What Is Superheat?

In any refrigeration system superheat is extremely important.  All technicians need to have a thorough understanding of what superheat is and its effects on the system.

Superheat is a measurement  of the temperature a vapour is above its saturation or boiling point.  Superheat can be measured in any vapour such as steam or refrigerant.

There are two important superheat readings that can be taken on a system.  The evaporator superheat and the system or compressor superheat.  You will get different results depending on where you take your measurements.  For this article we will focus on evaporator superheat.  This superheat is controlled by the thermal expansion valve.  

Evaporator Superheat is measured by connecting your low side Gauge Manifold to the outlet of your evaporator, then you will need to convert that pressure to a temperature using your pressure temp chart.  Then using a Clamp Thermocouple Thermometer take a temperature reading as close to the outlet of the evaporator as you can.  Generally near the TX Valve bulb is best.  

Subtract the converted pressure/temp from the actual temp reading taken from the thermometer and you have the superheat.  

There are a number of tools that can help with this process and give you readings in real time, this allows accurate changes in superheat to be made, such as Digital Gauge Manifolds with Thermocouples attached and Superheat Meters, links below.

FIELDPIECE WIRELESS 4-VALVE DIGITAL MANIFOLD WITH MICRON GAUGE SM480VINT

TESTO SMART PROBES A/C & REFRIGERATION SET (2X TESTO 549I / 2X TESTO 115I) 0563 0002 10

TESTO 557S SMART VACUUM KIT 0564 5572

The superheat readings you take show you as a technician the amount of refrigerant flowing through the evaporator and whether or not its the correct amount for the required load.

A superheat that is too high means there is not enough refrigerant flowing through the evaporator which gives a poor refrigeration result and inefficient system.  If the superheat is way to low you are getting too much refrigerant fed into the evaporator and this can result in damage to the compressor due to liquid floodback.

Superheat should be always be checked if the system has had a new compressor, tx valve change, refrigerant added to system or if the system is not refrigerating correctly.  This should be a check undertaken during preventative maintenance as its a vital indicator the system is working correctly and efficiently.

When checking superheat, the system needs to be under full load.  Superheat can be changed via the TX Valve adjustment stem.  Clockwise will increase the superheat and counter clockwise decreases the superheat.  

Be sure to check out our range of Fieldpiece and Testo Digital Gauge Manifolds.