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  1. #51
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    Re: Evap sizing tools



    DesA,

    You're simplifying what would happen too much. In the event of a failure the refrigerant would first dissipate into the water side of the HX, then have to travel through the pipework out of the heatpump, then through any pipework linking the HP to the UFH.
    The gas in the HP is only at 20bar because of the area it is confined in. Increase the area and the pressure will drop very quickly.
    If i was to bring the water in a system containing 3-400 litres. How man litres would I need to introduce to bring that to 10 bar? The same principle applies to the gas. Then you have air valves and pressure relief valves which are all points of escape.

    I'm not disagreeing that there is potential for failure. I'm merely pointing out that failure wouldn't be as disasterous as you imagine. It would be quite harmless actually.



  2. #52
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    Re: Evap sizing tools

    In the case of such a failure, assuming the pressure went into the water, which is hydraulically connected to the tubing, the first line of defense would be compressing any entrained air (assuming there is any) then the expansion tank, which would probably burst a bladder if it was too small but with acceptance volumes of 25-50L and the sudden condensing of the gasses, I think Bigfreeze is right and the most that would happen outside the system would be a PRV going off.

    Des, I just downloaded Coolpack and am starting the tutorials (I learn better hands on). In your diagrams, what isentropic efficiency did you use for the compressor? Given is 70% but I know this changes for different units and refrigerants. I think i am getting a better understanding of the issues but, man there is so much to learn.

  3. #53
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    Re: Evap sizing tools

    65% is a reasonable number for isentropic efficiency. Will vary depending on position in the heat-up phase.

    I'm not buying the pressure-diffusing arguments at all - doesn't follow the laws of physics, as far as I'm aware. Never know though, with today's 'new physics'.

    Ever seen a geyser go bang? (The roof-mounted geysers blow the whole jolly roof to bits).
    Last edited by desA; 27-12-2010 at 03:58 PM.
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  4. #54
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    Re: Evap sizing tools

    Quote Originally Posted by desA View Post
    65% is a reasonable number for isentropic efficiency. Will vary depending on position in the heat-up phase.

    I'm not buying the pressure-diffusing arguments at all - doesn't follow the laws of physics, as far as I'm aware. Never know though, with today's 'new physics'.

    Ever seen a geyser go bang? (The roof-mounted geysers blow the whole jolly roof to bits).
    Which law of physics would be broken in the scenario I just mentioned?
    Are you telling me that 4 or 5kg of refrigerant will be unloaded instantaneously into the system?
    Put your gauge line on a running system and crack the valve. What would happen? The answer, besides a cloud of gas, is not much.
    Put that same gauge line straight onto a bottle and crack the valve. Again, not much happens, no explosions, no dramatics.
    This isn't die hard, things don't go flying through the air anymore than a guy goes flying through the air if he gets shot.

    Every action has an equal an opposite reaction does not mean that said reaction must manifest itself in as dramatic a manner as the first.

  5. #55
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    Re: Evap sizing tools

    gasses are different than liquids for sure. I have absolutely no issue working with liquids at high temps and pressures and I have seen the results of gas water tanks with faulty controls AND faulty pressure relief valves. Hole in the building roof can result!!!!!
    Gases are a bit different and more explosive than water which more often than not, just leaks, but you have a limited volume to work with and the water acts as a big cushion to spread out and equalize the pressure.

    Regarding the efficiency, if it is around 65% peak, what would the average reduction be at the limits? 45%, 35%? I assume this will be reflected in the COP and I may have answered my own question?

  6. #56
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    Re: Evap sizing tools

    As far as isentropic efficiency goes, you may want to consult the various compressor manufacturer's tables, for the type of unit you use.

    Gas ruptures can be incredibly nasty - especially if sudden. Anyway, we digress.
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  7. #57
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    Re: Evap sizing tools

    Are discharge pressures of 20-30bar unusual? I cannot see pressures above that being healthy. Also, I assume the p2-p1 is dependent on the compressor and the temps at the time but are there accepted min/max values?

    Just soaking it up and learning. I appreciate all the chat.

  8. #58
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    Re: Evap sizing tools



    R410A.



    R134a.

    Which one would you consider to be the safer of the two options? For operating condition & off condition (settling pressure).
    Last edited by desA; 27-12-2010 at 06:38 PM.
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  9. #59
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    Re: Evap sizing tools

    the r134 appears better due to the ratio in pressures but does the compressor not like a higher ratio to be closer to full loading? I may have this concept wrong.

  10. #60
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    Re: Evap sizing tools

    actually by pressure ratio only the 410 looks better but again, I am not sure if the high pressure value is an issue for 410. COP is slightly better for 134

  11. #61
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    Re: Evap sizing tools

    I am playing with the coolpack but I cannot seem to get COPs about 2.7 at 0C and 1.6 at -20

  12. #62
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    Re: Evap sizing tools

    COP,hp = COP,r + 1
    A simple rule. Coolpack provides COP,r.
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  13. #63
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    Re: Evap sizing tools

    where does the extra 1 come from and how does it show up in the above graphs?

  14. #64
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    Re: Evap sizing tools

    Quote Originally Posted by MikeHolm View Post
    where does the extra 1 come from and how does it show up in the above graphs?
    The COP referred to in Coolpack is your refrigeration side COP. With a Heat pump you are concerned with the heat rejection, which is your Refrigeration capacity + your electrical load, hence the reference to COP +1

    i.e 12 kw heat pump, COP 4, 9kw refrigeration capacity, 3kw electrical load

  15. #65
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    Re: Evap sizing tools

    OK I get it. It isn't really '1' necessarily but power consumed plus power transferred and the rule of thumb is 1-3. Thanks.

  16. #66
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    Re: Evap sizing tools

    It is interesting that some of the processes in Coolpack allow you to change from cooling capacity to heating capacity and some don't. There is no explanation for this. For example, a 2 stage with liquid injection in the suction line cannot be changed to heating. There is something I am missing but I don't know what yet.

  17. #67
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    Re: Evap sizing tools

    http://en.wikipedia.org/wiki/Laws_of_thermodynamics

    http://en.wikipedia.org/wiki/First_l...thermodynamics

    These are links to the laws of thermodynamics. Once you have digested these, we can progress to the reasons for the COP,hp=COP,r+1 relationship.
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  18. #68
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    Re: Evap sizing tools

    Hi Des,

    After Christmas and moving my shop to a new location (not a small feat) I have digested the laws which I had not read before but are easy to understand as concepts. They can in some respects quite easily be compared to Newtons laws in the way the motion of objects and movement of energy interact in more sophisticated ways as more variables are included in the picture.

    Whats next?

  19. #69
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    Re: Evap sizing tools

    At least I hope I have digested them properly!!!

  20. #70
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    Re: Evap sizing tools

    How far are you progressing with CoolPack?

    Perhaps you could post snapshots of your simulations, so that we can discuss further?
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  21. #71
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    Re: Evap sizing tools

    I'll work on a few over the weekend, under different conditions.

    Thanks Des

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