Feature
Continued from page 35
2.
3.
4.
Convert this low pressure to temperature using a
pressure/temperature conversion chart or the relevant
refrigerant temperature scale on your manifold gauge.
Measure the temperature of the suction line as it
enters the condensing unit with an “accurate” digital
thermometer.
Subtract the converted pressures temperature from the
pipe surface temperature and that’s the superheat.
Example 1: If the R410A unit’s low pressure was 755kpa
and the suction pipe temperature was 12°C, then: 755kpa
converted to temperature is 2°C, so we then subtract 2 from
the 12°C pipe temperature which results in a superheat of
10°C, indicating in this case that it’s a little high and
requires lowering.
Example 2: If the R22 unit’s low pressure was 380kpa and the
suction pipe temperature was 4°C, then: 380kpa converted to
temperature is -1°C, so the difference between this -1 and the
4°C pipe temperature is a superheat of 5°C, indicating in this
case that it’s OK.
ADJUSTING SUPERHEAT
TX valve: Wind the valve stem IN to increase or wind it OUT to
decrease, then wait approximately 10 minutes for it to settle
before rechecking.
Accurators: Add refrigerant to decrease superheat
or remove refrigerant to increase Superheat, then wait
approximately 10 minutes for the system to settle before
rechecking.
EEVS
Electronic expansion valves (EEVs) can only be adjusted by
changing their internal programme parameters, which are
usually factory set. Chances are the problem is lack of system
refrigerant or a fault elsewhere.
An R134a
There are many influences associated with superheat
thermostatic
like evaporator airflow, evaporator return and supply air
expansion valve.
temperatures and condensing temperature. So if you’re
unsure, you need to contact the relevant supplier for
technical information.
The compressor discharge pipe temperature is in direct
relationship to the superheat. A ball park figure is that this
temperature on a scroll compressor is usually in the 60°C
to mid-70s range when the superheat is acceptable and the
operating conditions are normal.
As a crude rule of thumb, you will find that if the discharge
pipe is cool enough to hold your hand on then you can
almost guarantee that the refrigerant is flooding back (0°C
superheat). If it’s too hot to touch, then its superheat is
probably too high (starving).
You should be able to touch the compressor discharge pipe
for about half a second before the heat is unbearable.
Next time you set the superheat, measure the discharge
temperature and after a few settings, you should see a
pattern emerge.
HUMOUR
THE WARMEST PLACE
Archer: Do you know why it is always so hot in the corner of a room?
Timmy: Why?
Archer: Because a corner is 90 degrees! RACA
www.hvacronline.co.za
RACA Journal I June 2019
37