Author Topic: Swimming Pool Pumps and VFD's  (Read 5849 times)

Cary Austin

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Swimming Pool Pumps and VFD's
« on: November 03, 2013, 09:11:52 AM »
Hi,
There is currently a push in Australia for variable flow pumps for swimming pools, much the same as in USA. However the costs of the pumps or a VSD controller is around $900 and a variable speed pump upwards of $1000.
 
So a more cost effective solution certainly appeals, and the simplicity of the valve solution looks too good to be true!  The thought that a pump operates more efficiently with greater backpressure is very counter intuitive, but really shoots down the whole VSD argument.
 
A typical swimming pool setup has no pressure tank, with the pump feeding directly to the filter/chlorinator at full flow.   Are CSVs suitable for this kind of setup?  The pump we  currently operate is an Onga Leisuretime 750 (http://www.onga.com.au/objectlibrary/630), a fairly typical pump.   At 8 hours operation per day, this is circulating  3 x the amount water really required, so I think we want to reduce it from about 250 litres/minute (66gpm) to about a third of that.
 
So a couple of questions:
is a CSV suitable for this kind of application?
the reduced water flow may not be enough to operated the pool vacuum, so:
what is the variable range of a valve so we can continue to use the vacuum,
how easy is it to adjust,
or should be just bypass the valve when we want to vacuum?
Looking forward to hearing back from you.
Kind Regards,

Cary Austin

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Re: Swimming Pool Pumps and VFD's
« Reply #1 on: November 03, 2013, 09:13:04 AM »
Hello
Yes the fact that the horsepower or amperage decreases when the flow from a pump is restricted with a valve is very counter-intuitive.  Most people believe just the opposite.  They think using a valve to restrict the flow from a pump actually makes the pumps work harder.

It is also counter-intuitive that reducing the pump speed with a VFD, which also reduces the amp draw, actually increases the energy consumption per gallon of water produced.

VFD manufacturers use these two counter-intuitive principles to easily con many people into believing a VFD can actually save energy.  They also easily con government officials into thinking VFD’s will save energy and therefore mandating the use of VFD’s is the best thing for the people.

With a little research people will discover that nothing could be further from the truth.  As you have already discovered, VFD’s are very expensive.  What you will soon discover is that VFD’s are short-lived, not repairable, and therefore must be replaced on a regular basis at even greater expense.

Only if you change to a two-pump system, (small pump/large pump) will you have a way to find out how much energy is being wasted by the VFD when compared to using a smaller pump when needed.

A correctly sized smaller pump will use considerably less energy than a larger pump being slowed with a VFD to do a smaller job.  Standard across the line, full speed pumps will also last many times longer than a VFD or the pump it controls (destroys).

However, a single large pump using a Cycle Stop Valve can do smaller jobs just as efficiently as any VFD control.  You don’t have to reset the CSV to work at different flow rates.  You simply set the CSV to hold a steady 20 PSI, and it will change the flow rate from the pump to match any amount being used by holding the 20 PSI steady.

The CSV knows when you need more water as the pressure tries to drop to 19 PSI.  So the CSV opens up to deliver more water and get the pressure back up to 20 PSI.

The CSV knows when you are using less water, as the pressure will try to increase to 21 PSI.  So it reduces the flow from the pump to match the reduced flow required, by restricting the flow to maintain the 20 PSI constant.  This all happens faster than you can see as the pressure will barely dip or rise before the CSV makes the necessary adjustment to the flow rate.

The CSV will work fine without a pressure tank or pressure switch. You simply need to put a ball valve on the return line to the pool.  You can use the ball valve to restrict the flow to the jets as much as you want.  The CSV will see the pressure try to increase and will restrict the flow from the pump to maintain 20 PSI.  When the flow from the pump is restricted with the CSV, the amperage will also decrease in proportion.  When you need more flow for the vacuum or other things, simply open the ball valve, the CSV will see the pressure try to decrease, and will open to allow more flow from the pump. Of course when the flow increases, the amperage increases as well.  When people see this happen for the first time, they realize a VFD is just trying to trick a pump into doing something it already does naturally. 

The flow rate has always been proportional to the amperage with centrifugal pumps.  It is just that VFD salespeople don’t want you to know this.  Truth be told, many VFD salespeople don’t know this either.  They have just been “taught” that reducing the RPM saves energy and restricting with a valve “waste” energy, which makes sense to them because they don’t understand the basic counter-intuitive principles of a centrifugal pump.

It is rare for someone who understands the electronics of a VFD to also understand the mechanical principles of a centrifugal pump, and vice versa.

For the VFD and pump manufacturers, it is all about the money.  If they can make you think it saves energy, they can get the customer to purchase a more expensive, less dependable VFD controller.   If they can make you think it saves energy, you won’t mind that the pump and controller fails more often and you need to replace it about every 5 years instead of about every 20 years as with a standard full speed pump.

For our government officials, if they can make you think a VFD saves energy, they get re-elected for mandating the use and reducing the energy consumption of the entire country.

The homeowner gets stuck with increased pump replacement costs, more frequent failures and even further costs.  And if you have nothing to compare it to, you think the VFD is saving energy, because that is what you were told.  Then we vote the government officials back into office because they helped us save so much energy.  In reality, if the government officials can’t comprehend the simple, yet counter-intuitive principles of a centrifugal pump, how many more things are they getting wrong?

Here are a couple of links to articles from homeowners who have already figured this out, and an article I wrote to correct the US Department of Energy who still doesn’t understand.

Let me know if you have more questions.
Thanks
Cary

http://www.cyclestopvalves.com/vfd-energy_14.html

http://www.cyclestopvalves.com/pdf/letterseditor_27.pdf

http://www.cyclestopvalves.com/vfd-energy_15.html
« Last Edit: November 03, 2013, 09:15:18 AM by Cary Austin »