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mocars2

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Reply with quote  #11 
Dan - above in the - what I would change answer - you said

"might try and cut down a little on the turn around area between downspout sections. "

Why? And what is the size of the turn around now and how much smaller would you go?

solardan1959

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Reply with quote  #12 
Mocars2,
   Sorry missed your questions some how,
Quote:
So does the DS help with the cold drafts?


the new layout may cut down on the cold air pressure against the backdraft preventers but it hard to test or prove.  The downspouts were used just because I had them and I could not be bothered trying to tack down the screen at the time. 

Quote:
which is better - DS or Screen?


Experience tells me the screen will put out a higher temp than the downspouts with the same flow so I would use screen and next summer will probably swap back.  (plus if build new the screens cheaper)
Also, and most important is I want to try and make it all look better.  It's not bad but I want it to look like a manufactured unit when  I am done.  Most manufactured units you can't see what's in them but since I cant do that I may even go with a back pass for a nice clean look.  That would even give me one more layer of protection from the very cold air at night and might help isolate and keep out a little more of the cold.

Quote:

"might try and cut down a little on the turn around area between downspout sections. "

Why? And what is the size of the turn around now and how much smaller would you go?


I was thinking the turn around was a little bigger than it is, it's really not that bad and I probably would not cut it down much more than 6", (currently about 2 feet).  I think the curved piece would really help the flow turn around though.
gbwillson

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Reply with quote  #13 
Dan-

I agree that trying to match a return to the ducts is likely to create a bottleneck. I wonder if there is a "best" ratio for the size of a return? If your ducts are 30 sq. in., would return be better at 45 sq. in. or more? Rounding the outside curve sounds like it would help at it would smooth the flow.

Greg in MN[comp]
solardan1959

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Reply with quote  #14 
Greg,
   Another easy test, just put 2 downspouts in a capped channel like I did here with my "small long double downspout" with some Plexiglas on top, (semi air tight).

[image] 
  Then start about the width of the downspout and hook up a fan to one downspout.  Take your anemometer and read the flow on the other.  Slide back 1 inch and repeat the test,  Do this until the flow does not increase, and then a couple more times for the sake of a test.  I figure the above could be reduced by about a third but an additional test would then be to add a curved reflector at the peak point and see if the flow improves more.
  Since I already have this collector/setup, I think I may do this test in the near future as it may be a good guide.  I would think if the downspout doubles the best length for the cavity may double to.

Dan
Garage_Hermit

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Reply with quote  #15 
Dan,

Quote:
add a curved reflector at the peak point and see if the flow improves more.


I read the above with interest, and agree a lot with you !
The simple reason is, it LOOKS more efficient with a curved reflector !

I took a pencil & tried to figure how much curve...

That resulted in another flagrant requirement - for a barrier between the two DS !
I call this a "fence" (like on an a/c wing...)

Don't know why, but I feel "sure" that the dimensions must be related, somehow, geometrically.
Like, the length of the fence (l) will equate to a certain number of "r", say 1.5 or 3, or 4 R,
and that "l" "might" just *also* be proportional to the length of the DS (maybe...) (but that one will be a hard one to "prove" !)

 
best regards,

G_H


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(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
gbwillson

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Reply with quote  #16 
It might even make the air move faster, like the tail fins on a classic car![wink] Dan's skinny test box is a good way to quickly try out various return sizes and shapes. The test box may need to be bigger though. Many of the DS collectors have a good amount of space between tubes. Another thing to consider is unless you are running your collector at higher velocities, the air is not going to flow as if it were a racetrack. It will exit the DS tube and drift in the direction of the return tubes. 3CFM per SF is not a strong flow. With a multi-tube DS collector, the incoming DS tube that has the farthest distance to travel would therefore need to move at a faster speed to keep up with the other tubes. Gary noted this problem. He also noted the reverse whether air was push versus pulled through a collector. Perhaps if you played with the length of the ducts in relation to their distance from the return, or use fences of various lengths as GH suggested.

Greg in MN[comp]


Garage_Hermit

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Reply with quote  #17 
FWIW, some more thoughts on plenum configuration...

per http://www.amca.org/certified/education.aspx

Quote:
Essentially, duct turns or fittings change the performance of the fan and system when they’re located close to the fan, and the more fittings there are, the worse the problem becomes.


Thus, duct turns are bad news ! so one has to make the best of a bad deal !

Some potential test configurations in enclosed WP file.


G_H


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(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
Rick Stone

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Reply with quote  #18 
I can't open either of your links, check them please.

Rick Stone
Talmo, Ga.
mattie

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Reply with quote  #19 
Both links working here
Rick Stone

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Posts: 121
Reply with quote  #20 
Never mind, both are working now , they didn't yesterday, Must be the NSA screwing with me.
Very good DS flow diagrams! The video tells us that most solar air heaters are using  increased fan CFM to overcome system effects because compact design requirements but there is some room for inprovement.  It's interesting that in the video at 8:37 they slip in a political bias and push for the" New world order". Politics has gotten really out of hand sometimes.

Rick Stone
Talmo, Ga
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