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| heat exchanger efficiency question | |||
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| Posted by: magellan ® 11/11/2006, 04:49:25 Author Profile eMail author Edit |
Hello, A friendly discussion and disagreement about pellet (corn) stove efficiency amongst a group of us begs the answer to a few questions: 1) Is there a point where airflow volume through the heat exchanger tubes (in a tube type system) exceeds the ability of the heat exchanger to remove the heat? In other words, will more air always equal more heat extraction. It is my contention beyond the efficiency of the heat exchanger, increasing the convection fan CFM is an exercise in futility and only wasting electricity. 2) What is the most common and accurate method to measure overall efficiency of a stove? There is so much marketing fodder claiming this that and the other about effiency ratings. First there is combustion efficiency, then there is overall efficiency which is really getting to the core of the matter. There are no standards in the pellet stove industry, so the manufacturers take liberties in their advertisements. Thanks. |
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| Posted by: zekeman ® 11/12/2006, 08:52:55 Author Profile eMail author Edit |
1)Increasing the flow ALWAYS results in getting more heat out; however, increasing airlow costs fan power. I would say that you increase the fan speed to a point where an increment of fan power cost is equal to that which you gain from heating to get the most efficioent result. 2)output power/input power. Output power is the heating of the air in BTU/hr ,or
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| Posted by: magellan ® 11/12/2006, 17:44:30 Author Profile eMail author Edit |
There must be a point where the efficiency of the heat exchanger is peaked, no? By monitoring the exhaust stack temp on my stove the heat exchanger blower will lower the exhaust temp only to a certain point. Beyond that, no matter how much more air is put through the exchanger, the stack temp doesn't drop, or is negligible.
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| Posted by: zekeman ® 11/13/2006, 12:05:04 Author Profile eMail author Edit |
Like I said, you will always get more heat out if you increase the airflow, but as you noticed, this is a limiting problem, so that
even if you had an infinite flowrate,there is the resistance of heat flow across the heat exchanger that consists of three series thermal resistances. First, the interface there is a resistance at the interface between the hot gases and the inner wall of the xchanger. Second there is resistance of the xchanger wall, and third the resistance between the outer wall of the xchanger and the air that you are heating. You decrease this latter resistance when you increase the flowrate, but the other 2 are beyond your control. If you understand electricity, its like having a series circuit with 3 resistances and you start decreasing the third. Even if you bring it down to 0,(not practically possible) you still have current flow ( analog to heat flow)limited and due to the remaining resistances. |
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