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Post by Deleted on Mar 28, 2018 1:10:43 GMT -8
Hi everyone, I'd like to share with you some very important information about the heat extraction. The data is calculated with the spreadsheet of the MHA. The hypothesis are based on a good firing with a batch : - 500 ppm CO - 13 % mean O2 - the latent heat of the water vapor in the exit gases is not taken into account If the fire is slow, then the heat extraction efficiency will be much lower, even if the exit temperature is the same. mean exit temperature (°C) vs. heat extraction efficiency (PCI, in %)50 97,6 60 96,8 70 95,9 80 95,1 90 94,3 100 93,5 110 92,6 120 91,8 130 91,0 140 90,1 150 89,3 160 88,5 170 87,6 180 86,8 190 86,0 200 85,1 210 84,3 220 83,5 230 82,7 240 81,8 250 81,0 260 80,2 270 79,3 280 78,5 290 77,7 300 76,8 310 76 320 75,2 330 74,3 340 73,5 350 72,7
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Post by Orange on Mar 28, 2018 7:31:22 GMT -8
interesting I would think with a faster fire gas velocity goes up and extraction efficiency goes down.
so if the combustion efficiency is 97% and exit temp at 70C that would give total system efficiency of 93%?
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Post by Vortex on Mar 28, 2018 11:00:44 GMT -8
That's in line with what I'm seeing on the new stove. A faster hotter fire gives a higher overall heat output, than a slower lower temp one burning the same amount of fuel. The hot fast fire seems to drive the heat into the mass more effectively, whereas the slower lower one gradually leaks it out the chimney. 50*C is the average exit temp I'm seeing, goes up to 60 at the hottest part of the burn, and down to 40 at the coaling stage.
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Post by gadget on Mar 28, 2018 11:13:25 GMT -8
I agree that it seems counter intuitive that fast gas speed = more efficient heat transfer. In heat exchanger design face velocity decrease = increase heat transfer. There must be another mechanism at play here.
The real question is, how much more is there left in that water vapor??? Do you have those numbers for per say average moisture content??
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Post by Deleted on Mar 28, 2018 23:15:46 GMT -8
Hi all, gadget, It's approximately 13% loss from the water vapor You're right, in fact is not so much the gas speed that changes the heat transfer. It's the excess air. If you have a slow fire with very high excess air then a lot of air is drawn from your house, heated in your heater, and then wasted through the chimney. That's the most significant waste in my opinion. Much more than combustion efficiency. Orange, if you want to know the total efficiency, then it's combustion efficiency x heat extraction efficiency. Regards,
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Post by peterberg on Apr 1, 2018 1:51:41 GMT -8
In general, it's true that with a hotter fire more heat is accumulated. Depends all on the temperature difference between mass material and gases moving through the system. That is, IMHO.
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Post by boilermantim on Jun 9, 2018 12:42:17 GMT -8
In 1983 High efficiency gas furnaces (83%) lowered the flue gas temperatures from 450F to 300F. They did this by increasing the velocity of the gas causing more turbulence inside the heat exchanger and increasing the distance the flue gas had to travel, so it had the same time in the heat exchanger, but it contacted more surface area. Laminar flow of flue gases allow the hot gases to flow faster down the middle of the flue passage, while the cooler, slower gases stay stuck to the surface of the heat exchanger creating lower heat transfer. The advent of condensing heat exchangers removed the latent heat from the water vapor plus dropped the sensible heat in the flue gases down to 110F. They now operate around 95% efficiency.
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