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  1. #1
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    False Indication of Sight Glass when using R134a


    Hi Gents,

    I've heard that sight glass shall not be used to inspect refrigerant charge level in refrigeration systems using R134a. The reason is that R134a provides a false indication that the system is underchanged (bubbles) - when in fact it is not - leading to a refrigerant overcharge of the system.
    In this case, the only way to verify system charge would be using a set of manometers at low and high pressure side ports.

    Thanks and Merry Christmas !!



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    Re: False Indication of Sight Glass when using R134a

    Hi rhlg

    If you think about it all systems using a sight glass are overcharged, in a time before we all became lazy engineers sight glasses were fitted just before the TXV and not at the condensing unit.

    The reason behind this is the fact that you only need liquid refrigerant at the TXV and not a long liquid line full of refrigerant.

    we just thought it would be easy to service and see everything at once at the outdoor unit. Bubbles in sight glasses have caused big problems on extended piperun systems giving false indications of refrigerant charge within a system and because bubbles appear in the site glass it doesn't mean theres a problem.

    Sometimes the liquid receiver used to be fitted just before the TXV as well but it made more sense to locate everything together on the condensing unit to make our lives easier.

    Kind regards
    Lrac

    I bet that answer has forum members thinking i'm an idiot.

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    Re: False Indication of Sight Glass when using R134a

    Rhlg, bubbles in the sightglass has nothing to do with the fact that you use R134a.
    In fact, it has nothing to do with any refrigerant.
    Why do you see bubbles? There's only one reason.
    The person who said this to you doesn't understand the basic.

    LRAC, you're right, you only need full liquid just before the TXV. But it would be very hard to service a compressor if you should refill it with the sightglass just before the TXV.
    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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    Re: False Indication of Sight Glass when using R134a

    I agree. Refrigerant charge, disregarding of type, shall not be decided on sight glass appearance.

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    Re: False Indication of Sight Glass when using R134a

    Thanks for the feedback.
    I totally agree. The weight of the complete refrigerant charge is the most accurate way to charge the system.
    I've googled the subject and most of the technical articles have mentioned that in the days of R-12 refrigerant, the sight glass on top of the receiver-dryer would look clear when the system was right, indicating liquid refrigerant with no vapor bubbles. However, this is not applicable for R-134a . It uses a different type of oil, polyalkaline glycol (PAG). PAG oil does not fully emulsify the way the mineral oil used with R12 would in the past. The result is that the sight glass looks slightly cloudy even if the system is fully charged, except at very cool outside temperatures". The "sight glass charge inspection" can be used if the following conditions are met :
    • Ambient temperature below 95 deg F. >>
    • Humidity below 70%. >>
    • High condenser fan speed (>=1500 rpm).(It's normal for some bubbles to appear in the sight glass of R134a systems in low fan speed).
    • High-side system pressure below 240 psi (1640 kPa)>
    Summarizing: R-134a is very hard to adjust the recharge rate to so it's usually weighed into the system rather than depending on the sight glass method.

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    Re: False Indication of Sight Glass when using R134a

    I totally agree. The weight of the complete refrigerant charge is the most accurate way to charge the system.
    I disagree, measuring SC and SH is the most accurate way.

    Why do you see bubbles in the sight glass? What are these bubbles in fact?
    Last edited by Peter_1; 24-12-2006 at 04:06 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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    Re: False Indication of Sight Glass when using R134a

    For split system, condensing units and aeroevaporators, refrigerant weight method is not possible. SH and SC are always the best way to go.

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    Re: False Indication of Sight Glass when using R134a

    Thanks for the feedback.
    I totally agree. The weight of the complete refrigerant charge is the most accurate way to charge the system.
    I've googled the subject and most of the technical articles have mentioned that in the days of R-12 refrigerant, the sight glass on top of the receiver-dryer would look clear when the system was right, indicating liquid refrigerant with no vapor bubbles. However, this is not applicable for R-134a . It uses a different type of oil, polyalkaline glycol (PAG). PAG oil does not fully emulsify the way the mineral oil used with R12 would in the past. The result is that the sight glass looks slightly cloudy even if the system is fully charged, except at very cool outside temperatures". The "sight glass charge inspection" can be used if the following conditions are met :
    • Ambient temperature below 95 deg F. >>
    • Humidity below 70%. >>
    • High condenser fan speed (>=1500 rpm).(It's normal for some bubbles to appear in the sight glass of R134a systems in low fan speed).
    • High-side system pressure below 240 psi (1640 kPa)>
    Summarizing: R-134a is very hard to adjust the recharge rate to so it's usually weighed into the system rather than depending on the sight glass method.
    Modern R134a units use Ester (POE) oil, not PAG. PAG is not really used much anymore for refrigeration, since it absorbs moisture even easier than POE.
    "If Hannah was an air handler, I would be a condensing unit so I could open her TXV and pump my refrigerant through her coils." - a HVAC friend of mine

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    Re: False Indication of Sight Glass when using R134a

    Quote Originally Posted by Peter_1 View Post
    I totally agree. The weight of the complete refrigerant charge is the most accurate way to charge the system.
    I disagree, measuring SC and SH is the most accurate way.

    Why do you see bubbles in the sight glass? What are these bubbles in fact?

    Peter,

    Correct me if I'm wrong. Despite it's the most accurate way, raising the SC and SH is a procedure only valid for refinement of the refrigerant charge calculation used in the development of the system. Once you have refined the refrigerant charge, maintenance procedures are based on the charge weight. We're not going to raise the SC and SH every time you have replaced system components or servicing.

    Regarding the bubbles that I mentioned previously, I meant that you should add refrigerant until you stop seeing bubbles in the sight glass. However, what I've heard from some engineers it that, when using R134a , bubbles may continue to appear in the sight glass despite the system has the adequate amount of refrigerant what may lead to refrigerant overcharging event.
    Star882 have commented that this may be related to the use of PAG oil. According to him, modern R134a unit no longer uses PAG oil (they use POE instead). However, I could not find any article that confirms that we can rely on sight glass indication when using POE oil.
    I would appreciate if you have any material related to that. Thanks.
    Last edited by rhlg; 24-12-2006 at 03:51 PM.

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    Re: False Indication of Sight Glass when using R134a

    Quote Originally Posted by NoNickName View Post
    For split system, condensing units and aeroevaporators, refrigerant weight method is not possible. SH and SC are always the best way to go.

    I thought that SH and SC are only used during the development of the system, for refinement of the refirgerant charge calculated. Thanks for the information. The system I work on is VCS system with a dual evaporator system for a small vehicle. We have raised the SH and SC curves just to refine the refrigerant charge. For a regular maintenance the procedure that we have discussed is checking the charge by using the sight glass. I'm gonna study more deeper the systems you have mentioned (split system, condensing units and aeroevaporators) that does not allow refrigerant weight method.

    Merry Christmas to all of you and your families !!

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    Re: False Indication of Sight Glass when using R134a

    Bubbles in the sightglass are a physical consequence of the fact that your liquid is just on the line of the saturated curve where the smallest pressure drop will cause some flash gass.
    What are these bubbles? Flashgas, nothing more, nothing less.
    Flash gass has nothing to do with th used oil nor the used gass.
    If you're in the subcooled region, you have 100% pure liquid, that's pure thermodynamics.
    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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    Re: False Indication of Sight Glass when using R134a

    As Peter says the lack of bubbles in a sight glass is an indication of subcooled liquid so by measuring the amount of subcooling you can guarantee the lack of flashgas which is what will eventually damage the orifice seat in the TEV also cause the valve to hunt. The type of refrigerant used or the oil will not have any effect on the flashgas produced and hence the bubbles in the sightglass. If you are seeing bubbles in the sightglass this indicates to me that the system is short of gas and you need to check the subcooling and the superheat and add more refrigerant if needed.

    Ian

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    Re: False Indication of Sight Glass when using R134a

    I have noticed that in a modern R134a unit using POE, there are sometimes "microbubbles" in the sight glass, which show up as cloudiness or a "streaky" effect. I think those are bubbles of oil in the refrigerant. They're easy to tell apart from vapor bubbles, though.
    "If Hannah was an air handler, I would be a condensing unit so I could open her TXV and pump my refrigerant through her coils." - a HVAC friend of mine

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    Re: False Indication of Sight Glass when using R134a

    The fact the you are using the sight glass as a means to determine correct refrigerant charge is fine....assuming you have a correctly designed and installed system! their are so many factors that can affect a system that the appearence of the sight glass should one be only one of varios factors in determining gas charge. For instance if you have a system with multiple evaporators and one tx-valve if faulty or has a bulb not fitted correctly, during the commissioning process you will find it would take an excess amount of gas to clear the sight glass, due to the fact that one coil is being flooded with gas due to the faulty valve! I believe you should always look at the system as a whole whilst charging and never assume everything's perfect! Head pressure/evaporator load/ambient temps/cpr/eprs are all factors in a system! Maintaining a correct head pressure should be first and foremost if charging via a sightglass because it can influence the liquid level so dramatically!.

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    Re: False Indication of Sight Glass when using R134a

    Quote Originally Posted by Peter_1 View Post
    Bubbles in the sightglass are a physical consequence of the fact that your liquid is just on the line of the saturated curve where the smallest pressure drop will cause some flash gass.
    What are these bubbles? Flashgas, nothing more, nothing less.
    Flash gass has nothing to do with th used oil nor the used gass.
    If you're in the subcooled region, you have 100% pure liquid, that's pure thermodynamics.
    Hi peter1 Just to further your comment.... If you believe bubbles in the sight glass is due to flash gas then tell me this.... Most sight glasses are fitted directly after the liquid reciever, So without a liquid sub cooler fitted how would the refrigerant subcool by such a degree to cause flash gas when it is situated no more than a metre away from the condensor? It would be more likely to be over condensing to cause such a problem in wich case your head pressure should be corrected... ie cycling head fans or to the other extent lowering the head pressure!

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