Issue 6 - Sept 2005
Technical information for the Institute of Refrigeration Service Engineers’ Section
Welcome to this issue of Service Matters
Welcome to this issue of Service Matters in which we cover the mystery of superheat. For reliable and
efficient refrigeration the correct degree of superheat must be achieved in the evaporator. Too much and
you will have insufficient capacity, too little and you could carry liquid out of the evaporator into the
compressor leading to mechanical failures. Subcooling is also important and will be covered in a future
issue of Service Matters.
Stephen Benton
Superheat is the temperature of the vapour
above its boiling point.
Fig 1
As you can see by the diagram in Fig 1 water
boils at 100°C. Any heat added to the steam
above this temperature superheats it (and any
temperature removed from the water below this
temperature subcools it). Remember superheat
does not mean ‘Hot’ it simply means above its
boiling point. R134a boils at approx -26°C at
atmospheric pressure, so whenever R134a is
warmer than -26°C and at atmospheric pressure
it is superheated.
We calculate superheat by taking the difference between the
actual exit temperature of the refrigerant from the
evaporator (T2) and the saturated evaporating temperature
(T1). It should be between 5 to 7K. Fig 2 shows us this.
Superheated
100°C (Water boils)
Subcooled
Measuring Superheat
This is very simple, all you need are
the following tools: • A set of accurate gauges
• An accurate electronic
thermometer with a suitable
touch/contact probe
• A refrigerant comparator
(comparators are available
free of charge to all SES
members)
Fig 2
Service Matters is produced for the IOR Service Engineers’ Section by Cool Concerns Ltd
Please contact Stephen Benton with any feedback at [email protected]
For more details of Section membershipo see the web site at www.ior.org.uk
Issue 6 - Sept 2005
Follow the steps below (also see Fig 3 & 4):
1. Put you gauges on the suction pipe as close
to the evaporator outlet as possible. There
is usually a connection if you look.
2. Take the suction pressure and using your
comparator convert it into a saturated
temperature (T1). Check you are using the
‘gauge scale’ and NOT the ‘Absolute’
scale. When measuring superheats of
blends make sure you use the Dew or
Vapour scale.
3. Take the actual temperature using your
thermometer adjacent to the expansion
device phial or sensing probe (T2).
4. Take the saturated evaporating
temperature (comparator) from the
measured evaporator outlet temperature,
SH = T2 – T1.
Fig 3
Fig 4
Example Superheat - Q
•
•
•
•
R134a
Evap pressure, 1.5 bar g
Exit temp, 5°C
Superheat?
A superheat of between 5 and 7K will
ensure that all the liquid refrigerant is
boiled off in the evaporator and will not
carry over to the compressor.
5°C
Why is Superheat Important?
•• Optimum
Optimum
evaporator
evaporator
performance
performance
•
Zero
liquid
• Zero liquid
return
to
return to
compressor
compressor
Example Superheat - A
•
•
•
•
•
R134a
Evap pressure, 1.5 bar g
Evap temp, -4 °C
Exit temp, 5°C
Superheat, 5 –(-4) = 9K
5°C
• Open or close valve to correct
superheat (5K)?
Answer:
Open the valve to reduce the
superheat – check with
manufacturer’s instructions on
valve and always re measure the
actual superheat after every
adjustment. Allow time for the
valve to settle after adjustments.