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  1. #1
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    Setting the Refrigerant Pressure at the Compressor's Input



    Hi there!

    Please could someone explain the design process of setting the refrigerant pressure at the compressor's input ?

    e.g. I intend to use R290 refrigerant for an experimental cooler. The required evaporating temperature will be -40 C, which correlates to an evaporating pressure of 1.1 bar. How is this pressure set in the system ?

    Many Thanks in advance...



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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Hi, stephen

    Quote Originally Posted by stephen.t View Post
    Hi there!

    Please could someone explain the design process of setting the refrigerant pressure at the compressor's input ?

    e.g. I intend to use R290 refrigerant for an experimental cooler. The required evaporating temperature will be -40 C, which correlates to an evaporating pressure of 1.1 bar. How is this pressure set in the system ?

    Many Thanks in advance...
    ....by regulating valve on the liquid line before evaporator....this valve splits your plant into high pressure and low pressure part....

    ....from discharge valve on the compressor till to regulating (expansion) valve we have a high pressure part of the system...

    ....after regulating (expansion) valve till to suction valve of the compressor we have a low pressure part of the system...

    depending on how much is open/close regulating (expansion) valve compressor will create suction pressure i.e. evaporation pressure...

    of course...assuming complete system is in balance....otherwise we should face many problems

    Best regards, Josip

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Some general things about (thermostatic) expansion valves (TXV):
    - a TXV does not regulate the pressure(s) in a system, and should not be used to do so. A TXV regulates the superheat of your evaporator.
    - a (well dimensioned) TXV should not be 'regulated', it regulates itself. Avoid changing the factory setting. In 99,99% of the cases there is no need to do so.

    The system pressures depend on the external conditions (evaporator and condensing side) and on the design of all (main) parts of the system.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    If forgot something:

    In addition to the selected main components and the external conditions, any pressure regulting device (fan speedcontroller, EPR, CPR, ...) and to a certain extend the refrigerant charge will have an influence on the pressures in the system.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Quote Originally Posted by stephen.t View Post
    Hi there!

    Please could someone explain the design process of setting the refrigerant pressure at the compressor's input ?

    e.g. I intend to use R290 refrigerant for an experimental cooler. The required evaporating temperature will be -40 C, which correlates to an evaporating pressure of 1.1 bar. How is this pressure set in the system ?

    Many Thanks in advance...
    The real answer is by the correct design of the system.
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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Quote Originally Posted by BESC5240
    - a (well dimensioned) TXV should not be 'regulated', it regulates itself. Avoid changing the factory setting. In 99,99% of the cases there is no need to do so.
    Thank you, thank you, thank you.

    I wish more people realized this and would quit adjusting a TXV every time they see one.

    I hope someone listens to you as this is excellent advice!
    If all else fails, ask for help.


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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Choosing the correct compressor for the heat load with regards to temperature you are trying to maintain. The back pressure will constantly change as the heat load changes so you can't keep it at a constant back pressure.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    In a water chiller, the evaporator efficiency is very much dependent on the water flow. When insufficient water flow is established, the evaporating pressure suffers, and results in a lower suction superheating (liquid phase titration in vapour increases). Therefore, the TXV may be adjusted in case the water flow can't.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Quote Originally Posted by NoNickName View Post
    In a water chiller, the evaporator efficiency is very much dependent on the water flow. When insufficient water flow is established, the evaporating pressure suffers, and results in a lower suction superheating (liquid phase titration in vapour increases). Therefore, the TXV may be adjusted in case the water flow can't.
    That is not solution, that is patch! You need to establish proper water flow!!!
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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    us iceman also known as " tinkeritis " {adjustments of txv] regards butch

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Thanks to everybody for your answers

    To further clarify things in my mind... For a given system in which:

    - Both air temperatures (i.e. the one that "surrounds" the evaporator and the one that "surrounds" the condenser) remain unchanged
    - All the rest system components remain unchanged as well, except for the evaporator

    Would the following statement be correct:


    Small evaporator: less refrigerant can be released by the TXV, resulting in the compressor being able
    to obtain a lower input pressure.

    Large evaporator: more refrigerant can be released by the TXV, resulting in the compressor not being able
    to obtain such a lower input pressure.


    I am just trying to look at how the evaporator on its own effects the minimum pressure I can obtain at the compressor's input..


    Thanks in advance

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    You are trying to do a complex thing here, and I believe that you are starting in the wrong end.

    First you have to calculate the (heat) load on your system.
    Then you can choose evaporator.
    Then you can choose compressor.
    Then you can choose condensor.
    Then you will have to select other components, like TXV.

    Everything has to be balanced, it's not just changing one part of the system and then achieve different working conditions.


    Quote Originally Posted by stephen.t View Post
    Small evaporator: less refrigerant can be released by the TXV, resulting in the compressor being able
    to obtain a lower input pressure.

    Large evaporator: more refrigerant can be released by the TXV, resulting in the compressor not being able
    to obtain such a lower input pressure.
    Small(er) Evaporator will give a low(er) suction pressure if all other things are the same BUT... The duty will also drop.

    Look at it this way,
    The fire brigade are able to fill a swimming pool in an hour with one of their 2" hoses.
    If you try to do the same with your 1/2" garden hose, the pool will not get filled because the water is evaporating quicker than you are able to fill.
    (No, it is not true, it's only to illustrate what I'm on about, hopefully you will get the idea)


    SO... The starting point for any design is to determine what capacity you need, and at what conditions. This will give you the data you need to select the evaporator. Then you will have to design your system around this.
    Last edited by The Viking; 25-01-2008 at 02:06 AM.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Why do you keep trying to adjust pressures with evaporator size?

    Try adjusting the duty via the TXV.

    If, as above, you say that nothing changes around the condenser or the evaporator I would be tempted to say that the system is switched off
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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Brian,
    I think (from reading his original question) that he's new to the game and have gotten hung up on the saturated temperature/pressure. As a result he is trying to design the whole system based on a saturated temp of -40....
    But have still not looked at the duty?

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    I just could not resist myself from posting this. I agree 100% with BESC5240 regarding adjusting the TXV. I have been trying to make many a refrigeration technicians and engineers understand that they should not try adjusting the TXV. I
    I ensure that my technicians do not do this, and I am happy to state that the plants operate very efficiently. One should be careful in selecting the Refrigeration Controls and if it is done properly then one does not need to adjust any component.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Quote Originally Posted by The Viking View Post
    Brian,
    I think (from reading his original question) that he's new to the game and have gotten hung up on the saturated temperature/pressure. As a result he is trying to design the whole system based on a saturated temp of -40....
    But have still not looked at the duty?
    Oh, I agree
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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Quote Originally Posted by US Iceman View Post
    Thank you, thank you, thank you.

    I wish more people realized this and would quit adjusting a TXV every time they see one.

    I hope someone listens to you as this is excellent advice!
    And he can know it for sure US Iceman.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    The TXV has an adjustment knob or screw which is there for a function. Provided the maintenance man knows what he's doing, I don't see any reason why he shouldn't adjust it.
    And, don't rely on manufacturers quality system or testing alone.

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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Adjusting knob is there, if we want different superheat value than that standard factory adjustment. It should be adjusted at day of commissioning plant and if, for some reason, we want to change amount of superheat, but certainly not for service purposes.
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    Re: Setting the Refrigerant Pressure at the Compressor's Input

    Thanks to everybody for your answers

    The design process is much clearer to me now and you have highlighted some areas that I had not considered

    Best Regards
    Stephen.t

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