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gbwillson

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Reply with quote  #11 
Tj-

It takes very little airflow to overcome the natural tendency of warm air to rise, so feel free to orient your collector either vertical or horizontal. 

Last spring my friend Krautman and I found that a 1⅜" screen gap was ideal for a 16' collector with a high velocity of air flowing through. A smaller collector, or one with less airflow would likely be different. I would suspect a slightly smaller gap, but no smaller than 1". 

The reason to have the back screen layer slightly raised off the back of the collector box is twofold. First, it keeps the screen gap away from direct contact with an exterior surface. But the main reason is to allow small amounts of air along the boundary layer of the laminar flow to mingle with the rough screen layer. If the back layer of the screen gap were flat against the back of the collector, this mingling could not happen. Same thing is true if the back of the gap were a solid surface instead of an open weave. The rough surface of screen helps to break up the laminar flow which allow more of the air passing through the screen gap to pick up heat from the heated screen layers. At high airflow levels, the screens do keep MOST of the air between the layers. 

Question...Is there a technical reason you need to have the intake and exhaust at the same end of the collector? I only ask because my ZP sticks out more than 18' from the East side of my house. So the intake air has a long way to go before entering the collector, yet still produces great heat. It would also open up a lot more options for a collector build.

You mentioned the size of the garage, but how big is the area of the workshop you are heating?

Greg in MN


TJTurner

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Reply with quote  #12 
That makes sense, Greg. Interesting.

I was thinking of the input and output at the same end because of how I have to pipe into the garage, minimizing the ducting runs. But if you're getting good results with 18' runs, then I could easily do the same. I love this site. You guys have challenged and made me re-think several things I had as design constraints. So thanks! 

My workshop is 352 ft^2. The garage is larger. I'm just dumping heat, or looking to, into the workshop. I do have a south facing wall, which is 30' long by 10'+ high. My plan was to build a single collector I could bring through the window as a test bed. Then if that worked well, to make something more permanent that would attach to that wall. To do that, I have to bore through a brick facade. But the cinder block only goes about 8' tall. So I could bore through the brick and then pull air out and into the workshop through the ceiling.

I'm thinking of a 6" duct now. If I go that way, it looks like I can get a 6" duct fan that pushes a max of 470CFM. Is that too low with the duct losses for a ZP?

Has anyone ever tried perforated metal sheeting for the absorber in any design? That would be more expensive, but maybe absorb more heat than a screen?

Tj
stmbtwle

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Reply with quote  #13 
Perforated "soffit" has been tried, but I can't speak for it either way.

The advantage of the screen collectors is that they're not only effective, but COST effective, and in most cases that's what it's all about.

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Willie, Tampa Bay
SolarInterested

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Reply with quote  #14 
Tj Scott and Gary ran some tests

http://www.builditsolar.com/Experimental/AirColTesting/Index.htm
Gary said:
"Aluminum Soffit Collector:

In the performance area, this collector was basically tied with the screen collector, and there may be further optimizations that could be done to improve its performance from what was tested.

Its a relatively simple build, but harder and more expensive than the screen collector.

The pressure drop through this collector is low (which is good).

Performance rating: About equal to the base screen collector"

http://www.n3fjp.com/solar/comparisonhotair/comparisonhotair.htm
Scott said "The vented soffit appears to be a very good performer and is also a good choice.  I would choose the screen, however, because the screen appears to be performing slightly better, it is much cheaper, easier and quicker to build."

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TJTurner

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Reply with quote  #15 
Great info, thanks for the links SolarInterested.

Greg...

I think you've got me decided, to try the ZP. Do you have a good link/info about the most recent wisdom about dimensions? How far off the back surface is the bottom screen? How about the top screen from the glazing? And how are you doing the manifold design now to get a good constant airflow across the absorber?

I'm thinking I can do a 6" duct, with a 470CFM duct fan. Think that will work ok? Should I do a 1 3/8" gap in the screen with that CFM? Also it looks like the ZPs are mostly 4:1 ratio collectors. Would a 8'x4' be too wide?

Sorry for all the questions.

Thanks!
Tj
gbwillson

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Reply with quote  #16 
Tj-

I think almost any collector works better with an elongated configuration, or at least a design that keeps the air inside the collector box for a longer period of time. For example, you  normally wouldn't want the air on a 4'x8' collector to only travel 4' to exit the collector. However, with simple walls and vanes, you can change the path of the air flowing through the collector and greatly increase the distance and time it takes for the air to move through the collector box. Sounds like an easy solution for high heat output by making the air in the collector travel a long way and take a long time to reach the output.

Yes, most of the ZP's built so far are 4:1, but that's not to say you can't try something else. I do like the performance I'm seeing out of a unit of that size. But nothing says wider or longer can be an option. If anything, go longer as long as you have the fan capacity to extract the heat collected. The thing about a 2:1 ZP is that once you factor in the two manifolds, your ratio is more like 1.5:1. It still works, but some of the advantage of the ZP is lost. You could look at double pass version, where the air travels to the far end of the collector before returning to the near end. Another option would be a 2' x 16' ZP like dbc. It has the same area of collection as a 4x8 unit, but the air is heated over a much greater distance. Like a 4x8 collector, a 4:1 ratio unit is likely used due to the size of the materials sold. Otherwise you end up with lots of wasted materials. And it's not harder to build a larger unit, it just takes longer. 

A 4x8 unit or 2x16 unit are great starter collector sizes. They will heat a room or two very nicely. A 470CFM output fan will be overkill for a 32sf heater. But a speed control can dial that back as need. And should you upgrade to a larger collector, you will have a powerful fan available. There is no set distance for the screen to be away from either the back of the collector or the glazing. But it does allow air to "mingle" a bit with the screen fibers. It also keeps said air away from cold surfaces. I hope this helps...

Greg in MN

 


TJTurner

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Reply with quote  #17 
Thanks Greg, this is very helpful. 

What I was thinking was an initial portable/non-permenant collector that I can use along the side of my workshop and get the ducts in through the window. If that works well, then I thought I might upgrade to mounting something more substantial (like a 4'x16') collector along the south wall of the workshop. That requires boring through the brick wall of the workshop, which is why I was going to try the small collector first. Would you recommend trying it out at 2'x8' then as the initial collector? I want something that I can take down in the summer, and to convince the Dept of Treasury (the wife) to let me build the real big one next!

Tj
gbwillson

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Reply with quote  #18 
I don't think I would bother with a collector smaller than 32SF. Anything smaller might heat a very small bedroom or bathroom, but that's it. It certainly won't make a dent in your workshop heating needs. The common material size for sheet goods is 4x8, so you would also end up wasting half of the materials. If I had to choose between a 4x8 or 2x16, I would go with the longer unit. The air inside the collector is in contact with the absorber twice as long, so should be considerably warmer at the exhaust. And if you are considering a 4x16 unit in the future, the duct issues have already been worked out.

Since this is essentially a test unit, you might consider building the box entirely out of only polyiso insulation with a few reinforcement points. It sure would be light. Although being that it is somewhat exposed to the northern winds on the side of the house instead of being protected by the house itself, you may want to shield or reinforce the back so it doesn't bow inwards. And in the spring, the lightweight unit will be very easy to move out of the way. 

Greg in MN
TJTurner

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Reply with quote  #19 

Greg...great advice. I'll likely lean that way at first for certain.

I'm having a heck of a time finding polyiso near me. Is there a reason you guys use it over XPS? I have a sheet of 1" polystyrene left over from my floors, so of there's not a downside I'll use it. The melting point is around 240C, so it should be ok temp wise unless it has some creep properties at lower temp?

Thanks.

Tj

gbwillson

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Reply with quote  #20 
TJ-

You do NOT want to use XPS unless it has polyiso insulation covering it. The point at which it starts to deform is only about 212˚F, so while it may not totally liquify, it will trash your collector. In practical terms the only time this should be an issue is if the unit happened to stagnate. It's also flammable, but that is not likely to be an issue, even if a collector were to stagnate.

Most of the big box hardware stores carry polyiso sheets, or can order it. Another option is fiberglas panels. Where do you live? I'm sure we have someone in your area that has tracked down a source.

Greg in MN
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