Pipe thicknesses

[Mister Sheen]

Greetings peoples.

I've got 15mm copper piping throughout most of my east-west system, with 22mm coming down from the manifold to the loft floor. I did this because (a) I wanted to increase the stagnation volume in the event of a power-out, (b) decrease the surface area-volume ratio i.e. scope for heat loss and (c) as a novice I didn't have any experience with other pipe diameters!

I've also got a pressure expansion vessel on each of the two loops (overkill!) just in case I get heavy stagnation effects in both manifolds at the same time (bright sun overhead at noon -situation). The system is pressurised to 2 bar so as to (a) increase the system boiling point, (b) reduce the steam volume in/around the manifold, at any given temparature, on stagnation and (c) have a nice volume of water in the EVs to condense any steam that might enter it and avoid melting the rubber membrane. The EVs are oriented valve-down so that the membrane isn't holding the weight of that water.

Apart from the extra volume of circulating fluid to heat up (~2-3 litres compared with a  170 litre tank-full) is there any other reason for using lower pipe diameters?

And why 1 bar recommended system pressure?

Do I come across as being paranoid about stagnation?! lol

[dhaslam]

Large diameter pipes have a  bigger surface area  to lose heat  but the overriding reason for not using large diameter pipes (anything over 10mm) is that in a  start stop situation  you lose most of the heat in the  pipes each time circulation stops.     If the pipe run is very short then the large diameter pipe is advisable  to absorb the heat but not otherwise.   

I used 22mm to  the  home made panels because of the long run,  30 metres each way,  as well as  the size of the panels  30 square metres in total.  There is about 10C lost on the way at present.  I have to modify it in two ways,  first is to heat a small tank that will transfer  one full tank at a time and the second is to  use a mixing valve in reverse on the tank end to only allow warm water to transfer.  Otherwise the initial circulation would cool the tank.       

[alankelly]

Hi

I always though the smaller the better, so I went for 12 Twinsol for my install, as to fit 10mm copper pipe with all the bends required for my install. (ET tube collector is on the outside garage roof but my store in upstairs)

Just remember, just changing from 10mm pipe to 15mm increase the pipe cross sectional area from 78mm squared  to 176mm squared, meaning that the required solar fluid volume is over double, so in the winter time I presume you need to try and heat double the volume of fluid.

Also the surface area of the pipe increase by about an extra third (Circumference of a 10mm pipe is 31mm and for a 15mm pipe it is 47mm) so you have greater thermal losses due to a greater surface area for any given pipe run.

Best regards Al.  

[Richard Owen]

The important number here is the ratio of the volume to the surface area.

Or, to simplify things, the ratio of the circumference to the area.

15mm pipe has 50% greater circumference than 10mm pipe, but it has 225% greater area.

The greater the ratio of surface area to volume (i.e. the smaller the pipe) the easier it is for the pipe to lose heat.

The balance to be struck is the need not to lose heat (which argues for bigger pipe) against the need not to spend all the time in marginal conditions in start/stop (which argues for smaller pipe.)

[TwoHorsePower]

there is a useful explanation of the impact of pipe bore size and  'stop-start' and 'overnight' cooling in this interesting post

http://www.solarco-op.proboards.com/index.cgi?board=energyinfo&action=display&thread=36

.. and it mentions other benefits of small bore. think it fails to mention that insulation for small bore pipe is cheaper, and indeed so is the pipe itself!

regards

2HP

[micko]

Do I come across as being paranoid about stagnation?! lol

I think there are a lot of heat dump paranoids out there, myself included I spent lots of time setting up a heat dump, the system ran most of last year and never got close to dumping heat. I have 30 X 58mm tubes and a 300 L tank.
I have mounted the expansion tank the other way up to you, I was concerned that with stagnation the steam would hit the membrane rather than the liquid. Thinking about it won’t everything be at the same temperature even with steam in the system as it is closed?
What size system do you have? I was thinking of adding another panel to give me an East South system.

[wookey]

Richard - but you lose more heat due to the slow flow than you do due to the large area. We've been over all the sums 3 years or so ago and concluded that for best efficiency you should use the smallest pipe that wil still shift the max heat input at a sensible pump power. On most domestic systes that means 10mm pipe, and a low-power pump will usually suffice.

Use eric's simulator code to model this if you want to.

[Baz]

Is there a sort of graph for pump power, panel size/power, pipe dia, heat loss.
Probably needs an interactive spreadsheet thing.

[Philip R]

Use 10mm pipe, and your circulator will cavitate and heat your water through friction because its flow resistance will be not matched to the pump characteristic.

Use 15mm copper , your installation volume increases by approx 50% compared to 10mm ( Iam not taking into account the id of the pipe), but the cross sectional area increases by about 225% or flow velocity reduces to about 44% of that in 10mm pipe, the diff pressure reduces as the square of the quotient( assuming laminar flow) so about 20% of the differential pressure required to that of 10mm pipe.  So for a given flow rate, a marked reduction in pump power using 15mm compared to 10mm copper.

Using 15mm pipe means you can run the pump at minimum speed, i.e. the lowest electrical power. The cost of the energy (integrated pump power) should be factorred into the losses, and I would suggest that for a solar array up to say 80 tubes and / or maximum of 20 metres of pipework length including manifolds, then 15mm pipe is a good compromise in terms of pump power, insatallation cost and ease of insulation.

PhilipR

[Mister Sheen]

Thanks guys and merry christmas to one and all.

Seems like a case of swings and roundabouts so I think I'm in more-or-less the most appropriate zone as it were!

Robin

[ericw]

Hidden away on the Navitron home page

http://www.navitron.org.uk/pmwiki/pmwiki.php?n=Main.PipeSizingCalculations

You can use the spreadsheet referenced at the bottom to do your calculations.

[TwoHorsePower]

In support of comments from ericw and wookey,  I used ericw's model to calculate the expected pressure drop when planning by solar plumbing with a very long run of 10mm pipe, and from that to determine the suitability of a particular model of efficient low power 12v pumps. Several years on I am delighted with the results and happy in the knowledge that I haven't overengineered the pumping with an inefficient 35w 240v central heating derived 'solar' pump, and that furthermore i'm not losing heat unnecessarily with a high volume of water the solar loop pipework.

If you know the max theoretical power output of your panels, then you can check what deltaT can be expected from the flow rate predicted by ericw's spreadsheet. If your delta T is greater than say 10 degrees, then perhaps the flow rate may be regarded as insufficient (and you therefore need either wider pipe or a pump with greater head), but anything less than 10 degrees will be fine. The reason you don't want to be running continually with a larger deltaT than say 10 degrees is that panel efficiency drops significantly when the panel is being forced to run too hot by virtue of too low a flow rate.

If you are planning on a long run of 10mm pipe, as Philip R suggests, you may indeed run the risk of pump cavitation and you should therefore to do the sums first to be sure, though I would say that with short runs and most solar pumps you are almost always ok with 10mm pipe and do not need to do the maths.

regards

2HP





 

[desperate]

Just coz I'm an ornery git, I use 12mm pipe as a good compromise with no apparent cavitation issues, shifts all the heat from 60 tubes and is available in rolls of soft, easy to "cable" from here..................................to..........there.

Desp

[Iain]

Hi
Same as desperate

I use 12mm pipe as a good compromise with no apparent cavitation issues, shifts all the heat from 60 tubes and is available in rolls of soft, easy to "cable" from here............................

I used 12mm soft copper roll, 20m run. Runs 20 x 65mm tubes. Works really well and easy to fit.
Iain

[wookey]

I'm using 10mm pipe, on a 12m run with a 12W pump. Works nicely, just like that spreadsheet said it would.

PhilipR implies that you will get cavitation with 10mm. That's not the case unless you have a run longer than 40m and more than 40 tubes (or something like that - check with the spreadsheet if you are worried).