Feature
SUPERHEAT EXPLAINED
By Cyril Macquet-Maurel of Shosholoza Air Conditioning and Refrigeration
Checking and setting superheat at commissioning stage is important to
ensure the efficient running and long life of the refrigeration equipment.
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SO, WHAT EXACTLY IS SUPERHEAT?
Superheat is an indication of the amount of extra heat (or
superheating) the refrigerant vapour has received after it has
passed through its latent phase in the heat transfer coil.
WHY IS CORRECT SUPERHEAT SO IMPORTANT?
The compressor’s lifespan! If the superheat is set right then
there is no reason why the compressor won’t run for 20 or
more years. If it’s not set right and the compressor is flooding
or starving excessively, then expect as little as a year or two
before failure. The compressor needs the correct amount
of returning refrigerant to keep its motor windings at the
correct temperature and, most importantly, its oil at an
optimum.
hecking and setting superheat at commissioning stage is
critically important with split systems to ensure that the
refrigerant volumes are correct, particularly where additional
refrigerant charge is required.
While modern air-conditioning equipment makes
checking superheat as simple as plugging in a “Service
Interface” or reading the built-in display on a PLC control
board, there is still plenty of equipment in the field with older
style control systems that require the manual superheat
testing process outlined below.
POINTS TO NOTE:
1.
2.
HIGH SUPERHEAT: (STARVING)
In this scenario, the compressor will get too hot causing
its sump oil to overheat, slowly break down and lose its
viscosity (lubricating effect). If you don’t have the correct
lubricating effect, the compressor bearing will wear and
eventually seize.
LOW SUPERHEAT (FLOODING)
In this scenario, there is too much refrigerant coming back.
Some say that these new scroll compressors can withstand
liquid flooding, which they can, but it’s the oil that can’t.
What happens is that the sump oil becomes diluted with
the refrigerant and loses its viscosity (lubricating effect). If
you don’t have the correct lubricating effect, the compressor
bearing will wear and eventually seize.
www.hvacronline.co.za
An R404a
thermostatic
expansion valve.
3.
Bearing failure often gets reported as a winding failure
because the resulting locked rotors’ high current draw
can cause the winding to short out. It’s not until the
compressor is cut open for inspection that you realise the
seized bearing caused the winding to blow.
Superheat should be checked after the compressor has
run continually for at least 10 minutes to allow the system
refrigerant and oil to settle.
The indoor unit’s return air temperature and airflow
should be as close to normal operating conditions as
possible, as well as the outdoor ambient.
CHECKING SUPERHEAT
It is preferred to set superheat in a cooling cycle when the
indoor return air temperature is just above set point, for
example 23°C plus or minus a couple of degrees. The target
superheat temperature is generally 3°C to 5°C (4°C to 6°C for
electronic expansion valves).
1. Fit your LP gauge to the common suction line port situated
between the compressor and the reversing valve.
Continued on page 37
RACA Journal I June 2019
35