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  1. #1
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    control stepless screw compressors



    Hello
    I'm programing Carel controllers.
    I heed to program controller for screw compressors with steeples capacity like trane chhn06 etc.
    Because those compressors donít have a feedback from the slide valve, the problem is to know what is the load (%) of the compressors.
    Current is not enough because it changes depending on high pressure and other parameters.
    Does someone have an idea how to make calculation or formula to consider current, high pressure, outdoor temp and maybe another parameters to calculate the compressor load (%)?
    Thank you
    Ariel
    Last edited by chemi-cool; 09-03-2017 at 02:30 PM.



  2. #2
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    Re: control stepless screw compressors

    There is no certain formula. You can take into account delta T on evaporator.

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    Re: control stepless screw compressors

    Hi
    This the main reason normally we use step control
    In this type ofor compressor.

  4. #4
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    Re: control stepless screw compressors

    Hi, Ariel979

    welcome to RE forums...

    Quote Originally Posted by ARIEL979 View Post
    Hello
    I'm programing Carel controllers.
    I heed to program controller for screw compressors with steeples capacity like trane chhn06 etc.
    Because those compressors donít have a feedback from the slide valve, the problem is to know what is the load (%) of the compressors.
    Current is not enough because it changes depending on high pressure and other parameters.
    Does someone have an idea how to make calculation or formula to consider current, high pressure, outdoor temp and maybe another parameters to calculate the compressor load (%)?
    Thank you
    Ariel
    You not need feedback from slide position, because it is consequence and not cause i.e. giving you only information about approximate capacity in some moment what is not that important to you at all.

    you need to control only one value in most cases it is evaporating pressure i.e. temperature which you need for your cooling process. Signal form high discharge pressure you can use to reduce compressor capacity for some percentage to avoid shut down i.e. compressor trip due to high current i.e. overload.

    In simple words you need to control the most important value within your process ... like suction pressure, brine or water temperature (consider low limits to avoid freezing) for heat pumps that would be discharge temperature ...

    I do not know about formula but ... with ...if... then..else .. compare .. etc you can program your controller as you like ...

    there are some very important values about refrigerant which you need to include into your compressor controller, but all depends if your compressor is equipped with such transmitters.

    Of course, it will be much better (for discussion) to know more about your process and compressors.

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

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    Re: control stepless screw compressors

    Hello Josip
    Thank you for the respond
    How to make the proses I know
    The problem as I said with compressors that don’t have feedback!!!

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    Re: control stepless screw compressors

    Hi, ARIEL979

    Quote Originally Posted by ARIEL979 View Post
    Hello Josip
    Thank you for the respond
    How to make the proses I know
    The problem as I said with compressors that donít have feedback!!!
    You did not read my post complete ... you do not need capacity feedback it is telling you nothing ...

    ... you need capacity to do the job ... so it is irrelevant at what capacity compressor is running until job is done ... for example you can be at 100% compressor capacity and still above set point ... or v.v. your compressor is running at capacity of 60% and all temperatures are OK ...

    Refrigeration process is something alive and changes constantly i.e. every second we have some other values and we have to act accordingly ... i.e. reduce or increase capacity

    The only feedback for you must be from your set point i.e. suction pressure, discharge pressure, water or brine temperature etc. ... this is what we use to set compressor to run at some capacity .... compressor capacity is the consequence of the set point and we change capacity by request of set point and not v.v.


    example: suction pressure set point is 1,0 bar ... then we need something we call neutral zone (around set point) where we do not change capacity not up nor down - refrigeration process needs time ... usually we use it about 0,2 bar+/- what means you cover pressure from 0,9 - 1,0 - 1,1 bar ... where we do not change capacity ... but this depends on complete process

    then we have proportional zone ... usually 0,5 bar (0,75 - 1,0 - 1,25 bar) again around set point where we change capacity ... faster if we are close to outer borders of proportional zone and slower when coming close to neutral zone border ...

    Of course the most important is the time we use to check where we are regarding set point ... again we can check this every 30-60 seconds ... if we are within dead zone pressure no action i.e. no capacity change, but if we are at pressure of 0,8 bar then we issue signal-command to reduce capacity and vice versa if we are at pressure of 1,2 bar then we issue signal-command to increase capacity ...

    The length of both signals for capacity increase/decrease we issue in seconds ... for decrease maybe longer and for increase shorter signal and then wait 30-60 seconds to check the set point again and take or no take action ...

    Here you can read much more about ... it is not simple to explain screw compressor regulation here with short posts ...

    From page 80 - point. 7-Compressor regulation
    https://docs.google.com/file/d/0B1FB...5MzY5Y2Yz/view

    This one (UNISAB) is a special controller and very good for refrigeration compressors - Carel is another one controller, but control logic should be the same for any controller in refrigeration field to get a good control of refrigerant cooling or heating process with compressors.

    Hope this will be of some help to you.


    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
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    Re: control stepless screw compressors

    Hi,

    Late arrival here..

    Good advice All!

    How's about a calculation of Actual Running Amps versus Full Load Amps (inferring Load)

    Yes, it only applies when running & we will not see 0% neatly through 100% but I'd still expect the value to be repeatable and meaningful..

    Happy tinkering
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    Re: control stepless screw compressors

    Dear Josip
    Thank you for your detailed response
    I read youre post carfully
    I will ask my question in different way
    I have water chiller with tow screw compressors
    I want to start up the second compressor after the first one running at list 80% load, for good cop and to avoid unwanted starts.
    How can I know when the compressor is at this point?
    Thank you again
    Ariel
    Last edited by ARIEL979; 13-03-2017 at 02:01 PM.

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    Re: control stepless screw compressors

    the problem is the current is change
    from my experience the minimum and maximum is change in cold or hot day
    So if you want to knew that the first compressor is full load before startup the second compressor it's a problem

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    Re: control stepless screw compressors

    From my experience the minimum and maximum is change in cold or hot day
    So if you want to knew that the first compressor is full load before startup the second compressor it's a problem

  11. #11
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    Re: control stepless screw compressors

    Hi, ARIEL979

    Quote Originally Posted by ARIEL979 View Post
    Dear Josip
    Thank you for your detailed response
    I read youre post carfully
    I will ask my question in different way
    I have water chiller with tow screw compressors
    I want to start up the second compressor after the first one running at list 80% load, for good cop and to avoid unwanted starts.
    How can I know when the compressor is at this point?
    Thank you again
    Ariel
    Probably you read my post carefully ... but in my first reply to you I asked you more about your plant in question to give you some exact answer ... anyhow ...

    now we knew that you have two compressors running in parallel ... assuming you have enough electric power to run two electric motors and one or two condensers to reject all heat back to mother nature ....

    for this type of plant one logic can be to run both compressors in parallel ... starting first one and after few minutes start the second one ... then both will increase capacity until come to set point i.e. (suction pressure, brine temperature, discharge pressure ...) capacity regulation can be fast or slow when use timers ... or ...

    In this case you can program suction ramp timer to indicate how fast compressor is allowed to reduce suction pressure for 1K (to avoid water freezing or too high current consumption) until come to set point where capacity regulation start to be as programmed ... this I described before ...



    Quote Originally Posted by ARIEL979 View Post
    the problem is the current is change
    from my experience the minimum and maximum is change in cold or hot day
    So if you want to knew that the first compressor is full load before startup the second compressor it's a problem
    Of course, current is changing constantly what is normal ...

    Each compressor electrical motor has a plate with indicated max running current. To read it we use current transformer and then your program will issue signal which will interrupt compressor capacity increase when coming close to some limit or even to issue a signal to reduce compressor capacity if current consumption is too high ... 70-80% of max running current value you can use like an input signal to start the second compressor if you start one and after some time the second compressor ...

    There are many possible ways to control compressor capacity, but each plant is different and without more info about your plant it is not easy to give you, maybe, some better advice.

    Hope this will put some more light on your problem.

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

  12. #12
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    Re: control stepless screw compressors

    Quote Originally Posted by ARIEL979 View Post
    From my experience the minimum and maximum is change in cold or hot day
    So if you want to knew that the first compressor is full load before startup the second compressor it's a problem
    Use time delay.
    Assume that it takes compressor 1 min. to increase capacity from 0% to 100%. If suction pressure above set point for 2 min. second compressor can start and you know that first compressor is fully loaded.

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    Re: control stepless screw compressors

    His
    There is no way to find the position of that
    Best way use step 25 and 50 and 75 and 100

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