Rocket Mass Heater- MANIFOLD question

Hi,
 I have tried much internet searching on this subject, without luck.

Is there a certain size or CSA that needs to be observed in the manifold??
Also: half barrel vs. brick?
         I have lots of brick and only one barrel (paint still on!), so I was going to go with brick manifold.  I realize that maintenance would be easier with the 2 barrels, but is brick manageable?
     

Thank You!!
Staci

A rule of thumb is to make the manifold cross section three times the area of the rest of the system. This will compensate for common sharp, contorted layouts by reducing the flow speed and thus the friction in this area. The smoother and straighter your connection and transition from barrel to bench, the less you need to worry about manifold cross section.

The Wisners' half-barrel manifold base is clean and simple, but brick can be used just as well if you are careful about how you put it together. A smooth flow path is the most important consideration.

Hi Stacie;  I used brick on my transition area, works great. As glenn stated 3x the csa is standard but you can go quite a bit larger if you like (sort of a mini bell) and include an access to your horizontal pipe and an ash pit. A check of my posts will show you many pictures of my transition area.

When they say make the manifold cross section larger

Does this mean make it larger for a given length or just at the entry point?

The manifold should be large.  If it is large enough then your exit pipe can be standard diameter. If it is on the smaller side then you want to step down your exit pipe from larger down to flue size.

Ron Curtis wrote:
When they say make the manifold cross section larger

Does this mean make it larger for a given length or just at the entry point?

Hi Ron,
  From Ernie and Erica's book:

"The through-flow areas of the manifold need to be substantially larger than the systems cross-sectional area" at least 150% where the exhaust is changing direction, and about two to four times larger where it is merely flowing downward".

To my understanding, the manifold cannot be "too big".

Staci

Yes

I read that but what is the transition length of the larger area from larger to say 150mm duct

Is it just for a few inches it has to be larger before entering the ducting?

Hi Ron;  
Your terminology has me confused. (very easy to do...)  
Let me start by asking , are you planning on a small bell to transition or are you planning on using a double barrel set up to transition?
A small bell as in my picture (also what Staci has) needs no extra step down to enter the horizontal piping.
If using a sculpted double barrel arrangement as a manifold , you would want to start your 6" horizontal pipe with at least an 8" to 6" adapter, or even a 10" to 8" to 6"

What has me confused is the question "what is the transition length of the larger area"
 
 https://permies.com/t/61657/Flue-exhaust-transition-plenum-pictures   check out this post.

From HVAC duct design principles, a sharp entry to a duct from a large space will always cause a flow constriction; therefore, a tapered transition is always better. It only needs to be a few inches long, say from 10" to 8" over a 2-4" length.

Thanks for that

Hi Thomas - Barrel

I understand now about the transition - just wasn't sure how long the transtion had to last for

Ron, two details,

pipes don't work well with batches. The lightin g gets finicky.

And if you enlarge your flue pipe, to half barrel or else, gases slow in the flue, and get more exchange time. But, if you exceed a certain ISA (internal surface area) moisture in the flue gases will condense, and stall the draft completely.

ok thanks