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All installations and services must be performed by qualified service personnel.
The suction pressure and condenser subcooling are the most significant
readings. If a unit is suspected of having a low refrigerant charge, the unit should
be recharged using the suction pressure then topped off using subcooling. Any
additional refrigerant may cause damage or unintended problems.
The pressure/temperature chart on the wiring diagram is to serve only as a
guide. Pressures shown are realistic averages which will vary somewhat with
changes in air temperatures, air flowrate across the evaporator and condenser
coils and humidity – both inside and outside.
If a charge must be added to the system, connect to the suction service port of
the service valve and add a vapor charge only while the system is operating.
Addition of liquid refrigerant at the suction port may damage the compressor.
2. Definition of Subcooling:
Subcooling is extra heat removed from refrigerant below the saturation
pressure/temperature of the refrigerant. A certain amount of refrigerant
subcooling is usually beneficial and needed to help assure the best operation of
the cooling system. Subcooling takes place in the condenser. It is a function of
the amount of airflow into the condenser, the outside ambient air temperature,
the refrigerant pressure into the condenser, and pressure drop through the
condenser.
Refrigerant with low subcooling may partially change back into vapor, or “flash to
gas”, while flowing to the evaporator. The resulting gas bubbles can block or
interrupt flow through refrigerant controls causing the evaporator to “starve” for
liquid refrigerant. The consequences are poor evaporator efficiency, high
superheat, and generally poor cooling of the conditioned space. Unless very well-
insulated, long liquid lines from the condenser to the evaporator, specifically
liquid lines exposed to high ambient temperatures, are prone to reduce
refrigerant subcooling. To insure sub-cooled liquid refrigerant reaches the
evaporator under all conditions, it is usually advisable to take subcooling
measurements during the warmest part of the day. Test measurements taken
during this time would provide results typical of the most adverse conditions
under which the cooling system would operate. Installing a liquid line sight glass
at the inlet to the evaporator is a simple, but effective, way to prove liquid is
feeding the evaporator. If the sight glass shows all liquid flow without bubbles this
is a good indication that the amount of subcooling is adequate.
NOTICE: Do not attempt a superheat measurement when the ambient
temperature is less than 60° F. Low loading on the cooling system will
result in low or erroneous superheat measurements.
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