Liquid Quality vs. Subcooling

In this podcast episode, Bryan and Eric Mele talk about sight glasses, the significance of subcooling in refrigeration, and liquid quality.

While we measure subcooling quite often in HVAC work, we rely on sight glasses and liquid line receivers far more often in refrigeration. You need a sight glass to determine the liquid quality in a refrigeration system. Subcooling is one way to assure liquid quality without a sight glass or a receiver.

Subcooling refers to the temperature drop below liquid saturation. Head pressure can dictate subcooling, and several other factors can dictate the condensing temperature, including stacking. We use sight glasses because a clear sight glass can tell us that we have a full column of liquid (therefore subcooling) without hooking up gauges.

In HVAC, we care about having a certain level of subcooling because we want to make sure the refrigerant is fully liquid when it reaches the metering device; no bubbles should be present by the time it reaches the metering device. Like the suction line, the liquid line is a place where heat can be absorbed into the refrigerant. So, some manufacturers recommend insulating the liquid line to prevent heat from transferring to the refrigerant in the liquid line.

Unit orientation also affects subcooling. For example, you can shorten the liquid line sizing if your liquid line goes downhill to the air handler. Conversely, longer lines and uphill liquid lines require special considerations when it comes to subcooling.

Eric and Bryan also discuss:

  • Liquid line receiver fill standards
  • Subcooling and efficiency
  • Sight glass placement
  • Stacking liquid in the condenser
  • Pump down strategies
  • Mechanical subcooling
  • Flash gas
  • “Free” subcooling
  • Ambient temperatures


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2/7/22 at 10:15 PM

Great information. Thank you.



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