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Subcooling

The temp of the liquid line would never be below ambient under normal operating conditions. A restriction in the liquid line, an undercharge, a kinked line, or a plugged filter drier upstream can cause the liquid to flash into vapor, this would cool the line below ambient. This would not be a normal operating condition though.

Comments

  • TGO_54
    TGO_54 Member Posts: 327
    Subcooling

    On an air cooled condensing unit for a central air system can the measured temp of the liquid line (at the condensing unit) be lower than the ambient temp? If so by how much?
    Thanks

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  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Broken Thermometer?

    The purpose of the condenser is to reject system heat. PERIOD.

    The refrigerant leaves the compressor as a high temperature, high pressure supeheated vapor, which is hotter than the ambient temperature. As the refrigerant flows from the compressor to the condenser coil, heat transfers from the refrigerant to the air passing over the piping and condeser. Since high temperature flows to a lower temperature, the temperature of the refrigerant in the discharge line will drop.

    Given the laws of Physics, if the refrigerant in the line stopped flowing, it would eventually reach the temperature of the air surrounding the piping.

    Consider two glasses of water. Each glass contains 1 cup of water. The water in one cup is 50 degrees and the water in the other is 70 degrees. The air surrounding the cups is 40 degrees. If we mix the contents of the 2 cups, the water will be at a temperature of 60 degrees. If the 60 degree water reamins in contact with the 40-degree ambient, the water will eventually reach a temperature of 40 degrees. The water cannot cool to a temperature any lower than 40 degrees. Where would the water give up its heat to?

    So, in response to your question, if the liquid line temperature is lower than the outside ambient temperature you are defying the laws of Physcs. Since we know that such is not possible, either your thermometer is defective or there is a restriction in the liquid line.

    I would check the service valve on the condensing unit and make certain that the liquid line valve is open all the way. Simply cracking the valve open to allow refrigerant into the system on initial installation/startup can be the culprit for the problem your are experiencing.

    Keep me posted.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    To clarify

    If there are liquid line restrictions, the refrigerant will flash into a vapor, but this will occur AFTER The point of the restriction.

    Liquid line restrictions will result in refrigerant backing up in the liquid line, resulting in very low subcooling measurements and, in severe cases, can result in subcooling measurements of ZERO.


    Oops, check other posts in this area, as I have just let the cat out of the bag!
  • don_144
    don_144 Member Posts: 27
    now professor

    we dont call it backing... stacking is the prefer word.

    Welcome back!
  • TGO_54
    TGO_54 Member Posts: 327
    Thanks for the reply

    > The purpose of the condenser is to reject system

    > heat. PERIOD.

    >

    > The refrigerant leaves the

    > compressor as a high temperature, high pressure

    > supeheated vapor, which is hotter than the

    > ambient temperature. As the refrigerant flows

    > from the compressor to the condenser coil, heat

    > transfers from the refrigerant to the air passing

    > over the piping and condeser. Since high

    > temperature flows to a lower temperature, the

    > temperature of the refrigerant in the discharge

    > line will drop.

    >

    > Given the laws of Physics, if

    > the refrigerant in the line stopped flowing, it

    > would eventually reach the temperature of the air

    > surrounding the piping.

    >

    > Consider two glasses

    > of water. Each glass contains 1 cup of water. The

    > water in one cup is 50 degrees and the water in

    > the other is 70 degrees. The air surrounding the

    > cups is 40 degrees. If we mix the contents of the

    > 2 cups, the water will be at a temperature of 60

    > degrees. If the 60 degree water reamins in

    > contact with the 40-degree ambient, the water

    > will eventually reach a temperature of 40

    > degrees. The water cannot cool to a temperature

    > any lower than 40 degrees. Where would the water

    > give up its heat to?

    >

    > So, in response to your

    > question, if the liquid line temperature is lower

    > than the outside ambient temperature you are

    > defying the laws of Physcs. Since we know that

    > such is not possible, either your thermometer is

    > defective or there is a restriction in the liquid

    > line.

    >

    > I would check the service valve on the

    > condensing unit and make certain that the liquid

    > line valve is open all the way. Simply cracking

    > the valve open to allow refrigerant into the

    > system on initial installation/startup can be the

    > culprit for the problem your are experiencing.

    > Keep me posted.





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  • TGO_54
    TGO_54 Member Posts: 327
    Thanks for the reply

    We had two thermometers on the liqid line, mine and the reps. I had an ambient temp of 72 and a liquid line temp of 74. He had a liquid line temp of 69 and was using my ambient temp reading. It didn't seem right to me that the liquid line could be cooler than the ambient temp, but I wanted to check with you before I tell the rep to get a new thermometer.
    Thanks

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