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    Variable ratio in screw compressor



    Dear All,
    Hi,

    I want to understand what is Variable ration Slide, how it works in the compressor
    and what is 2.6, 3.0, 3.5, etc. Please participate in this discussion.

    Thanks in adavnce



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    Re: Variable ratio in screw compressor

    Last edited by RANGER1; 21-09-2014 at 11:11 AM.

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    Re: Variable ratio in screw compressor

    hi ranger, thanks your link.

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    Re: Variable ratio in screw compressor

    Vi ratio is the relation between suction volume and discharge volume and are connected to the compression mechanism. It shall be guarantee that the internal pressure on the end of the compression process will match with the external discharge pressure or the condensing temperature (negleting the pressure losses untill reaching the condenser)

    The main reasons for have the ideal Vi ratio are:
    - save energy shaft power
    - less heating of the oil
    - less noise


    The docs that ranger sent you are good "teachers" and explain deeply.

    Regards
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    Re: Variable ratio in screw compressor

    Thanks Dear Ranger 1
    what is 2.0 2.1 3.5 4.5 etc......

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    Re: Variable ratio in screw compressor

    Quote Originally Posted by shaaf View Post
    Thanks Dear Ranger 1
    what is 2.0 2.1 3.5 4.5 etc......
    Compression ratio's

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    Re: Variable ratio in screw compressor

    How do it calculate ?

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    Re: Variable ratio in screw compressor


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    Re: Variable ratio in screw compressor

    Thanks Ranger 1

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    Re: Variable ratio in screw compressor

    Sorry. I think these are volume ratios. Usually, they are from 2.2 to 5.0. 2.0 means that compressor suck 10m3 of gas and discharge 5 m3.

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    Re: Variable ratio in screw compressor

    Quote Originally Posted by Segei View Post
    Sorry. I think these are volume ratios. Usually, they are from 2.2 to 5.0. 2.0 means that compressor suck 10m3 of gas and discharge 5 m3.
    I agree with Segei.

    I take this post to say that my opinion is that the compression ratio should mean the same thing as the volume ratio. The author of one the articles names the compression ratio as it pression ratio but in fact the shall be distinguished.
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    Re: Variable ratio in screw compressor

    The volume ratio is entirely consequence of the geometry of the machine and is not effected by the gas being compressed or by the pressures it is being compressed from or to....

    The volume ratio is simply the trapped volume contained in one thread when thread is first sealed away from the suction port of the machine divided by the volume of the same thread when it is first exposed to the discharge of the machine.

    The compression ratio is the pressure of that same thread when it is first exposed to the discharge port compared to the pressure of that thread when it first sealed from the suction port. This is dependent on the compressed gas's characteristics and the extent of oil cooling, as well as on the volume ratio of the machine. The pressure of the suction thread will be exactly the same as the pressure of the upper cavity and either identical to suction pressure or a very little bit less should there be any flow restriction between the point of measurement for suction pressure and the cavity.

    In the meantime the thread pressure internal to the machine at the time it is first exposed to the discharge port can be higher than the discharge pressure or even a little lower. A machine whose internal pressure is higher than the discharge pressure is over compressing; and a machine whose thread pressure is lower than the discharge is under compressing. Both conditions will require increased power input to achieve the same flow of gas....So as suction and discharge pressures change, a fixed volume ratio machine will be over compressing at some conditions and under compressing at others.

    The pressure ratio is the fluid's pressure at the discharge end divided by the fluid's pressure at the suction end. Should a machine's thread pressures be in exact match with those, the compression process is near ideal. In reality, some stability is gained if the machine is over compressing by just a tiny extent.

    The variable Vi arrangements seek the ideal compression process by adjusting the point of last compression, that is modifying either the point of first compression or the point of last compression. The slide valve already changes the point of first compression; in almost all cases, the discharge port is modified by the valve adjusting the Internal Volume Ratio (Vi).

    Actually this is a lot easier to "see" in a rotary vane because the cavities are moving and shrinking in one direction only and the port locations and "edges" are well defined in terms of the passage of the "tips".

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    Re: Variable ratio in screw compressor

    Thanks Sterl,
    In variable Vi system how maintain same internal compression pressure which is system discharge pressure to prevent Under compression or over compression ?
    And which i understood that volume ratio is suction pressure divided by discharge pressure normally pressure in absolute unit.

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    Re: Variable ratio in screw compressor

    any images

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    Re: Variable ratio in screw compressor

    Hi, shaaf

    Quote Originally Posted by shaaf View Post
    Thanks Sterl,
    In variable Vi system how maintain same internal compression pressure which is system discharge pressure to prevent Under compression or over compression ?
    And which i understood that volume ratio is suction pressure divided by discharge pressure normally pressure in absolute unit.
    It is discharge pressure divided by suction pressure, otherwise Vi would be less then 1 ...

    ... actually Vi (internal volumetric ratio) is the ratio of the groove volume when suction finish (volume of suction refrigerant when compression starts) to the volume when the discharge port opens (volume of the same quantity of refrigerant at discharge port) ... of course Vi corresponding to the compression ratio changes according to refrigerant used (we have to use k= specific heat ratio of compressed gas) in formula to determine Vi ...

    Vik = pd/ps

    anyhow, suction and discharge pressure, in all systems, are determined by process and not by compressor...


    Actually, we can use compressors with fixed Vi or with variable Vi ... if we can keep operating parameters within design values no problem with fixed Vi ... this is possible on petrochemical plants with close control of suction and discharge i.e. condensing pressure at any capacity

    ... for cold stores with variable loads it is more practical to use compressors with variable Vi ... this is practical with manual variable Vi to change twice a year ... in autumn and spring time ... suction pressure remains, but usually condensing pressure varies a lot ... this is the way to save some energy and compressors are working in some optimum ...


    Best regards, Josip

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    Re: Variable ratio in screw compressor

    Volume ration is strictly a characteristic of the machine: Even if nothing was flowing through it, it would still have a volume ratio. If the tips and grooves were actually sealed without motion or oil: you could pour water in the suction port to just fill one thread and no more: that would be the volume available to the suction port. Flip the machine over and bring the same thread to just accessible via the discharge port and fill that with water and you would measure the volume of the thread when it is just exposed to the discharge port. If the suction volume were 3 meters cubed and the discharge volume were 1 meter cubed, the machine's volume ratio would be 3.0.

    Compression ratio is determined by both the fluid properties and the machine geometry.....If you measure the suction cavity pressure and (if you could) measure the pressure of the thread just as it was first exposed to the discharge port you would have the pressures of the compression process. THIS NOT THE SAME as a recip. which forms part of the confusion. Divide the internal thread pressure near discharge by the suction cavity pressure (in absolute units....) and you will have the compression ratio. The compression ratio is predictable from the compressor volume ratio, the characteristics of the gas being pumped, and the extent of cooling incorporated as the gas is being compressed...

    Pressure ratio is a characteristic of the circuit: and is simply discharge pressure divided by suction pressure, both in absolute units. In testing a compressor alone, this number will not include any pressure looses due to suction strainer, suction check valve, discharge check valve, coalescing elements and the like.

    So the compressor manufacturer's pressure ratio will not quite be the same as the circuit pressure ratio if the circuit pressure ratio is for instance, discharge header pressure divided by suction header pressure.....but they should be close. The compressor is always going to develop a slightly lower suction cavity pressure than that of the header and a slightly higher discharge port pressure than that measured at the header; so the machine's pressure ratio will be higher than the "header" ratio.

    Variable Vi addresses the departure of the compression ratio within the machine from the pressure ratio of the system. If the machine is over compressing, say making an internal P-ratio of 8, where the actual port cavity pressures have a ratio of 6: the machine is overcompressing by a factor of .33. This requires more power input per unit of mass flow: so the Variable Vi arrangement adjusts the machine's seal point for the suction fllow so that the discharge port is exposed earlier in the compression process, so the compression ratio decreases to match the pressure ratio....

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    Re: Variable ratio in screw compressor

    Thanks sterl sir

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    Re: Variable ratio in screw compressor

    Thanks all to Participants .

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