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**Frequently Asked Questions / Re: Increased back pressure and pump efficiency**

« **on:**September 29, 2022, 03:49:00 PM »

You previously stated that 'But back pressure has nothing to do with energy cost per gallon produced.' Your own equation (and the example we have hashed through) clearly shows that to be false. Increasing back pressure = increased energy per gallon produced.

When using a pressure tank to provide 5gpm, the pump is running at 25gpm, but only for 20% of the time, the pump never runs at 5gpm.

Since you keep bringing up VFDs or VSDs, isn't the situation the same as with a tank? A constant pressure VFD is only pumping against a back pressure of 50psi (assuming that is the setpoint) + the lift pressure. So even neglecting the impact of being closer to BEP at lower RPM, just the reduction in back pressure nets the same 35% reduction in energy as with a tank. If you include the increase in pump efficiency by operating closer to BEP, then the VFD looks even better, no?

If you have physics that shows otherwise, I would be keen to see it.

When using a pressure tank to provide 5gpm, the pump is running at 25gpm, but only for 20% of the time, the pump never runs at 5gpm.

Since you keep bringing up VFDs or VSDs, isn't the situation the same as with a tank? A constant pressure VFD is only pumping against a back pressure of 50psi (assuming that is the setpoint) + the lift pressure. So even neglecting the impact of being closer to BEP at lower RPM, just the reduction in back pressure nets the same 35% reduction in energy as with a tank. If you include the increase in pump efficiency by operating closer to BEP, then the VFD looks even better, no?

If you have physics that shows otherwise, I would be keen to see it.

Hydraulic HP = Head (ft) x Flow Rate (gpm) x (Specific Gravity)

3956

This is the formula for figuring Brake Horse Power. But head is irrelevant when the curve already shows the horsepower.

You are comparing the CSV working at 5 GPM and the system without a CSV working at 25 GPM, which makes no sense. When running the CSV at 25 GPM the pump will only see 4 PSI or 9' of head more than if there was no CSV, which is 524' instead of 515'. Do the math or just look at the curve and you won't be able to see any appreciable difference in horsepower with only 9' of head difference. I can even add a bypass that will keep the CSV from adding 9' of head when wide open if needed. But it is not worth the effort.

When using 5 GPM with the CSV the curve shows 2.5 HP, or 0.5HP per gallon, no need to do the math. We already know that without a CSV the pump will only be using 0.22HP per gallon, but will be cycling itself to death while that is happening. Actually it will be a little more than 0.22HP per gallon if you figure the number of cycles and add the inrush currents for each cycle.

I am not confused about VFD's but most people are. Even though the benefits of using a Cycle Stop Valve can far outweigh any loss of efficiency, of course restricting flow from a pump is less efficient. But it is the same thing when using a VFD. I wish others would do the math on VFD's and see that reducing the pumps speed also increases the horsepower per gallon used and is the same as restriction from a CSV.