That an elbow and it's associated mass will absorb the heat equivalent of 10 feet of run?

Chris, it sounds like you're confusing flow rates, with heat delivery.  The exhaust fan site that Nori found would not likely be talking about heat delivery - I think she's worried about maintaining good flow, and her RMH has a couple of unusual dimensions that might give her problems if she puts too much resistance on the outbound exhaust flow.

ElfNori wrote:
I'm building a 7" system and the flexible ducting I have is 6", so I won't be using it for this system.

- The difference in diameter will matter more than the bend, for RMH purposes.  Definitely don't use the flexible tubing for this project if it's smaller diameter.

ElfNori wrote:
I'm pecking away at my planning and understanding of RMHs.  Part of understanding is to read as much as I can on things smokey.  Today I ran across something on an exhaust fan page that says 90° elbows = 10' of straight run.  Is that what y'all see in your construction?

- Not that I've noticed.  We generally include about 20-30 feet of heat-exchange ducting (with 2 to 4 ninety-degree elbows or T-cleanouts), and another 10-20 feet of exhaust ducting (with 1-2 more elbows or T's), but I've seen and heard of systems with more than 60 feet total that worked under their own steam.

I can only guess that the exhaust fan is dealing with fluids at random temperatures that don't want to go where they're aimed.  A RMH ideally has two or three temperature gradients helping to draw the gas along.  I'm pretty sure that gases flow very differently when they're following a thermal gradient along a surface, than when they are being pushed by a fan.
RMH's have the up-and-down thermosiphon of the combustion unit itself, and then the upward (or downward) gradient of either a warm vertical chimney, or a cool horizontal one.  We routinely bring our exhaust chimneys back near the heat source before they exit the building, to give them a boost from the waste heat that would otherwise be soaking into an outside wall. 
Where it's not possible to do this, we try to provide a T so there is a horizontal exhaust option if the flue gas is heavy at that point, or the option to prime and forcibly heat the secondary exhaust chimney.

I'm sure that's all clear as mud in print.
I'd like to publish a study set of plans at some point, of stoves that worked and didn't work, so people can see just how they were put together.

I like the effect of a comparison test.  Maybe building a mockup, and plumbing it to 10 feet of elbows, then 10 feet of flexible tube, etc...

Problem is, you'd also need the same internal temperatures and fuel samples for a fair comparison.

I guess I can only say, we have identified some of the major factors in RMH design, but we don't have a total picture of how these other fluid dynamics rules might interact. 
My impression of fluid dynamics is that it's not terribly well understood by most of the industry, and that different fields just use different rules of thumb that give a decent 'best practice' in a limited range of situations.

I think the masonry heater rules of thumb are most likely to be relevant to Rocket Mass Heaters, since they are similar systems with gas going back and forth through 180-degree bends, and cooling as it goes.
The masonry heater guys don't tend to do more than 20-25 feet of 'intestines' (heat-eschange baffles) to leave the flue gas above 90 degrees (C or F, I'm not sure) for safe drafting up the chimney.  They do tend to include at least 4 right-angled bends, often more.
Masonry heaters rely on the chimney draft itself, so they need a fairly hot chimney.  Rocket mass heaters have this heat-riser thermosiphon 'pump' thing going on that allows pushing exhaust horizontally, and a very limited fuel/air intake that helps maintain the draft in the right direction without doors or air damping.

I wouldn't build an RMH entirely out of squiggly elbows just for the fun of it; I do think the elbows add some drag. 
But so do any number of other errors that most builders introduce, like dropping cob down the pipes and manifold, or raggedy recycled joints, or squashing the pipes while building it.

I think the easiest way to test the drag limits without building painstakingly identical systems would be to build one outdoor mockup, put as much straight pipe on it as you have on hand with elbows if needed to fit it into the available space (say, a 40-50 foot total length), so you're getting to the point where the drag is affecting the draft.  Then add 5-10 feet of pipe, or an assembly of 5-10 elbows, and see if your companion watching the fire notices any difference in draft.  Keep adding elbows until you get the thing to plug up. 

I'd love to know the results of such a test. 

I'll hazard a bet that in such a test, the final height of the exhaust, (not the number of elbows), would have the biggest difference on draft. 

Awesome post, Erica.  Thank you.

Would you ask Ernie what he meant about "recovery" after an elbow of flexible ducting?

Wasn't really my question. They throw a truckload of papers and books at you during the 4 or 5 year apprenticeship and you hold onto the info you need for your part of it; I'm a shop guy so I can design and build duct and fittings but I don't have a hand in designing systems so I didn't keep that set of papers on hand but a search for, "Resistance and Equivalent Length of Fittings" for either duct or pipe will get you sites with alot of the math already done. In the field there's more thought put into changing from square to round and vice versa and keeping equivalent areas. Again, from my perspective it's apples and oranges, hvac duct systems are about the destination and RMH's are about the journey. Seems to me you'd almost want to have the air slowed down in a mass heater; it's the point to lose heat through the duct.

Anyhow, thx for helping answer Nori's question better.

Chris

Erica, I did a search myself and cannot find a simple chart for resistance equivalencies; seems appendix 3 of Manual D has it. I can't just post it for legal reasons but if you want I'll dig it up, photocopy it and send it to you through the post.

Chris

I think I have a way to do a reasonably functional visual comparison of flow rates between different test setups.  We're currently snowed in and I still don't have comfy boots (hopefully coming soon to feet near me) with which to trek through the snow.  Once Wadly scores some rectangular tubing and the snow abates I'll get started on testing.  I'll also need to pick up a few things from the stove store.  I'll start cruising their junk pile for what I need.

I want to test as I build so I know the effects of:

[li]rectangular tube orientation - narrow side versus wide side toward fire[/li]
[li]head space - for my app this is important because my radiating tank is domed and the riser is rectangular[/li]
[li]flow change between elbow and straight ducting[/li]
[li]change in flow between elbow exit and straight exit[/li]

Because my system is so different I need to do a complete mockup before I start building.  I know what I want to do, I need to prove it's going to work before I start assembly.

Chris K e n d a l l wrote:
I'm a shop guy so I can design and build duct and fittings

any advice for somebody who isn't a "shop guy" but is trying to make some rectangular duct out of sheet metal?

Sure, I'll answer questions. First: buy gloves. and band aids. What wouldja liketa know?

Chris

thanks, Chris.  I'll start another thread.

Recovery after the elbow is when the flow fills the duct again. flow around the corner tends to be faster on the inside (smaller radii) and slower on the outside (larger radii) after the elbow the two speeds equalize and reestablish laminar flow. this can be used to slow a system down to allow more heat transfer; However it is a balance to keep proper function of the stove and slow the exhaust. To much slowing and you get smoke back to little and you have waste heat. figuring out what you want to do with the exhaust is a major design factor that should be considered at the beginning of the design process.
this will allow you to shoot for a particular result

That explains it!  Thanks.  I'll keep all that in mind as I work through my system's design.