In an R134a system, the high pressure gauge reads 143 psig and the liquid line temperature at the condensing unit is 95°F. What is the operating system superheat?

• A. 13°F
• B. 5°F
• C. 20°F
• D. 25°F

Answer: (A)

Superheat is the amount the refrigerant vapor leaving the evaporator is hotter than its saturated vapor temperature at the evaporator (low-side) pressure. To find it you look up the saturated temperature for the evaporator pressure and subtract it from the actual suction-line temperature.

With a high-side reading of 143 psig for R134a, the condenser temperature is about 118°F. The liquid line at the condenser outlet is 95°F, indicating the liquid is subcooled (about 23°F of subcooling). The evaporator (low-side) pressure for this operating condition corresponds to a saturated suction temperature of roughly 82°F. The suction line temperature is about 95°F, so the difference is 95°F minus 82°F, which equals 13°F. That is why the operating superheat is 13°F.