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Thread: Motor Efficiency Question

  1. #1
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    Motor Efficiency Question

    Strictly from an energy efficiency perspective.......which operating condition would be the most efficient?

    Two identical 40hp chw pumps.....One pump operating constant volume, the second pump running on a vfd at 60% meeting the flow requirement. Or.....both pumps operating at an equal vfd output percentage meeting the same flow requirement.

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    Quote Originally Posted by Artrose View Post
    Strictly from an energy efficiency perspective.......which operating condition would be the most efficient?

    Two identical 40hp chw pumps.....One pump operating constant volume, the second pump running on a vfd at 60% meeting the flow requirement. Or.....both pumps operating at an equal vfd output percentage meeting the same flow requirement.

    Both pumps running on the same VFD output. You can look at a pump curve to verify this, if one was at full tilt, the variable speed pump will not flow any water anything less than almost full speed...So you would be wasting energy and tearing up a pump...

    disclaimer; if the pumps in the discussion are piped parallel...


    GT Jets
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  3. #3
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    I would then need to assume that two pumps running at something less than maximum horsepower, satisfying flow requirements, would consume less electrical energy than one running balls to the wall and the other picking up the slack since horsepower is a cube function of the energy required? I'm still a bit bewildered, but accept.

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    GT Jets is correct. Assuming the pumps are in parallel they have check valves on the discharges. Either of the pumps would have to produce a pressure differential equal to or greater than the pressure on the other side of the check valve being created by the other pump.

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    Quote Originally Posted by Artrose View Post
    Two identical 40hp chw pumps.....One pump operating constant volume, the second pump running on a vfd at 60% meeting the flow requirement.
    If you run it like this then the result will be that one pump will always "dead head" against the other pump and will not move any water and will run hotter and hotter...... until something gives.
    If you run 2 pumps always run them at the same speed !!

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    Quote Originally Posted by Artrose View Post
    I would then need to assume that two pumps running at something less than maximum horsepower, satisfying flow requirements, would consume less electrical energy than one running balls to the wall and the other picking up the slack since horsepower is a cube function of the energy required? I'm still a bit bewildered, but accept.
    Well thanks...I think...

    The bewilderment may come from not understanding two things....

    The first would be the way a vfd actually saves energy....A vfd changes the voltage and the frequency sent to the motor slowing it speed, thus lowering horsepower required, in this particular scenario, the power consumption is not linear with RPM or GPM, in fact quite the opposite, the power consumption drops of way more dramatically than the RPM...

    The second is how two pumps in parallel effect each other, you really need to understand a pump curve to be able to understand how they would effect each other...


    GT Jets
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  7. #7
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    Ok, this is true for two pumps in parallel, then it would also be true for three, or four, or more pumps in parallel.... What would be an acceptable control strategy if you had two cv pumps and one vfd pump on a parallel secondary system?

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    Last edited by exwtk; 07-31-2010 at 03:14 PM. Reason: Duplicate post

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    Yes ... This is true for any number of pumps running in parallel. The rule is that only pumps with identical pump curves can run in parallel simultanously
    If you have identical pumps running in parallel at different speeds then you break the rule. The pump curve of the pump operating at reduced speed is lower and will not be able to overcome the head pressure created by the other pump(s) and will overheat if left running too long.

    The best strategy for your situation with 1 pump with VFD and 2 pumps without a VFD, would be to upgrade your system and get 2 VFD's for the CV pumps.
    If that is not an option, then I would run the VFD pump as your lead pump. Once the demand causes this pump to run at full speed for a certain amount of time, you start a CV pump. Once the CV pump runs the vfd pump needs to be locked in at 100 % speed.
    Not a very good way to do it... but it is the best you can do with what you got. For best overall savings and performance ... Get 2 VFD's
    Last edited by exwtk; 07-31-2010 at 03:14 PM. Reason: Duplicate post

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    Would the pump modulated by the drive waste more energy pushing open the lift check held down by the 100% pump?

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    Quote Originally Posted by miller_elex View Post
    Would the pump modulated by the drive waste more energy pushing open the lift check held down by the 100% pump?
    All energy consumed by the modulated pump is a waste !!
    The pump curve will be lower.. so it will produce a lower pressure.. so it can not lift the check valve held down by the 100% pump...so it will not move any water... so all energy put in the motor is eventualy used to heat up the water in the pump.

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    I think you all are missing the big picture. What controls a pump on a VFD? A set point of some sort. Either pressure or flow. If you use pressure for instance, you would ramp the speed of the VFD to maintain pressure (X). If the VFD is at 100% and can't maintain pressure (X) then you need to start another pump. And there are several ways to do this. One is to have a second pump with a VFD and have them run together at the same speed to maintain pressure (The most common). The other is to start a second pump across the line and change the speed of the first pump to maintain Pressure (X). Another way, which requires a high end PLC and programming and hardware. You disconnect the pump from the VFD and immediately switch the pump accross the line, Then connect the offline pump to the VFD and control the speed to maintain pressure(X). You can follow this routine for pump 3,4...Etc. Before you say "it can't be done" or "it won't work", I have seen it work and a system is currently in production that does precisely as I have described. They have been around for years. I don't work for either company so I am not trying to sell anything.Check it out!

    http://www.syncroflo.com/automatic-p...tem/the-eagle/

    or

    http://www.flowtronex.net/FT-Silent-Storm.asp

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    Quote Originally Posted by Doug-e-fresh View Post
    I think you all are missing the big picture. What controls a pump on a VFD? A set point of some sort. Either pressure or flow. If you use pressure for instance, you would ramp the speed of the VFD to maintain pressure (X). If the VFD is at 100% and can't maintain pressure (X) then you need to start another pump. And there are several ways to do this. One is to have a second pump with a VFD and have them run together at the same speed to maintain pressure (The most common). The other is to start a second pump across the line and change the speed of the first pump to maintain Pressure (X). Another way, which requires a high end PLC and programming and hardware. You disconnect the pump from the VFD and immediately switch the pump accross the line, Then connect the offline pump to the VFD and control the speed to maintain pressure(X). You can follow this routine for pump 3,4...Etc. Before you say "it can't be done" or "it won't work", I have seen it work and a system is currently in production that does precisely as I have described. They have been around for years. I don't work for either company so I am not trying to sell anything.Check it out!

    http://www.syncroflo.com/automatic-p...tem/the-eagle/

    or

    http://www.flowtronex.net/FT-Silent-Storm.asp
    This will work fine on jet pumps..................(very high pressure differentials)

    The pump impeller sizes if all the same (assumed) will not work the way you state.....

    The pumps in question were assumed to be controlled by a pressure differential sensor....


    GT Jets
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  14. #14
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    This will work fine on jet pumps..................(very high pressure differentials)

    The pump impeller sizes if all the same (assumed) will not work the way you state.....

    The pumps in question were assumed to be controlled by a pressure differential sensor....


    GT Jets
    The pumps are mostly vertical turbine and are in fact identical impeller sizes. They have a steep pump curves. Single impeller end suction pumps don't work well by this method. Someone may want to tell SyncroFlo or Flowtronex it won't work. They only been doing it for 20 years or so.

  15. #15
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    Quote Originally Posted by Doug-e-fresh View Post
    The pumps are mostly vertical turbine and are in fact identical impeller sizes. They have a steep pump curves. Single impeller end suction pumps don't work well by this method. Someone may want to tell SyncroFlo or Flowtronex it won't work. They only been doing it for 20 years or so.

    I never stated it wouldn't work...I said it wouldn't work with the types of pumps typically used in hydronic heating/cooling applications. The term steep impeller curves in your post verifies this.....

    Pump flows in a pump like the 1510 B&G are extremely non-linear therefore cannot be staged in a manner in which one pumps dynamic head overcomes another...........

    A stack pumps or a jet/turbine pump are a different animal...

    GT Jets
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  16. #16
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    A VFD can run more than one motor at the same time.

  17. #17
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    Quote Originally Posted by SomeOldGuy View Post
    A VFD can run more than one motor at the same time.

    X2...Only requires an electronic overload on each motor....See it often.

    GT Jets
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

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