Batch rocket greenhouse with bell bench

[jcfougere]

Hey all,

I've been spending some time reading through the forums, reading the Wisner's book 'the rocket mass heater builders guide' and now I'm working out logistics and considering what size system is best for my application. Of course a few questions have come up. I haven't been able to find the specific math involved for determining ideal bell ISA in a batch rocket design. Ive got Peters spreadsheet on proper dimensions of the batch box, riser, port size etc but I'm only familiar with traditional J tube designs when it comes to the thermal mass ducting part.

Ive attached a few drawings that show the building dimensions and an overhead of 2 design concepts. Also some pics of the actual greenhouse. The graph paper sketches are more or less to scale, each box 6"x6".

Build specifics:
-Greenhouse with a floor space of 178" x 94"
-primary purpose is to heat a horizontal mass that runs the length of the greenhouse in order to provide heat to seedling trays via conduction and convection
-secondary purpose to heat the air in the greenhouse via radiation from thermal mass and barrel on the heat riser
-will be fired in spring from Feb - April when overnight lows range between -5f and 25f
-also fired in fall during the months of sept-oct when lows hover around 25f
-batch rocket design to reduce feeding frequency
-horizontal bench with a series of bell chambers to maximize heat capture to thermal mass

We have 8 45gal drums to work with but I haven't seen them yet to measure sizes. I'm working with the assumption they are 2'x3'. I'm currently working out what size batch rocket we can fit, given that the ratios are very strict and we do have a bit of a space bottle neck vertically. The glass part of the roof is pretty low until it's 2-3' away from the wall, which puts the riser out 2-3' and the burn box another 1-2' further in to the greenhouse space.

If we can get away with a 6" system that will be roughly 72" of vertical (including a 3" gap at top of riser, a brick foundation, and 2' above the barrel for a kettle etc) which will require the back edge of the barrel to be 2' from the south wall. Then if the barrel is 2' in diameter and we have a batch box 17" long the entire burning unit will be about 6' from the south wall. Aligning the burn box towards the middle of the greenhouse will avoid blocking the door but as you can see we're a bit space limited even with a 6" riser. For a 7" system we're about an extra foot off the south wall, 8" system 2 more feet. Perhaps there is another easier way to fit this system in the footprint? The owners (I'm consulting and helping them build) want the chimney on the east side so it's not obvious from the road, and would prefer the thermal mass be on the south side.

Personally I think the easiest way to fit a batch system would be to stick the riser in the north east corner facing west, and then run the thermal mass in an L south and west, and vent out the west side(and not worry about who's seeing the flue). More heated bench run, easier to get in and out the door. Also easier to build a 7" or 8" sized system with the roof pitch on the north wall.

With all those variables I'd love to get the communities opinion on feasibility or if there are any glaring mistakes being made here.

Aside from the actual sizing and functionality, a couple big questions:

-how can I figure out the optimal Isa of a bell/pipe system, in order to figure out how big a bell can be, or how many bells I can have etc? I know draw is important and I'd like to get this right right off the bat. From my understanding too much ISA in the heat exchange area will reduce draw and cause issues firing up? Too small ISA and I'm losing heat out the chimney and wasting wood?

-when using a bell chamber system, can I link multiple barrels in to one giant bell chamber, or should they be sectional with a low port for cooler gas to pass through to the next bell? My idea was to have a run of brick under the bench, build up a 1' wide channel with brick about 6" high, then cut the 3' long barrels in to quarters and lay 3 of them down length wise to create a 9' long, 1' high and 1' wide bell, then cob it all in and finish the bench 2' high. Will this work? Of course all 6" ducting work being down low to allow cool air to pass.

-more basic question, in your opinion what size batch rocket is ideal for this design? 6, 7, 8? I know most designs look at heating a space but we really only need the seedling trays to stay warm, not the entire volume of the greenhouse.

-lastly, is 6" of space between the thermal mass and the flammable wall adequate? How much space would you allow behind the heat riser barrel? Just get a fire plate for the wall?

Thanks in advance for any help, feedback and comments.

If this is a success I'll be building another one on our farm next year and hosting a series of workshops to teach the basics!

Cheers,

James.


ibb.co/ceWdAq
ibb.co/bNSUGV
ibb.co/kZK4iA
ibb.co/n23Qqq
ibb.co/kj6dAq
ibb.co/n8r6wV

[peterberg]

Hi James, welcome to the boards.

As I understand it, you plan to build a mixture of bell/piped bench. Such a combination is hard to calculate, and a batch box is very picky about restrictions and system drag.

A bell bench is best built as just one stretch, feeding low and exhausting low. There's an ISA table on my site, see batchrocket.eu/en/building#bellsizing and also a quick rule of thumb. I find it tricky to advise you about the pipe part of your bench, let alone about elbows which in my opinion should be much wider than the pipe.

As for the question about size, start with a proper design without mixed technologies and what length of bench you would need. From there you will be able to determine the bell ISA and the system size of the batchrocket. Don't oversize the bell, in that case you'll need to fight the heater every time you start it up cold. Especially while the mass is still wet, provide for a way to heat the chimney first. This could be a primerbox or a paint stripper for example.

[wiscojames]

Hearing you say that, "the elbows should be much wider than the pipe", made me wonder about the feasibility of constructing a generously-sized cube made of bricks to act as a corner manifold. Would this help avoid the restriction? At some point it becomes a small bell. Hmmm

[peterberg]

Oct 29, 2018 13:15:33 GMT -8 wiscojames said:

Hearing you say that, "the elbows should be much wider than the pipe", made me wonder about the feasibility of constructing a generously-sized cube made of bricks to act as a corner manifold. Would this help avoid the restriction? At some point it becomes a small bell. Hmmm

Yes, that would deminish drag greatly. Why not build a tunnel instead of using a pipe?

[jcfougere]

Thanks for the input!

My lack of understanding with bells is mainly why I had defaulted to using pipe in addition to a bell, but it makes WAY more sense to just have one massive bell or "tunnel"!

I did some rough math and discovered my ISA would be 34 sq ft using the bell dimensions I was considering. It's about 15' of bench run, and the bell chamber would be 6" x 6" x 15" (estimated with a curved piece of barrel on top of a 12" x 6" x 6" channel).

Would I be correct to say that for a 6" riser and an ideal bell ISA of 57sqft I just need to make my bell chamber a bit larger and as close to 57sqft as possible?

It sounds like the 90 degree bend might be a problem area. The main goal is to fit as much heated bench in the greenhouse as possible, and match a batch rocket to size. Peter do you think with a 12" wide bell channel turning a corner will work similarly to a straight run? I'd round it off so it's as smooth as possible.

Thanks again for your help!

James.

[jcfougere]

Giving it some more thought and remembering Kirk's remodeling of his little rocket stove to a bell, would it be feasible to run the bell down towards the West side but have the low point for exhaust and the chimney on the East wall? Sort of a dead end bell? If the hot gas is high in the bell and cool air low, does it matter where the exit is so long as it's low?

Cheers!

James.

[coastalrocketeer]

With the exhaust at the bottom, it should be able
to be at the same end of the bell, but in order to work properly the bell’s cross sectional area of it’s internal opening needs to be significantly larger than the system CSA to allow gasses to slow and stratify effectively.

[jcfougere]

Thanks everyone I feel like I'm making really great progress conceptually and in design.

The advantage of being able to place the cool gas exit anywhere (so long as low) is fantastic!

Here's where we're at now:



The top down view is to scale, each square 6". Please forgive the attempts at 3d perspective, it's nowhere near to scale but shows the plan for cleanouts and chimney placement. The third sketch is a side cut of the bell to scale and each box represents 3".

I think based on Peter's numbers a 6" system will work great for our requirements, so the batch box, riser and the ISA of the bench bell will follow his math as close as possible.

The bench is going to be 2' wide and roughly 2' high(the height ultimately the flexible measurement to allow for correct ISA). The bell tunnel being 12" wide, 12" -15" high, and the ceiling made of a curved piece of metal with an unknown ISA just yet. I'm estimating the run of the bell will be 15 feet but in any case the bell will be following 57sqft recommendation for a 6" system.

I chose to place the chimney in the corner where there's extra room on the bench, and to facilitate a clean out that can access the bell and the outtake. The plan is for a exit flue to run straight up and out of the bench, turn out the wall, then turn again up for a vertical chimney. The other clean-out placed close to the barrel to access the manifold and around base of the barrel.

For construction of the bell and bench I was thinking of laying a full run of brick to level it all off, then laying a single high channel of brick to start the internal bell wall, followed by cob up to 12-15". At the top a piece of metal supported by both bell walls, then cob the remaining width and height of the bench.




Questions that have come up:

-With a 6" riser will I want a 6" exit to chimney as well?
-Will I need a clean-out at the far end of the ?
-Will the floor of the bell need to be smooth and would the plan for a brick floor be adequate?
-With a bench roughly 2'x2' and a bell 1'x1', I'm left with 6" of cob thickness on the sides and between 6-9" on top. Is this enough thermal mass? Too much or too little?


Thanks again for everyone's help and support!

James.

[peterberg]

Just to be sure: are you aware that the recommended ISA is meant as TOTAL. I.e. barrel, manifold and bench all together.

It's downright sensible to make the exit hole much wider (preferably not higher) and than taper to the 6" chimney size, like a funnel. In case you want to realize this with chimney pipe use a pipe reducer from 8" to 6" or a flat reducer that goes from, say, 10"x6" rectangle to the desired 6" circular. The flat reducer would be the best option in my opinion.

A clean out in the far end of the bench would be sensible, over time there will be a layer of very fine dust at the bottom. Be warned though that the end of the bench will be significantly cooler than closer to the barrel. That wouldn't be much different with the exhaust at the end by the way.

The brick floor would be adequate provided you smooth that surface out, filling the holes. The earth under bench is a gigantic heat sink, so it would be wise to dig a trench and fill that with insulation material of whatever kind. The brick floor will get only moderately warm, except for the manifold, so insulation under the bench could even be polystyrene foam.

In a greenhouse there isn't something like too much mass I'd think. In case you want to grow pot plants early in spring the top layer need to be at least 8" thick, preferably 10". The bench won't feel warm but there's a big difference between icy cold and just lukewarm as far as plants are concerned. I mean, it might be not warm enough for you to sit on for an hour. I'm just guessing here, don't have any experience with such a low-temperature bench.

[davidw]

I need to replace my prototype/test batch rocket in my greenhouse before the cold weather returns at the end of the season. I've used a firebrick-lined aircrete variation on the sidewinder design and a small drum (~30 gallon grease barrel) over a riser consisting of a length of insulated stainless steel chimney. (The chimney lining warped from high temperature but stayed intact.) A 6" stovepipe duct is connected to a hole cut just above the bottom of the drum and runs about 15' through a stone-filled bench (made from some of the numerous concrete patio slabs that came with my property) before exiting through an end wall into a 12' 8' vertical stovepipe chimney.

Considering that I have no need to heat the air the way the conventional riser and drum configuration does, I'm wondering if I can forgo the riser and use a horizontal equivalent of the insulated chimney length instead, going from there directly into the stovepipe through the bench mass. The stove itself provides enough space heating, supplemented with a small electric fan heater, but I'd like more heat in the bench. Is this feasible? It seems to me that the main function of a vertical riser is to complete the combustion before the exhaust gases reach a radiating surface (the drum) or a bell mass. Does it play an important role in maintaining the exhaust draft? I haven't seen evidence that it does, but that's the only design I've tried.

In short, is there any reason that a horizontal "riser" without a drum won't work?

[peterberg]

May 27, 2024 19:12:22 GMT -8 davidw said:

Considering that I have no need to heat the air the way the conventional riser and drum configuration does, I'm wondering if I can forgo the riser and use a horizontal equivalent of the insulated chimney length instead, going from there directly into the stovepipe through the bench mass. The stove itself provides enough space heating, supplemented with a small electric fan heater, but I'd like more heat in the bench. Is this feasible? It seems to me that the main function of a vertical riser is to complete the combustion before the exhaust gases reach a radiating surface (the drum) or a bell mass. Does it play an important role in maintaining the exhaust draft? I haven't seen evidence that it does, but that's the only design I've tried.

In short, is there any reason that a horizontal "riser" without a drum won't work?

To my knowledge, just a horizontal riser isn't expected to work.
The whole of the DSR3 development, lasting a full year, was geared to such a horizontal "riser". At long last, it worked, but the thing came out very specific in order to obtain satisfying results.

But there's another alternative now, called Shorty. It's the lowest core to date, with a very short riser. Normal riser would be as high as 10xbase figure, this one is 5.5xbase. Even the exhaust opening isn't on top. Construction is simpler as compared to the DSR3, a sidewinder version is expected to work although not tried as yet. Having said that, the proportions are specific again, very little room for error.

A brick bell with a Shorty core in it will come out quite low, so there will be more ISA available for the bench.

The chimney need to be a proper one, otherwise the core is reluctant to get up to speed. The vertical riser on the other hand is pushing the exhaust gases initially, it makes things easier to go.

[davidw]

May 28, 2024 10:31:38 GMT -8 peterberg said:

The chimney need to be a proper one, otherwise the core is reluctant to get up to speed. The vertical riser on the other hand is pushing the exhaust gases initially, it makes things easier to go.

Are you saying that having a vertical section immediately after the combustion chamber will create the draft better (or quicker) than relying on the vertical exhaust chimney for draft? I can sort of see that, but I'm a little surprised, considering that the exhaust has to immediately go down after the riser. I suppose that the drum will cool the exhaust, thereby drawing it down, but I'm trying to heat just the bench and avoid heating the air with the radiating drum. One idea I saw in a post on Permies (https://permies.com/t/60923/Wet-Tolerant-Rocket-Mass-Heater#1483306) is to put an exhaust fan at the end of the chimney. Presumably that would eliminate any issue with getting a draft started without a drum. I'll look into the Shorty design you mentioned.

[peterberg]

May 30, 2024 20:34:40 GMT -8 davidw said:

Are you saying that having a vertical section immediately after the combustion chamber will create the draft better (or quicker) than relying on the vertical exhaust chimney for draft?

Yes. Especially when the whole of the system is stone cold, a core with vertical riser is quicker.

May 30, 2024 20:34:40 GMT -8 davidw said:

I can sort of see that, but I'm a little surprised, considering that the exhaust has to immediately go down after the riser. I suppose that the drum will cool the exhaust, thereby drawing it down, but I'm trying to heat just the bench and avoid heating the air with the radiating drum. One idea I saw in a post on Permies (https://permies.com/t/60923/Wet-Tolerant-Rocket-Mass-Heater#1483306) is to put an exhaust fan at the end of the chimney. Presumably that would eliminate any issue with getting a draft started without a drum. I'll look into the Shorty design you mentioned.

In reality, you won't need a drum at all to induce draft. My own heater is stone(ish) mass all over and it works. That said, an exhaust fan will solve all issues with a smallish bell and a long bench. It will work even while the exhaust gas temperature is at room level.

Assuming using a fan, the bell could be closed almost directly above the Shorty core because the exhaust opening of this core isn't at the top of the riser. Building this bell out of insulating refractory materials should preserve most of the heat for the bench.

[davidw]

Thanks, Peter. Your statement about building the bell out of insulating materials addresses the biggest design question I had. That one design feature should go a long way toward preserving the heat for the bench. I probably won't have time to build the new stove before late summer, but I now have a much clearer idea of a promising design. I'll report its effectiveness when it's done.

I'll incorporate a chimney fan, so that I can be sure of a clean start. I definitely want to avoid future complaints from my wife about smelling like a ham hock after coming back from the greenhouse in cold weather. 

[Donkey]

Umm. Just a thought and I know I'm a major stove enthusiast but... Have you considered using chickens to heat the greenhouse? It's not as sexy as a Rocket Stove heated thing but in many ways, it's more robust.
I've never heard of chickens wrecking the plants with smoke and when it's cold, they will be inside heating your greenhouse, while you're tending to the frozen water lines.
Chickens have their drawbacks and you'd want to build the separation between them and the plants very much chicken-proof. They have many advantages too, like eggs and free winter heating that doesn't need attendance.

Either way, you have a cool project going. These folks gave you stellar advice, as usual (Love ya, Peter!). I'm looking forward to seeing your progress.