Results 1 to 4 of 4
  1. #1
    Join Date
    Mar 2012
    Location
    Fort worth
    Posts
    9
    Rep Power
    0

    A few questions reguarding component selection.



    I have a few questions about selecting correct components for system balancing, as well as interpreting compressor manufacturer's curves.

    First, from what I can tell the compressor is chosen based on its refrigeration capacity vs. that of the determined evaporator required. I am having trouble with determining what the reference evaporator inlet and exit conditions are that are used in this capacity calculation. If for example the chart gives a capacity curve and provides the suction/discharge pressure and the superheat/subcool, is the superheat in reference to the compressor discharge superheat or the suction? I am assuming the subcool is out of the condenser since this should be the only subcooled portion of the process, and this value is used in determining the quality of the refrigerant entering into the evaporator. Once this is figured out, can I then just divide the compressor's capacity at the given pressure delta and rpm by the enthalpy delta across the evaporator to get the mass flow rate of the compressor? I want to know this to verify that I am getting the required flow rate to the evaporator, and to be able to determine how to improve the system.

    Next, from the compressor's curve the power consumption is provided but how do I relate this to the heat of compression added to the system. I am assuming that the compressor power consumption is not the same as the compression work on the fluid due to inefficiencies in compression, but I am not sure how much. Is it just safe to assume that all of the compressor power consumption goes to compression work of the fluid so that the condenser is of adequate size, or is this overkill? With this assumption then the required heat removal of the condenser would be the total of the refrigerating capacity and the power consumption at the given rpm and delta pressure.

    Last, is it safe to assume that if the compressor's refrigerating capacity is provided just at the evaporator inlet and exit conditions provided by the testing curve, that it is possible to increase this capacity by increasing the subcooling since the mass flow should be constant?

    Thanks for your help in advance.



  2. #2
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    7

    Re: A few questions reguarding component selection.

    Welcome to RE forums!

    Next time please make your questions in separate paragraphs.

    , is the superheat in reference to the compressor discharge superheat or the suction?
    Suction! Discharge superheat has little or no relevance for compressor performance. It only affects discharge temperature (or reliability) but not the capacity of the compressor which is mainly given by the compressors input conditions and discharge pressure.

    I am assuming the subcool is out of the condenser
    Not necessarily you may have heat exchangers to subcool. There are many good old threads about subcooling, like:
    http://www.refrigeration-engineer.co...and-Subcooling

    Subcooling affects system performace, not compressor performance, and you should be careful to compare compressors without subcooling in their ratings! This is specially difficult in compressors with intermediate pressure ports.

    since this should be the only subcooled portion of the process, and this value is used in determining the quality of the refrigerant entering into the evaporator. Once this is figured out, can I then just divide the compressor's capacity at the given pressure delta and rpm by the enthalpy delta across the evaporator to get the mass flow rate of the compressor?
    The catalog gives you the subcooling and the superheat (or gas temperature and evaporation temperature=> calculate superheat) at the entrance of the compressor.

    The compressor capacity divided by the enthalpy difference at these conditions give you directly the mass flow trhugh the compressor at the given RPM. The RPM are implicit here they are not used for calculation.

    [QUOTE]I want to know this to verify that I am getting the required flow rate to the evaporator, and to be able to determine how to improve the system. [QUOTE]

    You need the flow rate of the evaporator if you want do design one but if you want to dimension one all you need is the capacity at the desired differential.


    Next, from the compressor's curve the power consumption is provided but how do I relate this to the heat of compression added to the system.
    Only in open compressors you can relate the heat of compression to the shaft power (minus losses).

    In semi hermetics part of the power input goes out of the system into the air and part goes to superheat the suction gas so you cannot relate unless you make more assumptions like temperature reise of the gas and how isentropic is your compression.

    In hermetics (minus orbital compressors) it is almost the same with less heat going to the air.

    In orbital compressors the heat transmission to the air is also important, like semi hermetics.

    I am assuming that the compressor power consumption is not the same as the compression work on the fluid due to inefficiencies in compression,
    Correct, but this relates with how isentropic is your compression and the type of compressor.

    but I am not sure how much.
    Neither am I! You can measure it but the problem is that it varies not only with compressor design but also with operating conditions!

    Is it just safe to assume that all of the compressor power consumption goes to compression work of the fluid so that the condenser is of adequate size, or is this overkill?
    It is safe, if this particular assumption is wrong you will only miss by about 1C differential in the working conditions of the condenser. But it all depends on your safety factors, if you enter all the compressor power into the condenser calculation you are playing safe and need less safety factor for the heat of rejection.
    Having a large condenser is more energy efficient and is easy to solve, small ones are a pain in...

    With this assumption then the required heat removal of the condenser would be the total of the refrigerating capacity and the power consumption at the given rpm and delta pressure.
    if by delta pressure you mean desired (or target) working conditions then yes.

    [QUOTE]Last, is it safe to assume that if the compressor's refrigerating capacity is provided just at the evaporator inlet and exit conditions provided by the testing curve, that it is possible to increase this capacity by increasing the subcooling since the mass flow should be constant? [QUOTE]

    It is only safe to assume that the compressors refrigerating capacity is: at the suction of the compressor! This may be way off evaporating capacity or useful refrigerating effect. You have losses in between.

    I think you should read old literature on this topic like:

    http://www.4shared.com/office/sKWvIp...n_Manual_.html

    Cheers!
    Last edited by aramis; 22-04-2012 at 11:16 PM. Reason: added "and discharge pressure (not superheat)." to first answer

  3. #3
    Join Date
    Mar 2012
    Location
    Fort worth
    Posts
    9
    Rep Power
    0

    Re: A few questions reguarding component selection.

    Thanks for your help, and also for the link to the literature. I originally intended for there only to be one question per paragarph, but I guess I got a little carried away.

    I did figure that the superheat reference on the compressor capacity curves was the suction superheat, but when I used these values to calculate the enthalpy difference accross the evaporator to determine the mass flow rate it was off when compared to some reference data that I found on the unicla compressor website. Thanks for verifying this.

    If you dont mind I was hoping to clarify this a little bit:

    It is only safe to assume that the compressors refrigerating capacity is: at the suction of the compressor! This may be way off evaporating capacity or useful refrigerating effect. You have losses in between.
    I was trying to see am I right in saying that after determining the mass flow rate of the compressor using the enthalpy difference accross the evaporator at the referenced values provided by the manufacturer, can I then use another method such as a counterflow heat exchanger between the suction line and condenser outlet to increase the system capacity. This seems to me like it may be a dumb question, but I do wonder what was ment by "the compressors refrigerating capacity is: at the suction of the compressor"

    Thanks again for the literature, there is a lot to digest there, and even covers some other issues that I was wondering about. One of these was the fact that when the compressor is not running that liquid would settle in the compressor so a solenoid in the liquid line or a bypass might be needed.

  4. #4
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    7

    Re: A few questions reguarding component selection.

    There are also several posts here about using suction-liquid line, this is a working day so please you look for them.

    I did not mean this type of heat exchanger but on liquid Line-air or Liquid Line-water or a subcooler with (dedicated) evaporated refrigerant.

    Liquid line-suction heat exchangers are generally misunderstood in the literature and practice.

    This heat exchanger does subcool the liquid with the effect of increasing system capacity, but it also increases suction gas specific volume which has the effect of decreasing compressor capacity therefore system capacity too. Besides increasing suction superheat increases discharge temperatures decreasing system reliability.

    In some systems this heat exchanger can account for other gains like decreasing losses (generally thermal) in the suction line, but again it increases pressure drop.

    So what really happens with these suction-liquid line heat exchangers is refrigerant, application, operation and system design dependent and you cannot assert it will increase system capacity a priori.

    What I meant by “the compressors refrigerating capacity is: at the suction of the compressor” is that the capacity tables published are at the suction of the compressor conditions which may differ from the actual outlet-of-the-evaporator conditions which means that compressor capacity not necessarily equals the useful refrigeration effect. It does on compact systems or systems with very little losses (both pressure and temperature or heat transfer) in the suction line.

    There are several design and control issues not to let liquid refrigerant in the compressor while iddle, like pumpdown, low charge, discharge and suction lines going down exiting the compressor before going up, suction accumulators, (oil separators), and discharge line going up condensers with poorly designed input collectors.

    In all of this I cannot see how a bypass can help but maybe younger generations like yourself can come up with something new!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •