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Thread: Sight glass flashing high load

  1. #1
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    Sight glass flashing high load

    Is it possible to have a txv full open cause sight glass to flash, regardless of sub cooling? I had a 16 ton r-22 unit where I measured 24 degrees sub cooling on roof. Go inside to evap (down 20 feet over 20 feet) and sightglass is flashing. I tried adding refrigerant, go back to roof to check subcool (no access to check just before expansion valve) and I had 42 subcool 330 psi head. Suction increased to 73 psi and superheat lowered to 7 degrees. Air handler is 100% outdoor air, ambient temperature was 90. Could expansion being wide open due to large load and component sizing cause this? I would think 40 subcool even with valve wide open I should have solid liquid? Non condensables or plugged drier crossed my mind, no temp drop across drier. Unit was cooling well though... if bms reading correct 30 td across evap. Thoughts?

  2. #2
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    A big restriction such as too small of a liquid line size or a kink in the line could cause the loss of subcooling. Running through a hot attic or space would also cause a loss.

    Is the txv a “put together” style? Like an alco TCLE or similar?

  3. #3
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    No, Twas a sporlan. According to line sizing chart, I think size is suitable

  4. #4
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    Flashing glass = NO sub-cooling.
    Measure the sub-cooling at the txv/sight glass. If measuring subcooling at the sight glass and still flashing you have non-condensables.
    Pump the circuit down into the condenser, run the condenser fans, once pressures stabilize compare the ambient to condenser saturated. If saturated is higher than ambient= non-condensables

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  6. #5
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    Where is the hi head coming from?
    Is there a receiver tank?
    What is discharge line temp?
    NonConGas does not get trapped in the cond coil due the cond coil /liquid line seal? On a system low on freon the NCG goes for their "joy rides" around the system and then you'll have activity at the SGMI.
    A 20' x 20' LL going downhill needs maybe 5-6*SC.
    Is there a suction press limiting device? The suction is low for a 90*F load

    What brand of unit is set up for a DOAS application?

  7. #6
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    "NonConGas does not get trapped in the cond coil due the cond coil /liquid line seal? On a system low on freon the NCG goes for their "joy rides" around the system and then you'll have activity at the SGMI.
    A 20' x 20' LL going downhill needs maybe 5-6*SC.[/QUOTE]

    Non-condensables will accumulate in the condenser and cause false sub-cooling readings. They remain as gas and do not leave the condenser during operation as long as there is a liquid seal at the condenser outlet. Old days they called the access valves on the top of condensers "purge valves". As long as you have a liquid seal non-condensables by definition stay in the condenser.

    Vertical static will reduce the required sc at the condenser, R-410a & R-22 typically need 2-4 *f sub-cooling at the txv under all loads to prevent hunting.

  8. #7
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    My comment about NCG had a Q mark at the end of the sentence, not a point making !!!!!! mark.

    I agree w/ your statement about NCG.!

  9. #8
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    Non condensables usually cause high side pressure to bounce, or not nessecarily?

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  11. #9
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    My experience with non-condensables in the system causing that big of a problem is that the gauge would flutter. If the equipment is matched properly, including the compressor in the condenser, the coils are clean, and air flow is good, which is the baseline for any diagnostics, then considering the low superheat, it looks like you either have non-condensibles in the system, or a bad refrigerant charge, such as a blend added to 22, or a blend that had a low side leak. I think that I would start by pulling the charge, evacuating the system well, and recharging the system.

  12. #10
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    I did not have opportunity to clean condenser so I wasn’t sure if that was cause of high head. No receiver. Did not get discharge temp, limited time. Mcquay acz.

  13. #11
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    Quote Originally Posted by ehsx View Post
    Flashing glass = NO sub-cooling.
    Measure the sub-cooling at the txv/sight glass. If measuring subcooling at the sight glass and still flashing you have non-condensables.
    Pump the circuit down into the condenser, run the condenser fans, once pressures stabilize compare the ambient to condenser saturated. If saturated is higher than ambient= non-condensables
    Thanks I’ll try non condensable Check next opportunity. Like I say, no access ports in liquid line except on roof. Would have been nice upstream and downstream of drier...

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    Quote Originally Posted by Shreadhead View Post
    Non condensables usually cause high side pressure to bounce, or not nessecarily?
    No reason to bounce, unless some new science not released.

    AGZ scroll compressors. Hit the pump-down switch, once down put in test mode (carel password 2001), turn fans on, stabilize and check ambient against condenser saturated for non-condensables.
    Filter drier, liquid line solenoid, sight glass, & txv should all be mounted at the evaporator. These units were designed for 14-18*f subcooling, excess will cause higher pressures. Because the evaporator is below the condenser you should be able to run lower sub-cooling due to static effect, unless the piping is undersized.

    Non-condensables will cause a false subcooling reading. Contractor should have installed assess taps at the evaporator. Line sets in excess of 60' are prone to refrigerant management and service pumpdown issues.

    Superheat should be checked at the evaporator, 7*f low if measured at the compressor. 8-12* at the evaporator, a couple * higher at the compressor depending on suction line size & routing.

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    I bet that system contains another refrigerant.....

    I've seen 407C do that.

    like the others said, close condenser service valves, and run the fans to check P/T relationship.

  16. #14
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    R-407c or any ref with glide: Use the dew point temperature on the pressure/temperature chart to obtain the evaporator saturation temperature for superheat, and the bubble point temperature to obtain the condenser saturation temperature to measure subcooling.

    With fractionalization you will have to increase the sub-cooling to compensate. SH & sc shift towards the lower pressure ref in the blend. That's why you need a full log sheet to verify charge on an existing system with a blended ref. Have to cross reference sh, sc, evap & cond approach. If you don't do a full log, most people just replace the charge, a waste of time and money.

  17. #15
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    This system does not need a receiver? The manual said there is a SG at the cond unit.

  18. #16
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    Quote Originally Posted by TechmanTerry View Post
    This system does not need a receiver? The manual said there is a SG at the cond unit.
    Due to condenser coil piping arrangement receiver not practical, should not be needed. When a remote evap is ordered or ordered less evap liquid line accessories were shipped separately.
    AGZ typically a package chiller, options for remote evap.

  19. #17
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    I would check for restrictions on the liquid line. If their are not temperature drops across any devices I would power off and check against ambient temperature to see what the P/T should be. What is the condenser split while running? If you split is high and there are no other restrictions it is most likely non condensables.

  20. #18
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    Quote Originally Posted by TechmanTerry View Post
    This system does not need a receiver? The manual said there is a SG at the cond unit.
    This is an a vintage, no SG. Just a split a/c, no receiver needed.

  21. #19
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    Thanks for the responses, my preference is to pull charge, replace drier and install access fittings, pull vacuum, and verify recovered gas matches up to PT chart before charging, possibly new refrigerant.

  22. #20
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    Quote Originally Posted by Shreadhead View Post
    This is an a vintage, no SG. Just a split a/c, no receiver needed.
    How is the cond unit "liquid seal" achieved?

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