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    COP and Superheat - does is it help?



    There any many references that say COP improves with superheat in some gases and not in others. R12 was classically described as an improver with SH whilst R22 was said to have worse COP values as SH increased.

    So far I have not been able to show this increase in COP and so ask if anyone knows of a good example/article that descibes it.

    All I have seen is that SH pushes the compression cycle into flatter entropy zones and hence greater compression work plus it reduces the compressor efficiencies and both are bad for COP.

    Also as SH increases so the quality increases which reduces the refrigeration effect. Combine all 3 effects and COP falls of very quickly with SH.

    Thanks

    Chef



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    Re: COP and Superheat - does is it help?

    Hi

    I think this is effective to all refrigerants, :

    All I have seen is that SH pushes the compression cycle into flatter entropy zones and hence greater compression work plus it reduces the compressor efficiencies and both are bad for COP
    .

    But if you found such refrigerant wich does not act like others, let me know

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    Re: COP and Superheat - does is it help?

    look for the program coolpack it is from denmark, here you can make calcs with sh and cop.
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by shooter View Post
    look for the program coolpack it is from denmark, here you can make calcs with sh and cop.
    Interesting as coolpack shows an increase in COP with increasing SH.

    Lets take some extreme values and have condensing at 35C and the evaporator at -10C so now we add 50C superheat and get an improvement in the COP.

    This means the evaporator is at 40C (thats +40C) which makes little sense. Not sure how they calculate the various states but in reality that cant possibly be true so just for fun I tried 100C superheat and my COP went up yet again.

    So coolpack does an energy balance and does not consider what the physical conditions in the freezer are, it is up to the user to change the evaporator temperature to allow realistic SH realisation.

    Bottom line is it has not solved the dilema.

    Chef

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    Re: COP and Superheat - does is it help?

    Have you played with useful and non useful super heat, and pressure drop which also shows as increase non useful super heat.

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    Re: COP and Superheat - does is it help?

    Back to my original question - there are several (many) references that show an increase in COP with SH but I am unable to find this.

    Coolpack gives unrealistic results as having 50C SH in a fridge system is not possible but it allows the calculations to continue. Hmmm. Concerning.

    If one assumes a box temperature and then adjusts evaporator temperature to include SH then the results are more reasonable and the COP does decrease. I wonder if everyone else uses the software that way?

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    Re: COP and Superheat - does is it help?

    Dear
    Effect on COP by increase in SH depends on the thermodynamic characteristics of refrigerant. If refrigerant have higher gamma value decrease the COP. Because increase in power consumption more as compare to improve in refrigeration effect. Super heat is mostly used for compressor safety not for improving the COP.

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    Re: COP and Superheat - does is it help?

    Yes with rising SH the COP goes up, but do not forget the return temp must be also very high, so yes it is correct what coolpack does.
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    Re: COP and Superheat - does is it help?

    Hi Chef,

    The answer to your question would surely be answered by running incremental (evap) SH trials on a complete system?

    Programs like Coolpack assume that as SH is varied, that all Te,sat & Tc,sat remain constant. Is this the case, in reality? What results does your own simulator predict?
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by shooter View Post
    Yes with rising SH the COP goes up, but do not forget the return temp must be also very high, so yes it is correct what coolpack does.
    So what you are saying is that 50C SH is feasable from a system where the ambient is maybe 30C. So you say more SH the more COP and Coolpack is right!!!

    You must explain how we build this thing.

    Maybe you could put the numbers into Coolpack yourself and then post a suitable design for the fridge you have.
    Last edited by Chef; 21-09-2012 at 04:57 AM.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    Hi Chef,

    The answer to your question would surely be answered by running incremental (evap) SH trials on a complete system?

    Programs like Coolpack assume that as SH is varied, that all Te,sat & Tc,sat remain constant. Is this the case, in reality? What results does your own simulator predict?
    To answer my question which was "Does anyone knows of a good example/article that describes COP increasing with SH" would be great.

    I really only want to know if you have any good referance material so doing an incremental SH trial on a complete system is a bit of an overkill.

    Programs like Coolpack assume that as SH is varied, that all Te,sat & Tc,sat remain constant. Is this the case, in reality?

    Well that is my point. If the predictions are not real how are we to apply them in everyday life.

    What results does your own simulator predict?

    Not what Coolpack does. As you say "is this the case in reality" and I am trying to get to the reality.

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    Re: COP and Superheat - does is it help?

    Come on chef, spill the beans, whats your giving that coolpack does not.

    No system really is even in steady state, there are very few exceptions.

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    Re: COP and Superheat - does is it help?

    [at]Chef : Chill-beans sent post-haste via an urgent ocean drop overhead. Be on the look-out.

    I stand by my view that there are always a few ways to arrive at an answer:
    1. Experimentation on real system.
    2. Controlled, accurate simulation, where all variable free to float to steady-state as SH varied.

    It would be interesting to see how your internal simulator stacks up in a controlled test situation. Precisely how SH is raised, will contribute (I'd imagine) as much to the system change, as anything. E.g. cutting back refrigerant mass-flow, or adjusting evap fan speed etc.

    Anyway, this thread promises to be an insightful one, if all parties can pull together on the same side of the investigation.
    Last edited by desA; 21-09-2012 at 08:48 AM.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by mad fridgie View Post
    Come on chef, spill the beans, whats your giving that coolpack does not.

    No system really is even in steady state, there are very few exceptions.
    Well as I said if you put 50C SH into Coolpack it just plods along and gives very unrealistic answers. It is not possible to continue adding SH and increase the COP ad infinitum so there is something wrong me says.

    What I see is a change in the evap temp so the box temp stays the same and this is real world so the SH is accounted for. My COP drops of quickly at 50C SH

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    [at]Chef : Chill-beans sent post-haste via an urgent ocean drop overhead. Be on the look-out.
    Please use Yellow Box Ticker code 47590023 for GPS location.

    I stand by my view that there are always a few ways to arrive at an answer:
    1. Experimentation on real system.
    Full system experimentation is massive overkill for my original question - does SH improve COP and is there any good articles about it. Even well founded opinions would be just as good.

    2. Controlled, accurate simulation, where all variable free to float to steady-state as SH varied.
    Thats exactly what I am doing but disagreements with other resources like Coolpack need to be addressed, understood and resolved. If my memory serves me well i believe you are a believer in SH being a COP riser so your input as to what conditions it will help is what we need.

    It would be interesting to see how your internal simulator stacks up in a controlled test situation. Precisely how SH is raised, will contribute (I'd imagine) as much to the system change, as anything. E.g. cutting back refrigerant mass-flow, or adjusting evap fan speed etc.
    What sort of controlled test situation, test against what, already tested it against Coolpack and it disagrees.


    Anyway, this thread promises to be an insightful one, if all parties can pull together on the same side of the investigation.
    If we are all on the same side then we will all agree and we will get nowhere, maybe you can explain a little more your comment here.
    My reply in blue of course.

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    Re: COP and Superheat - does is it help?

    I doubt that you are going to find the literature you may be searching for, on RE. There may be bits & pieces scattered throughout academic literature, but perhaps not quite in the form you are looking for.

    A few thoughts;
    1. If someone has ready access on a test rig (heat-pump, or refrig circuit), we can run up a set of parallel trials - for hard comparison purposes. (Thinking here of MF, as my test lab is still packed away in SE Asia. Will only have access to that in a few months again).
    2. You (Chef) then run trials using your simulator.
    3. Coolpack 'snapshots' are taken along the test trajectory.
    4. Suggest we also use Emerson 'Select 7.6', or 'Duprex 3.2' as additional comparators.
    5. We bring all findings to the table, & compare notes along the exploration trajectory.

    We could all learn a great deal from this exercise & it would be fun, to boot.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: COP and Superheat - does is it help?

    Ok, I am struggling with the question and the answers.
    If we have refrigerant that has fully boiled, then we add further heat, and if the heat is useful, then at this point only the system must be more efficient.
    So we have 2 different points we have same pressure on with some addition energy and one with a lot of additional energy.
    Is the question or the implied answers about what happens in the compressor or what is happening to the system.???????? ???????? ???????

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    Re: COP and Superheat - does is it help?

    if super heating from liquid line make better performance or not is depend of two thing

    1. If high or low specific heat to increase refrigerant up/down one degree in gas or liquid state (not phase change) .

    2. how much latent heat in phase change between liquid and vapor

    For example ammonia have very high latent heat on phase change ie. low mass flow comparing to cooling power and low specific heat so even small amount of joule transfer in SH-exchanger with liquid line, punish system badly on COP and make very heat high side output gas temperature after compression.

    Other side, take example of R125 or HFO1234yf is have very low latent heat (need high mass flow to make some cooling power and have high specific heat) you _need_ SH-exchanger with liquid line to make any acceptable COP (or you take with you so much heat in warm liquid line in to evaporator so most of cooling power goes to cool liquid itself down to evaporator temperature and you have low quality of refrigerant - ie. very little of mass refrigerant stay in liquid state i evaporator after expand and most already vaporize without any use for cooling room) and also to make secure with SH at refrigerant are dry after compression step so you not have refrigerant steam in exhaust side and wash away oil from inside cylinders, cylinder head, valves etc. inside compressor and wear out this to fast.

    Propane (R290) and isobutan (R600a) have lite of this problem with steam in exhaust if not have some degree SH in suction line with compressor with low heating of suction gas before entering compression step (to many mistake done about not respect this fact and make short life equipment depend of bad lubrication in cylinder head in the end) and winning some COP if carefully balanced between high pressure side temperature and make liquid side cooling so close as possibly to evaporator temperature before expansion - in this way you can in theory make same or slightly better performer with propane compare to R134a without SH-exchanger with liquid line.

    - but if you use pure R125 and HFO1234yf this problem is much worse, risk to working with wet steam inside compressors and COP going down badly if not using heavy grade transferring SH-exchanger with liquid line - all depend of low latent heat compare to high specific heat

    For refrigerant as R12 and R134a have very small improve COP with SH-exchanger with liquid line and for bigger equipment not invest it in most case - exception white wares as fridges and freezer for for kitchen and houses ie. mostly put in capillaries inside suction line or welded/pressed/soldered parallel some distance (but often in little to late step for best performance ie. liquid already starting to vaporize in capillary line before put together with suction line).

    R22 have slightly punish on COP and rise hot gas temperature very fast if try SH-exchanger with liquid line
    Last edited by xxargs; 23-09-2012 at 12:30 AM.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    I doubt that you are going to find the literature you may be searching for, on RE. There may be bits & pieces scattered throughout academic literature, but perhaps not quite in the form you are looking for.
    I am looking for it in any form so happy to get any reference out there.

    A few thoughts;
    1. If someone has ready access on a test rig (heat-pump, or refrig circuit), we can run up a set of parallel trials - for hard comparison purposes. (Thinking here of MF, as my test lab is still packed away in SE Asia. Will only have access to that in a few months again).
    The prospect of a test rig showing COP increases for a few degrees of SH is unrealistic, unless it is at JPL or similar. My rig is set up for cap tubes not for SH and COP trials so that is out as well.
    2. You (Chef) then run trials using your simulator.
    Already done that and the results dont agrree
    3. Coolpack 'snapshots' are taken along the test trajectory.
    That is where I got one comparison from as already posted.
    4. Suggest we also use Emerson 'Select 7.6', or 'Duprex 3.2' as additional comparators.
    OK you can do that bit and see if you find COP going up or down in those programs
    5. We bring all findings to the table, & compare notes along the exploration trajectory.
    Table the results from 4 above so we can see if thay all predict the same thing
    We could all learn a great deal from this exercise & it would be fun, to boot.
    Answers in blue

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by mad fridgie View Post
    Is the question or the implied answers about what happens in the compressor or what is happening to the system.???????? ???????? ???????
    It is about the system I suppose as it concerns the COP - what happens in the compressor is only part of it.

    Differant programs and differant answers is a concern. If I fiddle Coolpack with the evaporator temperature it gets closer

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    Re: COP and Superheat - does is it help?

    xxargs - a very detailed reply and thankyou for the input.

    You are bringing in a SH-exchanger but initial tests were not on a system with a SH exchanger and that will come later.

    For refrigerant as R12 and R134a have very small improve COP with SH-exchanger with liquid line and for bigger equipment not invest it in most case - exception white wares as fridges and freezer for for kitchen and houses ie. mostly put in capillaries inside suction line or welded/pressed/soldered parallel some distance (but often in little to late step for best performance ie. liquid already starting to vaporize in capillary line before put together with suction line).

    Even if the liquid is vapourising in the capillary the addition of SC will still be felt just the same if was applied before the capillary. Any cooling of the fluids in the tube will have an effect no matter where they occur.

    The main question in this case is how effective is the capillary tube in a suction line acting as an effecient heat exchanger as the area of the tube is so small it cant really have an effective heat transfer between the 2 fluids. It may help a little but this cant be called a SH-exchanger.

    I realise various gases have differant properties when doing these calculations and at the moment I am looking at R134a, R12 and R22. Although R12 is defunct it is still the standard for all comparisons.

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    Re: COP and Superheat - does is it help?

    I think we need to look at an extreme example.

    We have fluid that needs to be cooled single pass 50C to 5C. So we need a evaporation temp below 5C, say 0C

    We could have then up to 50K of useful superheat available. Lets say 24K superheat as this practically in range of most compressors.

    Will this give a better COP than say having a conventional 6K Sh, and does vary due to the refrigerant?

    And if it is better, what effect does this have on the design of the evap heat exchanger.

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    Re: COP and Superheat - does is it help?

    Right then, Chef. Please set the example/s & lets get cracking.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    Right then, Chef. Please set the example/s & lets get cracking.
    Hello friends, Chef, MadF and surely DesA (greetings to Pam)...this seems again to be a very intersting thread.
    Just read very fast this interesting thread..asmost threads started by you guys.
    Chef, I did the same excercise a long time ago with the Bitzer software (and I think we discussed this once with DesA)
    Result of this is now that in fact, we just build a DX-GSHP for my daughter's new house where we're using this theory (as far as I'm following correctly your explanations) We assembled yesterday this heatpump and we took several pictures of it.
    We will add additional and controlled energy to the suction gasses before they enter the compressor to achieve - we hope- much higher discharge temperatures at a low HP and an increased COP due to the much higher and controlled SH. We therefore will 'sacrifice' low temperature water to gain high much higher temperature water. Perhaps the additional costs will be lost if we see once running that the theory was wrong but anyhow, we tried it at least.
    Last edited by Peter_1; 24-09-2012 at 09:29 AM.
    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: COP and Superheat - does is it help?

    I try some writting her (Im not native English - so excuse for misspellings word and grammar)

    If we take AC for cars in desert area with 2 degree C of evaporator and 70 degree C in condenser temperature no SH or SC on pipes before condenser/evaporator

    compressor assume 1 in isentropic effectivity for more easy calculation (ie. ideal situation...)

    (i know - is ugly way to put numbers)



    R134a Temperature
    C
    Pressure Mpa Enthalpy kJ/kg Entrophy kJ/kg Density kg/m^3


    without SH
    exchanger








    2 0,31 251,62 0,93 15,46

    Cold gas from evaporator

    77,04 2,12 291,04 0,93 106,5 -39,42
    work to the compressor

    70 2,12 156,14 0,54 996,25 134,9
    heat from condenser

    2 0,31 156,14 0,58 29,66

    After ex-valve
    quality 0,51551

    2 0,31 251,62 0,93 15,46 -95,48
    Cooling power from evaporator






    3,42
    Heating-COP






    -2,42
    Cooling-COP






    266,86
    kg/hour refrigerant for 10 kW heat






    17,26
    m^3 i suction volume per hour for heat






    95,87
    cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW heating






    267,87
    liter liquid refrigerant before pressure loss in TXV for 10 kW heat
    377,04 kg/hour refrigerant for 10 kW cooling
    378,46 liter refrigerant/hour for 10 kW cooling power
    24,38 m^3 i suction volume per Hour for 10 kW cooling
    135,46 cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW cooling


    and for HFO1234yf (new refrigerant for Car to replace R134a)

    HFO1234yf Temperature
    C
    Pressure Mpa Enthalpy kJ/kg Entrophy kJ/kg Density kg/m^3


    without SH
    exchanger








    2 0,34 201,99 0,74 18,83

    Cold gas from evaporator

    70 2,04 232,02 0,74 137,88 -30,03
    work to the compressor
    Q = 0,95 => steam in vapor out from compressor

    70 2,04 141,07 0,53 882,47 90,95
    heat from condenser

    2 0,34 141,07 0,57 29,86

    After ex-valve
    quality =
    0,62384

    2 0,34 201,99 0,79 18,83 -60,92
    Cooling power from evaporator






    3,03
    Heating-COP






    -2,03
    cooling-COP






    395,82
    kg/hour refrigerant for 10 kW heat






    21,02
    m^3 i suction volume per hour for heat






    116,76
    cc efficiency volume on compressor on 50 Hz (3000 rpm) for 10 kW heating






    448,54
    liter liquid refrigerant before pressure loss in TXV for 10 kW heat
    590,94 kg/hour refrigerant for 10 kW cooling
    669,64 liter refrigerant/hour for 10 kW cooling power
    31,38 m^3 i suction volume per Hour for cooling 10 kW
    174,31 cc efficiency volume on compressor on 50 Hz for 10 kW cooling



    You lose COP going from R134a to HFO1234yf and volume flow on liquid line almost doubling compare to R134a for same cooling power and system need bigger pipe with more volume inside or accept higher flow rate and compressor have almost 30% bigger pumping suction volume for same cooling power.

    If done HFO1234yf system witch suction SH with liquid line, is possibly make better COP and lower mass and volume flow (but how this work in car in hot temperature and maximum fan to cooling down compartment short after start and suction gas already hot from evaporator before SH-exchanger, this is different story)



    HFO1234yf Temperature
    C
    Pressure Mpa Enthalpy kJ/kg Entrophy kJ/kg Density kg/m^3


    with SH-
    exchanger








    2 0,34 201,99 0,74 18,83

    Cold gas from evaporator
    2(g) -> 53 (g) 0,34 252,37 0,91 14,96 50,36 heat from SH- exchanger

    53 (g) -> 110 (g) 2,04 293,37 0,91 92,37 -41
    work to the compressor

    110(g) -> 70 (liq) 2,04 141,07 0,47 882,47 152,3
    heat from condenser
    70(liq) -> 39(liq) 2,04 91,33 0.32 1048 -50,36 heat to SH- exchanger

    39(liq) -> 2 (liq/g) 0,34 141,07 0,57 29,86

    After ex-valve
    quality =
    0,317

    2 0,34 201,99 0,79 18,83 -110,66
    Cooling power from evaporator






    3,71
    Heating-COP






    -2,7
    cooling-COP






    237
    kg/hour refrigerant for 10 kW heat






    12,6
    m^3 i suction volume per hour for heat






    70,03
    cc efficiency volume on compressor on 50 Hz for 10 kW heating






    226,48
    liter liquid refrigerant before pressure loss in TXV for 10 kW heat
    325,32 kg/hour refrigerant for 10 kW cooling
    310,4 liter refrigerant/hour for 10 kW cooling power
    17,3 m^3 i suction volume per Hour for cooling 10 kW
    95,97 cc efficiency volume on compressor on 50 Hz for 10 kW cooling

    With good heating transfer SH-exchanger seem HFO1234yf working very good - better than R134a without SH-exchanger and almost all this is depend to make cooler liquid line before TXV and quality of evaporated mass going from 0.62 of liquid mass for self-cooling to only 0.317 of liquid mass and have almost double of usable liquid mass in evaporator to absorb heat.

    But i real world you have heating from compressor easily exceed maximum 110 degree C of hot gas temperature from compressor, SH-exchanger not work perfectly and we have also starting process before we have any cool suction gas from evaporator to feed SH-exchanger...


    Interesting notice how this look in using DiMetylEther (short DME) in car-using situation - this single refrigerant is very similar to R12 an R134a in pressure at same temperature

    DME Temperature C Pressure Mpa Enthalpy kJ/kg Entrophy kJ/kg Density kg/m^3


    with out SH -
    exchanger








    2 0,29 522,82 1,93 6,26

    Cold gas from evaporator

    84,87 1,81 613,14 1,93 35,6 -90,32
    work to the compressor

    70 1,81 264,72 0,91 578,47 348,42
    heat from condenser

    2 0,29 264,72 0,99 15,56

    After ex-valve
    quality =
    0,49706

    2 0,29 522,82 1,93 6,26 -258,1
    Cooling power from evaporator






    3,86
    Heat-COP






    -2,86
    Cool-COP






    103,32
    kg/hour refrigerant for 10 kW heat






    16,5
    m^3 i suction volume per hour for heat






    91,65
    cc efficiency volume on compressor on 50 Hz for 10 kW heating






    178,62
    liter liquid refrigerant before pressure loss in TXV for 10 kW heat
    139,48 kg/hour refrigerant for 10 kW cooling
    241,12 liter refrigerant/hour for 10 kW cooling power
    22,27 m^3 i suction volume per Hour for cooling 10 kW
    123,73 cc efficiency volume on compressor on 50 Hz for 10 kW cooling


    DME have high latent heat compare to most other refrigerant except ammonia, so not need so much liquid refrigeration mass to give lot of cooling power

    (i hope is not made to much mistake on all calculation and numbers above and make wrong conclusion depend of them in later text here.)

    Some compare:



    for 10 kW cooling at 2 degree C for evaporator and 70 degree C on condenser R134a HFO1234yf HFO1234yf with SH exchanger and 110 degree C hot gas from (ideal) compressor DME
    (dimetyl ether)
    mass flow kg/hour 377 591 325 139
    volume flow liq liter/hour 379 669 310 241
    cc on compressor at 50 Hz (3000 rpm) 135 174 96 123,7
    suction volume m^3/hour 24,38 31,4 17,5 22,3
    cooling-COP 2,42 2,03 2,7 2.86


    DME not winning to have SH exchanger

    DME have almost same burning heat as methanol/etanol in air - around half of energy per kilo compare burning propane/butane - Need also higher lever of leak gas blend in air before lowest explosion level compare to HC (around 3.5 percent in air, HC have 2 percent limit) and if make design right on for example car-AC with smaller pipes (going from 8 mm to 6 mm on liquid line ex.) etc. it needs only 150 - 180 gram DME refrigerant compare to standard small/medium car with 470 gram R134a load for same cooling power..

    ie. load is close to allowed maximum HC-refrigerate mass inside fridge and freezer for indoor using (max 150 gram HC) with free placing and no demand of extra ventilation.


    With HFO1234yf you need going up some size on pipes and bigger load of refrigerant to take more mass-flow and if using SH-exhanger with some usable area inside, take also lot of liquid volume. I expect around 30% more refrigerant with HFO1234yf compare to R134a for same cool-power.

    But.. last time i look in new car with new HFO1234yf filled - newer see any trace of SH-exhanger in cooling loop, evaporator seems not enforced/rugged. TXV still inside compartment area with O-rings-sealing as standard in many car, and build pretty same as ordinary R134a-system, but now we have flammable refrigerant with poison fumes if burning...

    So they either take down cooling capacity a bit or work with high mass flow and lower COP in system, so after pretty tales about carbon oxide foot-print and so on, seems in the end cheating and only using cheapest solution provided and payed from refrigerant makers in time had possibility to make serious move to non synthetic and nature and more effective existed refrigerant...

    ---

    Is very lively discussion over year if people slightest thinking to using HC as propane and isobutane-blend in car-AC - but now is flammable refrigerant is provided from 'right side' - seem now OK even if refrigerant burning make poisonous hydrofluoric gas in some percent in smoke (0.1 percent hydrofluoric gas in breathing air is deadly even in very short time as couple of seconds).

    As consumer as me want have some choice between refrigerant in car in case of burning and for me is very easy to select between car with 700 grams of HFO1234yf and giving poison smoke if burning compare 150 gram of DME for same cooling power and with burning have same smoke as over Trangia stove burning ethanol...
    Last edited by xxargs; 24-09-2012 at 12:44 AM.

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    Re: COP and Superheat - does is it help?

    OK here is the standard set of conditions to start us of.

    It is a simple freezer with just 4 components, the evaporator, the compressor, the condenser and the expansion device. There is no HX between suction/liquid at this time.

    Freezer box temperature -20C +/-0.1C
    Condensing temperature 30C and no SC
    SH out of the evaporator is 3C
    Compressor is 50% efficient at all points in the cycle (for the time being)
    All lines that need to be are insulated.
    You may assume good heat transfer coefficients in both the condenser and evaporator of 1C delta T.

    We will use R134a for this stage.

    So we need to show the COP and any increase in COP when we add a further 5C to the SH giving a total of 8C SH out of the evaporator.

    At great risk of being controversial no one is allowed to use an exolube or endocube or whatever it is called to improve efficiency.
    Last edited by Chef; 24-09-2012 at 05:15 AM. Reason: forgot the gas

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    Re: COP and Superheat - does is it help?

    xxargs - that is a lot of data you posted there but I have to ask how is it really relevant to the question in hand. We are looking for the truth of whether SH increases the COP and the I suppose a comparison of gases is critical eventually I was hoping to do that later - ie after we see whether the COP goes up or down with one or 2 gases and then see if others act differently.

    I also understand the relevent inherrant safety about differant gases and at a real life level it is important but at a conceptual level we could even choose a gas as UrFl to see what the numbers predict. I mean Uranium Flouride is not your everyday fridge techy gas and has some enviromental issues.

    If you have any data that shows how the COP improves with the SH rise it would be great. IChoose any gas you like and we can adapt the base line rules to suite.

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    Re: COP and Superheat - does is it help?

    Hi

    Pete, yes done this trick many times, very highly suited to one pass water heating systems with very low flow rates, big water temp splits and long thermal length on the heat exchanger.

    Not sure if this what Chef is discussing, as the benefits the heat side for sure.

    Chef, just a little confusion. you have cond and evap at delta T of 1C, it is not possible to have useful superheat higher than 1C, without the use of an liquid suction heat exchanger. Or is this just a typo and should read 10C delta T

    xxargs, great English spelling and grammar (I wish mine was as good, and good info as well. Keep up the work!

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    Re: COP and Superheat - does is it help?

    Mad - The whole idea to set the dT to 1C is make sure excessive SH cannot be added as seems to be the case in Coolpack.

    I do not know what the other programs predict but as DesA is in charge of the comparisons we shall see.

    Your idea of a 50C liquid to be cooled in a counterflow exchanger is a bit outside the realm of the discussion at the moment but may be relavanrt later. The same rules will apply when this is analysed I suppose but is it realistic.

    Just want to look at stuff you can but of the shelf.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Chef View Post
    Mad - The whole idea to set the dT to 1C is make sure excessive SH cannot be added as seems to be the case in Coolpack.

    I do not know what the other programs predict but as DesA is in charge of the comparisons we shall see.

    Your idea of a 50C liquid to be cooled in a counterflow exchanger is a bit outside the realm of the discussion at the moment but may be relavanrt later. The same rules will apply when this is analysed I suppose but is it realistic.

    Just want to look at stuff you can but of the shelf.
    But in coolpack, you should split the superheat between useful and non useful. So in your example you can only have 1K useful and 2K non useful. (increasing to 7K non useful)

    So increase the system superheat, then you must say it is the non useful superheat that has to increase. (exclude the S/L heat exchanger)

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    Re: COP and Superheat - does is it help?

    If I put some values in Solkane, then COP only increases with a usefull superheat (in the evaporator which is of course normal) If you add Sh along the suction line, then COP decreases fast. As long as the energy to superheat doesn't come from the room or the cooled product itself, then the COP will allways decrease. That's what my elbow is saying me.
    Last edited by Peter_1; 25-09-2012 at 06:06 AM. Reason: typo error
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    Re: COP and Superheat - does is it help?

    So taking what MAD says and no SH (usefull) is allowed outside of the evaporater I get these numbers.

    3C SH gives COP 1.776
    8C SH gives COP 1.558

    This is for a box temperature of -20C in both cases and dT is 1C.

    This was done by putting the TXV bulb at the outlet of the evaporater and setiing my dT to 1C, then as I vary the SH control I get the COP values as above so I am still seeing a reduction in COP as I increase my SH.

    Peter - you get an increase in COP using Solkane (?) so we are working at odds here or inputting different values or something is wrong?

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Chef View Post
    So taking what MAD says and no SH (usefull) is allowed outside of the evaporater I get these numbers.

    3C SH gives COP 1.776
    8C SH gives COP 1.558

    This is for a box temperature of -20C in both cases and dT is 1C.

    This was done by putting the TXV bulb at the outlet of the evaporater and setiing my dT to 1C, then as I vary the SH control I get the COP values as above so I am still seeing a reduction in COP as I increase my SH.

    Peter - you get an increase in COP using Solkane (?) so we are working at odds here or inputting different values or something is wrong?
    Without do the numbers your results look correct!

    Peter,s COP only increases if the superheat is useful, in other words energy is removed from the product or product cooled area.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by mad fridgie View Post
    Without do the numbers your results look correct!

    Peter,s COP only increases if the superheat is useful, in other words energy is removed from the product or product cooled area.
    My SH is useful as it comes from the evaporator but just because it useful in terms of taking heat away from the box/product does mean it is useful in a cycle as the COP falls.

    The more you increase the SH inthe the TXV the further down the evaporator pressure gets depressed and so COP falls off.

    My initial comment still stands - I have not seen a COP increase with SH.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by mad fridgie View Post
    Peter,s COP only increases if the superheat is useful, in other words energy is removed from the product or product cooled area.
    Isn't that the same what I said MF but in other words? COP will decreaseas long as SH doesn't comes from the room or product
    Last edited by Peter_1; 25-09-2012 at 06:04 AM.
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    Re: COP and Superheat - does is it help?

    Solcane
    with 50% compressor effficiency
    Troom = -20C, Te = -21C SH suction line is 3K -> COP 1.95
    Troom = -20C, Te = -21C SH suction line is 8K -> COP 1.91
    This seems normal although numbesr are a little different .
    Last edited by Peter_1; 25-09-2012 at 06:07 AM.
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Chef View Post
    ....
    The more you increase the SH inthe the TXV the further down the evaporator pressure gets depressed and so COP falls off.
    ..
    Chef, the SH isn't created in the TXV but at the end of the evaporator and then further not usefully outside the cold room. If it comes from the room, even from the suction line inside the room, then energy that needed to be removed from the room was removed, so this energy was useful.
    So with the energy needed to evaporate the refrigerant, we add some additional energy also taken from the room to become a total energy retracted versus needed energy. If we add some more energy,then TE will also slightly increase, TC as well but influence of Te is much bigger resulting in a COP increase.

    In fact, I guess this explanation wasn't usefull :-) for you at all and it's me not following for 100% due to a 'language-misunderstanding-barrier' from my side?
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Peter_1 View Post
    Solcane
    with 50% compressor effficiency
    Troom = -20C, Te = -21C SH suction line is 3K -> COP 1.95
    Troom = -20C, Te = -21C SH suction line is 8K -> COP 1.91
    This seems normal although numbesr are a little different .
    The SH must be taken from the Troom and at your conditions this does not seem feasible.

    So you have Troom at -20C and Te at -21C but how does Troom add 8C to the SH - it is not possible unless Troom was at -11C. My guess is that both Coolpack and Solcane (do not know this program) both allow you to vary the SH outside of practical limits.

    If you want an 8C SH from your room at Troom=-20C then Te has to be -29C. That will indeed reduce the COP.

    Your results show a drop in COP but not nearly enough for the extra 5C.

    Also I get 1.776 and you get 1.95 so a difference here as well.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Peter_1 View Post
    Chef, the SH isn't created in the TXV but at the end of the evaporator ?
    I got this and the bulb is positioned at the end of the evaporator and is set to 8C so the TXV and the bulb act together to control the SH. That is how it was input to the program. It seemed to understand.

    So if my temperature is set to -20C then the system keeps on running till the Te is -29C and the SH is 8C.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Chef View Post
    So you have Troom at -20C and Te at -21C but how does Troom add 8C to the SH - it is not possible unless Troom was at -11C. My guess is that both Coolpack and Solcane (do not know this program) both allow you to vary the SH outside of practical limits.

    If you want an 8C SH from your room at Troom=-20C then Te has to be -29C. That will indeed reduce the COP.

    Your results show a drop in COP but not nearly enough for the extra 5C.

    Also I get 1.776 and you get 1.95 so a difference here as well.
    Of course Chef, in a room at -20C, you can't add 8SH in a normal, practical way. But you can do it of course with an in-line heat exchanger p.e. But I thought you guys were more discussing theoretical values to prove that Coolpack calculates something wrong


    I also noticed the different values for the COP both program gives you. Solkane is from Solvay and can be downloaded for free at their website. It has a nice graphical interface, better than Coolpack but less features than Coolpack.
    Honeywell also made something similar and I tested it some time (in SI units) but found very strange results when calculating cycles. Seems that the authors of this program doesn't care about there mistakes although it was nice made.
    Last edited by Peter_1; 25-09-2012 at 07:36 AM.
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    Re: COP and Superheat - does is it help?

    Peter sorry for stepping on your toes! Not my intention!

    Coolpack and the like in my opinion is to used by those who understand the refrigeration process. So the issue of what the superheat is, is determined by the knowledge on the person imputing the data.

    I agree you can almost insert what ever data you want and it will calculate it.

    The areas in Coolpack that I use, do not reference the load itself, (ie room temp), so that being the case, they level of superheat useful or not, can be what ever you want, coolpack also does not give warnings about very discharge temps (even though is does calculate the temp), which is also a practical issue

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    Re: COP and Superheat - does is it help?

    MF, no you didn't step on my toes, not at all, I only have sometimes doubts if I understand correct what I read.
    Ever tried Solkane?
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    Re: COP and Superheat - does is it help?

    Honeywell link http://www.honeywell-refrigerants.co...mber_108269853 very similar to Coolpack but more graphics
    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: COP and Superheat - does is it help?

    Peter - I downloaded a copy of Solkane and it is very nice stuff, lots of respect for the authors.

    When I put the numbers in that I use it came back with exactly the same answers so very pleased with that agreement.

    So at this point I cant yet see where SH improves the COP and as MAD points out if you enter the wrong data into Coolpack it will give the wrong answer. It does not seem to check if what you are trying to do is reasonable. Solkane is also similar in it allows unrealistic SH to be added although the COP remains pretty constant. The basic reason seems to be neither consider the box temperature which in a real scenario is essential.

    The main idea of this thread is to find out if SH ever increases COP and is not intended to show up Coolpack or Solkane but as a corrollary to the thread it does appear that care should be exercised in their use.

    So my only intention is determine if SH ever improves COP and so far I have not seen any conclusive evidence.

    Now there may be many reasons why SH is a good thing but that can be looked at later.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    Right then, Chef. Please set the example/s & lets get cracking.
    Ready when you are.

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    Re: COP and Superheat - does is it help?

    Hi folks,

    Apologies for my late entry into the debate. Have been frenetically busy of late. I'll review the post above & make comments along the way. Fascinated to see how Peter's trials work out in practice.

    What Peter & Chef's (& hopefully MF's) systems will be able to provide is the following vital information:
    1. What happens to Te,sat / Tc,sat / Tc,disch when ONLY evap SH is altered?
    2. How does the METHOD of altering SH affect Te,sat / Tc,sat / Tc,disch?
    2.1 For instance, if SH is altered by trimming refrigerant mass flow?
    2.2 If SH is altered by adjusting evap fan speed?
    2.3 If SH is altered by other means?

    If Peter / Chef / MF could provide practical, experimental data, then this would be the place to begin the COP calculations.

    From my own work in my Laos lab, I noted that as SH was adjusted, that other variables in the system automatically re-set themselves, as the system balanced itself. An adjustment of one part, affects the whole system.

    I would also agree with the discussion of 'useful' SH. In other words, the SH rise has to come from useful evaporator effect ie. extraction of energy from atmosphere - NOT heat-ingress along the suction line.

    Anyway, I feel that the experimental feedback will be absolutely vital, before going into detailed calcs via Coolpack / Solkane / etc.

    [at]Peter : Pam says 'Hi'... Please send our regards to Kristien.
    Last edited by desA; 29-09-2012 at 10:57 AM.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: COP and Superheat - does is it help?

    Ah DesA - I have already posted the paramters to use for the comparisons.

    Also put up my results for the 3C and 8C SH cases and it pretty well shows the COP falls off with increases in SH.

    "Anyway, I feel that the experimental feedback will be absolutely vital, before going into detailed calcs via Coolpack / Solkane / etc."

    I think we have to get away from the experimental feedback, the complexity of a test rig to produce results reliable enough to determine COP would be an awesome rig with massive amounts of sensors. SO experimental is out and lets stick to hear say or gut feel.

    Detailed calcs in Coolpack/Solkane? How detailed can you get in those programs!

    Lets see what you come up with to support your ideas and then we can compare.

    Oh and fans on the evaporator are not allowed but changing the mass flowrate or even the charge is fine. Whatever you choose.


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    Re: COP and Superheat - does is it help?

    Chef,

    My thoughts are as follows:
    1. Performing a purely academic exercise on a fixed set of values may not be representative of what happens in real life.
    2. Obtaining feedback from the real rigs out there - Peter, MF, your systems - will allow us to set more realistic calculation regime, for comparison.

    I'm pretty eager to have more solid feedback from the real-world rigs, before we can proclaim a 'cast-in-stone' result of precisely how COP does vary, in reality. Apologies if this was not they way you wanted to go.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
    Rarefied Technologies ( SE Asia )

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    Re: COP and Superheat - does is it help?

    DesA - This is my initially posted question.

    "So far I have not been able to show this increase in COP and so ask if anyone knows of a good example/article that describes it."

    Really that is all I want and comments and points of view along the way are very illuminating, but your continued insistance on testing is unfounded. I cannot ask anyone to go to the lengths of running real life tests to satisfy such a simple question.

    Of course it is an interesting question.

    Your comment that performing acedemic calculations would be invalid without testing, seems, presumptuous at best.

    If we are not able to calculate, predict and understand theoretical results what qualifications do we have to interpret and make conclusions about test data.

    More to the point "if" it was decided to do testing to prove a theory, one must already have a premise to base the test upon, ie we must already have some insight into how the test parameters are to be set to prove the concept either way.

    Maybe you could propose such a test schedule and your reasons for such a test.

    You never know - with some some promising paramters to work to I may even update my capillary test rig.

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    Re: COP and Superheat - does is it help?

    Hi,

    "First" "Mads Rig", ah is not longer a test rig, is chunking away nicely cooling milk on a farm.

    However I understand both of your concerns, measuring a test, is only any good, if you know what your measuring against, but us in the praactical side, also know that steady state theory does not really apply to any great extent to what really happens.

    Also care has to be taken, in understanding what your require from the system. Peters example (my understanding, please correct me if I am wrong), was focusing on the heating side of the loop (even though the cooling is still important).

    Back to practical testing, it would be difficult with engineers tools and test gear to accurately see some of the changes you would hope to see. (either the machine needs to be really large, or test equipment needs to be of very high quality (My test gear was better than most ref engineers would have but not up to the equipment that full test labs have.

    I think that DesA should throw some calcs into the mix (using his expertise in heat transfer, which may have more of an influence, than just small changes is superheat) I think that could give a starting point, us dummies, then could apply some of our practical knowledge and observations.

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