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bubbles in sight glass

Hello All

Why do I commonly see bubbles in the sight glass of refrigeration units when they are properly charged.  And if its because the larger size of the sight glass lets some of the liquid flash, why then dont I always see it. And if you say thats because sometimes there is more subcooling, than you might be right!


  • TechmanTechman Member Posts: 2,144

    are not normal in any ACR application. The system has to be low on freon or the head press is not being controled at its minimum allowable pressure.
  • Eugene SilbersteinEugene Silberstein Member Posts: 349
    Agree on Disagreement?

    If an air conditioning or refrigeration system is operating close to its intended target temperature, there should be no bubbles in the sight glass. In other words, if the load on the system is relatively low.

    However, if the system is operating under high load, such as in the case of a hot pull down on a refrigeration system or an air conditioning system that is turned on when the indoor ambient is high, bubbles will very likely be present in the sight glass. This is due to the metering device (namely the TXV) opening wide to lower the increased evaporator superheat caused by the high load conditions. This increases the flow rate in the liquid line and can result in some flashing.

    An old school of though was to charge systems to a full sight glass all the time. In the case of a hot pull down, charging to a full sight glass will result in a system overcharge. This overcharge often goes undetected since TXV systems typically have receivers that store this excess charge.

    So, if your system is operating with bubbles in the sight glass and there is a large load on the system, no worries. However, if the system is operating with low load, then you should not see any bubbles in the sight glass.

    Hope this helps.
  • charliechicagocharliechicago Member Posts: 68

    Thanks for the response Mr Silberstein.

    When you say that "this increases the flow rate in the liquid line" do you mean that the "extra" liquid leaving the liquid line through the txv causes a slight pressure drop in the line and causes flashing? Would i theoretically see bubbles in the entire liquid line as well?

    Thanks, Michael.
  • TechmanTechman Member Posts: 2,144

    when I grow up!
  • jttechjttech Member Posts: 1
    heat pump during cold days sometimes strains and swooshes

    Heres our situation. We have a heat pump outside, this is a 2 heat / 1 cool type unit. Im concerned as occasionally the unit will shut off and on or switch cycles in that it makes a loud bang.

    As I was standing out side by it the other day I heard the compressor straining away, the fan ontop of the unit was not running. All of a sudden I heard this loud swooshing sound as if a valve opened up and a large volume of liquid was released. Then the fan on top of unit started running. I noticed too a few air bubbles flowing by the sight-glass

    I noticed inside also by the furnace at times the liquid lines feel cold and we get cold air out of the plenem. I have the thermostat set at heat only.

    Inside temperature is set at 72 dgrees farrenhiegt and the out side temperature hovering around 5 to 10 degrees farenhieght.

    Is this behavior of the swooshing something I should be concerned about? Also is there maybe a setting on the thermostat that can be adjusted. For instance can we lower the number of times the compressor is allowed to cycle as it is set at 5 c/ph.

    Any help is appreciated.

  • TechmanTechman Member Posts: 2,144
    edited December 2013
    Heat Pump

    That swooshing noise,outdoor fan not running/then running, indoor supply air plenum air temp sometimes cool/cold , sounds like a HP going thru a "defrost cycle" to melt frost off of the outdoor unit. Are you a H.O. ( home owner ) or a tech? Check the electric heat (2nd stage heat) during a defrost , and also ck the Freon pressurers during the defrost. How many stages/strips of electric heat do you have? The bubbling in the SG is a "it dependes " situation, mostly no. And for your wallets sake ,I hope those "air bubbles" are not air bubbles but "Freon bubbles".
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