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Superheat is the temperature of the vapour
above its boiling point.
As you can see by the diagram in Fig 1 water
boils at 100C. Any heat added to the steamabove this temperature
superheats it (and anytemperature removed from the water below
this
temperature subcools it). Remember superheatdoes not mean Hot it
simply means above itsboiling point. R134a boils at approx -26C
at
atmospheric pressure, so whenever R134a iswarmer than -26C and
at atmospheric pressureit is superheated.
Fig 1
We calculate superheat by taking the difference between
theactual exit temperature of the refrigerant from theevaporator
(T2) and the saturated evaporating temperature
(T1). It should be between 5 to 7K. Fig 2 shows us this.
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
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 andefficient refrigeration the
correct degree of superheat must be achieved in the evaporator. Too
much andyou will have insufficient capacity, too little and you
could carry liquid out of the evaporator into thecompressor leading
to mechanical failures. Subcooling is also important and will be
covered in a futureissue of Service Matters.
Stephen Benton
100C (Water boils)
Superheated
Subcooled
100C (Water boils)
Superheated
Subcooled
Measuring SuperheatThis is very simple, all you need arethe
following tools: -
A set of accurate gauges An accurate electronic
thermometer with a suitabletouch/contact probe
A refrigerant comparator
(comparators are availablefree of charge to all SESmembers)
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Fig 3
Why is Superheat Important?
Example Superheat - A
R134a Evap pressure, 1.5 bar g Evap temp, -4 C Exit temp, 5C
Superheat, 5 (-4) = 9K
Open or close valve to correct
superheat (5K)?
5C5C
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. Thereis usually a
connection if you look.
2. Take the suction pressure and using yourcomparator convert it
into a saturated
temperature (T1). Check you are using thegauge scale and NOT the
Absolute
scale. When measuring superheats ofblends make sure you use the
Dew or
Vapour scale.3. Take the actual temperature using your
thermometer adjacent to the expansiondevice phial or sensing
probe (T2).
4. Take the saturated evaporatingtemperature (comparator) from
themeasured evaporator outlet temperature,
SH = T2 T1.
A superheat of between 5 and 7K willensure that all the liquid
refrigerant isboiled off in the evaporator and will not
carry over to the compressor.
Fi 4
Answer:
Open the valve to reduce the
superheat check with
manufacturers instructions on
valve and always re measure the
actual superheat after every
adjustment. Allow time for the
valve to settle after adjustments.
Example Superheat - Q
R134a
Evap pressure, 1.5 bar g Exit temp, 5C
Superheat? 5C5C
Optimumevaporatorperformance
Zero liquidreturn tocompressor
Optimumevaporator
performance
Zero liquid
return to
compressor
