lower speed pump

[Greenpool88]

I have a 16,000 gallon kidney-shaped pool, with jacuzzi tub overflowing into it. There is one return line to the tub and 3 to the pool. All lines are 2 inch, except from manifold (lines from skimmer, pool drain, tub drain) to pump and pump to filter where the line is 1.5 inches. The pump is a single speed 1 hp and the filter is a 24-inch sand filter. There is a heater as well that I never use. 14 cents per kWh in the second tier.

Okay, I was thinking to update my motor or pump/motor to something efficient and was thinking of a 2-speed type. When looking at the Whisperflo 1hp 2speed performance curve, I see that at 3450 rpm the pump can deliver the gpm for high heads quite well. However, to be energy efficient I have my eye on the lower speed (half of full speed), but the curves do not go above 20 feet of head. I assume that my effective head is perhaps double of 20 feet. If my assumption is correct, then the lower speed would never move my pool water. Is this a correct interpretation? Thanks!

 

[Melt In The Sun]

Welcome to TFP!
Head is a function of the speed that the water is flowing through the pipes...faster movement means more friction against the walls of the pipe. Your system may have 40' of head at high speed, but at low speed it would be much less. A good rule of thumb is half the flow rate of high speed for 1/4 the power usage.

 

[Greenpool88]

Ah, okay, I forgot about that. So, if 40ft head at high speed, what is the rule of thumb for half the speed? Is it 1/4 as well?

 

[Greenpool88]

Ah, I didn't think about the head changing for different flow rates! So, is there a rule of thumb for calculating head by reducing speed? Is it 1/4 as well? That is, if 40 feet at high speed, would it be 10 at lower speed? If so, then I can see how this pump would work for me at half speed.

 

[mas985]

Head follows pump affinity equations so 1/2 RPM = 1/2 Flow = 1/4 Head. So if normal head loss is 40' of head, low speed will be 10'.

Power actually flows the 1/8th rule but for a two speed pump, low speed loses 50% efficiency so in practice, the low speed of a two speed pump is about 1/4th the energy consumption.

Pump Affinity Equations:

GPM B = GPM A * (RPM B / RPM A)

Head B = Head A * (RPM B / RPM A) ^ 2

Hydraulic HP B = Hydraulic HP A * (RPM B / RPM A) ^ 3

 

[Greenpool88]

Thank you very much for this information! Now, those plots make total sense to me!

 

[Greenpool88]

Since all my piping is 2-inch, except around the pump where it is 1.5 inch, does the 1.5 inch piping control my calculations - that is, is it like the weakest link defining the strength of a chain? Is it odd that there is 1.5 inch piping only in this one part of the system?

 

[jblizzle]

Not exactly. The smaller pipe will just add as little more head loss.

 

[JasonLion]

No. If you only have a little 1.5" pipe, then it only makes a small difference. The more 1.5" pipe the more of a difference it makes. If most of the system is 2" and you only have a foot or two of 1.5" pipe, then it only makes a tiny difference.

 

[Greenpool88]

Thank you!