How to calculate change in head with lower HP pump.

[jay jay]

Hello, I measured my current pumps vacuum -7.5 "hg, and pressure 15 psi, to determine head, 43.13'.
The current pump is a 1hp Hayward superpump. Looking at the pump curve this puts the flow around 50 gpm.
If I were to replace this with a 3/4hp superpump, the flow at the same head (43.13') would be around 38 gpm.

Now I would think that since the 3/4hp pump has less flow, that the head would also be less, therefore, the flow would be higher than 38gpm.
Is there a way to calculate what the new head would be?

Reason is the 1hp pump is failing and will need to be replaced. And at the same head, adjusted run times for the same turnover, and kwh calculations, the energy use comes out to be about the same for both pumps. This can't be right since flowing water slower should create less head. Shouldn't the 3/4 hp use less energy? Or am I thinking this all wrong?

Now I would love to get a 2 speed pump or a VS pump. But, total lack of funds makes that impossible right now.

Pump run time is 6 hr/day. 120vac. Current reading of 11.5 A at the motor. $0.15 KWH. Cost around $40 a month.

Does anyone know how to figure this out? Thanks.

 

[JamesW]

You plot the system curve on the same graph as the pump curve.

The system curve shows the head loss at each flow rate. The curve starts at 0,0 and curves up, whereas the pump curve starts high and curves down.

Where the system curve crosses the pump curve will be the operational point.

Head loss 2 is roughly head loss1 x (flow2/flow1)^2.

For example, if you double the flow, the head loss is 4 times as much.

 

[mas985]

You can use the spreadsheet in my signature which can calculate the plumbing curve from the current pump's operating point and then determine the new operating point from any other pump.

But really turnover is irrelevant and not something you should really be concerned with. It just doesn't matter.

The only runtime that is needed is that which is necessary to generate and distribute chlorine which isn't very much.

 

[jay jay]

Ahh. The system curve was what I was missing. Will be doing some calculations and checking out the spreadsheet tonight. Speaking of the spreadsheet, I read through that section before but missed the link.

I agree about the turnover, was just using it for the calculations because it was an easy number to use.

Thanks for the info. Hope you guys don't mind if I come back with a question or 2 after doing some research.

 

[jay jay]

Just going to update this post with my results. Maybe it can help someone in the future. I used the head loss calculations and also the pump spreadsheet.

I ordered a 3/4 HP superpump to replace my late 1990's 1 HP failing superpump. Here's what that was based on.

22,000 gallon pool. Being in NJ, the pool is only opened from the end of May until the beginning of September. Average of 4 month a year usage. One water turnover per day (I will experiment on cutting back run time). $.15 KWH electric.

1.0 HP - 60 GPM - 6 Hrs - $34 month
3/4 HP - 51 GPM - 7 Hrs - $24 month
1/2 HP - 44 GPM - 8 Hrs - $22 month

If I can reduce the run time to only a 1/2 of turnover per day, the cost should be under $13 per month to run the pump.

Thanks.