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View Full Version : hi new to this i have a couple of questions about presure temp







superduke
21-01-2013, 03:39 PM
hi there ,

I am studying at the moment and have loads of questions the first one is about pressure temp

on my chart for pressure temp it tells me that at 0oc the pressurre of r134a is 2.98 bar
which is straight forward enough , but if my gagues tell me that the suction pressurre is 2.98 bar and my temp is zero in my evaporator were it is changing phase what will my air off be , it wont be 0oc

is there a rule of thumb to tell you what your air off will be

for example i want my freezer down to minus 10oc what pressurre would i want my r134a at to get me that temp

Rob White
21-01-2013, 05:09 PM
hi there ,

I am studying at the moment and have loads of questions the first one is about pressure temp

on my chart for pressure temp it tells me that at 0oc the pressurre of r134a is 2.98 bar
which is straight forward enough , but if my gagues tell me that the suction pressurre is 2.98 bar and my temp is zero in my evaporator were it is changing phase what will my air off be , it wont be 0oc

is there a rule of thumb to tell you what your air off will be

for example i want my freezer down to minus 10oc what pressurre would i want my r134a at to get me that temp

The evaporator is always the coldest and we normally work to a Rule of Thumb of 10degs difference.
The evaporator will be about 10 degs colder than the air off the evaporator.
If you want a case temp of -10 degC then you will be evaporating at approximately at -20 degC.

But in your calculation you have mixed up gauge pressure and absolute pressures.

We measure scientifically in pascal's (named after the guy who invented pressure :D )
and there are two pressures there is the absolute pressure and then there is gauge pressure.

Gauge pressure would show zero if you open your gauges up to the air you are stood in (under normal atmospheric conditions), but absolute would read 1 bar (or 100,000 and a bit pascal) so if you are looking
on a compariter don't confuse the two pressure.

Gage pressure is always the lower of the two and that is the one you use to measure the working system.
Your 0 degC at 2.9 should be 0 degC at 1.9 (2 bar to you and me) when reading on a working system
using gauges.

Your "for example i want my freezer down to minus 10oc what pressurre would i want my r134a at to get me that temp"

Case temp -10 degC = evaporating at about -20 degC = pressure of 0.3 to 0.4 barg.


Regards

Rob

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superduke
03-02-2013, 05:07 PM
aha that is superb , thank you very much .

superduke
03-02-2013, 05:08 PM
and bar gauge as well i had not factored that in .

Rob White
03-02-2013, 05:41 PM
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Nice one.

Regards

Rob

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