Superheat
Superheat is how many degrees a vapor sits above its saturation temperature at its pressure. Take the suction pressure, read its saturation temperature off a PT chart, and subtract it from the suction line temperature. It tells you the evaporator has boiled off all its liquid before the vapor reaches the compressor.
From the lesson The Basic Refrigeration Cycle
Why it matters on the exam
Superheat and subcooling come up on the exam, most of all in Type II. Reading them right is how a technician judges a system’s charge without guessing.
Key points
- Where. Measured at the evaporator outlet or suction line, where the refrigerant should be all vapor.
- How. Suction line temperature minus the saturation temperature for the suction pressure.
- Its opposite. Subcooling is the same idea on the liquid side: saturation temperature at the condensing pressure minus the liquid line temperature.
Common trap
Subtracting the wrong way round, or using the gauge pressure as if it were a temperature. Convert the pressure to a temperature first, then subtract.
Question 1 of 5
An R-410A system's suction pressure gives a saturation temperature of 40°F, and the suction line reads 52°F. What is the superheat?
Tap an answer and I’ll tell you why.
Question 1 is written for this page and checked by a person. The others are practice questions from the MindMax app. None are from a past exam.
Where it comes from
Lesson 1 in Refrigeration Fundamentals · Core Section
The Basic Refrigeration Cycle
- Name the compressor, condenser, metering device and evaporator and state the function of each
- Trace the refrigerant's changes of state around the cycle
- Identify which parts of the system are the high side and which are the low side
- Explain that heat is absorbed by boiling refrigerant at low pressure in the evaporator
- Define superheat and subcooling
Key terms in this lesson
- Subcooling
- How far a liquid is below its saturation temperature at its pressure.
- Superheat
- How far a vapor is above its saturation temperature at its pressure.
Learn more
FAQ
The low side. You take the suction pressure and the suction line temperature at the evaporator outlet. Subcooling is the high-side reading, taken on the liquid line at the condenser outlet.
The vapor leaving the evaporator is barely above its saturation temperature, so liquid refrigerant may be reaching the compressor. Common causes are an overcharge, low airflow across the evaporator, or a metering device feeding too much.
Fixed-orifice systems, with a piston or capillary tube, are charged by superheat. TXV systems are charged by subcooling, because the valve holds the superheat steady itself.