Thermo Pride MODEL OPA - Manual (Page 45)

All installations and services must be performed by qualified service personnel.
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Refer to Example 3, in Appendix B: Calculations of this manual, for a sample
calculation of how to figure the degree of superheat for the evaporator. In
general, Thermo Pride recommends an operating evaporator superheat of
between 5
° and 12°F.
4. Definition of Subcooling:
Subcooling is extra heat removed from refrigerant below the saturation
pressure/temperature of the refrigerant. A small 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 or saturated refrigerant 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.
5. Condenser Subcooling:
Before taking subcooling measurements, allow the system to stabilize through
continuous operation for at least 15 minutes. To determine subcooling, measure
the refrigerant pressure at the liquid line as it leaves the condenser. It is
preferable to measure pressure of the liquid line at the evaporator, but most
manufacturers, including Thermo Pride, do not include test ports (or Schraeder
valves) at the evaporator. Typically, it would be necessary to compensate for the
pressure drop through the line connecting the condenser and the expansion
valve. However, due to the close proximity of these components, the pressure
drop will be small and can be neglected.