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gbwillson

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Reply with quote  #21 
Most of the ZP's built so far use a 4to1 ratio, as in 4'x16'. There are also a couple of 5to1 as well, with a skinny 8to1 being the most recent built. I do like the idea of the pipes, but your pipe or tube would have to be connecting to the duct you will be using. So in my case, either 6" or 8" ducts for the amount of air needed. Are you suggesting connecting a duct directly to the said tube or pipe? Either that's gonna be a big piece of pipe, or you will be greatly limiting the amount of air through the collector because of the reduction. Does this also mean the air enters the collector from the side?

Greg in MN

Rick H Parker

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Reply with quote  #22 
Area of a 6" duct 27.274 in sq. 8" 50.265 in sq. On the 4'X 16' your 1" gap area is 4' X 1" = 48 in sq. That partly explains why you need such a large duct.

"Does this also mean the air enters the collector from the side?"

Can be side, back, top, bottom or end.   Feeding from the end of the pipe looks like the way to go, that would distribute the Dymainic Pressure along the length of the slotted pipe and just Static pressure would be pushing on the exit slot. Static pressure is constant the entire length of the pipe. Producing an even flow across the collector, at least that is the theory.

Either that's gonna be a big piece of pipe, or you will be greatly limiting the amount of air through the collector because of the reduction.

Its not the size of the pipe that matters. Your own deflector proves that. It is the area of the cross sections that matters.
The slot along the length of the pipe serves the same function as your deflector.

Dropping down to a smaller pipe would create a pressure head. Your own design does that so nothing loss there. Also it is unavoidable, as with electronics or any other flow through a opposition to flow, Force = Rate of displacement * opposition to displacement.

Air pressure is Kinetic energy be stored as potential energy.  If 6-8" and the fans your using are creating too high of a pressure head, it indicates overkill.  It would stand to reason 3" and less powerful fans might be a better match to the collector.

What type of fans are you using with them 6-8" ducts and what are their Open CFM rating?  Also could you link them to a specific size collector with a specific gap and its performance. I'm trying to guesstimate the velocity of the air flow.

When it all boils down, velocity per length of collector is going to be the key to controlling the output temperatures. There is a practical length limit for a given pressure and resistance. I don't know what it is but, you might be exceeding it or pushing the envelope. It would explain your runaway temperatures.  I think your going to find long panels need to be split into two panels and plumb in parallel. The 4X16 would effectively become a 8X8 done this way. The air flow would travel only 8 feet instead of 16 but you can still get the 4X16 foot print stacked end to end.

All the more reason I think an inexpensive air velocity sensor would be a very useful tool for Solar Thermal Air experiments.


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Rick H Parker
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gbwillson

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Reply with quote  #23 

Rick-

I like your idea of a pipe or tube for a smaller build. Not sure you could effectively move enough air for a larger build. A person can certainly build a smaller collector/fan/duct setup. But a smaller setup will net you less BTU output. So it depends on a persons needs and expectations from a collector. In most cases, a longer collector will outperform a square collector having the same square footage. But a longer collector has greater resistance, and a long ZP would be no different. But the ZP also has shown to have very low resistance. Most of the ZP’s built so far are outputting over 400CFM, which is 23MPH using 6" ducts or 13MPH using 8" ducts. With one ZP output being used as a part time hair dryer(sorry Bert). So we are certainly pushing the boundaries with the 16'-20' long ZP's. And for anyone considering building their own large ZP, I would suggest using 8" ducts, with 6" ducts being the minimum, especially if the duct runs are long.

There are a wide variety of fans being used for all types of collectors. Some are store bought, though many are scrounged from various sources, including old furnace fans, and kitchen exhaust fans. Every build, duct, and fan setup is different being DIY, so a find or state a specific collector/duct/fan combination would be unlikely. The output air temp is the best indicator of proper air flow. Too hot, and you need more air. Too low, and you need to reduce your air output. The lower the output temp is almost always the efficient. High output temps are wasting heat. A person’s comfort level of the incoming air is subjective. A large amount of lukewarm air is far more efficient than a small amount of hot air when entering the home. And since my fans and incoming air are not on the main floor, I'm fine if the incoming air is in the low 90˚F range.

I’d love to have a way to monitor and log airflow and temps automatically. But I'd need a kit and a video to put it all together.  A quality anemometer, and both wired and remote thermometers to collect performance data, will work fine for most people.

Greg in MN

Rick H Parker

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Reply with quote  #24 
"Every build, duct, and fan setup is different being DIY, so a find or state a specific collector/duct/fan combination would be unlikely."

I beg to different, you could be specific about one of your builds and leave data from all other builds out of the discussion.

I’d love to have a way to monitor and log airflow and temps automatically. But I'd need a kit and a video to put it all together.  A quality anemometer, and both wired and remote thermometers to collect performance data, will work fine for most people.

I'm trying to ferret out the conditions and requirements so I can taylor a solution. I need a static target, otherwise I'm just spinning my wheels.

"I like your idea of a pipe or tube for a smaller build. Not sure you could effectively move enough air for a larger build."

The question being could one push 400 CFM through a 3" pipe?  For a EL of 100' of 3" pipe, 12 - 15 PSI pressure differential or one atmosphere would do it. 

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Rick H Parker
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gbwillson

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Reply with quote  #25 
Okay, let's try these figures for your calculations:

425CFM output
1.25" screen gap
50' EL of 8" ducting
???

Greg in MN


Rick H Parker

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Reply with quote  #26 

“1.25" screen gap”

Insufficient data to calculate Equivalent Length. The Equivalent Length of the screens could be calculated from experimental data. Could calculate Equivalent Length it if you knew the Moody Friction Factor, Minor Loss Coefficient, the cross-sectional area and perimeter length of the cross-sectional area.

"50' EL of 8" ducting"

50’ of straight 8” duct is equivalent to 50’ of 8” duct.

Equivalent Length is applied to components that have a higher or lower opposite to flow per length then the reference, bends, reducers, smaller pipe, larger pipe, ect. In this case the reference is 8” duct.

To say something has a Equivalent Length of X ft of 8” duct is to say is to say it has the same opposite to flow as X ft of 8” duct.
Another way to view it is, Equivalent Length is Equivalent resistance.

Another way to approach this is to determine the resistance of all the components then calculate pressure drops and flow the same way one calculates voltage drops (pressure) and current (flow) in a electrical resistive circuit. The math is the same, just different units of measurements.

R =∆P/V

Where
R = resistance,
∆P= the pressure difference.
V= flow.

Looks like Ohms law, works like Ohms law because it and Ohms law are both applied versions of the same physics equations. That is how I can say the math is the same.

Calculations are some what more complicated then electronics because, in electronics the "pipes" are considered to have no resistance and electrons always has the same mass.


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Rick H Parker
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Rick H Parker

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Reply with quote  #27 

There is one thing I cannot figure on these Zero-Pass. If you want the air to flow between the screens why are you using wire mesh screens instead of solid aluminum screens? Solid aluminum screens would have a much lower opposition to airflow then wire mesh screens.


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Rick H Parker
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Bert

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Reply with quote  #28 
There's been studies that shown a mesh increases the efficiency of the collector. The rougher surface will help the airflow scrub off the heat.
Also the front screen will let sun hit the back screen.
You could use a solid surface to replace the back screen but may need to rough up the surface a bit.


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Bert K.
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Rick H Parker

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Reply with quote  #29 

Also the front screen will let sun hit the back screen.

Has anybody considered that the top screen would be a glaze both by physical means and by thermal means. There would not be much of a temperature difference between the upper and lower screens. Therefore the heat would tend to conduct toward the cooler center between the two screens. It like putting a negative charged plate between two positive charged plates, energy flow will be toward the center.

"You could you a solid surface to replace the back screen but may need to rough up the surface a bit."

 Or use a textured aluminum sheet, if it really is the surface texture responsible for the increased efficiency. I not entirely convinced that is the case but I won't say it is not.


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Rick H Parker
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gbwillson

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Reply with quote  #30 
Rick-


I kept getting interrupted. Late to post I see. 


If the screen was a solid sheet, it would no longer be a ZP-type collector, it would be called a backpass-type where the air passes behind a solid sheet of metal. If you look at a screen, or mesh, from a very shallow angle(<1˚), it almost becomes solid surface both visually and to air flowing by. Keeping the air between the screen layers helps isolate the warmed air from the cold glazing. I say almost, because being an open weave and less than smooth, allows the air to intermingle slightly on either side of the screen surface. At higher velocities, very little air passes outside of the screen gap. Studies have shown that the courser the mesh or screens, the better they are at breaking up the laminar flow, at the cost of increased drag. We use window screen as it is cheap, and easy to work with. Whether the screen is made from aluminum or fiberglas makes little difference in performance due to the low mass of the screen. The top-most layer of screen is open-weave enough to allow the sun to pass through and still warm the lower screen(s).This allows the air passing through the collector's screen gap to be warmed from both top and bottom. Most ZP builds double up the bottom screen layer, so more of the sun's ray's stay where wanted, in the screen gap. 

Hence the reason for our ZP test. You can build a ZP collector that allows you to adjust the size of the screen gap, but you can't compare two differing gaps, at a given flow rate, at the same time. At a given flow rate, the air passing between the two screen layers may be too far apart for effective heat transfer and performance would be diminished. Too close, and flow resistance increases. Too little airflow also allow air to pass outside the screen gap only to be cooled by the glazing. Studies have shown that higher velocities are more efficient at extracting heat from a collector. A lot of lukewarm air will give you far more BTU's than a little hot air.

Late edit:Yes, heat does flow towards cold, which is why the proper screen gap is so important. Any rough surface material could be used for the back layer. Have you priced a sheet of aluminum lately? And if directly on the back of the collector, it wouldn't matter much if it was aluminum or something else unless it stood up more a textured surface. I've thought about having the back screen layer attached directly to the back insulation of the collector. But I still think the little bit of air space behind the back layer(s) helps somewhat.

Weather permitting, we may have a few answers soon...

Greg in MN




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