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  1. #1
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    Question How to control volume ratio


    Hi all,

    I am trying to control screw compressor
    I know how to control capacity but
    I do not know how to control volume ratio.
    When it should be moved.

    thanks



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    Re: How to control volume ratio

    Quote Originally Posted by keerati View Post
    Hi all,

    I am trying to control screw compressor
    I know how to control capacity but
    I do not know how to control volume ratio.
    When it should be moved.

    thanks
    By using movable discharge port with assistance of "volume ratio slide valve". When the condensing pressure is dropped this valve moves towards the suction valve to open the discharge port earlier than the end of full compression process and vice versa. I wish it is clear

    Mohamed

  3. #3
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    Re: How to control volume ratio

    Since the purpose of the compressor is to control suction pressure this is fixed. If the discharge pressure increases, the volume ratio increases. For those times when the discharge pressure decreases, the volume ratio also decreases.

    If you have a compressor that has a manual volume slide, the slide position adjustment is only necessary to achieve the lowest motor amps for that operating condition.
    If all else fails, ask for help.


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    Re: How to control volume ratio

    The algorithms to control variable Vi are fairly complex due to the control on both the capacity control as well as the Vi control. The compressor control is looking at both the suction pressure and the discharge pressure. It is looking at the combination of the pressures based on the refrigerant k value.

    In an automatic control, you can use an infinitely variable system or a step control.

    Most screw compressor panels use a time proportion type control to fire the load and unload solenoids and use the same approach for the Vi slide valve.

    Vi = volume at suction/ volume at discharge or
    Vi = (Ps/Pd)^k.

    If it is a manual adjust screw, look for a chart from the compressor manufacturer that list the pressure ratio and the appropriate Vi. You will manually turn an adjusting screw X number of turns to set the Vi. At the correct Vi, the compressor will draw the fewest amps as compared to over or under compression.

    Ken

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    Re: How to control volume ratio

    Hi Mohamed,US Ice man and Ken,

    Thank you very much for you answer.
    Now, I am clear about concept.

  6. #6
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    Re: How to control volume ratio

    We need volume ratio control only when compressor is fully loaded. If compressor is part loaded, Vi control will be useless.

  7. #7
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    Re: How to control volume ratio

    Quote Originally Posted by Sergei View Post
    We need volume ratio control only when compressor is fully loaded. If compressor is part loaded, Vi control will be useless.
    I do not understand how you do not benefit from Vi control at less than full load. The Vi is controlable at all load conditions.

    When a screw compressor unloads we are in effect merely shortening the effective length of the rotor that traps the gas for compressor. The slide valve moves back and the mesh point between the rotor moves back trapping less gas in the compression cycle.

    You still have the same issue with the Vi and either over or under compressing the the gas it we are not at the ideal Vi for the compressor.

    If you have ever watched a auto Vi compressor in operation, you will see that when the slide valve volume control moves to a new position, the Vi slide will also move to a new position to optimize the Vi at the new load condition.

    Ken

  8. #8
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    Re: How to control volume ratio

    Quote Originally Posted by TXiceman View Post
    I do not understand how you do not benefit from Vi control at less than full load. The Vi is controlable at all load conditions.

    When a screw compressor unloads we are in effect merely shortening the effective length of the rotor that traps the gas for compressor. The slide valve moves back and the mesh point between the rotor moves back trapping less gas in the compression cycle.

    You still have the same issue with the Vi and either over or under compressing the the gas it we are not at the ideal Vi for the compressor.

    If you have ever watched a auto Vi compressor in operation, you will see that when the slide valve volume control moves to a new position, the Vi slide will also move to a new position to optimize the Vi at the new load condition.

    Ken
    Vi control determines position of Vi stop to optimize the effective length of rotors(to get optimum Vi). Capacity of compressor will increases when slide valve moves to Vi stop. At certain point slide valve will touch Vi stop and we have 100% loaded compressor and optimum Vi(optimum effective length of rotors). When compressor unloads, slide valve moves from Vi stop. Capacity of compressor will be less than 100% and compressor will have internal Vi(not optimum). This internal Vi is major reason of inefficiency of screw compressors at part load. At 90% load the difference between optimum Vi and internal Vi isn't significant and inefficiency isn't significant either. At 20% load we have significant difference between optimum Vi(at current operating conditions) and internal Vi and inefficiency is pretty significant.

  9. #9
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    Re: How to control volume ratio

    Sergi, your understanding of Voume index control is a lot different than mine. Vi doe not effect the active or effective rotor length, but it does effect the discharge port configuration. The discharge port is what determines the Vi.

    If you will notice that on a fixed Vi compressor, the notch in the slide valve (discharge end) and the discharge housing is very different on a 2.6Vi and a 5.8Vi. As a fixed Vi compressor unloads, the slide valve moves back into the discharge port and attempts to come close to maintaining a fixed Vi.

    The suction volume control changes the effective rotor length...not the Vi block. When you change the suction volume on the compressor you have to change the Vi in order to maintain correct Vi.

    Vi = V suction/ V discharge

    Ken

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    Re: How to control volume ratio

    Hi, Ken.
    I mentioned about variable Vi. Vi control determines position of Vi stop and consequently determines position of slide valve, because they touch each other at full load. This position of slide valve determines position of discharge port as well as compressor Vi(optimum). When compressor unload Vi will change and Vi stop doesn't have influence on Vi at part load.
    My statement is easy to test. Compressor with variable Vi should be unloaded(slide valve) manually to 50-60%. Check electrical load on the motor. Manually change Vi from maximum to minimum and back to maximum. Electrical load will not change. It means that Vi stop doesn't influence on compressor Vi when compressor is part loaded.

  11. #11
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    Re: How to control volume ratio

    you are greatful!thanks a lot!

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    Re: How to control volume ratio

    i'v learned a lot,very good!

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