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  1. #51
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    Re: COP and Superheat - does is it help?



    [at]Chef : My recommendation for linking the proposed cycle operating points to reality, is that a purely academic review (dead easy to perform) will presume certain parameters to remain constant, whereas, in reality they move as the SH 'raiser/lowerer' will affect the rest of the system. No single variable remains locked as SH is altered, in reality.

    Presuming parameters to remain constant, when in fact they change, may (will) lead to incorrect final conclusions.

    This was the reasoning behind my suggestions. No problem if this is ignored - feel free - it is your thread, after all. But, don't complain when the final 'comparisons' turn out to be implausible, in practice.


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    Re: COP and Superheat - does is it help?

    It's a pitty I never made this test rig http://www.refrigeration-engineer.co...retical+sensor
    This could have been very useful now. I found after this post someone who could program this alll and send it via ZigBee to a laptop and then project it with a beamer
    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?

    Quote Originally Posted by Peter_1 View Post
    It's a pitty I never made this test rig http://www.refrigeration-engineer.co...retical+sensor
    This could have been very useful now. I found after this post someone who could program this alll and send it via ZigBee to a laptop and then project it with a beamer
    My rig was very similar, using a SCADA, I did not have all the inputs going into it ( i am thick when comes to electronics)
    What i was lacking was high end "energy measurements" and "refrigerant mass flow devices".
    So it was excellent for see change, but not good enough to give definitive performance (estimate +/-5%)
    I would tie the figures in with the comp manufactures data.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    [at]Chef : My recommendation for linking the proposed cycle operating points to reality, is that a purely academic review (dead easy to perform) will presume certain parameters to remain constant, whereas, in reality they move as the SH 'raiser/lowerer' will affect the rest of the system. No single variable remains locked as SH is altered, in reality.

    I do not know what the acedemic reviews presume till I read them But as it is dead easy to perform we hope to see some good stuff posted

    Presuming parameters to remain constant, when in fact they change, may (will) lead to incorrect final conclusions.

    It is a bit of a circular comment, if parameters remain constant how can they change.

    This was the reasoning behind my suggestions. No problem if this is ignored - feel free - it is your thread, after all. But, don't complain when the final 'comparisons' turn out to be implausible, in practice.

    Of course your testing idea has merits, lets imagine I want to upgrade to a new gas so you would propose testing all available gases till a good one is found and ignore the massive amount of data provided by the manufacturers/suppliers.

    Or better still I have to install a split into a new build in Raynes Drive. So rather than suppliers selection information coupled to weather and solar data I have to buy a house and test various splits over the seasons to make sure I have the right unit. Seems right to me!!!!!

    Oh and I wont complain if the results are of the mark but if you can possibly make a contribution it would be appreciated.

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    Re: COP and Superheat - does is it help?

    Peter mentioned:
    It's a pitty I never made this test rig http://www.refrigeration-engineer.co...retical+sensor
    This could have been very useful now. I found after this post someone who could program this alll and send it via ZigBee to a laptop and then project it with a beamer
    [at]Peter:
    When you say 'program this all', do you mean as in generate a computer simulation?

    If so, than this may help us to establish the movement of variables with our modification of SH. This could possibly also then assist to settle the COP discussion. Not sure mind you, unless the author manages to correctly model compressor efficiency changes, evap & condensor changes, as the cycle changes.

    Would be nice to see in action.

    I had also in mind trying to locate a dynamic simulator for an hvac system. This may, however, also suffer from the same inaccuracies.

    This is where a simple circuit will be so useful. (My heat-pump lab is currently in moth-balls in Laos. Would have run some hard trials & sorted most of this out by now... )

    Peter, or MF, are you perhaps able to run some trials with your rig/system/s? It should take one day, at most, to determine what we need - at least, close enough for us to clarify Chef's dilemma.
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    Re: COP and Superheat - does is it help?

    Chef commented:
    Oh and I wont complain if the results are of the mark but if you can possibly make a contribution it would be appreciated.
    SH is never controlled directly, but indirectly. Other parameters e.g. TXV, fan speed, etc are controlled, or adjusted directly.

    Once we have established the system settling point (Te,sat; Tc,sat; Td,comp; compressor power input etc) (via tweaking of a direct variable), we will be able to compute COP & track it against SH. That will then provide some level of answer to your question. No?

    What will also be important is the adjustment methodology:
    1. Adjust controlled variable;
    2. Leave system to settle into steady-state operating point;
    3. Take readings;
    4. Repeat, as required.

    At that point, I'll be calculating away, you betcha...
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    Re: COP and Superheat - does is it help?

    I have a little program that pretends it is a fridge. A pretty simple one with the condenser at the back, a plate inside as the evaporator, a capillary tube and compressor. No HX.

    The inputs are ambient, box contents, temp setting of the thermostat and Temp cycle (on/off temps)

    Other inputs that define the fridge are more or less fixed for that model like compressor characteristics, evaporator and condenser dimensions, heat transfer coefficients, heat leakage rates etc insulation, even convection constants.

    The outputs are mainly time based and position based. So with a stable and cycling fridge turning on and off the results show, for instance, box temp v. time or any system variable v. time but more importantly and what I really want is the refrigerant mass in each component.

    So as the fridge cycles on and off one sees a nice drop in box temp and in the off cycle a slower rise as heat is lost. Now a snap shot every minute (or whatever) shows where the refrigerant is located and how it migrates around the system during cooldown and warm up.

    By changing the thermostats temp cycle for instance from 5C down to 1C shows an increase in the number of cycles and all is as one would expect. At some point the cycle is so short that pressure equalisation cannot be achieved and so starting loads become large.

    Throw in 4 litres of warm water and cycle times now extend (after a longer initial cooldown period)

    Now in changing system charge, or some of the fridges components sizes and especially the cap tube the SH can be varied and the response of the whole system can be studied, BUT many sources suggest more SH increases the COP and so far all I have seen is a decrease in COP.

    So if it is true that COP falls as SH rises it helps to have more confidence in the programs results, but if that assumption is wrong then I need to find out where the problem lies and fix it.

    Hence my question

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    Re: COP and Superheat - does is it help?

    Thanks Chef for a very, very interesting review of your fridge simulator.

    If I may ask, does your simulator determine an evap SH / Te,sat / Tc,sat / Tcomp,disch?

    If memory serves correctly, you are something of a guru on setting up pretty realistic simulators using EES platform.

    I could possibly assist on the evap heat-transfer (condenser, as well), if this would assist in modeliing the Te,sat / SH characteristics of the evap. Once these are in place, then the simulator may be able to give us even deeper insights into the expected system balance points.

    I'd say that publishing of the various simulated system parameters for critique, would allow a few folks to assist towards a better understanding of what is really going on. We'd all benefit from the experience, I'm sure.
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    Re: COP and Superheat - does is it help?

    I would tend to look at this another way.
    Imagine the rig, is steady state, -20C box temp, Te -26C sh 6K (100% useful, no more useful available)
    So it must be that a part of the evap is only used superheating.
    So this then brings in DesA (you chose a evap), what would be the new Te if the SH was only 1K "the net cooling load remain constant"
    Ignore other system changes at this point.
    I think this will prove the practical point about SH

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    Re: COP and Superheat - does is it help?

    Yes it does Te, SH, Tc Tcompdisch etc plus a few more.

    Input and output variables include, Uc, dTc, ambient at condenser, convection constant, condenser dimensions, Qc, dPc, SC, RMc (refrigeration mass cond) heat transfer coeff air side.


    Evap variables are similar and also include velocity versus evap position, boiling scheme, Ue V. position, dPe V. position, Qe V. position, RMe V. position ......etc

    Other inputs include pressure drops in suction line, valves in comp, discharge line. Line dP can either be input or calculated from system conditions and pipe dimensions and K factors for fittings.

    Comp efficiency V. Te/Tc and SH

    Cap tube length, diameter, coil size and number of coils, Isenthalpic or adiabatic expansion, Fanno recovery efficiency, # of tubes, K Epsilon vorticity, kinematic relaxation, . Too much output data to list here really.

    System wide variables include insulation heat transfer properties, fridge size, thermal mass, box contents and thermal mass and heat transfer properties.....etc

    It is a pretty long list really and not sure it will help discover a solution to the SH question.

    MAD - With a box of -20C and SH=1C all available used I get Te=-21C but the Qe goes up and so I cant keep the load consatant or the same as with Te=-26
    Also have to change the cap tube or it will not even allow that to happen.

    Here is a fun plot of the fridge inside temperature distribution, plate is at the top of course and blue is cold.

    Fridge temp dist.JPG

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    Re: COP and Superheat - does is it help?

    Very cute.

    I would suspect that, in order to answer your original question/s, & check your simulator realism, you'd be better off building a test fridge & comparing variable responses... Anything else may end up being pure speculation - honestly.
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    Re: COP and Superheat - does is it help?

    DesA - I believe this is an important topic and deserves a proper hearing.

    There appear to be many engineers looking for a resolution on this topic.

    Please do not keep repeating an old mantra that has been shown to be outside the scope of a discussion thread.

    Also there seem to be outstanding questions for you to answer that may go along the right track to progress the answer.

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    Re: COP and Superheat - does is it help?

    Hi Chef

    you certainly pick interesting topics, beyond me

    but from my limited understanding without the calcs

    the lower the superheat the higher the cop

    as mass flow through the compressor increases with lower superheat

    the condenser does less de-superheating, resulting in more subcooling

    resulting in more cooling effect through the evaporator

    gonna be a long learning curve rewiring my brain if you's now say thats not the case ?

    R's chillerman
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    Re: COP and Superheat - does is it help?

    Chillerman - lower SH and higher COP is what I see as well so not trying to get you to relearn.

    Increased mass flow at lower SH and less desuperheating are two good points. Also there may be increases in compressor efficiency and steeper slopes on the specific entropy curves adding to the mix.

    You also mention more SC also adding in there.

    There may also be a reduction in pressure losses but I would have to study that more but from intial thoughts dP will fall as SH falls because dP is a function of velocity squared compared to just density.

    So we have at the moment
    • Increase in mass flow
    • drop in desuperheating
    • increase in compressor efficiency
    • decrease in specific entropy
    • increase in SC
    • Decrease in pressure drops



    Nice one Chillerman, may be fun to which one is more influential

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    Re: COP and Superheat - does is it help?

    Here is a snip from a paper that makes it even more complex, it seems it can decrease or increase or not affect the COP.

    SH comment.JPG

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    Re: COP and Superheat - does is it help?

    Chef:
    DesA - I believe this is an important topic and deserves a proper hearing.
    There appear to be many engineers looking for a resolution on this topic.
    Agreed.

    Please do not keep repeating an old mantra that has been shown to be outside the scope of a discussion thread.
    I suspect that we'll - with respect - have to agree to disagree.

    Also there seem to be outstanding questions for you to answer that may go along the right track to progress the answer.
    There certainly are a lot.

    What I'd like to suggest first off is that you reveal a lot more of the software platform you are developing. This can then form the substantive basis for discussion & comparison, in the absence of a robust comparative test platform.

    What will also be of major interest in the 'why' of high superheat leading to increased COP is precisely how the condenser operates & how heat is extracted. In this regard, MF can contribute an immense amount of experience, based on his earlier heat-pump projects.

    In other words, in raising evap SH - if it leads to 'increased' Tc,sat , will almost certainly have a COP reduction impact. If raising SH, allows a technology 'tweak' to be used such that Tc,sat is lowered, then COP rises.

    In order for your simulator to reflect this, you will need to model the condenser in split form.
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Chef View Post
    Here is a snip from a paper that makes it even more complex, it seems it can decrease or increase or not affect the COP.

    SH comment.JPG
    I suspect that this may be due to the enthalpy (h) response with pressure - more accurately as enthalpy difference response with pressure. The effect at low P (evap side), may be different to the high P (condenser side), affecting COP response - as the cycle migrates over its range of operation.

    Interesting perspective...
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    Re: COP and Superheat - does is it help?

    DesA - here are the outstanding questions from previous posts that would help, if we keep leap frogging and missing stuff out we will lose vital comments and information.

    Quote Originally Posted by mad fridgie View Post
    I would tend to look at this another way.
    Imagine the rig, is steady state, -20C box temp, Te -26C sh 6K (100% useful, no more useful available)
    So it must be that a part of the evap is only used superheating.
    So this then brings in DesA (you chose a evap), what would be the new Te if the SH was only 1K "the net cooling load remain constant"
    Ignore other system changes at this point.
    I think this will prove the practical point about SH
    Quote Originally Posted by desA View Post
    [at]Chef : My recommendation for linking the proposed cycle operating points to reality, is that a purely academic review (dead easy to perform) will presume certain parameters to remain constant, whereas, in reality they move as the SH 'raiser/lowerer' will affect the rest of the system. No single variable remains locked as SH is altered, in reality.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    I suspect that this may be due to the enthalpy (h) response with pressure - more accurately as enthalpy difference response with pressure. The effect at low P (evap side), may be different to the high P (condenser side), affecting COP response - as the cycle migrates over its range of operation.

    Interesting perspective...
    I dont really understand what you are saying here, maybe you could rephrase it so we can understand what the article is alluding to.

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    Re: COP and Superheat - does is it help?

    I better clear up the evap proposal.
    as an example the box requires 500w of cooling (steady state), this being achieved with an evap (simple) which is running at -26C and 6K SH.
    We could say that 20% (as an example) of the surface area of the evap is being used for superheating. (in my opinion poorer heat transfer)
    So if we reduce the superheat to 1K, then for example only 4% of the surface area is used for superheating. So we now an additional 16% which is wetted and has better heat transfer.
    So to maintain our 500w steady state will the evap temperature in increase to reach the 500w equalibrium.
    (at this point I have excluded all other parts of the system) So for example the evap temps goes from -26C to -24C (no clacs done)
    Now lets look at the rest of the system, to ensure the equalibrium of 500 watts and the new Te, then the system mass flow would reduce (by means of reduction in comp swept volume) Do we know get a better efficiency?
    Or if the system remain the same, would we end up with a new equalibrium point, lets say Te being -25C, but we now have a net cooling duty of 520w.
    What would these figures do to the system COP.
    So it all comes down to what effect SH has on an existing evap performance. Then we can determine if SH on an energy level is beneficial or detrimental.
    In depth knowledge on heat transfer coefficients, is not my field. (a little knowledge may be dangerous???)

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    Re: COP and Superheat - does is it help?

    I'm not sure I understand the dilemma.

    If the load is held steady, then the only way the SH at the evaporator outlet can be increased is to reduce the mass refrigerant flow... thus dropping the saturation temperature and reducing the COP.

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    Re: COP and Superheat - does is it help?

    According to the TECHNISOLVE program, as increase the superheat decrease the cop except r12.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by moideen View Post
    According to the TECHNISOLVE program, as increase the superheat decrease the cop except r12.
    Thankyou moideen, a result, what kind of result do you get for R12.

    I do not know TECHNISOLVE so by how much and a few conditions.

    So far it seems that COP goes down as SH is increased but ther are are also some exceptions. This is exactaly what we are looking for so thanks.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by mad fridgie View Post
    I better clear up the evap proposal.
    as an example the box requires 500w of cooling (steady state), this being achieved with an evap (simple) which is running at -26C and 6K SH.
    We could say that 20% (as an example) of the surface area of the evap is being used for superheating. (in my opinion poorer heat transfer)
    So if we reduce the superheat to 1K, then for example only 4% of the surface area is used for superheating. So we now an additional 16% which is wetted and has better heat transfer.
    So to maintain our 500w steady state will the evap temperature in increase to reach the 500w equalibrium.
    (at this point I have excluded all other parts of the system) So for example the evap temps goes from -26C to -24C (no clacs done)
    Now lets look at the rest of the system, to ensure the equalibrium of 500 watts and the new Te, then the system mass flow would reduce (by means of reduction in comp swept volume) Do we know get a better efficiency?
    Or if the system remain the same, would we end up with a new equalibrium point, lets say Te being -25C, but we now have a net cooling duty of 520w.
    What would these figures do to the system COP.
    So it all comes down to what effect SH has on an existing evap performance. Then we can determine if SH on an energy level is beneficial or detrimental.
    In depth knowledge on heat transfer coefficients, is not my field. (a little knowledge may be dangerous???)

    Very clever MAD but having a variable speed compressor does not really fit the original specs for the problem.

    OK it was not specifically excluded but it makes it more complex and how are we going to get peer results from other contemporary programs, non of them have variable speed compressors as the input.

    I dont know if that can be included - but if it does indeed show a COP increase it would be interesting.

    Have you got any more results that show this is possible?

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    What I'd like to suggest first off is that you reveal a lot more of the software platform you are developing. This can then form the substantive basis for discussion & comparison, in the absence of a robust comparative test platform.

    If you are conversent with macro, finite element methods, finite difference time slices and Newton Raphson derivatives and second order approxamations for N+1 setups it may help.
    Eulerian flows in tetrahedral elements would be even beter.

    The program is written to discover where the refrigerant mass is located in a cycle and how it moves plus to show efficiency with varying charge, cap tube dimensions etc.

    My results so far show COP always decreases with SH increasing which is not in line with some published data and so this is the question. Has anyone got any evidence COP increases with SH.


    What will also be of major interest in the 'why' of high superheat leading to increased COP is precisely how the condenser operates & how heat is extracted. In this regard, MF can contribute an immense amount of experience, based on his earlier heat-pump projects.

    You have lost me there, the bit about how the condenser operates? Please advise on this one.

    In other words, in raising evap SH - if it leads to 'increased' Tc,sat , will almost certainly have a COP reduction impact. If raising SH, allows a technology 'tweak' to be used such that Tc,sat is lowered, then COP rises.

    This is a 4 component fridge in its simplest form, no technology tweaks allowed, but if you are saying raising the SH can raise the COP - as you state above - what is the basis for this and some reference would be nice.

    In order for your simulator to reflect this, you will need to model the condenser in split form.

    What is a split form condenser???????????
    Seems you have a way to raise COP with increased SH - sounds interesting.

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    Re: COP and Superheat - does is it help?

    Chef:
    DesA - here are the outstanding questions from previous posts that would help, if we keep leap frogging and missing stuff out we will lose vital comments and information.
    ???
    Last edited by desA; 10-10-2012 at 08:18 AM.
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Chef View Post
    Thankyou moideen, a result, what kind of result do you get for R12.

    I do not know TECHNISOLVE so by how much and a few conditions.

    So far it seems that COP goes down as SH is increased but ther are are also some exceptions. This is exactaly what we are looking for so thanks.
    dear chef,

    find the attachment.in the technisolve program using r12 the cop increase slightly as increase the SH.
    Attached Files Attached Files

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    Re: COP and Superheat - does is it help?

    DesA:
    What I'd like to suggest first off is that you reveal a lot more of the software platform you are developing. This can then form the substantive basis for discussion & comparison, in the absence of a robust comparative test platform.
    Chef:
    If you are conversent with macro, finite element methods, finite difference time slices and Newton Raphson derivatives and second order approxamations for N+1 setups it may help.
    Eulerian flows in tetrahedral elements would be even beter.
    One sincerely doubts that your simulation would require the use of Eulerian flows modeled on a tetrahedral mesh. FE/FD methods are well known - as is Newton Raphson convergence techniques. (Not always the most stable, mind you - fairly dated by now). That is, unless you're attempting to accurately model two phase flow evaporation/condensation regimes. You may want to save some level of energy, by reading up varous legacy books on the topic (e.g Collier/Thome & Thomes' later works).

    I suspect that you're attempting to show off a tad... gotcha...

    Back onto topic, then, dear fellow.

    Chef:
    The program is written to discover where the refrigerant mass is located in a cycle and how it moves plus to show efficiency with varying charge, cap tube dimensions etc.
    Useful. Try experimenting with RefSim (Technisolve). May prove to be a useful back-check. This software will balance the system temps, as various process parameters are adjusted. Even allows real compressors - performance tables & all, to be used. There are ways to tweak the parameters further, in order to study what you are looking for. Allows TXVs.
    Last edited by desA; 10-10-2012 at 08:33 AM.
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    Re: COP and Superheat - does is it help?

    Handbags at dawn????????

    But back on the subject.

    We have to be careful that we are using the superheat for. As Des has stated increased suction superheat can improve the benefits for a heat pump application, where the heat side becomes the priority, but my understanding of the original question was related to the cooling side, but in either case w change on aspect of the cycle then it follows that all other areas change as well. So it still my belief it may not be the software that wrong, but how any data is in putted.
    Also we have to consider why require superheat (normally to stop chance of liquid flood back and/or compressor oil dilution, so should the superheat that is required, be useful "direct", non useful (energy directly picked up from out side the refrigerate envelope, or just a pressure drop.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by chillerman2006 View Post
    Hi Chef

    you certainly pick interesting topics, beyond me

    but from my limited understanding without the calcs

    the lower the superheat the higher the cop

    as mass flow through the compressor increases with lower superheat

    the condenser does less de-superheating, resulting in more subcooling

    resulting in more cooling effect through the evaporator

    gonna be a long learning curve rewiring my brain if you's now say thats not the case ?

    R's chillerman
    Glad you jumped in before me C.M.

    Read the comments. "Chef" continues to ask: is there proof that increased (useful or not) superheat increases C.O.P? I don't qualify for this level of discussion however, any amount of superheat, say 15K on a cooling cycle cannot increase C.O.P. as there is less duty performed by the volume of saturated refrigerant in the evaporator to start with. Increase S.H = less work done and longer running cycles. Negates C.O.P. During any duty cycle.

    Maybe you are thinking, if the increased discharge temperature is raised with higher superheat, then the transfer of heat through the condenser is maximised, increasing performance. (Oversized condenser)

    If one were to use increased superheat in a reverse cycle situation, then my answer is yes.

    Just my humble response guys.
    Last edited by mikeref; 10-10-2012 at 10:00 AM. Reason: Getting carried away with too much tech!! Another rum thanks..
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    Re: COP and Superheat - does is it help?

    Handbags at dawn????????
    Slide-rules...

    MF:
    So it still my belief it may not be the software that wrong, but how any data is in putted.
    Agreed.
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by mikeref View Post
    Glad you jumped in before me C.M.

    Read the comments. "Chef" continues to ask: is there proof that increased (useful or not) superheat increases C.O.P? I don't qualify for this level of discussion however, any amount of superheat, say 15K on a cooling cycle cannot increase C.O.P. as there is less duty performed by the volume of saturated refrigerant in the evaporator to start with. Increase S.H = less work done and longer running cycles. Negates C.O.P. During any duty cycle.

    Maybe you are thinking, if the increased discharge temperature is raised with higher superheat, then the transfer of heat through the condenser is maximised, increasing performance. (Oversized condenser)

    If one were to use increased superheat in a reverse cycle situation, then my answer is yes.

    Just my humble response guys.
    The increased discharge temp will increase the amount of heat rejected on the de-superheating part of the cond, but when phase change is occurring "condensing in this case" greater energy is transferred, but saying that at what point is increase temp better than phase change.

    Get carried away, makes us all think!!!!!!!!!

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    Re: COP and Superheat - does is it help?

    MF:
    at what point is increase temp better than phase change.
    This will be a function of the type of heat-exchanger surface. For instance, a plate unit will respond differently to a tube-in-tube, or shell-&-tube unit.
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    Re: COP and Superheat - does is it help?

    Are we talking about evaporator outlet SH?... or compressor inlet SH?

    It makes a big difference.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    This will be a function of the type of heat-exchanger surface. For instance, a plate unit will respond differently to a tube-in-tube, or shell-&-tube unit.
    Agree, when comes down to the specific of heat exchangers, more knowledge is required than what i have.
    This why i though you could do a small simulation on an evap, and see how the SH effects the performance.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Gary View Post
    Are we talking about evaporator outlet SH?... or compressor inlet SH?

    It makes a big difference.
    Agree.

    I believe, chef is talking about evap superheat, but increasing evap SH will increase discharge SH, and is there then a benefit to the system as a whole.
    If we have a fixed simple system any change within the system must also effect to some degree the rest of the system.
    One thing that is also confusing, if you look at the nominal performance of a compressor (not design detail), the rating always shows high suction superheat, 100% useful, which in many cases is outside the working envelope and there is no practical way of getting this SH as useful.

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    Re: COP and Superheat - does is it help?

    As I see it, if we raise SH by lowering the saturation temp (reduced mass flow), there will be a reduction in the COP. The discharge heat is just sensible heat.

    If there is increased load, there will be a higher saturation temp, higher SH... and higher COP.

    If the load is held stable and the evap outlet SH is held stable, but we add heat to the suction line... then the added heat is a bonus and the COP will rise.

    The answer to the question is... maybe.

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    Re: COP and Superheat - does is it help?

    Outside the working envelope? In discussing COP, the entire low side is the working envelope.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by desA View Post
    One sincerely doubts that your simulation would require the use of Eulerian flows modeled on a tetrahedral mesh. FE/FD methods are well known - as is Newton Raphson convergence techniques. (Not always the most stable, mind you - fairly dated by now). That is, unless you're attempting to accurately model two phase flow evaporation/condensation regimes. You may want to save some level of energy, by reading up varous legacy books on the topic (e.g Collier/Thome & Thomes' later works).
    You asked me to reveal a lot more about the software platform - so I did.

    In a coiled capillary tube one cannot use cubic meshes (they dont go around bends) and so tetrahedral is the best choice. For each mesh one solves the conservation of mass plus the momentum equations. Now add to that the energy equation and Oh gosh, bless my soul, it is called the Eulerian formulation.

    And strangely enough as Eulerian maths involves derivatives the Newton Raphson is the recommended first approach to a solution. And this is exactly what I do in my cap tube and evaporator.

    Come on DesA - any first year student would know that. That would of course be in an engineering discipline and not psychotic behaviour

    So if you ask a question - read and respect the answer.

    So now that you are done throwing your toys out of the pram maybe, as you say old boy, we can back to the task at hand.

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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Gary View Post
    If the load is held stable and the evap outlet SH is held stable, but we add heat to the suction line... then the added heat is a bonus and the COP will rise.

    The answer to the question is... maybe.
    So if the added heat to the suction line can be described as SH you saying the COP will rise. Where do you get the heat from.

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    Re: COP and Superheat - does is it help?

    desA
    One sincerely doubts that your simulation would require the use of Eulerian flows modeled on a tetrahedral mesh. FE/FD methods are well known - as is Newton Raphson convergence techniques. (Not always the most stable, mind you - fairly dated by now). That is, unless you're attempting to accurately model two phase flow evaporation/condensation regimes. You may want to save some level of energy, by reading up varous legacy books on the topic (e.g Collier/Thome & Thomes' later works).
    Chef
    You asked me to reveal a lot more about the software platform - so I did.

    In a coiled capillary tube one cannot use cubic meshes (they dont go around bends) and so tetrahedral is the best choice. For each mesh one solves the conservation of mass plus the momentum equations. Now add to that the energy equation and Oh gosh, bless my soul, it is called the Eulerian formulation.

    And strangely enough as Eulerian maths involves derivatives the Newton Raphson is the recommended first approach to a solution. And this is exactly what I do in my cap tube and evaporator.

    Come on DesA - any first year student would know that. That would of course be in an engineering discipline and not psychotic behaviour

    So if you ask a question - read and respect the answer.

    So now that you are done throwing your toys out of the pram maybe, as you say old boy, we can back to the task at hand.
    Dear Chef,

    Many cap tube simulators will tend to use a 1D model. Refer here to the Indian lecture notes you mentioned some years ago. With the length-to-diameter ratio for a cap tube being so large, the radial/centrifical effects would/could generally be neglected. The numeric scheme for such 1D model could use a number of discretisation approaches - some more stable than others - 1st order, 2nd order, etc. A description of one approach is offered on the Technisolve website - that program offers 3 different solutions to the cap tube length. Without being able to dissect your methodology, I would have to take your word that you have a new, novel approach to solving the matter.

    The reference to Eulerian flows & tetrahedral meshes would, in most engineering academic circles, infer the use of a CFD approach to problem solution (FV/FEM/FD...). Given the current lack of theoretical knowledge in the two-phase area, using such an elaborate approach would have been both overkill, & in all likelihood, very inaccurate. Still a lot to be learned in this area. Packages like Elmer may be of use to you here. The name Euler has been attached to many, many areas of maths/science.

    I will stand my ground academically against you, as I've been in the simulation & heat-transfer arena all my working career. I am prepared to argue all the way through defence of the Navier Stokes & Energy equation sets, if needs be. This is my stock-in-trade, if you will.

    My preferred approach, however, is to engage in a healthy debate, via this forum & the wonderfully helpful contributors & colleagues. We can all stand to learn a huge amount from each other, even if we don't each, individually have each & every answer. The collective approach is both fun & informative - we all learn a huge amount. In many aspects of this business, much can be said about it being part art-form, experience-based, & academic.

    On a public-domain web-forum, the 'shoot the messenger' approach, generally leads to tears & discomfort for the other forum members reading the 'kill-shots' & 'return-shots'. Perhaps you would, in future, be so kind as to take this kind of attack offline via PM? I'd certainly be most happy to discuss further with you off-line.

    Should we agree to disagree & get back to the topic at hand? I'll bet that we are both spoiling the thread by our bison head-butting approach.

    Take care,
    desA
    Last edited by desA; 11-10-2012 at 09:15 AM.
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    Re: COP and Superheat - does is it help?

    Gary:
    As I see it, if we raise SH by lowering the saturation temp (reduced mass flow), there will be a reduction in the COP. The discharge heat is just sensible heat.
    Makes sense - if Tc,sat remains substantially the same. Does it?

    Gary:
    If there is increased load, there will be a higher saturation temp, higher SH... and higher COP.
    Please expand on this. How is the load increased, for instance? Fan speed increase? More meat in the freezer?

    Gary:
    If the load is held stable and the evap outlet SH is held stable, but we add heat to the suction line... then the added heat is a bonus and the COP will rise.
    Can we show this, in practice? What happens to the compressor operation as the suction line flow is heated - reduced density - higher volumetric flowrate - higher flow velocity - higher line pressure drop...?
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by Gary View Post
    Outside the working envelope? In discussing COP, the entire low side is the working envelope.
    No it is not, all compressors have an operating range which includes maximum suction superheat temps, so adding superheat useful/non useful has its practical limits. Even though there is quite a bit of theory, there still has to be some level of practical application

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    Re: COP and Superheat - does is it help?

    RefSim feedback

    This process simulator allows a designer to tweak various system parameters & see how others change. I've found it very useful over the years.

    RefSim package relies on the evap program DXC, which fixes SH at a normative value of 7.5K. It cannot be adjusted further.

    There is no way to adjust suction line heat addition, or removal. I was hoping to coerce RefSim via this route. Didn't work.

    RefSim can export to a Mollier chart program, with a snapshot of the last system balance. In Mollier, the SH can be adjusted, See an earlier reference in post #77. The problem with this is that Mollier takes, as given, the values of Te,sat / Tc,sat / SH / SC - it does not re-balance via RefSim to compute the new system balance point - if SH varied within Mollier itself.

    Basically, I expect Te,sat to lower as SH is raised off a fixed operating condition. For the condenser, I expect the increased SH to raise Tcomp,disch. For single condenser, this would then take up a little more of the condenser surface area with de-superheating, displacing some condensing area. With reduced condensing area, Tc,sat would also rise a little to compensate. This assumes that the reduction in refrigerant mass-flow around the system (with increased SH achieved by throttling TXV a tad), does not auto-compensate in the condenser, so re-balancing available condensing area.

    Haven't thought through the compressor efficiency implications yet.

    So, if my logic is correct, with unchanged compressor efficiency, a rise in SH leads to Te,sat reduction / Tc,sat rise / COP reduction. (Please holler folks if I'm missed something obvious).

    COP reduction can be compensated for in the condenser by oversizing the surface area, forcing a reduction in Tc,sat. Alternatively, use a de-superheater/condenser combination.

    These logic twists, assumptions & guessing, is why I honestly advocate performing some runs on a real system.
    Last edited by desA; 11-10-2012 at 02:23 PM.
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    Re: COP and Superheat - does is it help?

    I do not know of a design program that does circular reference calculations.

    So your example is good and what you would expect, where manual determination of steady state points is the only method of calculation.

    The problem with testing is that the rig has to be big enough that low grade test equipment can see the difference or the requirement for high end test equipment/chamber (calorific meters and so on)

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    Re: COP and Superheat - does is it help?

    DesA wrote
    Basically, I expect Te,sat to lower as SH is raised off a fixed operating condition. For the condenser, I expect the increased SH to raise Tcomp,disch. For single condenser, this would then take up a little more of the condenser surface area with de-superheating, displacing some condensing area. With reduced condensing area, Tc,sat would also rise a little to compensate. This assumes that the reduction in refrigerant mass-flow around the system (with increased SH achieved by throttling TXV a tad), does not auto-compensate in the condenser, so re-balancing available condensing area.


    At the moment the general concensus of views seem to be along these lines. But it is probably the most promising area to try to show if there are (strange) conditions where SH can increase COP. If for instance the condenser did compensate more in improving COP than the evaporator/suction reduces the COP for a given rise in SH.

    The design of such a condensor may be implausible but for the discussion imagine a condensor operating with a very large dT - say 40C and now the SH is increased, Te drops and mass flow drops, now Tc falls (maybe if the reduction in mass flow has a greater effect than increase in disch temp) and more than compensates for the SH increase. It also sounds like it would be unstable but a line worth thinking about.

    Also your synopsis of the solution methods for cap tubes is noted and the two phase flow part is of course key and it seems the method I have adopted is working just fine. However as stimulating this topic is may be it should be addressed elsewhere.

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    Re: COP and Superheat - does is it help?

    In the condenser, I believe that the main variables to investigate for each gas type would be the latent heat & specific heat. The relative values of these could/would play a role in how the condenser surface reacts to changing Tcomp,d (via raised SH) & changing refrigerant mass flow.

    In other words specific heat would affect the de-superheating portion & latent heat the condensing portion. For now I've neglected the sub-cooling impact.

    As mentioned earlier, the type of condenser e.g. plate, shell-&-tube, tube-in-tube - will also react slightly differently to the de-superheating/latent heat loads.

    The thing to watch when adding condenser surface is not to force a 'temperature pinch' where the secondary cooling fluid (water, air etc) temperature somewhere inside the condenser reaches the same temperature as the refrigerant Te,sat. At that point, the condenser dynamics change & Tc,sat will literally jump upwards - losing effective high-side control (very, very non-linear). I've seen this with compact plate condensers.
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    Re: COP and Superheat - does is it help?

    Quote Originally Posted by mad fridgie View Post
    The increased discharge temp will increase the amount of heat rejected on the de-superheating part of the cond, but when phase change is occurring "condensing in this case" greater energy is transferred, but saying that at what point is increase temp better than phase change.

    Get carried away, makes us all think!!!!!!!!!
    There is a limit to how high discharge temperature can go before gas, oil and compressor break down. That much is obvious so enter liquid injection. Looking at volumetric efficiency and preserving the operating system with the trade off being less work done by the evaporator, but regulated injection is able to make better use of compressor generated heat than what could be obtained from the evaporator..

    Counter the loss of refrigerant going to the evaporator with an increase in refrigerant quantity. More refrigerant circulating means more Phase change.
    Regulate the head pressure by bleeding off high pressure vapour,(from a receiver) into the suction line without raising suction, or lowering head pressure significantly..
    Lower head, lower current draw, increased superheat in suction line, higher C.O.P...
    Everyone is happy. Please, pick my theory to pieces but don't ask me to do the maths.
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    Re: COP and Superheat - does is it help?

    Mikeref:
    There is a limit to how high discharge temperature can go before gas, oil and compressor break down. That much is obvious so enter liquid injection.
    Liquid injection into compressor suction line, or compressor where liquid injected internally?
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    Re: COP and Superheat - does is it help?

    A few useful simulation links are listed below. If anyone has links to other useful refrigeration simulators, where the Te,sat / Tc,sat variables are derived, as various process parameters are manipulated, these would be most useful.

    RefSim explained

    http://www.coolit.co.za/refsim/index.htm
    http://www.coolit.co.za/refsim/artic...sim/refsim.htm

    A Maple approach
    http://www.engr.mun.ca/~yuri/Courses...rigeration.pdf
    Last edited by desA; 12-10-2012 at 04:52 PM.
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