superheat

[Shajan John]

can anybody tell what is the normal suction superheat of R134a gas in central air conditioning system..???

[Rob White]

.

Normal is very vague :D

But if it is working with an expansion valve, most
valves come pre-set to about 6 - 8 degs of superheat.
That is measured out of the evaporator. If you measure the
suction closer to the compressor the superheat will be higher.

Regards

Rob

.

[Shajan John]

6-8 deg f or deg c ... txv is using in this chiller, am measuring suction near compressor..

[moideen]

6-8 deg f or deg c ... txv is using in this chiller, am measuring suction near compressor..
centigrade.suction superheat

[Brian_UK]

It is degrees Kelvin not C or F.

[moideen]

It is degrees Kelvin not C or F.
Dear brian, please check example in DEG.C.
Suction line temp = 12C.
Saturated suction temp = 4c.
Superheat = 8c.
In deg.Kelvin also same value .as per my understanding Kelvin is positive value. it is better to use in minus temp to calculate superheat.

[nike123]

Use in conjunction with Celsius[edit (http://en.wikipedia.org/w/index.php?title=Kelvin&action=edit&section=3)]
http://upload.wikimedia.org/wikipedia/commons/thumb/2/22/CelsiusKelvinThermometer.jpg/220px-CelsiusKelvinThermometer.jpg (http://en.wikipedia.org/wiki/File:CelsiusKelvinThermometer.jpg)

A thermometer calibrated in degrees Celsius (left) and Kelvin (right).


In science and in engineering, degrees Celsius and kelvins are often used simultaneously in the same article (e.g. "...its measured value was 0.01028 °C with an uncertainty of 60 µK..."). This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures and the magnitude of the degree Celsius is exactly equal to that of the kelvin.[10] (http://en.wikipedia.org/wiki/Kelvin#cite_note-sib222-10)Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states, "a temperature interval may also be expressed in degrees Celsius", the practice of simultaneously using both "°C" and "K" remains widespread throughout the scientific world as the use of SI prefixed (http://en.wikipedia.org/wiki/SI_prefix#List_of_SI_prefixes) forms of the degree Celsius (such as "µ°C" or "microdegrees Celsius") to express a temperature interval has not been widely adopted.[4] (http://en.wikipedia.org/wiki/Kelvin#cite_note-res313-4)


Wikipedia ........

[Rob White]

Dear brian, please check example in DEG.C.
Suction line temp = 12C.
Saturated suction temp = 4c.
Superheat = 8c.
In deg.Kelvin also same value .as per my understanding Kelvin is positive value. it is better to use in minus temp to calculate superheat.


You are right up to a point Moideen but what Brian said is correct :)

When we explain a difference in temperature we explain it as degrees Kelvin
that means you are talking about a difference between two temperatures and not
the temperature in Deg C.

If the saturation temp is 0 deg C and the suction is 8 deg C then the difference is 8K
(8 degrees Kelvin).

To avoid confusion temperature difference is always expressed as Degrees Kelvin.

Regards

Rob

.

[moideen]

You are right up to a point Moideen but what Brian said is correct :)

When we explain a difference in temperature we explain it as degrees Kelvin
that means you are talking about a difference between two temperatures and not
the temperature in Deg C.

If the saturation temp is 0 deg C and the suction is 8 deg C then the difference is 8K
(8 degrees Kelvin).

To avoid confusion temperature difference is always expressed as Degrees Kelvin.

Regards

Rob

.
absolutely correct.i agree with you.

[Shajan John]

17 psi suction and suction line temp is 7-8 degree C.... R134a gas.. FILTRINE CHILLER using for MRI machine

[moideen]

17 psi suction and suction line temp is 7-8 degree C.... R134a gas.. FILTRINE CHILLER using for MRI machine
suction superheat= 15.5K

[Shajan John]

is it ok sir for this chiller

[moideen]

Check the suction superheat value when pull down is complete and steady full load operation has been established. 15.5 is high. Bring down to 6-8k.

[Shajan John]

pls mention in deg c or deg f........

[nike123]

pls mention in deg c or deg f........
It is already explained earlier in this thread.


This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures and the magnitude of the degree Celsius is exactly equal to that of the kelvin.[10 (http://en.wikipedia.org/wiki/Kelvin#cite_note-sib222-10)

[Glenn Moore]

Hi 17psi = (minus) -8 degrees C, using R134A
Suction pipe temperature 7 to 8 deg C
So (minus)-8 Saturation temperature to +7 to 8 deg C actaul pipe temperture= approx15.5 degK superheat
R134A is a very stable refrigerant and normally I would expect to get a steady superheat of between 5 and 8 deg k using a thermostatic expansion valve
If using an Electronic expansion valve (stepper valve) I would expect a superheat of between 4 to 7deg C superheat.
Your evaporating Pressure/Temperature ie 17psi/ (minus)-8 degC is low for an air conditioning system as AC systems normally work in the positive temps ie +5 deg C not in negative temps ie minus 8
By opening your expansion valve a few turns you should see the suction guage rise a few psi and the suction pipe temperature should fall a few deg C.
So in reality you should see a suction pressure of about 27psi/0 degC with an actual pipe temperature of about 6 degC .So 0degC to 6 degC = 6degK actual superheat
This all depends if the system is fully charged , the drier is clean etc etc
I always fit electronic expansion valves as they always give superior control,a more stable suction condition and give better system efficiency by running with a lower superheat condition thereby utilising the evaporator surface more efficiently
Are you cooling air or water or some other fluid?
Kr Glenn