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Post by jliebler on Mar 19, 2021 11:07:41 GMT -8
It appears to me that the recommended ISA is based on a necessary output temperature of the exhaust flow, about 80 degrees C. ,with a totally cold mass heat extractor. In a bell the pressure will be the same every where so flows out of the bell will be proportional to flow area approximately. I propose a small opening to inject hot gases into the chimney flow After a while the two flows will mix and become the proportional average temperature. this flow is what exits the chimney. With the recommended ISA and no bypass the gas flow will be cooled to 80c. But if we increase the ISA more cooling will occur. For example let's assume we oversized the bell and our output temperature is 43C. our flue is 7" rectangular and we add a 1 1/2 dia. hole injecting un cooled gas at 1000C, the flow out of the chimney will be about 80C. What I don't know is how much ISA will cool the output of my 8" system to 43C. It seems to me that more cooling followed by hot gas injection can actually increase efficiency and result in dryer exhaust.
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Post by Dan (Upstate NY, USA) on Mar 20, 2021 6:44:15 GMT -8
Lots of variables there.
For ease of experimentation you should just make a big bypass that you can throttle, with maybe like a guillotine valve/door.
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Post by travis on Mar 20, 2021 22:19:12 GMT -8
I agree that’s pretty complex, but if you experiment enough with it let us know what you find!
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Post by Karl L on Mar 20, 2021 23:01:06 GMT -8
It seems to me that more cooling followed by hot gas injection can actually increase efficiency and result in dryer exhaust. Can you say why you think the efficiency might increase and that the overall exhaust will be dryer? Thanks! |
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Post by jliebler on Mar 21, 2021 3:19:26 GMT -8
My reason for suspecting efficiency could be greater by using hot gas bypass is that the greater cooling of most of the flow will capture the heat of vaporization from more moisture in the gas.
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Post by pianomark on Mar 21, 2021 5:03:42 GMT -8
I'm no expert, but it seems to me that the efficiency of heat recovery between the core and the stack will determined by temperature into bell vs. temperature out top of stack. If you cool some portion of the gases to 43 C in a bell and then add heat to get the stack to 80 C, I don't think you have gained any efficiency. Smarter folks here may correct me if I am wrong. Also consider that to "capture the heat of vaporization" will require condensation (inside the bell). That creates a whole new problem. Again, I could be wrong, but I suspect you are overthinking this.  |
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Post by Karl L on Mar 21, 2021 5:23:42 GMT -8
My reason for suspecting efficiency could be greater by using hot gas bypass is that the greater cooling of most of the flow will capture the heat of vaporization from more moisture in the gas. As I understand it, it may be possible to extract the latent heat of vaporisation by the codensation of the water vapour in the exhaust gases, but if that condensate is not removed, then it will eventually have to evaporate, and then absorb the same amount of heat it previously deposited. In which case, for this to work you'd need to have a condensate drain of some kind, just as in a condensing gas boiler. |
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Post by jliebler on Mar 21, 2021 14:31:36 GMT -8
I can easily add a condensate drain as part of my base slab. Would be interesting to collect and measure the water out. Ordinary fire brick would not be the best choice for the lower portion of the bell as their surface gets wetted and holds the moisture however soapstone brick or slabs or ceramic tile would capture much less water.
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