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04-18-2011, 12:14 AM   #16
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Quote:
 Originally Posted by JJboy PT chart indicate the temp of the environment where is the refrigerant. The pipe temperature it's the measured temp
I think technicians check the suction pressure on the system and compare it with the pressure on the PT chart to see if their actual reading on the system is off and they do the same thing with the discharge pressure. Am I right sir?
I mean for example at 72F ambient/environment temperature the PT chart for R22 shows the pressure as 125 but when the tech actual reading is way below 125, this means that the suction pressure in the system is low. May be there is some restriction in the system. Please correct me if I am wrong. Again, thanks for all ur help.

 04-18-2011, 12:24 AM #17 Member     Join Date: Oct 2010 Location: Austin - Texas Posts: 1,402 Rewards Points: 500 Yes, with these values ​​we can determine if the unit is undercharge, overcharge, with restrictions, contaminated. etc __________________ **Always kill the power ** Hot air rises, but heat will always move from higher to lower temperatures. ** Real man shoot in manual. ** If it ain't grounded, it ain't dead.
 04-18-2011, 06:02 AM #18 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,574 Rewards Points: 6,112 The pressure and temps on the PT charts are saturation pressures and temps. They are not suction or discharge temps or pressure. this is the pressure the refrigerant will exert at X degrees provided that both liquid and vapor are in the container(as said earlier). The chart is used to determine the amount of subcool or superheat the system is operating at. Not what it should be operating at. Take a new cylinder of R22, at 32°F it will have 58PSIG in it. At 83°F the same tank with the same amount of refrigerant in it will have a PSIG of 180. The reason is that it is a saturated refrigerant. Meaning that the refrigerant is existing in the cylinder as both liquid and vapor. As the refrigerant warmed up, more of the liquid boiled off to vapor, and the vapor takes up more space then the liquid did, so the pressure increased. But the total mass in the cylinder remained the same. You may want to read up on "saturation".
04-18-2011, 01:27 PM   #19
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Quote:
 Originally Posted by beenthere The pressure and temps on the PT charts are saturation pressures and temps. They are not suction or discharge temps or pressure. this is the pressure the refrigerant will exert at X degrees provided that both liquid and vapor are in the container(as said earlier). The chart is used to determine the amount of subcool or superheat the system is operating at. Not what it should be operating at. Take a new cylinder of R22, at 32°F it will have 58PSIG in it. At 83°F the same tank with the same amount of refrigerant in it will have a PSIG of 180. The reason is that it is a saturated refrigerant. Meaning that the refrigerant is existing in the cylinder as both liquid and vapor. As the refrigerant warmed up, more of the liquid boiled off to vapor, and the vapor takes up more space then the liquid did, so the pressure increased. But the total mass in the cylinder remained the same. You may want to read up on "saturation".
Thanks beenthere for the detailed explanation. One question: when does the ambient temperature used on the PT chart? I mean for an ac system in a room.

04-18-2011, 02:28 PM   #20
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Quote:
 Originally Posted by Samart Thanks beenthere for the detailed explanation. One question: when does the ambient temperature used on the PT chart? I mean for an ac system in a room.
Ambient temperature has nothing to do with a PT chart. It has to do with the refrigeration pressure and temperature.

The only real reason I use a PT chart for is when my gauges are on a circuit and it's a refrigerant that my gauges don't give me a temperature for. Say 407c. I get my pressure because that will always be the same on any Gage. I then go to my PT chart and find the corresponding temperature for that pressure under that refrigerant.

You may be talking about a psychometric chart? Where you have dry bulb, wet bulb, enthalpy and other items on the chart.

 04-18-2011, 05:20 PM #21 Member   Join Date: Oct 2007 Location: Manitoba, Canada Posts: 400 Rewards Points: 380 When we put guages on your system and check the pressures we are seeing what the operating characteristics are in your system. When my suction guage says 60 Psi on an r-22 system in an outdoor ambient of 75F it is telling me that the refrigerant in the evaporator is boiling off at about 34 F. This tells me that you are getting pretty close to max cooling out of the coil. As the load on the evaporator rises the suction pressure will also rise. Airflow through the evaporator is key for this to work properly. The discharge pressure tells me how hard the compressor is working. Typically the discharge should be approximately 30F above ambient temps on the chart, so if it were 75F outside I would add 30 F to make it 105 F and the corresponding pressure would be about 210 psi. of course the condition of the condenser coil (clean or dirty) would impact this pressure significantly. I would also use these pressures and temps to calculate subcooling and superheat values to check the charge in your system as well as things like compression ratio of the compressor, which would give an indication of it's efficiency etc. Getting too deep. Hope this is what you were looking for.
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04-18-2011, 06:36 PM   #22

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As the others said, ambient temp and the PT chart don't really get used together with a refrigeration system/circuit. The only time it is really used with ambient temp, is when your checking the calibration of your gauges.

Perhaps if you explain what type of charging problems concerns you have, we can better tell you how to check charge of a system.

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