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gbwillson

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Reply with quote  #11 
You make as much sense as anyone else around here...

There is an old rule of thumb with a solar heater that you wanted roughly 3CFM of fan output per SF of collector. The depth of a collector is normally not considered. So for a 64sf collector you would want 3CFM of airflow x 64sf of collector = 192CFM of airflow out of the tailpipe(NOT the free-air rating of the fan).

A more basic way to calculate airflow needs is you want a collector that gets very hot, and enough airflow the keep the output temp rather cool. Pretty much any collector will extract more BTU's /hr if the CFM is higher than 3CFM/sf. The problem is, most collectors with don't or can't operate efficiently at high CFM output levels. High CFM's tend to cool off the absorber faster than the absorber can be heated by the sun. On a basic 2-screen collector, running at high CFM, the air passes through basically the same area  on the screen layers. The sun can't keep up when it comes to heating the screen, so at higher CFM's, the 2-screen falters. Collectors like the ZP and downspout, heat the air continuously as it passes through the collector, so higher output CFM can be achieved for better efficiency. 

The ZP, for example, needs a LOT of air to keep the collector from getting too hot. How much? More than double the rule of thumb 3CFM/sf of collector. At 400CFM output, in a 6" duct, the air is moving over 33' per SECOND! So air moves through the collector in less than half a second. Keep in mind, that while the ZP has low resistance compared to some other collector designs, the ducting and fan will be a major factor in the CFM output as well. 

Again, you want a collector design that gets very hot, and a fan that moves enough air to make it efficient. Personally, I like to have a bit more fan capacity than needed, so I can use a speed control to better adjust the flow rate. 

Greg in MN

rustythread

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Reply with quote  #12 
Greg,  I'm kinda slow on this, and me in the ducting. For simplicity, I'm thinking about these questions in terms of theory excluding drag, turbulence, and other efficiency-limiting factors. 

If I have a colloctor made entirely of square 6" ducting with the intake and output both the same square 6" size, I can see that the output cfm will be the same cfm as in the body of the collector (disregarding for simplicity the effects of drag or turbulence).  But if I have the same intake and exhaust size, but a collector body which is 12" square, wouldn't the average velocity of the air in the 12" body be theoretically 1/4 the velocity in the exhaust?  [It's been nearly 60 years since high school physics, and I couldn't spell Bernoulli without looking it up...]

If my theoretical assumption should be that  all the air movement in a 2-screen ZP is between the two screens,  I would have the  cross-section and total volume of just that portion of the collector as one basis for calculations.  If I use the cross-section and total volume of the collector inside the glazing, I would have the theoretical other basis for calculations, and the practical answer would probably lie somewhere in between.  I.e., if the cross-section of the body of the collector is not the same as the cross-section of the exhaust the velocities will not be the same, nor will the transit time of the air through the collector. 
   Is the practical bottom line  that I should size my intake and exhaust for the same cross-section as the body of the collector and just use the twice-the-usual-3cfm /square foot  as a guide?

Beginning to wonder if I need 3 separate 8 x 8 collectors, just due to limitations on available fan motors and the catastrophe if it's all one big unit and the motor fails.

Sorry if this is becoming a PITA. Not so intended.  Thanks again for your thoughts.







gbwillson

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Reply with quote  #13 
As Olivia Newton John once sang, "Let's Get Physical..." Oh never mind!

Don't overthink this, or it will never get built! I'm not sure where you are getting the idea building a collector using 6" ducting. Ducting on a ZP system is often the biggest restrictor of good air flow. I only mentioned 6" ducting above as a comparison. But I also mentioned previously that you should use 10" or even 12" round ducts to connect to your collector. This is so you system can breath freely and allows the use of smaller fans, which are quieter and use less energy.

That being said, a 8' tall x 28' wide ZP, with the intake at one end and the exhaust at the other end will be the simplest build. I do think you will want a second fan at some point, but hold off until you get the system up and running with your recycled furnace fan pushing air through the system. Once you have a better idea of the output flow and temperature, you can add a second booster fan, if needed, of the appropriate capacity. You can always cover part of the collector temporarily until you acquire a secondary fan if temps are too high. But this is mostly an issue in the spring and fall when mild temperatures can dramatically raise collectors temps.

Not all airflow inside a ZP is between the screens. But it should start out that way as it leaves the intake manifold. My tests have show that the vast majority of air moving through the ZP stays between the screen layers, But a little mixing of the air close to the screens is essential as it helps break up the laminar flow for better heat transfer.

Any space inside a ZP collector above and below the screen gap is to allow a bit of air mixing near the screen surface and to keep the warm screen layers away from a cold surface. 

And the manifolds are nothing more than a place where the incoming air enters the collector, swirls about, and exits the manifold through the only opening, the screen slot. 




rustythread

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Reply with quote  #14 
Greg:  appreciate the reply.  I used a 6" square duct & 12" collector  only as an example for discussion; no danger of my actually going that small on the buil

Thanks
stmbtwle

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Reply with quote  #15 
"Don't overthink this, or it will never get built!" (Greg)

Have to agree 100%! Even a failed attempt is a learning experience. The next one will be better. But if you wait for the "perfect" design...

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
rustythread

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Reply with quote  #16 
I expect it will be spring/summer before I get to actual building, due to pressing nature of other items, so I have some tiime to try to get it planned as well as  feasible.

I don't expect to try to get anything perfect--too many unknowns  and  variables--but I'd like to get it as good as I can, since I hope not to have to re-do it once it's up and running.  The suggestion for covering part of it as may be necessary untill bigger fans arrive   will  be the key to getting the fans rated, installed, and adjusted.  Gave me one of those "dope-slap" moments with its sheer simplicity.  Thanks for that!

Could use another "dope slap" on the layout of intake and exhaust manifolds for ZP--can anyone point me to existing drawings so I can get my head around the proper concepts?

The wealth of knowledge on this site, and the active assistasnce, are very much appreciated.

Thanks


gbwillson

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Reply with quote  #17 
Rusty-

I can say with all honesty that during every new collector build I think something I want to try or a different way of doing something. It NEVER fails! Sometimes I'm lucky enough to be at the stage of the build where I can make a change. But more often than not, I have to make a mental note for my "next" collector. 

As previously mentioned, it sounds like you already have a large furnace fan, which should be more than enough to get you buy until you find out exactly what you are outputting.

Below is a drawing of the side view of a ZP. The main point is that regardless of the depth the collector box, air is prevented from leaving the box except through the screen slot. 

ZeroPass Intake Detail.jpg 
Here is a view from the front or top:
ZP Top view.jpg 
This is not to scale, but it varies based on your intake/exhaust ducts. So if you use 10" ducts, make your manifold a couple of inches wider than the ducts. You can place the ducts anywhere inside the manifold if you need to based on the install. You can also place extra screen in the manifolds to capture more heat. The air in the two manifolds swirls violently before exiting. Air passes through the entire collector in a couple of seconds or less. So make sure the air has a clear path to and from the screen gap and is not blocked by any screen.

Greg in MN


rustythread

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Reply with quote  #18 
Thanks!  The pictures help a great deal in trying to grasp the ideas!  Back to the drawing board...
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