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gbwillson

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Reply with quote  #41 
NEW MATH? No. The test bed isn't 48" wide. The scrap wood we used for the back was just over 32" wide. 2x6's are used for the side walls. So, 3-9" wide channels, plus 4-1.5" sidewalls adds up to 31.5" total. My initial choice would have been to rip a piece of plywood for a 24" wide test collector. But that would have made for 6.5" wide channels which would have made them too narrow for the stapler. Even the 9" wide channels were hard to work on. Craig had some scrap chip board, so we used it.

Greg

Rick H Parker

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Reply with quote  #42 
Not new math, same old math I been using for 50 years.

3*9" + 4*1.5" = 33" .... closer only 1.5" different from your 31.5". Even closer to the just over 32".
It looks like you got 3/4" screen mounting rails. If so your active channel width would be 9" - 2 * 0.75 = 7.5".



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Rick H Parker
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gbwillson

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Reply with quote  #43 

Hi gang-

 

Testing is done! Just a quick recap: My neighbor Craig and I wanted to explore the optimal screen gap. So we built a test collector with three channels with various screen gap sizes. We wanted to find the gap size that produced the most BTU's. Of the few ZP collectors that have been built, the screen gaps range from 1", up to roughly 1.5". I didn't count Seatec's 8 layer ZP since it is really a different beast altogether. We tested the following screen gaps: 1", 1.25", 1.375" and 1.5". 

 

Day 1 of testing: We compared the 1", 1.25", and 1.5" screen gaps. All three channels were outputting 73 CFM’s @ 9.5MPH. Three readings were taken, with the average below.

1" screen gap---------1,796BTU's Produced

1.25" screen gap-----2,341BTU's

1.5" screen gap———2,076BTU's

 

The 1.25” gap produced 30% more BTU’s than the 1” gap

The 1.5” gap produced 16% more BTU’s than the 1” gap

The 1.25” gap produced 15% more BTU’s than the 1.5” gap

 

So in this round, the 1” gap was the loser, and the 1.25” gap was the winner!

 

Day 2 of Testing: Since the 1” gap clearly lost round 1, we eliminated it from competition and changed it to 1.375”, which is right between the two best performers from day one:1.25” and 1.5”. 

Again, we matched the CFM output at 65 @ 8.5MPH, tested three times, and averaged the three below. Being that the difference between the screen gap sizes tested below is now only ⅛”, we expected the results to be closer together, and they were.


1.25” screen gap————2,201BTU’s Produced

1.375” screen gap———-2,489BTU’s

1.5” screen gap————-2,099BTU’s

 

The 1.375” gap produced 19% more BTU’s than the 1.5” gap

The 1.25” gap produced 5% more BTU’s than the 1.5” gap

The 1.375” gap produced 13% more BTU’s than the 1.25” gap

The 1.375” gap produced 39% more BTU’s than the 1” gap.

 

Conclusion:Clearly you can have a gap that is either too large or too small, and as this test shows, the sweet spot is a 1.375” screen gap. I expected that the 1.5” would be too large as the screens would be too far from the main flow of air. I didn’t expect the 1” gap to perform so poorly. My thought were as long as you had the same amount of air moving through the 1” gap, the closer screens would transfer heat more effectively. Perhaps at a lower CFM the 1” gap might perform better. Testing screen gaps in increments smaller than ⅛” is impractical as it would be difficult to tighten a screen so it has less than a 1/16” amount of sag. 

 

One last note:We compared the airflow entering the intake manifold from the end versus the back of the collector. The CFM results were the same. It was thought that air entering the manifold from the end had an easier entry into the screen gap since it is pointing directly at the gap. A deflector was NOT needed for end entry for our testing. Entry via the end often requires the use of extra elbows, along with straight or corner boots, which add a LOT of EL to the ducting. Entry via the back is far easier and looks better too as it hides the ducts from sight and keeps them out of the sunlight as well. But either way works.

 

Greg and Craig in MN







Rick H Parker

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Reply with quote  #44 
"the sweet spot is a 1.375” screen gap."

At what velocity?

It is a good practice to specify your test conditions so that others can duplicate, compare and with some luck, build useful equations.

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Rick H Parker
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gbwillson

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Reply with quote  #45 
My bad...I had it, just forgot to share. Added above.

Greg 
mranum

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Reply with quote  #46 
Thats some great data Greg. Thank you for taking the time and energy to put it all together! I for one appreciate it! When I get some time I may have to open my collector up and widen out my screen gap a bit and see what I get for results.
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Rick H Parker

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Reply with quote  #47 

All three channels were outputting 73 CFM’s @ 9.5MPH. Three readings were taken, with the average below.

1" screen gap---------1,796BTU's Produced

1.25" screen gap-----2,341BTU's

1.5" screen gap———2,076BTU's


CFM in ft^3/minute = Cross Section area in ft^2 * velocity in ft/minute.
9.5 mile/hour = 836 ft/minute.

73 foot^3 / minute / 836 ft/minute = 73/836 * ft^3/ft * minute/minute = 0.087 * foot^2 * 1 =    0.087 foot^2 cross section area.

(1" = 1/12 =  0.083 ft.)
0.087 ft^2 / 0.083 ft. = 1.048 ft (12.576") active channel width.

(1.25" = 1.25/12  =  0.104 ft.)
0.087 ft^2 / 0.104 ft = 0.837 ft (10.044") active channel width.

(1.50" = 1.50/12  =  0.125 ft.)
0.087 foot^2 / 0.125 ft = 0.696 ft (8.352") active channel width.

Clearly the CFM and the velocity cannot be the same on three different channel sizes ... unless your allowing the air that passes through the screen to exit on top of the screen, in that case the three channel sizes would be the same. 

You said you measure the velocity of all three channels then balanced them, did you just assume the CFM was the same? If your using an CFM/ Anemometer, the CFM must be calibrated to the cross section. CFM is calculated from the velocity, there is no direct way to measure CFM. If the CFM/ Anemometer was not recalibrated for the cross section of each channel, that would explain the discrepancies.

If you can get me the actual width of the active channel ( the distance between the screen mounts) CFM per channel can be calculated from the air velocity of each channel.


Any idea what went wrong or did I find what I am missing? the exit construction perhaps?


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Rick H Parker
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mranum

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Reply with quote  #48 
Is there a reason to CONSTANTLY critisize peoples efforts? Everything here is DIY at home, in a basement or shop. Nothing is done in a lab, results are never pristine or exact. Constructive crticism is great but overall thats not what I read. I just read posts by someone who's goal is to pick apart everything. Someone who knows everything but has done nothing and to me thats just plain being a dick.
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Bert

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Reply with quote  #49 
It's obvious that Rick has major problems. Let's just ignore him.

Greg,

Thanks for doing this test. 99% of people here appreciate it.
It looks like 1-3/8" worked out the best in this test. I'm close at 1-1/4".  

I wonder if a wider collector would effect this?  

Do you think that the gap could be wider if a middle screen was added? Seems likely. I'm wondering if that would decrease air resistance in extremely long collectors like mine.



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Bert K.
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Rick H Parker

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Reply with quote  #50 
"Is there a reason to CONSTANTLY critisize peoples efforts?"


It is not criticism, it is peer review. Peer review is how teams accomplish things.
Egotistical individuals who are seeking glory instead of accomplishment do have a problem with peer review, team players do not.

Greg comes across as a team player.

Two others here clearly are not.

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Rick H Parker
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