Pump Efficiency

[chem geek]

Mark,

OK, I think I'm beginning to understand this. So at lower flow rates there is less frictional loss. OK, so let me work backwards from the optimal flow rate for the solar panels which is 4 GPM per panel and I have 12 panels so that means 48 GPM. The minimum recommended per panel is 3 GPM because efficiency drops, but that gives a minimum of 36 GPM.

So, at 48 GPM (instead of 65 GPM) the frictional loss in a 2" (inner diameter) pipe is 3.9 feet of head per 100 feet (instead of 6.8 feet of head per 100 feet -- that's a heck of a difference!). With my 290 feet of total round-trip run length, this is a loss of 11 feet of head (instead of 20) and I add 0.87 PSI or 2 feet (it's lower than before due to the lower flow rate) for a total loss for the solar panels of 13 feet of head which is about 6 PSI. That's a whole lot better than I have currently (with my current pump, the solar difference is 13 PSI or a head of 30 feet) and is closer to what you say is normal.

So now I can look at (full-rated) 3/4 HP pump curves to see if they can handle the base 15 PSI (yes, I know this will be less as well) plus this 7 PSI increment or 22 PSI total which is about 50 feet of head (instead of about 65 feet of head I had before). It looks like most 3/4 HP pumps will readily handle the 50 feet of head and even if priming needed another 15-20 feet, they could handle that as well (though at a slower GPM, which is fine for priming). After priming, the pump at 50 feet of head delivers 66 GPM which ironically is where I'm at today. So in practice what will happen is that the higher flow creates more friction and as you pointed out it's where the two curves intersect that will be what happens and this will clearly be somewhere between 50 and 60 feet of head, which is 66 to 51 GPM. The turnover will be slightly longer at around 5 hours instead of 4 (with the solar on), but that's fine.

Now, with the solar off, the pump will output less than before (it's a smaller pump, after all) so the GPM will be less than the previous 90 GPM and the feet of head will be less than the previous 45, but there is no question that whatever it settles down to, the pump will be fine. The bottom line is that a 3/4 HP pump should work fine for my pool system with the existing plumbing.

Gee, this is fun. Just goes to show how truly geeky I am. Did I roughly do the above correctly and would you say that I probably could get away with a 3/4 HP pump to replace my current 1 HP pump? If so, then it looks like I will save at least 500 Watts which translates to over $300 per year in savings. Looks like that would pay for even a high-quality expensive pump (e.g. the Hayward Northstar) in just a little over a year for the pump cost plus more years of operation to pay for the labor (which I have a feeling might add up to quite a bit, but even so this seems worth it).

Richard

P.S.

If I can find a 2-speed pump that was 3/4 HP and 1/2 HP or a variable speed pump (expensive! about $1000) that I could set and if it could be triggered by the same switch that the solar valve triggers on, then I could be even more optimal and save even more money when the solar was off. Any suggestions? I looked up the Pentair (that you mentioned) WhisperFlo that is 3/4 HP (full-rated) and it looks like the high-speed will work perfectly with the solar system, but the low-speed curves may not be good enough for the solar off situation, mostly due to the longer lines to the pool, though as you say at low GPM this should not be a problem. The pump curves are at this link. I noticed in these specs that the efficiency at the high-speed is not good (2-speed 3/4 HP full-rated is 14.6A * 115V = 1.68 KW which is almost as bad as what I have now; only the low speed 4.7A * 115V = 540 W would save money). I need at least 33 GPM flow rates to have an 8-hour turnover and that implies no more than about 12 feet of head (about 5 PSI). I'm not sure that will be attainable -- the pipe head might only be 3 feet of head or less (at 33 GPM), but I have no idea what the filter head will be at that GPM (actually, I just looked up some typical charts and the filter head is minimal -- even at 90 GPM it's 3 PSI and at normal lower GPM it's less than 1 PSI). If I just figure 1/3rd the amount of head (based on head loss tables of 50 GPM vs. 90 GPM) then I should be at around 5 PSI (12 feet of head). Maybe this will work out OK after all though it seems right on the edge and has longer turnover (8 hours vs. 5 hours) when the solar is off.

OK, now this is probably insane, but the Pentair IntelliFlow at this link automatically adjusts the speed to maintain a constant settable flow rate. It's an expensive pump (about $1500 online) but it does look like it could save a substantial amount of money. I could optimize for the 4 GPM per solar panel so 48 GPM with the solar on and it would automatically slow down to maintain that same rate with the solar off. It looks like it could save at least half or more of my main pump costs and that would be at least $500 per year. My main concern is how reliable this is since it's an expensive "toy" to break! I'd love a long warranty with this guy, but only comes with a 1-year warranty. I thought about using this single pump for the pool sweep as well (which would turn on with a valve), but I think the flow/pressure requirements for the pool sweep are quite high (which is why it has a dedicated 3/4 HP booster pump) so would be incompatible with the much lower flow rates in the rest of the system. Oh well...

The last radical thought I had was going down to a 1/2 HP pump since most have pump curves that can output 50 GPM at feet of head of 22 (Hayward Maxflo), 30 (Pentair SuperFlow, Hayward SuperPump), 40 (Pentair Challenger), 45 (Hayward Tristar), 50 (Pentair WhisperFlow). At the lower flow rate, the total head with the solar on will likely be less than 30 feet. Anything from the Pentair Challenger up should work out just fine, even with an extra 15-20 feet at lower GPM to prime the solar. I just have to be careful since some 1/2 HP pumps don't seem to use any less electrical power than 3/4 HP pumps though most do use less.