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Scott Davis

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Reply with quote  #11 
Hi Spokes, I used a Fanteck radon fan for my downspout collector.  I think it is rated at something like 160 CFM, though the corners in my downspout collector are too restrictive and I'm only getting about 80 CFM out of it.
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gbwillson

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Reply with quote  #12 
Found the following link about increasing the efficiency of double back pass collectors. Most of it is way over my head,   but they are talking about efficiency rates as high as 80%, so it should be worth the read.

Greg, MN
Scott Davis

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Reply with quote  #13 
Hi Greg,

For some reason the link didn't come through.  It sounds interesting!

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gbwillson

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Reply with quote  #14 
Opps! Let me try this again...


http://www.wseas.us/e-library/conferences/2010/Japan/ICOSSSE/ICOSSSE-68.pdf
gbwillson

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Reply with quote  #15 
Thanks GH,

When I scrolled down the page and saw all the number crunching, the first think I thought of was Nick, Nick, Nick. (No offense  to Nick, but with many of his answers I need a secret decoder ring to understand) [biggrin] With a lot of these "academic" papers I read the conclusion first. If I can't understand that, the rest of the page won't make any sense either. I did note as you did that they measured the weight of the air in their calculations. So what do we multiply the 1765 by, feet per second?

Greg, MN




Garage_Hermit

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Reply with quote  #16 
FWIW, some random observations from an (admittedly) cursory read of this interesting paper:

  • A finned dual-flow backpass absorber is more efficient than a flat-plate absorber.
  • Full-length fins are more efficient than short fins.
  • A long-fin backpass collector is between 2 and 8 percent more efficient than a flat plate model.
  • The input flow is next to the glass, and has no fins.
  • The reverse backflow is next to the insulation, and has fins.
  • The increase in efficiency is proportional to air flowrate: more air = more efficient
  • The test collector dimensions were (approx.) 4 ft by 8 ft.
  • Sunlight was simulated using halogen lamps
  • The test collector returned an efficiency of over 75 percent, at an airflow rate of 125 cfm
  • Variable air blower, controlled by a rheostat
  • The fins are 6 cms deep.
  • The overall depth of the collector is not stated…
  • The intake channel appears (schematically) to be somewhat narrower than the fins (which are 6 centimeters).
  • Intake channel is assumed to be 5 cms deep (two inches).
  • The backpass channel is deeper than the fins, therefore it is about 8 centimeters deep.
  • Insulation appears to be about 3 cms thick (approx. half the height of the fins).
No doubt I missed a lot of other stuff...

G_H

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solarusmc

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Reply with quote  #17 
G_H

Hats Off to you Sir!
I see you've created your first solar hot air 'monster' eh?
Way to go! [wink]

I followed the link GBwillson provided and attempted to understand the calculations they described in their findings utilizing that back pass finned design.

well, it's just not happening. [confused]

If I didn't see those drawings showing the fins and air flow patterns my visit to the pdf file would have been a complete loss.  [smile]

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Pat B. Warwick, Rhode Island Rest Assured! Comments and/or suggestions I make here at the forums on 'your' projects as well as my own have all been carefully and scientifically calculated by the 'seat of my pants'
gbwillson

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Reply with quote  #18 
GH-

Higher output speeds, which lowers output temps, makes sense, as a result of less loss through the glazing and other leaks. The air coming from the vents from my high efficiency forced air furnace is lukewarm at best. Unless your air output is in a location where you can feel the breeze, having a higher fan speed would make a collector more efficient. And since many of us are away from home during the hours anyway, it would be a great way to add the extra heat to your home simply by turning up the fan. A 40% improvement is worth the extra electricity and breeze.

Greg, MN
solarusmc

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Reply with quote  #19 
Greg, I'm with you on increasing the fan speed..

I run 6 inch 250 CFM booster fans on each one of my 4ft x 6 ft flat plate hot air collectors.
My temp rise is 30 to 45 degree increase from input air..

very satisfied so far with the additional air flow into the room.


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Pat B. Warwick, Rhode Island Rest Assured! Comments and/or suggestions I make here at the forums on 'your' projects as well as my own have all been carefully and scientifically calculated by the 'seat of my pants'
gbwillson

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Reply with quote  #20 
Pat-

That's a lot of fan for the size of your collectors. Do you know how many CFM's you get out of each collector? I forgot to mention, that if you increase your fan you will also increase the noise too. I guess we each have to find our own balance between comfort and efficiency. Since this year is for experimenting, I won't be cutting any openings in my new fiber cement siding. So I'll most likely have both the intake and exhaust in the basement workshop. That reminds me, I need a filter of some sort. Downstairs I couldn't care less how loud or cold the collector was blowing. I'm usually wearing earplugs and my workshop is cool by choice.

Greg, MN[wave]
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