|
Post by ericaus on Nov 15, 2015 0:54:10 GMT -8
Hello Peter, First off I'd like to say thanks very much for sharing your extensive research here. I for one really appreciate the work you have done and I'm currently designing a system using your figures.
Would you mind elaborating on how you calculate the bell ISA? I have used the CSA ( in metres) of several size units, but the figures don't come out to match what you have here. Thanks for any help in understanding these ratios. Eric
|
|
dvawolk
Full Member
DSR2 125mm open system (actual project)
Posts: 255
|
Post by dvawolk on Nov 15, 2015 1:33:28 GMT -8
ISA dimension of a bell is simply the total inner surface area of a bell (where gasses can touch this surface) minus all the bottm horizontal surface minus all outside surfaces of a burner if they are exposed to heating gasses... In some cases you should minus also possible other surfaces that dont take away heat from gasses, no matter the reason ( they may be bad heat conductor or they may be as hot or even hotter as gasses...) Also read glossary of terms. donkey32.proboards.com/thread/732/glossary-termsIt would be a good idea to print this glossary, just to have it at hand when reading/writing to the forum..
|
|
|
Post by peterberg on Nov 15, 2015 1:34:17 GMT -8
Calculation of the bell is done as follows: take all the internal surfaces into account, excluding the floor of the bell and the firebox itself. Pillars built inside the bell, for example to support the top slabs are also surface area. Irregular wall area will also enlarge the numbers, can even be done without enlarging the bell. ISA means simply Internal Surface Area, this is used because the volume of the bell won't extract heat, the walls and ceiling does. The footprint of the bell is determining the volume to area ratio. The circle has the largest, next one is the square, next is rectangle, and the shape of an E or W has probably the smallest ratio of volume to wall area.
Volume slows the gas velocity down which allows gravity to do its work. Area extracts heat, surprisingly enough steel isn't any faster as compared to brick in this particular situation.
Edit: I see Klemen has answered just a bit earlier than me, his explanation is just as good as mine.
Eric, welcome to the boards.
|
|
|
Post by ericaus on Nov 15, 2015 12:17:19 GMT -8
Hello Peter and Klemen, Thanks very much for both your replies. I am familiar with the terms ISA and CSA, but I'm thankful for the further explanation. My initial question was more regarding how the ratios and calculations were performed. I'm probably best to give an example from your initial post Peter. You say to take 1% of the 6 inch riser CSA and have this divided into the CSA of the system riser under discussion. So the CSA (in square metres) of the 6 inch system (150mm) is 0.075 by 0.075 by 3.14. This gives a CSA of 0.01766 metres squared. So one percent of this is 0.0001766. Now let's say that we are looking at a 10 inch system. The metric CSA of this in metres again is 0.125 by 0.125 by 3.14. This gives 0.04906 metres squared. So now if we divide 0.04906 by 0.0001766 this gives a figure of 277.8. By extrapolating your initial figures above, I would come up with something around 13 square metres for a 250mm (10 inch) system. Can you tell me were I'm going wrong here?
I'm currently drawing this system up in Solidworks and I'm going down today to purchase the barrel. I've come across a ERW pipe of 12mm wall thickness and 700mm diameter. It's 4.5 metres in length, but the ends are rough oxy cut. I found a place that will saw cut these ends off so that sealing both ends with gaskets and flange plates won't present any problems. My biggest challenge will be getting the barrel inside the house and standing it up vertical. It weighs in at just over 900Kgs. This week I'll be ringing around to find someone that can 3D laser cut the the firebox hole and the two flue holes. The top hole (200mm down from the top on the side) being for the bell bypass for cold start ups.
Thanks again for the help Peter and Klemen. I'll post pictures and drawings if anyone's interested.
|
|
|
Post by peterberg on Nov 16, 2015 2:16:02 GMT -8
This sounds like a very hefty system! As far as I know, a 250 mm batch box hasn't been built yet. Now for the calculations of the bell. This is recently revised as you understand and I think your calculation is right. That is to say, uptil a 200 mm system it seems to be right but the size you are planning for is enormous.
According to the numbers this 13 m2 is correct, time will tell whether this is true or not. Since you are planning for a bypass there wouldn't be too big of a risk, I would say. The bypass is tricky that high up in the bell, maybe it isn't too late to recommend a lower position? Somewhere halfway the height wouldn't be as thermically challenged as compared to the top.
And yes, I am very interested in hearing more and seeing pictures. Maybe even a video of the finished product in action? I think it will look something like a true rocket at that size.
|
|
|
Post by satamax on Nov 16, 2015 3:01:27 GMT -8
Guys, i'm not far.
Latest build is 220mm. I have a double bell. 10.6m² total. I haven't accounted for any surface bellow the exhaust top. So in reality, i have a bit more.
First bell is 4.25, and has the riser in it. A hole at the very bottom, leading through pipes with two 90° elbows, to the second bell. Which is 6.35m² above the flues. The first bell seems to shed tons of heat, and the second, is lukewarm untill the first reload. Now that i'm covering it with bricks, i might have a bit more heat in there. I have 8m of uninsulated chimney behind. Well, it's pozzolan cement elements, with an air gap on most of the surface, so it might be considered a bit insulated. No bypass.
So i find the 13m² rather conservative, but sound i think.
|
|
|
Post by peterberg on Nov 16, 2015 4:48:16 GMT -8
Yes, it is conservative. A single bell serving a 220 mm system would be, according to the same method, 10.7 m2 of ISA. Now you are doing the same with a double bell so, still according to the numbers, that should be 9.1 m2 ISA. But you are in a cold and dry winter climate, high in the Alps and utilizing a long chimney although uninsulated. Leave one of those helping factors out and you probably have a problem in your hands. But it appears to run very well, so congratulations!
I'd like to recommend sizes and numbers which will work in nearly every environment. The basis for this is my own heater which employs an insulated chimney stack of only 4 m, of which 3.2 m are outside the building envelope. And oh yes, there's also 2 m of bare pipe plus an elbow in the living room. When the thing wasn't dry yet it was a real stubborn pig to get it going.
|
|
|
Post by satamax on Nov 16, 2015 5:00:41 GMT -8
Yep Peter. I think the most important is the chimney. For the temps, ok, it's dry, but i have only 5C° of difference in temp between inside and outside when i light it up!
|
|
|
Post by ericaus on Nov 16, 2015 12:03:45 GMT -8
This sounds like a very hefty system! As far as I know, a 250 mm batch box hasn't been built yet. Now for the calculations of the bell. This is recently revised as you understand and I think your calculation is right. That is to say, uptil a 200 mm system it seems to be right but the size you are planning for is enormous. According to the numbers this 13 m2 is correct, time will tell whether this is true or not. Since you are planning for a bypass there wouldn't be too big of a risk, I would say. The bypass is tricky that high up in the bell, maybe it isn't too late to recommend a lower position? Somewhere halfway the height wouldn't be as thermically challenged as compared to the top. And yes, I am very interested in hearing more and seeing pictures. Maybe even a video of the finished product in action? I think it will look something like a true rocket at that size. Hello Peter, Thanks again for the reply. I'm happy to take your advise on the position of the bypass. I will make that amendment. The only other deviation that I think I've made to the system is in the roof of the firebox. I was going to make this arched to suit off the shelf fire bricks that are readily available here. These are 230mm by 115mm by 75/65mm. This forms an arch with a 330mm radius, which would add slightly more volume to the firebox unless I lower the sides to stay with the calculated volume. I will be constructing the firebox from 12mm plate and line it with standard fire bricks. The sizes (internal) I've got are 720mm long, 540mm high and 360mm wide. What do you think about the arched self supporting top? Do you think that I need to lower the sides to reduce the volume back? Thanks again. Eric
|
|
|
Post by peterberg on Nov 16, 2015 12:28:27 GMT -8
I think there's no need to lower the sides because of that shallow arch. With all batch boxes it is best to leave at least 50 mm or more above the fuel anyway. The dimensions you quote are really enormous for a batch box. Where do you need all that power for?
|
|
|
Post by ericaus on Nov 17, 2015 0:50:51 GMT -8
Thanks again for the feedback Peter. This unit will be heating around 700 cubic metres in an open plan, cathedral ceiling house. The insulation is quite poor and can't easily be improved due to the limited ceiling space. The floor to the apex (ridge beam) is around 6 metres I think. I can post some pictures of the intended installation if interested.
This project won't happen over night due to the out sourcing and fabrication requirements. The bell (barrel) has been transported to the company that is going to cut the rough oxy cut ends off. That happened today. Should I start a separate thread on this build?
|
|
|
Post by peterberg on Nov 17, 2015 0:59:01 GMT -8
Yes, a separate thread is best, I will move the relevant posts to that location when you create it.
|
|
|
Post by ericaus on Nov 18, 2015 0:34:24 GMT -8
OK Peter, I'll do that when the barrel arrives here. I got a phone call today saying it's ready for pick up. I can't find anyone that has the capacity for the 3D laser cutting of the firebox hole and the two flue outlets. Seems I'll be doing that work myself here.
|
|
|
Post by dustinmattison on Nov 30, 2015 4:27:37 GMT -8
peterberg So if I have a water tank that is 55cm X 55cm inside the bell which is 1 meter wide X 2 meters tall (cylinder), does that mean my ISA is about 2m squared for the water tank and about 7m squared for the bell (cylinder) (total ~9 meters squared)? I want to build a bell water heater like the Sun Dog Safe Way to Heat Water example. I have an 8 inch J tube. Dustin
|
|
|
Post by peterberg on Nov 30, 2015 6:58:40 GMT -8
Dustin, A water tank is cooling the gases much faster than a masonry wall would do. How to calculate the bell and what size the tank could be is something I don't have experience with.
|
|