Results 1 to 20 of 20
  1. #1
    Join Date
    Oct 2005
    Location
    IRAN
    Age
    53
    Posts
    878
    Rep Power
    19

    Thumbs up Frost on the suction line



    Hi there,
    Please have a look at the drawing. Everything is explained there. Any Ideas?
    Cheers
    Last edited by lana; 04-04-2007 at 12:31 PM.


    Even Einstein Asked Questions

  2. #2
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,302
    Rep Power
    25

    Re: Frost on the suction line

    When the suction line temperature is below 0C you can have frost. This has nothing to do with superheat or liquid returning from the compressor.

    Stop and think about this. If the evaporator temperature is -42C, the suction line will be colder than 0C (where frost starts to form). Even if you have 30C of superheat, the line can still be frosted since the suction line is still below 0C.

    At low temperatures frost does not indicate liquid flood-back or any particular problem. Frost simply shows up when the surface temperature of the pipe is below 0C.

  3. #3
    Join Date
    Oct 2005
    Location
    IRAN
    Age
    53
    Posts
    878
    Rep Power
    19

    Thumbs up Re: Frost on the suction line

    Quote Originally Posted by US Iceman
    When the suction line temperature is below 0C you can have frost. This has nothing to do with superheat or liquid returning from the compressor.
    Hi US Iceman,
    If there is a cold vapour flowing, I think there will be only droplets of condensed moisture on the pipes not frost. I have seen many -40°C systems where there is no frost on the accumulator and the suction line (parts without insulation). Again I think that, only liquid has high latent heat therefore has the capacity to turn the water into frost.
    Correct me if I am wrong.
    Thanks.
    Even Einstein Asked Questions

  4. #4
    Join Date
    Oct 2005
    Location
    IRAN
    Age
    53
    Posts
    878
    Rep Power
    19

    Thumbs up Re: Frost on the suction line

    Quote Originally Posted by Marc O'Brien
    No, not true at all. And in hot climates where you're typically condensing above 50°C, adjusting superheat to clear the suction line frost can quickly lead to a compressor replacement (overheating). Talking -40°C systems here.
    Hi Marc,
    The question I have is why accumulators always frost where there is liquid present?
    I have built an ice machine with accumulator (te=-15°C). At the end of the cycle there is liquid flooding into the accumulator. At this point frost can be seen at the bottom of the accumulator. But during whole cycle it is completely free of ice.
    Appreciate your responce.
    Cheers
    Last edited by lana; 16-01-2006 at 03:35 PM.
    Even Einstein Asked Questions

  5. #5
    Join Date
    Oct 2005
    Location
    IRAN
    Age
    53
    Posts
    878
    Rep Power
    19

    Thumbs up Re: Frost on the suction line

    Quote Originally Posted by Marc O'Brien
    I just downloaded you pdf and now understand your question...

    Ambient dew point variations?

    Suction line retriction?
    You mean there might be a restriction in the suction line which causes this problem (frost)?
    Cheers
    Even Einstein Asked Questions

  6. #6
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,302
    Rep Power
    25

    Re: Frost on the suction line

    Quote Originally Posted by lana
    I have seen many -40°C systems where there is no frost on the accumulator and the suction line (parts without insulation).
    Frost can form at 0C. If warm air is circulating past (or a warm ambient with no air circulating past) the component, the frost will melt at different rates. If the suction line temperature is below 0C and liquid is present, the frost forms during this condition since the liquid refrigerant is boiling and removing more heat than that being exchanged to the air.

    I think your next comment answers the question.

    I have built an ice machine with accumulator (te=-15°C). At the end of the cycle there is liquid flooding into the accumulator. At this point frost can be seen at the bottom of the accumulator. But during whole cycle it is completely free of ice.
    During normal operation the accumulator has higher superheat and since the accumulator temperature is probably close to 0C, the frost melts due to ambient conditions.

    However, when the end of cycle is approached some liquid is flowing back into the accumulator. When the evaporating temperature is -15C and liquid is leaving the evaporator, the liquid is boiling and keeps the accumulator cold enough to freeze the frost.

    In your drawing, you say the coil has a lot of superheat. Must be if the outlet header is not frosted. However, the trap at the outlet can hold liquid. If the pressure drop through the suction riser is too high, any liquid in the trap will flash and the returning two-phase mixture could produce the localized frosting on the lines.

    There should also be an inverted trap on the top of the suction riser. The drawing does not show one.

    It seems like you have more than one problem here.
    Last edited by US Iceman; 16-01-2006 at 06:14 PM. Reason: text editing

  7. #7
    Join Date
    Oct 2005
    Location
    IRAN
    Age
    53
    Posts
    878
    Rep Power
    19

    Thumbs up Re: Frost on the suction line

    Hi US Iceman,
    You are right. There are more than one problem here and there is no inverted trap. OK thanks for the frosting theory but what about the restriction and the high pressure drop? If there is high pressure drop then the compressore will run at lower te and gives less capacity which is the case here. Will this make the compressor wear and tear and overheating worse?
    Cheers.
    Even Einstein Asked Questions

  8. #8
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,302
    Rep Power
    25

    Re: Frost on the suction line

    If there is high pressure drop then the compressor will run at lower te and gives less capacity which is the case here.
    That is correct. If you know the design evaporating temperature, determine the evaporating pressure. Now, what is the suction pressure? The difference is the dP of the suction line. Have you calculated the dP for the installed piping lengths, and does this agree with the measured value?

    I seem to remember from your drawing that you had a suction line filter. How many cores does it contain and what are the pipe connection sizes? Depending on the number of cores and connection sizes, the suction filter can greatly contribute to an excessive pressure drop.

    I have attached a file with excerpts from the Sporlan bulletin for your review on this matter.

    As you are probably aware of, any increase in suction pressure can significantly increase the refrigerating capacity of the system. Therefore, it is important that you verify the cause of the high losses.

    The suction riser may be too small for the required load also.

    Will this make the compressor wear and tear and overheating worse?
    Absolutely... The suction superheat at the compressor has a direct impact on the capacity of the compressor, the discharge temperature of the gas, and can contribute to abnormally higher oil temperatures. With the addition of the high suction line pressure losses, the increase in pressure ratio also reduces the volumetric efficiency and further increases the discharge temperature. In essence, everything is going the wrong direction!

    All of which can significantly reduce the life expectancy of the compressors due to a decrease in oil viscosity, potential copper plating and acid generation.

    If the suction superheat at the compressor can be adjusted to a reasonable value, you should see noticeable results.

    One additional question about the evaporator... Are the coil circuits properly designed and is a refrigerant distributor being used? The high evaporator superheat and occasional flooding sounds like an imbalance in the liquid feed to the evaporator coil???
    Attached Files Attached Files

  9. #9
    Join Date
    Oct 2005
    Location
    IRAN
    Age
    53
    Posts
    878
    Rep Power
    19

    Thumbs up Re: Frost on the suction line

    Thanks US Iceman,
    Very good explanation as always.

    With the installed pipe size, I calculated the suction pressure drop as 3.5°C drop in saturation temp.?!!
    I always size the suction line for 1°C drop in the saturation temp. The design te is -40°C but according to the suction pressure I have -44°C therefore there is a large pressure drop. There is no doubt there is an unacceptable pressure drop here.
    About the evaporator I have to say that it has a distributor and the size is also correct (I checked it) and I had the same evaporator installed in another double-stage system with perfect results. I don't see any problem in the evaporator section.
    Anyway, I have given my suggestions to the owner about the piping lets see what happens.
    Thanks again.
    Even Einstein Asked Questions

  10. #10
    Join Date
    Aug 2005
    Location
    iraq
    Age
    67
    Posts
    130
    Rep Power
    19

    Re: Frost on the suction line

    hi us iceman
    from your post i understend that : in -42 evaporator temprature we not should be worried if we see ice in sution line,,,,is that true?

  11. #11
    Join Date
    Aug 2005
    Location
    iraq
    Age
    67
    Posts
    130
    Rep Power
    19

    Re: Frost on the suction line

    to us iceman
    from your explanation ,,,i understand that if there ice on suction line for -42 evaporator is it normal condition ,,is that right?

  12. #12
    Join Date
    Aug 2005
    Location
    iraq
    Age
    67
    Posts
    130
    Rep Power
    19

    Re: Frost on the suction line

    hi us iceman
    according to your replay , i understand that it is not harmfull if we see ice arround the suction pipe in -42 evapotator temp.
    is this right?

  13. #13
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,302
    Rep Power
    25

    Re: Frost on the suction line

    afeef,

    Sorry for not replying sooner. I have been out of the office for several days and may have missed your question.

    To answer your question... Yes.

    One thing we have to remember is that ice formation by itself does not indicate liquid flooding back to the compressor.

    If the suction superheat is present at the compressor suction valve then the suction line is dry. Even if the suction line temperature is below 0C.

    Frost is not a true indication of liquid flooding. It only means the suction line temperature is below 0C.

    If the suction superheat is zero and you have frost on the suction line you could experience liquid flooding.

    The superheat determines if liquid flooding is occurring, not the fact that frost/ice is present.

    Another way to check for liquid flooding on low temperature systems is to check the discharge superheat at the compressor discharge service valve.

    If the compressor has some small quantity of liquid entering the compressor, the discharge temperature will be much lower than normal.

    The discharge superheat is determined by: Discharge temperature minus the saturation temperature of the gas at discharge pressure.

    For a quick check, the actual discharge temperature can be used.

  14. #14
    Join Date
    Dec 2005
    Location
    Zagreb
    Posts
    190
    Rep Power
    19

    Unhappy Re: Frost on the suction line

    If we have 50 C condesation temperature how smal can be discharge superheat so we can conclude that liquid enters the compressor.

    Renato

  15. #15
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,302
    Rep Power
    25

    Re: Frost on the suction line

    Renato RR,

    If you are condensing the refrigerant at 50C, the actual discharge gas temperature (at the compressor discharge valve) should be greater than 50C.

    The way to compare this is using the data from a properly functioning system. If the suction superheat is normal, measure the discharge temperature. The discharge temperature could be at 76C or higher. The difference of 76C-50C is the discharge superheat.

    If the discharge temperature is much lower, or you can see it decreasing rapidly, the suction superheat is too low or the compressor may be experiencing a little liquid flood back.

    This is very important especially for screw compressors. If the discharge temperature is not high enough, refrigerant can be in solution with the oil. This reduces the oil viscosity.

  16. #16
    Join Date
    Feb 2001
    Location
    Tampa Florida
    Posts
    1,076
    Rep Power
    25

    Re: Frost on the suction line

    The evaporator super-heat is very
    high therefore 4 rows of the coil and also the evaporator outlet header are free of any frost. Also
    the vertical part after the oil trap is free of frost. But after this point (the orange lines indicated
    above) including the suction filter are frosted to the compressor inlet ??!!! Any idea?
    Hi, Lana. We really need to discuss the pipe temperatures instead of whether they collect frost or not. Are the early piping runs in the evaporator actually -40 deg? What is the temperature of the pipe where the TEV bulb is clamped?

    Does the riser not frost because it is in the conditioned space and then the horizontal run to the suction filter exist in a non-conditioned space?

    Just tell us the temperatures of the piping and it will become evident what you have.

  17. #17
    Join Date
    Oct 2005
    Location
    IRAN
    Age
    53
    Posts
    878
    Rep Power
    19

    Thumbs up Re: Frost on the suction line

    Quote Originally Posted by Dan
    Hi, Lana. We really need to discuss the pipe temperatures instead of whether they collect frost or not. Are the early piping runs in the evaporator actually -40 deg? What is the temperature of the pipe where the TEV bulb is clamped?

    Does the riser not frost because it is in the conditioned space and then the horizontal run to the suction filter exist in a non-conditioned space?

    Just tell us the temperatures of the piping and it will become evident what you have.
    Hi Dan,
    The pipe temperature was measured in the horizontal section and it shows -23°C which in turn gives 17°C superheat. The temperature of the bulb can not be measured because it goes behinde the evaporator and there is no access. And yes the horizontal section goes to the machine room. I think what we have here is high suction pressure drop and not adjusted TEV.
    Cheers
    Even Einstein Asked Questions

  18. #18
    Join Date
    Aug 2005
    Location
    iraq
    Age
    67
    Posts
    130
    Rep Power
    19

    Re: Frost on the suction line

    thanks us iceman
    very nice explanation ,,,on the counter way, if the suction pipe have no frost on it (ubove 0 evapotator temp.) this is not means that flooding is not exist,,,
    am i righr?

  19. #19
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,302
    Rep Power
    25

    Re: Frost on the suction line

    afeef,

    If you are above 0C for the saturation temperature of the refrigerant, the line will not frost. You can still have flooding back to the compressor however, without frost being present on the suction line.

    This is why it is important to check the evaporator superheat and the compressor suction superheat. The evaporator superheat shows how the TXV is adjusted and operating.

    The compressor suction superheat shows how much additional heat is picked up in the suction line.

    Does this help you?

  20. #20
    Join Date
    Aug 2005
    Location
    iraq
    Age
    67
    Posts
    130
    Rep Power
    19

    Smile Re: Frost on the suction line

    thanks a lot us iceman
    its clear now
    afeef

Similar Threads

  1. Suction gas <-> liquid line heat exchange and performance gains
    By DaBit in forum Technical Speculations
    Replies: 30
    Last Post: 18-05-2010, 01:52 PM
  2. Iced suction line - Question from Rockfun
    By Brian_UK in forum Air Conditioning
    Replies: 8
    Last Post: 14-06-2006, 07:24 PM
  3. Bearing trouble - rotary compressor
    By rosetennis in forum Industrial Compressors
    Replies: 6
    Last Post: 18-05-2006, 01:26 AM
  4. Measure suction line pressure drop with thermometer
    By Steve Wright in forum Fundamentals
    Replies: 7
    Last Post: 17-09-2005, 11:05 PM
  5. Minimal required R134a speed in suction line
    By DaBit in forum CPU Overclockers
    Replies: 32
    Last Post: 08-05-2002, 05:19 AM

Posting Permissions

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