Sizes for single bell systems

[peterberg]

Hi all,

During the course of this year there has been some new experiments conducted regarding bell size. In my own opinion, those experiences point strongly to a smaller Internal Surface Area (ISA) than assumed before. In order to keep the gas velocity in the heater high enough to get a good burn there should be at least an end temperature of 80 degrees Celsius (176 F) in the chimney stack directly after the bell.

Recommended bell size for a 150 mm (6") system is down to 5 m2 (53.8 sq ft)now.

All other sizes can be calculated from the riser cross section area of the respective heater. Just take the csa of your riser, and divide it by 1% of the 6" riser's csa .
That would mean a 180 mm (7.09") system could drive 7.15 m2 (77 sq ft) of bell ISA, taken into account all surfaces excluding the floor and the firebox itself.
The same proportions applied to a 200 mm (8") system would end up with 8.85 m2 (95.3 sq ft).

All the numbers assume the heater is coupled to an adequate chimney stack. Better stack, larger ISA, worse stack, smaller ISA, a bypass and/or a primer box of some kind. Also, the idea is that inside the bell system is very little drag. When the drag is increased by a not very favourable exhaust opening which is situated in a corner close to the floor for instance, the bell should be smaller in order to keep gas velocity adequate.

In short:
Recommended bell size is
150 mm system, 5 m2
180 mm system, 7.15 m2
200 mm system, 8.85 m2

Understandably, all those figures are just guidelines, subject to tolerances depending on circumstances as laid out above.

[photoman290]

i am slightly confused about this. if this is the volume of the the bell shouldn't it be in cubic feet?

[satamax]

Internal surface area, or the surface of each wall in the bell. Exept the floor.

[photoman290]

is that the total ISA of all 5 walls combined. ?

[peterberg]

Yes, left, right, front, back and ceiling. The floor won't heat as much because the exhaust opening is just above that.

Volume in terms of heat extraction is a misleading thing. Try to calculate a bell with a round footprint and compare that with other shapes. For example, a square, rectangle or a H or E shape. When all the volumes are the same the surface area will differ over a wide range, the circle being the smallest. After all, the internal surface of a bell takes up the heat.

[photoman290]

thanks peter i can see how the volume can change depending on the shape. i am using a single oil drum. i read that you need 4 times the diameter of the flue between the inlet and the outlet so 6 inch flue 24 inch drum. i also read that having the inlet one diameter higher than the exhaust is a good idea. is that still valid.?

[peterberg]

Hmmm. That would be very good, since I assumed the csa should be used to calculate whether the expansion is large enough. In your example that would be just over 16 times the 6" inlet csa. It looks like a definition which is simple but that won't be that way when the bell doesn't sport a cylindrical shape. In your case, the ratio between the pipe and the barrel is very favourable I would say.

Placing the inlet higher than the exhaust is still valid, unless those are very wide apart. In that case it wouldn't make any difference. When the inlet stream is coming in through the bottom of the barrel there's another situation, though. The incoming stream is vertically in this case and will ingnore the exhaust opening altogether despite the fact that the inlet opening is lower than the exhaust. When both openings are in the side of the barrel, it would be best to create the height difference of one inlet diameter in my opinion.

All is about the placement of those openings, where they are in the bell and how close they are to each other. Hard to give a hard definition which will fit every situation.

[photoman290]

i will go for having the drum vertical with the inlet one flue pipe (150mm) higher than the exhaust. i wonder if anyone has done any CFD analysis of bells. i know david in mongolia did some stuff on the shape of rocket burn chambers. i don't know if i can climb the learning curve, cliff? of learning how it do it myself with one of the free CFD programs.

[photoman290]

dropped off a oil drum and a couple of grill to pipe adapters off to my mate to weld up for me. will post up results when i get them back later in the week.

[dustinmattison]

Peter, I am not sure if this question was answered somewhere else on the forum; can the bell system be used to move gasses through a bench? Currently, I plan to build one to heat water inside the bell, but can it also be used to heat a bench? If so, do you have any pictures or diagrams? I am having a hard time conceptualizing this.

Dustin

[radek]

dustinmattison here is 18cm batch box with a bench made as a bell. s101.photobucket.com/user/snake11112/library/18cm%20batch%20box%20rocket%20heater%20with%20bench

[dustinmattison]

radek thanks for the picture. Now it is more clear. I assumed that the heating of the bench could use the tubing like a RMH. Would it work to use the bell connected to tubing inside the bench? (the way the RMH do) 



Dustin

[satamax]

Dustin, it's possible.

Matthew Walker has done it. Tho He used 6 inch batch box, with an 8 inch heat riser and tubing i think. And added a bell at some point.

donkey32.proboards.com/thread/1361/converting-8-6-batch


Mind you, if you have a real drafty chimney, you could do it no problem. The isue being, if the friction in the tubes slows down the gases too much, a batch of the normal type wouldn't work too well. But with the latest slow burning discoveries, It could even be done. Remember that i don't know anything about the latest, slow burning, short riser type of batch rockets.

[oxbowfarm]

I'm sorry, but I'm really getting confused about how to properly design a bell system. This thread very authoritatively states that minimum bell ISA should be approximately 5 meters sq for a 150 mm system. But in the Bell Explanation? thread, matthewwalker states a single bell made from 6 half barrels is possible, which seems much larger than that.  Or is my math very wrong?  

[peterberg]

Your math is wrong. In my old workshop I used a three barrel "tower" with a 6" or 150 mm batch box inside it. Those three barrels together were nearly 5 m2. So, 6 half barrels should be the same internal surface area.