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Posted by Boiler Guy on November 20, 2007 at 11:55:04:
In Reply to: Re: Preheat boiler return water from stack exhaust? posted by Sean on November 20, 2007 at 11:06:38:
What would happen: my guess is you would spend a little time and money with very little to show for the effort. Wrapping the coil around the outside of the stack would provide very little heat transfer. Large boilers use an economizer to recover waste heat in the flue gas into condensate or feedwater. These units utilize finned tubes inside of the gas flow because water can absorb heat at a higher rate than flue gas can release it. Using a bare coil puts you at a significant disadvantage right from the start. I would anticipate the flue gas flow at the inside of the stack wall to probably be laminar so the inside heat transfer rate would be extremely low, that is why an economizer is located inside of the stack. The wrapped copper coil would not have a metallurgical bond to the stack meaning even the smallest air gap will have a significant impact on heat transfer. A wrapped coil will only contact the stack in a tangential manner meaning the very small contact area combined with air gaps would provide for very low heat transfer. For all a conventional boiler economizer has going for it from an engineered product standpoint it typically results in an efficiency gain in the range of 3% to 5%. An industrial boiler is also going to be less efficient than your low pressure steam residential unit making the economizer potential gain better than a residential boiler. I would guess a copper coil wrapped around the stack might get you at best a one percent gain, just shooting from the hip in three minutes here.
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