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  1. #1
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    Liquid pumps on VSD - flooded systems



    Hi All

    I have been thinking about fitting a VSD to liquid pumps on flooded systems where the number of system users, (tunnels, spirals, glycol chillers etc) change over the course of the production day. On most plants there are at least two pumps with two being needed for full load but one ends up being "oversized" during the non production times. If one of the pumps was speed controlled to maintain say 2.5 bar over LP instead of it going up to 4 or 5 bar over LP when only the holding freezers are running power savings would be made which would over time recover the cost of the upgrade.

    Has anyone done something similar or are there any traps in this?



  2. #2
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    Re: Liquid pumps on VSD - flooded systems

    I think the amount of power savings you might realize is very small. Refrigerant pumps are relatively low power anyway, so putting a VFD on a refrigerant pump does not seem cost-effective to me.

    On the other hand, if you can solve some operational problems by using a VFD then it might be more worthwhile from an aggravation viewpoint.
    If all else fails, ask for help.


  3. #3
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    Re: Liquid pumps on VSD - flooded systems

    IF you are using centrifugal pumps the power consumption goes down when the head rises. This is not to say that controlling the speed will not have an effect on power consumption, just that the savings might not be that great in comparision to the overall load.

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    Re: Liquid pumps on VSD - flooded systems

    I think it would be a mistake as what would pump do if it cavitates,speed up?It would also change pump dynamics if you changed speed to much.
    If bypass line is sized correctly why worry.

  5. #5
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    Re: Liquid pumps on VSD - flooded systems

    If you are using multiple pumps wouldn't you have to run them all at the same speed? It seems to me that if you slowed only one down that the other would overcome it and maybe slow the flow down more than desired. I have only seen a vfd on an Ammonia pump once. It was because the designer missed a chiller and the pump was too small. They ran it at 70 hertz. It actually worked but who knows how long the pump will last.

  6. #6
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    Re: Liquid pumps on VSD - flooded systems

    I just give example of one plant.
    Compressors - 1000 BHP
    Condensers - 100 BHP
    Evaporator fans - 120 BHP
    Ammonia pumps - 10 BHP
    Why to save energy for ammonia pumps? You can save more energy by improving operation of compressors, condensers and evaporator fans. How to do that? This is another story.

  7. #7
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    Re: Liquid pumps on VSD - flooded systems

    Quote Originally Posted by Sergei
    Why to save energy for ammonia pumps? You can save more energy by improving operation of compressors, condensers and evaporator fans.
    Which is exactly what I tried to say. Even if you saved some astronomical amount of say 50% of the pumping energy, it only amounts to 5 HP. Big deal.

    You can save many times that by lowering the discharge pressure 5 psi.
    If all else fails, ask for help.


  8. #8
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    Re: Liquid pumps on VSD - flooded systems

    I have seen a VSD fitted to a liquid pump to prevent the pump from drawing too much current, usually when a system change has allowed the liquid pump to pump too much volume at a low head and would cause the pump to exceed it FLA. The VSD would sense the overload condition and reduce the motor speed to prevent and overload trip.

  9. #9
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    Re: Liquid pumps on VSD - flooded systems

    Quote Originally Posted by Mike W
    I have seen a VSD fitted to a liquid pump to prevent the pump from drawing too much current, usually when a system change has allowed the liquid pump to pump too much volume at a low head and would cause the pump to exceed it FLA. The VSD would sense the overload condition and reduce the motor speed to prevent and overload trip.
    Some good points to consider. Installing a VFD (or VSD) can provide some benefits that are not really energy conservation related.
    If all else fails, ask for help.


  10. #10
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    Re: Liquid pumps on VSD - flooded systems

    Quote Originally Posted by Mike W View Post
    I have seen a VSD fitted to a liquid pump to prevent the pump from drawing too much current, usually when a system change has allowed the liquid pump to pump too much volume at a low head and would cause the pump to exceed it FLA. The VSD would sense the overload condition and reduce the motor speed to prevent and overload trip.
    I have concerns about this approach. Demand is high, and VSD reduces supply. I think that pump can loose the prime.
    Some pump manufacturers have special valve on discharge side to prevent overflow and overload.

  11. #11
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    Re: Liquid pumps on VSD - flooded systems

    VFD's can help to solve some minor operating issues, but they will not correct major design flaws. If the pump does not have the proper valves and controls VFD's will not correct that.

    Say the pump is controlled by the VFD by a pressure signal. As the pump discharge pressure drops, what do you do?

    If the pump is cavitating increasing the speed will not help.

    If the discharge pressure drops because the pumping system was not properly commissioned or installed, an increase in speed will make the problem worse.

    On the other hand, if the pumps are over sized you can slow the pump down and reduce the NPSHR to help prevent cavitation.

    VFD's can cause more problems than they correct if they are not properly applied also.

    Food for thought...
    If all else fails, ask for help.


  12. #12
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    Re: Liquid pumps on VSD - flooded systems

    Thanks for the feedback - like I said, its just something I'd been thinking about and I shall think about it no more!

  13. #13
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    Re: Liquid pumps on VSD - flooded systems

    Hello, While we are on the subject of saving energy here, I will talk about my situation at my plant! I am part of the utility/refrigeration crew at my plant, my plant engineer has expressed that my job is to save energy every chance i get! during the weekend we do every thing that we can to save energy! In my plant we must maintain 75 psi air pressure, but normally run at 120 psi, so we lower the set point! also turn off all compressed air valves that we can! We have an ultrasonic air leak detector and you would be amazed that the human ear cannot detect some of these leaks! Also if no one is in the plant turn off all lights, even emergency lights! less heat means less refrigeration load! If you have air handling units, lower room pressure set-points so that your supply fans are not working as hard if they are run thru VFD's! A lot of this stuff is non production specific, but every little bit helps!
    I would like to hear some energy saving tips from others, if you have any!

  14. #14
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    Re: Liquid pumps on VSD - flooded systems

    Quote Originally Posted by Zolimer
    I would like to hear some energy saving tips from others, if you have any!
    You mentioned everything except the refrigeration system... what are you doing with it?
    If all else fails, ask for help.


  15. #15
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    Re: Liquid pumps on VSD - flooded systems

    Did you think if installing a PM valve on pump discharge to control pressure?

  16. #16
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    Re: Liquid pumps on VSD - flooded systems

    Hi Guys:

    We extensively use VFD's in our plant production lines for speed control, with the added benefit of inherent power factor correction.
    We have a VFD controlled pump as a primary, as we can ramp up slowly on pulldown, reducing cavitation problems until we stabilize the system. With wide swings in demand, this has helped us when pressures suddenly change or we are starting up (we have frequent power outages). Since many pump manufacturers offer two speed pumps or 2 motor speed selections for pumps, there is obvioulsly a lot of legroom for performance modification via speed, and in a wide load swing application bears looking at from a performance standpoint. Controlling it effectively is another can of worms.

    I agree energy savings are insignificant. Cost of a VFD in a Ul panel if required in your area can be prohibitive for energy savings alone. We build our own here (UL panel shop) and I figure even a 5hp panel is about 2 grand before the electricians install.

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