Balanced vs unbalanced pool solar collectors?

[Tomfh]

I have a question regarding rooftop pool solar collectors that I would be interested to hear your opinions on. (I’m a structural guy and am a bit out of my depth on fluid mechanics.)

Basically, is it worth balancing your rooftop collectors?

See attached images for what I’m talking about:

I’m about to replace mine, and am wondering whether to bother with balanced plumbing this time around. I have a balanced system at present and it’s annoying having an extra pipe looping back. I’d prefer just the two pipes.

The system is 40mm header pipes, 7m long, with 168 of 6mm ID tubes, each 20 metres long (10m out and back).

My instinct it that the resistance of the 20m long 6mm tubes is so great compared to the short lengths of 40mm header tubing that the flow will balance itself pretty well across all the tubes regardless. The white arrows in figure 21 seem like an exagerration to me. But I am not a hydraulic engineer.

Let me know what you think.

 

[3DDave]

A small difference in resistance to flow can produce significant changes in resulting flow distribution.

You have about 8000 mm^2 of small tube section and 1300 mm^2 of feed tube area. I'd have to dig out the formula book and guess what flow rates and pressures you might have, but it seems like the possibility for imbalance exists.

OTOH - you might save the money up front, see if it works well enough and, if not, add the extra plumbing later.

What's missing, for me, is what the change in energy gathering occurs if it's unbalanced. The lower the rate, the longer the dwell time, so the higher the outlet temp on that 20m line. However at the higher temp the line is radiating more heat so there's a loss from that. OTOH too high a flow rate means a smaller temp delta at the other end of the loop, decreasing the efficiency of heat transfer. Balancing means it's easy to alter the flow rate and know the entire array will have predictable heat gathering changes.

Were it me and I decided on the unbalanced approach, I'd still leave fittings in place to make converting to a balanced one as simple as possible later on - maybe a ball valve at each end of the header and unions with the untaken choice capped off.

 

[1503-44]

I'd balance them. You're only talking about one pipe length or so. Probably costs less than adding a valve or two and then never being able to get the valves adjusted right.

 

[LittleInch]

It will be aa bad as your diagram.

You need the header to be at least twice the area of the branches to avoid the short circuit.

Which is about 150mm diam.

With 168 tubes there 8s no practical way to "tune" the system.

If it isn't broke don't fix it...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tomfh]

Ok, thanks for the feedback.

I will install it as a balanced system.