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  1. #1
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    Pressure ratio vs compressor input



    Hi,

    Most of my refrigeration knowledge is theoretica, and i sometimes run into issues when converting that to practical issues.

    So, a compressor operates between set-points, which are designer/manufacturer designed. Now, if due to ambinet conditions, the pressure in the condenser changes (assume that it drops), why does it result in a reduction of the compressor work. Shouldn't the compressor be doing the same amount of work, since it is still compressing (or trying to compress) upto the designed setpoint?

    If someone installs an electronic expansion valve, and allows the condenser pressure to drop, why would that result in a decrease in compressor consumption? that should only happen if some sort of controller is telling the compressor to adjust its pressure ratio.

    Sorry for the long winded question. I am really desperate to understand how compressor set-points work in such a system.

    Thanks.



  2. #2
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    Re: Pressure ratio vs compressor input

    Mk2
    The refrigerant compressor has a fixed volume (unless unloaded) but the quality of the refrigerant gas varies with load, ambient conditions and other factors
    Other types of compressors don't have varying quality of vapour so you can't really compare the 2
    Last edited by frank; 21-09-2015 at 09:12 PM.
    I'm back on the Pale

  3. #3
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    Re: Pressure ratio vs compressor input

    Hi mk2coupe85 !
    First of all welcome to this valuable forum !
    I discovered this forum about 7 years ago when i was novice to refrigeration . I have learned too much simply reading posts in this forum and I can asure you that this is indeed the best place where you can find an answer for every question about refrigeration .
    I will try to clarify with my own words some aspects of compressors and refrigeration .
    A compressor is a kind of pump that move a nearly fixed volume of gas in each revolution from one pressure (level) to a higher one (level) . For this work to be done it consumes some energy . If the the temperature at the condenser decrease or the air flow increase than the condensing pressure will decrease to a lower level so the compressor needs less power .
    We cannot regulate the condesing pressure thru the expansion valve , the valve regulate the evaporation pressure instead . It let the exact amount of liquid refrigerant to pass in the evaporator satisfying the cooling effect and protecting the compressor from liquid refrigerant flooding .
    The working points are pressures that are derived from many quantities in a real system , mostly tem9eratures and

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    Re: Pressure ratio vs compressor input

    All to do with compression ratios, high CR high power consumption, Low CR less power consumption.
    As per internal combustion engines High CR high power generated.

  5. #5
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    Re: Pressure ratio vs compressor input

    You could compare a compressor to a bike tyre pump.
    When tyre is flat it is easy to pump air in.
    As the pressure in the tyre increases, it gets harder to pump.
    Also less & less air gets pumped in the higher the tyre pressure with each pump.
    In a compressor the higher the differential pressure across it, the less efficient it becomes due to clearance volume at top dead centre of piston stroke.
    As ambient & evaporator loads change the refrigeration system reaches a new "Balance".

    http://nptel.ac.in/courses/112105129...cture%2025.pdf

    this is one of many links explaining balance
    Last edited by RANGER1; 22-09-2015 at 03:26 AM.

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    Re: Pressure ratio vs compressor input

    Hi mk2
    A refrigeration system is designed with a set pressure drop across the expansion device be it a capillary, TX valve or expansion valve. If you look at a tx valve selection chart you will see it will have the suction pressure on one axis and condensing pressure on the other. You will notice the capacity of the same valve will vary depending on the pressure difference across the valve, suction to discharge, the bigger the difference the bigger the capacity. So if your condensing pressure drops then the capacity of the valve will drop and in doing so will starve the evaporator of liquid this will cause the suction to drop and hence the work done by the compressor will drop because the gas flow is reduced.
    Paul
    Born to fish, forced to work

  7. #7
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    Re: Pressure ratio vs compressor input

    Another link which may be of help



    http://www.danfoss.com/nr/rdonlyres/.../pf000f202.pdf

  8. #8
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    Re: Pressure ratio vs compressor input

    The system is designed for two pressure temperature levels. Evaporating temperature/pressure is derived from the temperature level required for the process and the requirement always remain same. The condensing temperature / pressure is derived from the worst ambient conditions and the type of condenser used. The expansion valve is selected to suit these two pressure levels. Now you are talking about two things. One: lower ambient temperatures which if condenser is not controlled will definitely result in lower condensing pressure. Two: use of electronic expansion valves, where in you can adjust the flow going to evaporator irrespective of pressure differential available, by programming in such a way that there will be desired flow of refrigerant.
    In such situation it is desirable to operate the system is lowest condensing pressure possible taking advantages of the ambient conditions. Of course, there may be limitation from the oil supply temperature in case of screw compressor.
    As explained by others, as the compression ratio is reduced there is reduction in power consumption.

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    Re: Pressure ratio vs compressor input

    I will give it also a try... the valves (suction and discharge) inside compressor are not manually operated. They open if the pressure differential over the valve becomes bigger than 0. Its pressure operated.
    Let's first look to the discharge valve.
    On the outside valve, we have condensing pressure, let’s say 10 bar. At the inside, we have the pressure inside the cylinder. The piston moves up and compresses the gas. As soon the gas pressure inside the piston becomes more than 10 bar, this valve will lift up. This happens at a certain height of the piston inside the cylinder.
    The piston moves back down, volume increases for certain volume of gas, so pressure decreases. As soon pressure in the cylinder becomes 10 bar, the valve closes. This happens at a certain height of the piston inside the cylinder.

    Suppose now outside temperature rises and also condensing pressure rises to 11 bar.
    The next stroke, the piston moves up again and compresses the gas till the gas inside the cylinder is at the same pressure at the pressure on the other side of the discharge valve, in this case now 11 bar. Because the condensing pressure is now a little bit higher (11 bar compared to 10 bar), the piston must move a little bit higher to compress a little bit more the gas above the piston. So the valve will open a little bit later because there’s a time lapse compared to the first stroke.
    Pistons moves down again and pressure decreases due to the larger volume. The piston now is a little bit higher inside the cylinder when the valve closes because it now has to decent not as low as at 10 bar. We reach sooner 11 bar inside the cylinder. So the valve closes sooner than the first time. So, at 11 bar, it opened later and closed sooner. So there was less time to let escape gas to the condenser at 11 bar.
    Compressing higher means also more power for the motor. Opening later means less gas can flow out of the cylinder.

    Now the suction valve, a little bit the same story. Above the suction valve at the inlet of the compressor, we have the suction pressure pushing on the valve. Let’s say 3 bar. The piston moves down from condensing pressure. Pistons moves further down until gas expands and we reach less than 3 bar inside the cylinder. The suction valve then opens. Gas flows in the cylinder. Pistons moves up again and starts its compression cycle. As soon pressure rises above 3 bar, the suction valve closes because pressure inside the cylinder becomes more than 3 bar (the evaporating pressure) which stands on the other side of the valve.

    Suppose now a new situation where suction pressure drops to 2 bar.
    Piston moves down again after a compression cycle. Piston must now move down a little bit lower before pressure drops to 2 bar (it expands ater to 2 bar than to 3 bar) So the suction valve opens later than in the situation at 3 bar. Gas flows in and pistons moves up while gas flows in the cylinder starting its compression cycle. Piston reaches now a little bit sooner 2 bar inside the cylinder as long as the pressure inside the cylinder becomes larger than the 2 bar on the opposite of the valve. So valve closes already at 2 bar.
    So at 2 bar, it opens later and closes sooner, so less gas can enter the cylinder.

    So higher high pressure and lower low pressure are negative effects on the cooling effect in a refrigeration cycle.

    For the rest, I want to refer to the example with the air pump of your bicycle. This explain s it very good. And the electronic valve has nothing to do with condensing pressure. It only maintains a stable DT across the evaporator whatever the pressure differential may be across the valve.
    Hopes this clarifies it a little bit.
    Last edited by Peter_1; 28-09-2015 at 06:17 PM.
    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: Pressure ratio vs compressor input

    More information, but not hearing back from original poster!

    http://nptel.ac.in/courses/112105129...cture%2019.pdf

  11. #11
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    Re: Pressure ratio vs compressor input

    Thanks Ranger, this is the link to all the chapters of this course you just posted .
    http://nptel.ac.in/downloads/112105129/
    Forgot the link, sorry
    Last edited by Peter_1; 28-09-2015 at 01:01 PM. Reason: Forgot the link
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  12. #12
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    Re: Pressure ratio vs compressor input

    Quote Originally Posted by Peter_1 View Post
    Thanks Ranger, this is the link to all the chapters of this course you just posted .
    Sorry for repeating, forgot about that one.

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