Heat loss along 15mm & 22mm Cu pipe

Question:

My question concerns the insulated return pipe from a solar collector. I intend to run copper pipe to the hot water tank/heat exchanger but can’t decide what size pipe to use (the anticipated hot water flow will be perhaps 50 lt/hr).     Comparing a 5m run of 15mm against 22mm copper pipe, which is more efficient (less heat loss)?     22mm pipe that has a ‘high’ volume to surface area or a 15mm  pipe that has lower volume, relatively higher surface area BUT has a faster transit time. I would be most grateful for any suggestions. Richard England

Response:

The smaller the better (less surface, less time in the pipe), BUT there’s a limit to it when it comes to resistance. I have a table at home about these limits, and I can look it up in the evening. The minimum diameter depends on the flow and on the length of the pipe, but with 50 l/h and 5 m I think 15 mm is enough. I have 18mm pipes in my system, which can pump more than 300 l/h over about 25 m of pipe – although I know this is on the limit and I should have used 22 mm to reduce the strain for the pump, but the whole system works as expected. Christian

– Hide quoted text — Show quoted text -> My question concerns the insulated return pipe from a solar collector. > I intend to run copper pipe to the hot water tank/heat exchanger but can’t > decide what size pipe to use (the anticipated hot water flow will be > perhaps > 50 lt/hr). >    Comparing a 5m run of 15mm against 22mm copper pipe, which is more > efficient (less heat loss)? >    22mm pipe that has a ‘high’ volume to surface area or a 15mm  pipe that > has lower volume, relatively higher surface area BUT has a faster transit > time. > I would be most grateful for any suggestions. > Richard > England

Response:

– Hide quoted text — Show quoted text -> My question concerns the insulated return pipe from a solar collector. > I intend to run copper pipe to the hot water tank/heat exchanger but can’t > decide what size pipe to use (the anticipated hot water flow will be > perhaps > 50 lt/hr). >    Comparing a 5m run of 15mm against 22mm copper pipe, which is more > efficient (less heat loss)? >    22mm pipe that has a ‘high’ volume to surface area or a 15mm  pipe that > has lower volume, relatively higher surface area BUT has a faster transit > time. > I would be most grateful for any suggestions.

Larger pipe means less pumping work to get the flow.  And higher flows through collecters and heat exchangers means better heat transfer.  As far as heat losses go, put some insulation on the pipe.  Putting 5mm of foam sleeving around it raises the diameters to 25 and 32.  That means the difference in surface area becomes a factor of 1.28 The pressure drop through a pipe varies inversely with the diameter raised to the *fifth* power.  So the difference in pressure drop for (22mm/15mm)^5 = 6.7 times more for the smaller pipe.  Mind you, the pressure drop in 5m of pipe is probably only a tiny fraction of the pressure drop in the whole system. But insulation costs are a one time thing (here in the US, foam sleeves for common pipe sizes are very cheap), whereas pumping power and low flow are issues that you will have forever. daestrom

Response:

Christian> The smaller the better (less surface, less time in the pipe), Christian> BUT there’s a limit to it when it comes to resistance. I’d think you would want to model your system as having some sort of SEER ratio: how much heating do you get for the amount of electricity expended running all the bits (pumps and controllers). Pipe diameter comes down to trading heat loss against pump power. The most important variable is probably the length of that return pipe (5m), and the combined flow rate through all the panels (50 l/hr). It probably also matters how much insulation you have on the pipe, and how well installed it is.  (Did you tape over the insulation, or otherwise seal gaps? Is it protected from the weather?) Finally, systems at different SEER ratios might use different pipe diameters. >From http://www.anotherurl.com/therm/howmuchenergy.htm

urethane foam pipe insulation, 40mm outside diameter, 15mm inside, R-6 per inch. R-6/inch is .042 W/m^2-C Rcyl = ln(outer radius/inner radius)/(2*pi*length*conductivity)      = ln(20/7.5)/(2*pi*5*.042)      = 0.743 Assume a fluid-to-air temperature delta of 40 C, you get P = 40/0.743 = 53.8 watts lost urethane foam pipe insulation, 46mm outside diameter, 22mm inside, R-6 per inch P = 71.5 watts lost So, can you save 17.7 watts from the pump by going to a larger pipe? If so, use the bigger pipe. You can also get 22mm thick insulation, which I would seriously consider using, and I’d use on the outbound pipe as well (if there is any recirculation going on).  It’s not clear to me if the O.P.’s flow rates will save much power going through 22mm pipe versus 15mm. 15mm: 10.4 cm/s  <- this is already somewhat slow 22mm:  4.4 cm/s

Response:

iain-3, that link is nice – I will calculate my system (although I will for sure NOT change all the pipes… )) Christian – Hide quoted text — Show quoted text ->From http://www.anotherurl.com/therm/howmuchenergy.htm

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