|
Post by fireboxfrankie on Mar 11, 2015 9:49:32 GMT -8
hi, i'm new to this forum as of today, i've built a number of ceramic stoves in the past, starting first with kits then designing my own using the same principles, i love the simplicity of rocket stoves and especially love the peterberg batch, i hope i'm not in the wrong place for asking a few basic questions , if so apologies and please direct me to a more appropriate thread, i reckon i have understood quite clearly the 'engine' of the peterberg batch but being a novice to rocket stoves needed a few clarifications beyond this point. I wanted to build an 8 inch model to heat a well insulated space of 300 cubic meters. Am i right in understanding after the flue gases have travelled up the riser and down the barrel, through the 10.6 inch diameter outlet neat to the bottom of the barrels there is still enough heat energy left to travel down a heated bench section, if so what length of bench can i run and approximately what mass could it heat, could the system cope with a u bend at the end of the bench bringing the flue gases back to a chimney situated near the firebox. Also how critical is it that the riser is round or octagonal, is building a square riser a definite no no, and lastly, probably being a bit stupid with this one, peter in your three barrel test hosted on masonry heaters of north america there is no insulation around the riser that would create that thin gap down the inside of the barrels, i take it that's just because its a test
many thanks for a stunning forum, wish it was so open with ceramic stoves, eagerly awaiting a reply, was planning to start building the stove in a month or so
firebox frankie from a very wet wales
|
|
|
Post by peterberg on Mar 11, 2015 14:56:37 GMT -8
Hi Frankie, welcome to the boards. As you can see, I've moved your first post to a new thread. It should be moved anyway because it has nothing to do with the Reference Library. So it's not done to offend you, only to keep the forum reasonably tidy. Now for your questions: Be very careful with an 8" batch box, it's like heavy artillery. In order to understand what it's like, please read this thread: This heater is placed in a 200 m2 shed with high ceiling which is not very good insulated and it is capable of heating it very fast. For your wel-insulated space of 100 or 120 m2, a 6" batch box would be perfectly adequate in my opinion. It depends what you need the bench for. When the space isn't a living area but a workshop I won't recommend a bench. The best riser is one with no mass at all, high heat resistant and highly insulating. In my three barrel tower the riser is made of a round ceramic fibre duct which is exactly that. Round or octagon is not much difference, square is second choice at a distance. The reason for that is the so sought after turbulence is starting immediatly after lighting the thing, which isn't the case in the square riser. You are right about there isn't a narrow gap between the riser and the barrel in my test heater. In fact, there is plenty of room so the speed of the gases is very low which in turn give the gases more time to lose its heat. The gaps mentioned in the Rocket Mass Heaters book are minimum values, in my opinion larger gaps are extracting more heat. For example the top gap of a 6" system according to the book is about 1.5". I tried one as large as 4' without adverse effects, it worked even better. Admittendly, this heaters do need quite a good chimney.
|
|
|
Post by fireboxfrankie on Mar 12, 2015 14:27:21 GMT -8
Hi peter, thanks so much for your reply, i hope i don’t bombard you with too many questions but i really, really want to do this, your double rams horn flame patterns are just like the vortex’s i created in my ceramic stoves but the build so much simpler and more fun. First the sizing 6 inch or 8, the space is 7.5m by 13m and 3.5m high, new build insulated to an average u value of 0.14, this opens out to an adjoining existing room through a 2m by 2m aperture which is 4m by 7m, same height but with a u value of 1.4, sorry quite a lot larger than what i mentioned before, the max distance people will be from the stove will be 12m, but usual distance will be 6m. The stove will be centrally placed and built into a dividing wall; will a 6 inch system still suffice? I have spent another evening looking at the recommended thread and others. With your three barrelled testo the insulated riser appeared to reach near the top of the third barrel, on permies it seemed much shorter and then there is the 10 times the diameter of the flue pipe calc, which one should i go for? I really want to build a big mass heater, having experienced the lovely heat from my large ceramic stoves, it is a living and office area, so firstly, using the round insulated riser could i then replace the barrels with a brick construction plastered with cob and then not use the third barrel on the floor lined with brick but instead run the ducting into a long bench , if so what length of bench can i run and approximately what mass could it heat, could the system cope with a u bend at the end of the bench bringing the flue gases back to a chimney situated near the firebox, would you advice to use round ducting in the bench or can this be brick channels. And lastly from the top of the firebox what length of chimney would you recommend, assuming it’s well positioned near the apex of the roof and no down draughts. Thank you again for your superb advice, hope i’m not asking too much frankie
|
|
|
Post by patamos on Mar 12, 2015 20:17:43 GMT -8
Hi Frankie,
thought i'd chirp in a bit.
The length of bench run is contingent upon a few factors. Eg. how much heat you will be shedding before the bench, How tall the chimney is. Whether or not you include a start up bypass damper…
Generally though you can send the heat down a bench and back. If you sift back through Peter's threads on rocket siphon you can see early experiments with 'bell chambers' in place of flue runs. For such a heat harvesting system on a 6" batch box Peter has recommended roughly 6.2 m sq. of internal surface area (ISA) total - including the ISA of downdraft chambers outside the riser.
If you build a hollow bench with a divider down the centre line the divider is only included once in the calculation. Make sure the 180 return at the end is larger than the CSA of the chambers so that gasses don't accelerate too much going round the corner...
my two bits worth
|
|
|
Post by fireboxfrankie on Jun 10, 2015 11:20:45 GMT -8
Hello fellow rocketeers
Took me a while to get back to you, but finally a proposal, what do you all think, if there any threads that i still need to read please let me know.
I want to build a 6 inch system using the petersberg batch box dimensions, the firebox out of fire bricks joined to the round riser of pumice insulated with superwool, i would like to extend the depth of the firebox from 432cm to 500cm, simply because all our wood is cut to this length, is that an issue?
The bottom of the firebox is 60 cm above floor level, the 1080cm riser flowing into a bell that is round, made of solid engineering bricks laid flat, (10 cm thick), the diameter is 58cm, roughly a barrel and the height is 218cm from the floor, about two and half barrels high. The internal surface area would be 4.16m2, it would end up a wee tad less if i were to discount the area below the exit into the bench. The gap from the top of the riser to the inside top of the bell will be 130cm.
Flue gases would then travel down and back up bench made of standard 4 inch concrete blocks, (not sure about the blocks heat retaining capacity?), total channel length would be 7 meters, channels would be rounded at the base and ends, the internal surface area of the channels within the bench would be 1.68m2. The divider wall is only included once in this calc also i have only included the upper two thirds of 6 inch CSA for the entire bench run, correct?
From the bench into the chimney that rises alongside the bell, no more than 50 cm between them, made of pumice sections 5 metres high exiting the roof alongside the apex, the upper 2 meters would be have additional insulation.
To start i would use a bypass located 60cm down from inside top of the bell, straight into the chimney, is this in the right place?
The weight of all materials directly in contact with the flue gases is very roughly working out at 2 tons
The stove would weave its way through a large concrete block dividing wall approx 10m long and 3.5m high within a timber frame building hopefully creating on huge radiator.
Reckon thats all for the moment, may have a couple of small qestions to askon the back of this huge long list, once again thanking you again for this truly awesome forum
take care frankie
|
|
morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
|
Post by morticcio on Jun 10, 2015 13:29:58 GMT -8
Think you have some typos with measurements in cm and mm you might want to check -
If you mean 500mm for length of the firebox that will be okay but remember to add 50mm to 75mm to allow for gap between the wood and door/port at each end. You will end up ramming it in and dislodging the bricks or breaking the glass on the door otherwise.
Use firebricks for the courses level with and above the top of the riser exit. As you have a 130cm gap this will mean over half the bell needs to be make of firebrick. I'd suggest using 100% firebrick for ease of construction.
If I have got your measurements correct, then the diameter of your bell will make things a bit tight internally...
External diameter of the bell = 580mm or 23" Width of brick = 100mm or 4" Internal diameter of bell = 380mm (580mm-(2x100mm))
Riser diameter = 150mm Riser wall thickness = 2 x 30mm min Riser insulation = 2 x 25mm min
With the riser external diameter at best 260mm this leaves you only 60mm either side of the riser - will this cause a restriction?
Plus with the bell at over 2 metres high an awful lot of your heat will be above head height. Is there any reason you can't widen and shorten the bell?
|
|
|
Post by shilo on Jun 10, 2015 23:17:01 GMT -8
the tunnels and elbows will create friction. so your calculation is problematic. you probably can go with it because the bypass. your box should be the length of the wood plus 2"
|
|
|
Post by fireboxfrankie on Jun 11, 2015 1:36:58 GMT -8
thanks morticco and shilo
yep sorry ill try and keep everything in cm for clarity, so the firebox will be 50cm plus the advised 5.0 to 7.5cm
the 58cm diameter of the bell is an internal dimension, my mistake again, so external will be 78cm in brick, plus the cob, that helps with internal dynamics i trust?
suppose the bell is quite high because the bottom of the firebox 60 cm is above the floor, so firebox and riser means the top of the riser is just over 200cm. Allowing for what i understand to be the ideal top gap of 10cm to 15cm,(although i think peter stated it could be more), the top of the bell ends up at that height, I could lower the whole firebox. By the way do i understand correctly that riser height is from the top of the firebox to the top of the riser and not from the bottom of the firebox?
flue gases will have to drop something in the region of 175cm from the top of the riser into the bench section, is that feasible? if not perhaps another reason to drop the firebox.
what kind of temperatures can i expect at the top of the riser and top of the bell?
shilo you mention friction in the tunnels and elbows, i really want to make a heated bench so would it be better to do it another way? Have a big open chamber under the whole bench with no channels? Make the 180 degree elbow a smooth curve? Use pipes instead of rounded masonry, or do think for sure i will be all right because of the bypass? have i put the bypass in the right place, 60 cm from the top, or should it be lower down the bell?
do you both reckon my chimney is high enough at 5 metres?
thanks guys, very much appreciated
|
|
morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
|
Post by morticcio on Jun 11, 2015 4:13:32 GMT -8
Do you need a bypass? On my 15cm PvdB batch box bell I didn't install one. This works without one even though the exit from the bell to the flue is underneath the firebox. Okay, I have to remember to start the fire towards the back of the firebox using kindling. Once this gets going I then add a few more bits at a time. If I add too much it will stall and smoke.
I am using 4 metres of 20cm ID twin wall insulated flue - an extra metre would help a lot. Ideally I wanted it to be 5 metres of 15cm ID twin wall, but that is all I had. I would recommend using an anti-downdraught cowl.
|
|
|
Post by fireboxfrankie on Jun 11, 2015 12:09:09 GMT -8
hiya, the fire will be lit by several people, some of whom will not be so attuned, hence a simple user friendly system. Will go for 5 metre chimney, perhaps a touch more. Does you stove go from bell to bench and then flue, or just bell to flue?
|
|
|
Post by shilo on Jun 11, 2015 12:44:33 GMT -8
riser height is from the bottom of the firebox to the top of the riser. you can do a little bench from 2 half barrels. 60cm downdraft is possible but it a lot and a big challenge to the stove. the ideal top gap is at least 30cm 5 meter is good if the roof is only 4.5 meter.
|
|
|
Post by fireboxfrankie on Jun 12, 2015 4:54:24 GMT -8
bit confused by your response shilo, when you talk about 60 cm down draught being challenging, firstly by that i take it you mean from the top of the riser to exit into the bench, i thought id seen loads of stoves where the down draught is easily 100cm, similarly with the bench thought i had seen may benches a similar length to what i had in mind a 3.5 metre bench, meaning a max of 7 metres in channels, when you say a little bench out of two half barrels length, diameter and configuration are the barrels, could you shed a bit more light and if there are some threads i have missed please let me know
thanking all
|
|
|
Post by shilo on Jun 12, 2015 6:03:22 GMT -8
I refer to "The bottom of the firebox is 60 cm above floor level" search here for matt half barrels system thread.
|
|
|
Post by patamos on Jun 14, 2015 8:52:03 GMT -8
As a general rule you want the gasses to move at a even pace through the flue runs. When using galvanized pipe (or the like) it is difficult to widen out the turns. So the gasses accelerate. At the same time the added friction and shifts in direction create turbulence. Plenty of heaters have been built with them, but this is one of the factors that affects over all draw through the system.
The system is most prone to stalling when cold because the expansion pressures created by combustion are minimal. The bypass damper temporarily eliminates much of the resistance to flow in the system. Which makes it easier for multiple users to get it going. Peter has mentioned elsewhere that a properly designed heater should not need a bypass. I respect his point, and/but many of the old Grundofens with flue runs around 75% the height of the chimney were built with them. So the matter is one of choosing between the challenge of cold starting or the technical aspects of building in the added feature.
Regarding Matt's half barrel approach, we discussed a while back the idea of having the gasses run first through a flue pipe to the far end of bell. In longer bells this can help even out the temperature. Alternately, you could stack a line of bricks along the centre line of the bell and have the gasses run along the front half first (where you may want the most heat to be radiating into the room) then turn around and come back along the back side. This makes for a sort of hybrid flue/bell harvesting.
my two bits worth
|
|
|
Post by fireboxfrankie on Jun 23, 2015 9:35:01 GMT -8
thanks all
so have upsized the system to 7 inch, the firebox will be within the bell, which will be double skinned, the ISA of the bell is working out at just under 6m2, this exits into a bench which i will adapt depending on what the system can cope with, either a short section of masonry channel with the option of extending or just a simple blind bench which again can be adjusted, i will build a by pass just to be safe, hopefully wont need it,
i was going to build the firebox out of firebrick and cast the riser, does it need to be split down the length for expansion or can i get away doing it in one, two more questions, what the latest on the ISA for a seven inch system? and whats the advantage of putting superwool between the two skins as opposed to a 5mm air gap
have got the base of the wall down and mapped out the base of the stove, try and upload some pics next time, small ones...
ta very much
|
|