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  1. #1
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    Smile Crazy suction line



    Hello

    This is a small custom built dairy / drinks fridge running on R404 1.5kg, set point +5c. Superheat fine at about 6 or 7k, low sub-cooling at 2-3k

    IE Alco TEV 01 Orifice

    LP 4.2 bar / -4c
    HP 14 bar / +32c
    Ambient +22c

    Not perfect, obviously, but working reasonably well.

    My problem-

    With 7k superheat at 4.2 bar the pipe temperature measured at the phial is about +4c. It's then about a 10 metre pipe run to the condenser in 3/8 insulated tube where it comes off the service valve into a 1/4 suction line to the compressor (it's a small system).

    Can anyone explain why the 1/4 suction line from the service valve to the compressor is fluctuating around 0c and freezing up?

    Compressor pulling too much vapour through the narrower pipe I guess, maybeit's normal and nothing to worry about?

    Thanks
    Last edited by Contactor; 01-10-2011 at 08:49 PM.



  2. #2
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    Re: Crazy suction line

    SP 5c, lp -4c sub cooling 2-3k.
    Initial thought is that it is short of refrigerant, however have you checked the state of the evaporator and condensor?
    Grizzly

  3. #3
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    Re: Crazy suction line

    Contactor,
    Does the TX valve have an external equalizer? does it have any ice on it?
    Maybe measure superheat downstream of E E connection if installed.

  4. #4
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    Re: Crazy suction line

    Both clear, but dt across them both was also low. I tried adding more 404 but the HP shot up, it's got a receiver but maybe the charge is more critical than I thought.

  5. #5
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    Re: Crazy suction line

    Valve is internally equalised and is frosted.

  6. #6
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    Re: Crazy suction line

    SCT is 10K above ambient (32C-22C=10K), indicating an unusually light load. I would suspect insufficient evap airflow.

  7. #7
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    Re: Crazy suction line

    Quote Originally Posted by Contactor View Post
    Hello

    This is a small custom built dairy / drinks fridge running on R404 1.5kg, set point 5c. Superheat fine at about 6 or 7k, low sub-cooling at 2-3k

    IE Alco TEV 01 Orifice

    LP 4.2 bar / -4c
    HP 14 bar / - 32c
    Ambient - 22c

    Not perfect, obviously, but working reasonably well.

    My problem-

    With 7k superheat at 4.2 bar the pipe temperature measured at the phial is about 4c. It's then about a 10 metre pipe run to the condenser in 3/8 insulated tube where it comes off the service valve into a 1/4 suction line to the compressor (it's a small system).

    Can anyone explain why the 1/4 suction line from the service valve to the compressor is fluctuating around 0c and freezing up?

    Compressor pulling too much vapour through the narrower pipe I guess, maybeit's normal and nothing to worry about?

    Thanks
    Am i reading this wrong? phial at 4C, then 10 m pipe run to the condenser. Shouldn't this be, temp. leaving evap. at +4C then 10 m insulated pipe run to suction service valve, then line size reduction to 1/4" as it enters compressor? Me thinks you should take a pressure reading at evaporator outlet, and again at compressor inlet. The 1/4" service valve is acting as as an EPR valve.
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

  8. #8
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    Re: Crazy suction line

    You're reading it right.

    I can only take a pressure at the suction service valve 4.2 bar -4c (approx)

    Evaporator pipe temp +4c (measured at phial, approx)


    It's a long run and its on a warm wall, so the refrigerant is highly superheated before it hits the 1/4 suction line.

    The 1/4 suction is freezing up, could the EPR effect be responsible for de-superheating by so much?

    Thanks
    Last edited by Contactor; 29-09-2011 at 02:40 PM.

  9. #9
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    Re: Crazy suction line

    I would think that if the compressor suction inlet is designed for 1/4", it would be getting full flow... unless there is a restriction where the 3/8" connects to the 1/4". Perhaps the 1/4" end was not properly deburred?

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    Re: Crazy suction line

    is the 1/4" pipe not acting as a secondair expansion....like a capillarity, so thats why its freezing up??

    change the pipe in 3/8" and see what you get then.

    ice
    ps; the temp discrytion in your post is verry misleading whit the " - " before the numbers. are they pos or neg ?

  11. #11
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    Re: Crazy suction line

    Ice, I don't think you'lg get expansion through the 1/4 as the refrigerant is (I think) 100% superheated vapour, I may be wrong.

    I've edited the first post to clarify temps, apologies.

  12. #12
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    Re: Crazy suction line

    But I did wonder if the 1/4 is restricting the flow of vapour, and because it's so close to the compressor intake the pressure in the 1/4 is much lower than the LP saturated along the rest of the pipe, Gary was basically saying the same thing about the 1/4 being restricted.

    Or maybe the 3/8 was blocked when brazing with silver rod into the steel service valve.

    I think i'm getting there, thanks all.

  13. #13
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    Re: Crazy suction line

    Just jumping in into this one and haven't read all posts very thoroughly. Some remarks/questions.
    1.5 kg refrigerant for a 1/4 suction compressor seems a lot. 1/4 suction will be something around 1/3 HP or even less.
    I should say something around 500 to 750 gr.
    But this doesn't explain your phenomena.

    Then, you're measuring a certain SH somewhere on the outside of the tube. I do it also this way like you do but that's not 100% correct. We have to test this for the Belgium F-gas certification.
    Measure once just after the TEV (inlet evaporator) and/or somewhere at an evaporator bend in the 1st part of the coil. You should measure there exactly your evaporating temperature. Well, you will see that you will measure a temperature at least 4 to 6 K higher. I've demonstrated this already - at least - 500 times.
    So, you have an instrument fault and a fault due to the thermal resistance of the copper.

    So you measure 7K on the outside but the gas itself inside the tube is at 2K SH. With some liquid droplets in the flow.
    I guess that these droplets collect at your 1/4-restriction and that this too small tube acts as a second expansion for the not superheated droplets.

    There's no other explanation, you suppose your gases leaving at 2°C to 3°C and 10 m further, you have again 0°C. If they were 100% superheated and there should be a small expansion afterwards, you never can get lower temperatures then the gas temperature leaving your evaporator. Or there should be a really huge expansion like in a Joule-Tomson turbine.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  14. #14
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    Re: Crazy suction line

    I also have to add, you have an Alco with an orifice 1.
    I bet that this valve is too big for your evaporator and that someone screwed in the SH-needle a lot.
    Or the system is hunting , so adding at regular times some liquid into the suction.
    What's the type of your compressor Contactor?
    Did you make this cabinet and have you ever serviced it before?
    You should solder once some schraeder valves at 1. outlet evaporator 2. just before the service valve and 3. inlet compressor
    Mmmm, interesting problem.
    Is there anyway a problem for your customer or is it more just out of curiosity?
    Last edited by Peter_1; 02-10-2011 at 08:07 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.

  15. #15
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    Re: Crazy suction line

    Either faulty measurements or mixed refrigerants.

  16. #16
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    Re: Crazy suction line

    Quote Originally Posted by Peter_1 View Post
    I've demonstrated this already - at least - 500 times.
    So, you have an instrument fault and a fault due to the thermal resistance of the copper.

    So you measure 7K on the outside but the gas itself inside the tube is at 2K SH
    Hi Peter

    I have always been concerned about inaccurate readings & always use strap probes which are fitted long before readings are taken.

    As I am insulating the probe from abient temperatures, how close should I expect my readings to be to internal temperatures, just allowing for thermal resistance ?

    R's chillerman
    If the World did not Suck, We would all fall off !

  17. #17
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    Re: Crazy suction line

    Quote Originally Posted by Peter_1 View Post
    I guess that these droplets collect at your 1/4-restriction and that this too small tube acts as a second expansion for the not superheated droplets.

    Or there should be a really huge expansion like in a Joule-Tomson turbine.
    thnx Peter_1, for explaining better what i was saying before. i am just starting to teach, so i have to learn to describe things better.......lol

    Ice

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    Re: Crazy suction line

    I've seen a couple of grossly overcharged systems do something similar, its like there is so much liquid in the system for its volume it can't get the evaporating temp low enough, the suction line ends up being part of the evaporator. Tend to have very low discharge superheat too.

    I'd look for faults that some one might have tried to correct by adding refrigerant, like blockages, faulty tevs etc and has been said the correct refrigerant.

    Jon
    Mostly found in Oxfordshire, UK :)

  19. #19
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    Re: Crazy suction line

    I should follow Monkey Spammers' advice, connect the liquid receiver to your R404a bottle and push as much as possible in your bottle. Monitor SH and LP.
    To do it really good: recover all and refill till you got a SH of 7K.

    To go on further on Money Spammers story: I had a client with a counter (12 m/36ft) in truck connected to a Unithé Hermétique of 2HP/R134a. Compressor was some days previous to our visit newly installed by a colleague.
    High pressure was 10 to 15K over ambient - don't remember correct numbers right now - and LP was high (in the range of evaporating at 5°C to 8°C for an ambient of 20°C) and SH was 'normal' (don't remember either the right numbers but this isn't important now)
    It all were symptoms of leaking suction reeds inside the compressors (not pumping very well) but good discharge reeds.
    I know that these units run with +/- 4 to 5kg of refrigerant.

    We finally recovered the refrigerant with then intention to do a test on vacuum pulling with the compressor to check suction reeds. Surprisingly, believe or not but we recovered more than 20 kg of refrigerant. We recharged with +/- 4kg and all pressures were normal again.

    Some questions I had were never solved: why was SH 'normal', why was discharge not extremely higher, where in the system went all that huge amount of refrigerant, why did compressor not tripped on overload,..

    I saw the original installer afterwards and he told me he charged till a full sightglas, finding it strange he needed that much refrigerant.
    Last edited by Peter_1; 02-10-2011 at 12:52 PM.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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