Pipe Head Loss and Temperature Dependence

chem geek

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Mar 28, 2007
12,082
San Rafael, CA USA
#1
I started up my solar heating this week and noticed that the energy usage on the Pentair Intelliflo pump was around 1650 Watts when it normally would be around 1500 Watts. The temperature of the water is currently 57F while our normal temperature when it gets fully heated is 86-88F. I thought that the difference might be due to the increased kinematic viscosity of the water at lower temperatures so I plugged the numbers into a spreadsheet I wrote for this (a while ago) here. Sure enough, at the 48 GPM flow rate I have (so using the 50 GPM column in the spreadsheet) I get a pipe head loss of 4.14 feet per 100 feet at 57F and 3.82 feet per 100 feet at 88F so would expect a 1500 Watt at 88F to go to 1500 * 4.14 / 3.82 = 1625 Watts assuming no significant change in pump efficiency (i.e. output power of Head times GPM to input power roughly the same).

At least for me, it's a really great feeling when chemistry and physics work out in a nice way to predict reality. Thank you to all of you experimenters and fellow inquisitive folks out there that have helped us figure out what really goes on in our pools. I reviewed a public draft copy of APSP-11 described here, here, here and here and was very pleased to see that it had information on the chlorine/CYA relationship, at least in a strong qualitative sense. I submitted comments to fill in more detail including a more quantitative sense and to show the true chlorine vs. pH graphs when CYA is present.

Richard
 

mas985

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May 3, 2007
12,181
Pleasanton, CA
#2
chem geek said:
I started up my solar heating this week and noticed that the energy usage on the Pentair Intelliflo pump was around 1650 Watts when it normally would be around 1500 Watts. The temperature of the water is currently 57F while our normal temperature when it gets fully heated is 86-88F. I thought that the difference might be due to the increased kinematic viscosity of the water at lower temperatures so I plugged the numbers into a spreadsheet I wrote for this (a while ago) here. Sure enough, at the 48 GPM flow rate I have (so using the 50 GPM column in the spreadsheet) I get a pipe head loss of 4.14 feet per 100 feet at 57F and 3.82 feet per 100 feet at 88F so would expect a 1500 Watt at 88F to go to 1500 * 4.14 / 3.82 = 1625 Watts assuming no significant change in pump efficiency (i.e. output power of Head times GPM to input power roughly the same).

At least for me, it's a really great feeling when chemistry and physics work out in a nice way to predict reality. Thank you to all of you experimenters and fellow inquisitive folks out there that have helped us figure out what really goes on in our pools. I reviewed a public draft copy of APSP-11 described here, here, here and here and was very pleased to see that it had information on the chlorine/CYA relationship, at least in a strong qualitative sense. I submitted comments to fill in more detail including a more quantitative sense and to show the true chlorine vs. pH graphs when CYA is present.

Richard
Interesting results. This is probably why Pentair thought it necessary to include temperature compensation in the Intelliflo calibration. Generally I ignore water temperature in head calcs since in most cases it has a fairly minor impact on the estimates. Even the with your situation the delta is not that much. From the pump affinity equations, 10% change in wattage should be approximately a 6.6% change in head and 4.9% change in RPM or flow rates. This is less than 2.5 GPM out of 50 GPM.

Also, the pump affinity equations dictate that you need to adjust the power by 1500 * (4.14 / 3.82) ^ (3/2) or 1692 watts. Wattage is not directly proportional to head but by the power of 3/2 or there abouts. The relationship is a lot more complex than that because of the efficiency change in the pump as head changes while affinity equations assume constant efficiency.

Like you said though, it is nice to see confirmation of theory.
 

chem geek

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Mar 28, 2007
12,082
San Rafael, CA USA
#3
Mark,

I thought that Output Power = GPM * Head * factor where the factor is 0.188165 if head is in feet and power is in watts. That's directly from the physics of moving a volume of water against a force (pressure). The Intelliflo presumably kept the GPM the same (it said it did) so then the output power would be proportional to the head. If the efficiency were constant, then the ratio of output power to input power is constant (since that's the definition of power efficiency).

I understand how the pump affinity laws get power factors when looking at RPM (which applied to a system curve results in changes to both GPM and Head), but assuming that it is GPM that is kept constant (the pump did show that a higher RPM of 3110 vs. 2980 was needed for the cold water vs. warmer water), what's wrong in my analysis above? For RPM, the exponent for my data is 0.53 such that 2980 * (4.14 / 3.82)^0.53 = 3110 so is roughly a square root relationship between head and RPM. So at constant GPM, the head is proportional to the square of the RPM and this is consistent with the head formula that matches the Intelliflo pump curves (from their charts) quite well:

Head (feet) = (RPM/350)^2 - (GPM^2)/470

Richard
 

mas985

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May 3, 2007
12,181
Pleasanton, CA
#4
My bad, I neglected the constant GPM. I am curious though, if the difference in estimate could be accounted for in the change in efficiency. Based upon the RPMs and power (both reported by the Intelliflo), the drop in efficiency is about:

Delta Efficiency = 1- 1650 / 1500 * (2980/3110)^3 = 3.2%

So with the efficiency change included 1500 * (4.14 / 3.82) * 1.032 = 1678.

Now on the high side so I suspect it is simply the accuracy of the Intelliflo power and RPM reporting and inaccuracy of setting a constant GPM all it seems less than 2% error which is not too bad.

Anyway good job putting that all together.