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  1. #1
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    Is suction/Discharge gauge stable ?



    Hi guys,

    Yesterdays I was talking with my friend about inverter compressors.
    I hadn't experience about this system up to now. I have a question, Suppose that suction pressure is on 70 PSI and discharge pressure is on 230 PSI (R22) and compressor are working on 100% capacity.

    When inverter control system decrease compressor speed (By decreasing frequency), Do compressor's suction and discharge gauge change ?

    I can understand that when speed changes displacement of compressor changes too, But will it influence the pressure of suction and discharge gauge ?

    Do any of friends have this experience to share with me ?

    Thanks
    Last edited by A.Mortezania; 04-11-2016 at 07:39 AM.



  2. #2
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    Re: Is suction/Discharge gauge stable ?

    Hi Mortezania,
    When compressor speed is changing, it changes controlled by some kind of algorithm, normally it tries to maintain suction pressure constant. Also in the condenser normally there's some kind of control of the ventilation or the cooling fluid (ex. water) so that the condenser pressure is maintained near the set-point established.
    So in the most common situation both, suction and discharge pressures are maintained aproximatly constants.
    The are other situations with floating evaporator and/or condenser pressures, and of course in that situations those pressures are not constants.
    It all depends on the algorithms of control.
    Regards,
    CDuque

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    Re: Is suction/Discharge gauge stable ?

    Quote Originally Posted by cduque View Post
    Hi Mortezania,
    When compressor speed is changing, it changes controlled by some kind of algorithm, normally it tries to maintain suction pressure constant. Also in the condenser normally there's some kind of control of the ventilation or the cooling fluid (ex. water) so that the condenser pressure is maintained near the set-point established.
    So in the most common situation both, suction and discharge pressures are maintained aproximatly constants.
    The are other situations with floating evaporator and/or condenser pressures, and of course in that situations those pressures are not constants.
    It all depends on the algorithms of control.
    Regards,
    CDuque
    Dear CDuque,

    First of all thanks for your reply.

    I understand that when compressor's speed changes, It means we have lower displacement and lower compressor refrigeration capacity.
    I know this, Also some controller for example decrease speed of fan of condenser/evaporator to decrease also capacity of evp./cond. to match it with compressor.
    I can understand it clearly.

    My question is other thing. Imagine evp./cond. is constant and we do not control capacity of them, Just we have variable compressor by changing speed of rotation of compressor (Inverter system means changing frequency).

    Twice I say my question again, Now compressor is working 100% , Suction is 75 PSI and discharge is 230 PSI.
    We decrease frequency and compressor rotation, For example now compressor are working on 50% of capacity, Is suction/discharge constant or it will change ?

    Thanks.

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    Re: Is suction/Discharge gauge stable ?

    Of course it will change. Half the speed, half the capacity.
    Pressure will change. Its about keeping temp and not pressure.
    That's the whole thing about refrigeration > changing temperatures!!

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    Re: Is suction/Discharge gauge stable ?

    Quote Originally Posted by chemi-cool View Post
    Of course it will change. Half the speed, half the capacity.
    Pressure will change. Its about keeping temp and not pressure.
    That's the whole thing about refrigeration > changing temperatures!!
    Dear Chemi-cool,
    You told half speed means half displacement and half capacity.
    OK. It is right.

    But in the other hand, Compare 1/4 hp compressor (for example for fridge) with 40 hp Copeland compressor for cold rooms.
    Very clearly displacement of these 2 compressors are not same, But when both of 1/4hp and 40hp compressor work in same evp./cond. temperature suction and discharge gauge are same, Isn't it ?

    My mean, capacity of compressor do not mention pressure of suction and discharge, two compressors with different capacity but both of them while work in same condition, pressures are same.

    Sincerely yours.

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    Re: Is suction/Discharge gauge stable ?

    Maybe the pressures are the same but what are the temperatures?
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    Re: Is suction/Discharge gauge stable ?

    Hi Mortezania,

    When we say that an equipment has some X capacity we are only saying that in certain conditions the "resulting" capacity will be X. In other conditions the capacity will be different.

    Catalogs state conditions as a photography but real life and real equipments work like a video, always changing and always trying to find an equilibrium state.

    For instance think of an installation working stable, with a constant evaporating temperature and a constant condensing temperature. If you only reduce the working speed of the compressor, the capacity of the system will reduce. Then the evaporator will no longer be able to remove all the heat from the cold chamber, the temperature will increase. In that situation the evaporating temperature will also increase reducing the heat load, and, as the compressor will have a better volumetric efficiency it's capacity will also increase and in a certain moment it will compensate the heat gains at the evaporator and be again in a stable condition.
    That will be the new equilibrium situation, with an higher evaporating temperature. In the condenser the same kind of adjustments occur. This is true if the conditions do not change again before the equilibrium is achieved.

    The question is that this is what constantly occurs in real life, everything is always changing, we never give enough time to get stable conditions, so installations are always trying to adapt to the new situations. Now and then the systems conditions pass over the limits and protections go on. That's when clients scream and call the maintenance.

    Regards,
    CDuque
    Last edited by cduque; 05-11-2016 at 05:45 AM. Reason: clarification

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    Re: Is suction/Discharge gauge stable ?

    .

    You need to seperate the evaporating - suction temperatur from the discharge - condensing temperature.

    The purpose of the inverter is to maintain correct suction - evaporating temperatures and as that
    temperature changes the inverter speeds up and slows down accordingly.

    The condensing temperature is dictated by the ambient temperature around the condenser.

    In cooling mode the evaporating temperature is most important and the compressor speeds up and down
    to maintain that correct evaporating temperature, as the as the evaporator works and it achieves correct
    temperatures the suction pressure drops so the inverter controler slows the compressor down, this
    reduces capacity and evap temps then increase to the correct levels.

    The condensing temp is going to be approximately 15K above the ambient temp.

    So in answer to your question yes the pressures rise and fall as the compressor speeds up and down
    but only because the evaporating temperature rises and falls as the refrigerated air space cools and warms.

    Rob

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    Re: Is suction/Discharge gauge stable ?

    Quote Originally Posted by Rob White View Post
    .

    You need to seperate the evaporating - suction temperatur from the discharge - condensing temperature.

    The purpose of the inverter is to maintain correct suction - evaporating temperatures and as that
    temperature changes the inverter speeds up and slows down accordingly.

    The condensing temperature is dictated by the ambient temperature around the condenser.

    In cooling mode the evaporating temperature is most important and the compressor speeds up and down
    to maintain that correct evaporating temperature, as the as the evaporator works and it achieves correct
    temperatures the suction pressure drops so the inverter controler slows the compressor down, this
    reduces capacity and evap temps then increase to the correct levels.

    The condensing temp is going to be approximately 15K above the ambient temp.

    So in answer to your question yes the pressures rise and fall as the compressor speeds up and down
    but only because the evaporating temperature rises and falls as the refrigerated air space cools and warms.

    Rob

    .
    Dear Rob,
    Thanks for your reply.

    As you told, Yes , I think it is correct answer, Speed changes to maintain suction pressure constant.

    I know condenser temp. and pressure is as result of ambient temperature but if compressor's speed decreased, For example capacity of compressor changed form 12 KW to 9 KW and if capacity of condenser (fan speed) doesn't change, It will cause decreasing condenser/discharge pressure. Because capacity of condenser is much more than "compressor capacity + input power".

    By this, When compressor speed decrease to maintain suction pressure, Speed of fan of condenser also must change.

    Sincerely yours.
    Last edited by A.Mortezania; 05-11-2016 at 12:27 PM.

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    Re: Is suction/Discharge gauge stable ?

    Quote Originally Posted by A.Mortezania View Post

    By this, When compressor speed decrease to maintain suction pressure, Speed of fan of condenser also must change.

    Sincerely yours.

    Yes, it is a fine ballance. The inverter controling the evaporator, compressor and condenser
    speeds are designed to give best performance without harming the evaporator and condenser
    temperatures too much.

    But you are correct when you say that condenser and compressor capacity are closely linked.

    Regards

    Rob

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  11. #11
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    Re: Is suction/Discharge gauge stable ?

    Your good with the words Rob.
    Well and delicately put!
    Thanks Grizzly
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