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Total subcooling encompasses liquid subcooling from the
condenser's 100% saturated liquid point to the metering device.
This means total subcooling includes condenser subcooling, and
any subcooling that takes place after the condenser up to the
metering device. (Liquid can subcool in the receiver, filter drier, and
liquid line before it gets to the metering device.)
In the case of a capillary tube metering device, liquid subcooling
occurs through the entire length of the capillary tube. However,
for simplification, we will assume that total subcooling ends at the
capillary tube entrance as it is difficult to measure temperature at
the capillary tube outlet in a service environment.
Liquid sub-cooling is normally measured at the liquid line
service valve. The manufacturer will usually specify the required
subcooling. Always go by the manufacturer’s requirements.
TO MEASURE LIQUID SUBCOOLING
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SUPERHEAT AND SUBCOOLING RELATING TO
REFRIGERANT CHARGE
Proper performance of heat pumps and air conditioners are
determined by many factors, one of the most important ones
being proper refrigerant charge. In order to control something, we
need to be able to measure it accurately.
Expansion valve systems can be charged by using the
subcooling method. Liquid subcooling is required so that only
liquid enters the expansion valve with no bubbles present. Vapour
bubbles in the refrigerant will cause low refrigerant flow. Low
refrigerant flow will cause a loss of capacity and efficiency in a
cooling system.
68
RACA Journal I January 2020
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Attach a gauge manifold to the liquid line service port and
take a pressure reading.
Use a temperature-pressure chart to convert the pressure to
the saturated condensing temperature of the refrigerant.
Attach an accurate digital thermometer to the liquid line. It’s
usually best to insulate the thermometer probe so that it’s not
affected by the ambient air. The temperature shown by the
thermometer should be lower than the saturated condensing
temperature. The difference between the measured liquid line
temperature and the saturated condensing temperature is
the liquid subcooling.
Add refrigerant to increase subcooling.
Recover refrigerant to reduce subcooling. Note that if the
subcooling and superheat are correct, and the suction
pressure is low, the system probably has low air flow. Correct
the airflow problem and check the charge again.
When charging by the subcooling method it is important to
check the suction superheat as well. If the expansion valve is
faulty or incorrectly adjusted there can be a very low suction
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