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bpricedo

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May 20, 2012
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Okay, I've been reading about pipe size, dynamic head, head loss, flow rate, etc. it is often confusing and very daunting, but I'm trying to understand. One thing is puzzling me: looking at flow curves for a two speed Whisperflo, specifically the low speed curve, none of the shown dynamic head values are above 22 or so. Does dynamic head drop this much when flow rates decrease? If it doesnt, how do these speeds work against higher dynamic head values?

http://www.pentairpool.com/images/produ ... ve_new.gif
 
So, you are saying a system with a pump that flows 90 gpm at 50 ft of dynamic head will only have 15 ft of dynamic head at 40 gpm (using curves B and G, which is for the 1 1/2 hp up rated pump, I believe)?
 
Your systems actual dynamic head would need to be figured out for each flow rate to be exact, but in general, the drop in head from high to low will be in that range.

The pump curves you see are what the pump can pump at different head pressures...but in most cases if you know your system/piping head loss at high speed, and if the pump curve shows the pump can operate there with an appropriate flow rate for the equipment, then usually the low flow curve will also work.

One caveat that I can think of is at pump startup. For example, I have solar on my roof and I have to prime the panels on high since they won't prime on low. Once the panels are primed then I can run on low. The same might be true for priming a pump that is above the level of the pool.
 
According to the pump affinity laws, flow rate is proportional to the motor RPM and dynamic head is proportional to the square of the flow rate/RPM, so if the full speed operating point is 90 GPM @ 50 ft of head, the half speed operating point will be 45 GPM @ 12.5 ft of head.

I have a write-up on this in the Hydraulics sticky in my signature.
 
This came up because the PB agreed to give me a 2 speed upgrade for no charge but he wasn't sure how much benefit I would get. I wanted to be able to show him on the flow curve that it was still possible to get usable flow on low speed.

I really have no idea about the true dynamic head of the system but I will give more info: it is approx a 27-30k gal pool. There will be 2 skimmers and a drain with 2 in pipe each returning to the pad. The pad will be about 30 ft from the pool but the far skimmer may run around 80-90 ft to get to the pad. They want to run a single 2 in line and loop for the returns (5 returns, 1.5 in each). They want to put in a 2.5 hp up rated Whisperflo. The pad and pool waterline are about same elevation. There will be two bubblers on a tanning ledge about 70-80 ft from the pad on a 1.5 in pipe separate from the return loop. The filter is a TR 100 sand filter, 98gpm max, 140k gal/24 hr turnover. Auto pilot SWCG. No heater.

I just want a valid argument to take to the pool builder about pump sizing, I am thinking the 2 hp up rated 2 speed Whisperflo would be more than enough. Seem okay?
 
In my mind the 2 HP is way too much pump unless you need it fro something else than the pool. I don't have all the details about your setup but from what you have given me, here is a comparison between the 2 HP Up rated and 1 HP Uprated pumps:

[tr:1oyte0ea][td:1oyte0ea] Summary [/td:1oyte0ea][td:1oyte0ea] [center:1oyte0ea]WFDS-24[/center:1oyte0ea] [/td:1oyte0ea][td:1oyte0ea] [center:1oyte0ea]WFDS-28[/center:1oyte0ea] [/td:1oyte0ea][td:1oyte0ea] [center:1oyte0ea]WFDS-24[/center:1oyte0ea] [/td:1oyte0ea][td:1oyte0ea] [center:1oyte0ea]WFDS-28[/center:1oyte0ea] [/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Pump Speed (rpm) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]3450[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]3450[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]1725[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]1725[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Water Flow (gpm) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]82.7[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]97.7[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]41.1[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]48.5[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Suction Head Loss (ft) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]3.0[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]4.1[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.8[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]1.1[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Pad Head Loss (ft) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]32.9[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]45.9[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]8.1[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]11.3[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Solar Head Loss (ft) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.0[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.0[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.0[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.0[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Return Head Loss (ft) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]8.4[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]11.5[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]2.3[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]3.1[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Jet/Eyeball Head Loss (ft) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.9[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]1.3[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.2[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.3[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Total Head Loss (ft) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]45.2[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]62.8[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]11.4[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]15.8[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Input Power (Watts) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]1592[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]2380[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]373[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]434[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Energy Factor (Gallons/Watt-hr) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]3.12[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]2.46[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]6.61[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]6.69[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea][/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Suction (in hg.) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]2.6[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]3.6[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.7[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]0.9[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][tr:1oyte0ea][td:1oyte0ea] Filter Pressure (psi) [/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]14.9[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]21.2[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]2.8[/center:1oyte0ea][/td:1oyte0ea][td:1oyte0ea][center:1oyte0ea]4.4[/center:1oyte0ea][/td:1oyte0ea][/tr:1oyte0ea][/table:1oyte0ea]

Note that the WFDS-28 is about 20% less efficient than the WFDS-24 and it also comes very close to the filter maximum flow rate. You really don't need much pump to run just the pool. For example, I am using a 1/2 HP two speed pump to run my pool with both solar and a suction side cleaner at the same time.
 
The PB actually spoke to the Pentair distributor about this (not sure if being a distributor makes him an authority, but who knows). They felt like it would be okay, but said I would need to pay special attention to filter backwashing and keeping skimmer baskets clean as the effect on low speed circulation would be more pronounced. Also that skimming action may not be as good on low vs high speed. I am actually impressed that they are discussing this seriously and not just blowing me off.
 
Just wanted to come back and thank you, Mark, for putting that chart together for me. Sorry it took awhile but I was consumed by getting these bids' equipment lists narrowed down so I could really see the difference in cost between PBs. You really helped me and I appreciate it.
 

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