AC compressor question.
09-30-2015, 10:43 AM
#76
Operational (compressor running, condenser fan running) high side pressures at idle are "typically": for R12 refrigerant 2 times ambient plus 15%, and for R134a refrigerant 2 times ambient plus 20%. This is a rule of thumb.
So for example, on a 90f ambient day using R12: 2x90f=180psi x 1.15 = 207psi, and R134a would be 2x90f=180psi x 1.2 = 216psi. Again, a rule of thumb.
Pressures rise above idle. A common check point used is between 1200 and 2000 rpms.
An example of this pressure operational pressure and temperature relationship, taken from the 993 shop manual, is noted below for R134a.
On a 90F ambient day (32.2C) the range high side pressure, the shop manual suggests at 2000 rpm, ranges from 217 psi to 275 psi (15 to 19 bar).
Auto manufacturers typically give a wide range.
And, yes, the chart is not completely linear which could be related to something particular with the 993 model, such as the 2nd condenser fan speed operating at given data value.
So for example, on a 90f ambient day using R12: 2x90f=180psi x 1.15 = 207psi, and R134a would be 2x90f=180psi x 1.2 = 216psi. Again, a rule of thumb.
Pressures rise above idle. A common check point used is between 1200 and 2000 rpms.
An example of this pressure operational pressure and temperature relationship, taken from the 993 shop manual, is noted below for R134a.
On a 90F ambient day (32.2C) the range high side pressure, the shop manual suggests at 2000 rpm, ranges from 217 psi to 275 psi (15 to 19 bar).
Auto manufacturers typically give a wide range.
And, yes, the chart is not completely linear which could be related to something particular with the 993 model, such as the 2nd condenser fan speed operating at given data value.
09-30-2015, 03:51 PM
#77
Chronic Tool Dropper
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Griff--
The 928 WSM has a very similar chart for factory-filled R-134a cars. And it's consistent with some other R-134a diagrams. That sharp spike increase in pressure after the dip is why many believe that R-134a is a 'problem' in cars originally fitted with R-12. That sharply increasing rate continues beyond the range of this chart, where it highlights the need to have effective condenser performance and also a working overpressure protection switch for the times when condenser performance is reduced.
A Molinear diagram for R134a is essential for really understanding and appreciating the non-linear relationships among state, pressure, and temperature in refrigerants. Of course that non-linear characteristic is why we can use them as refrigerants. A quick online search for "R134a Molinear diagram" will net readers more than they probably want to know about this stuff. But it will also perhaps add some guidance on what happens and why certain recommendations are made. "There's a reason for everything.", right?
The 928 WSM has a very similar chart for factory-filled R-134a cars. And it's consistent with some other R-134a diagrams. That sharp spike increase in pressure after the dip is why many believe that R-134a is a 'problem' in cars originally fitted with R-12. That sharply increasing rate continues beyond the range of this chart, where it highlights the need to have effective condenser performance and also a working overpressure protection switch for the times when condenser performance is reduced.
A Molinear diagram for R134a is essential for really understanding and appreciating the non-linear relationships among state, pressure, and temperature in refrigerants. Of course that non-linear characteristic is why we can use them as refrigerants. A quick online search for "R134a Molinear diagram" will net readers more than they probably want to know about this stuff. But it will also perhaps add some guidance on what happens and why certain recommendations are made. "There's a reason for everything.", right?
09-30-2015, 07:55 PM
#78
Hey Doc,
OmG! "Molinear diagrams". Scary as turbo compressor maps and adiabatic heat processes.
Frankly, I don't have a clue as to whom, where or how the 993 manual came up with the funky up and down spike at 30C. But we do know that near 17.5 bar or 254 psi they decided to crank up the condenser fan to high speed.
Maybe if they had variable fan speed control that was linear with the pressure we would not see the hiccup on the chart.
However, in any case as temperature rises so does pressure, and visa versa, so I kinda leave it there. The chart is a graphical aid, at least the red line, in understanding the P&T relationship.
It's my preference when explaining things to keep it simple, hence:
1) A common problem most DIY's don't see, or are aware of, is the presence of residual air in a system that has been vacuumed down. Its normal to think after pulling a vacuum when you see the low side gauge reads -30inHg that that part of the job is done. Residual air can still be present. Moisture can still be present. And residual refrigerant trapped in the refrigerant oil, still present. Air raises pressures and can be easily determined by knowing what the 'bogey' target weight of refrigerant should be in the system. To know this you need to know what the anticipated operational high side pressure should be at a given ambient, hence a 'chart' as noted in post 59, P&T. The other tool you can use is the 'rule of thumb' in post 76 Rule of Thumb. So, for example, if you know you should have about 39 oz in the system, its 90f outside, you only got 24 oz in and you are seeing 300 psi instead of 216-238psi, you got some cold out of the vent, the evap outlet pipe is sweating, your low is 30-40 psi.... you either got air in the system or condenser function issue. To determine which you verify if the condenser function is 'normal' (aka a stock or better system and the fans are running)... all of which is to help guide Ducman through his question in the OP.
2) Before I blame a TEV (exp valve) for abnormal high side pressures or excessive low side pressures, I review the evacuation procedure and weight of refrigerant put in. Again using the P&T chart or rule of thumb I can more easily determine if we have a "mechanically" failed TEV or a TEV that has moisture frozen in it mimicking the same. And, if anyone wants to know 'how the heck can you have moisture in a system that was pulled down to -30inHg ?" It's rather simple. You do it quickly such as here:
The video is kinda sublime but its effective in point.
Hence is why it is necessary to pull several 'vacuums' with refrigerant or nitrogen 'purges' in between to raise the pressure again to change the phase of the 'water' from a solid back to liquid.
3) Drier sight glasses- with R12 and mineral refrigerant oil it was common to see bubbles telling you there is air in the system. With R134a refrigerant and PAG or Ester the refrigerant/oil flow tends to be cloudy or milky white, you can't really see bubbles often. So, back again, to the P&T chart, rule of thumb and count on the weight of refrigerant you put in vs. what the bogey should be.
I'm just going on our working experience here. With respect to why so many have problems with R134a? Experience tells me they are learning the basics (how to use a gauge, pump, and what a P&T relationship is). And, Ducman took some good notes and documented his data. Bravo!
I had R134a in all of my sharks (1978, 1984, 1989), all running stock condensers and fans. Every shark blew ice cold. Lovely cruising machines.
Its all dogmatic to what you experience.
OmG! "Molinear diagrams". Scary as turbo compressor maps and adiabatic heat processes.
Frankly, I don't have a clue as to whom, where or how the 993 manual came up with the funky up and down spike at 30C. But we do know that near 17.5 bar or 254 psi they decided to crank up the condenser fan to high speed.
Maybe if they had variable fan speed control that was linear with the pressure we would not see the hiccup on the chart.
However, in any case as temperature rises so does pressure, and visa versa, so I kinda leave it there. The chart is a graphical aid, at least the red line, in understanding the P&T relationship.
It's my preference when explaining things to keep it simple, hence:
1) A common problem most DIY's don't see, or are aware of, is the presence of residual air in a system that has been vacuumed down. Its normal to think after pulling a vacuum when you see the low side gauge reads -30inHg that that part of the job is done. Residual air can still be present. Moisture can still be present. And residual refrigerant trapped in the refrigerant oil, still present. Air raises pressures and can be easily determined by knowing what the 'bogey' target weight of refrigerant should be in the system. To know this you need to know what the anticipated operational high side pressure should be at a given ambient, hence a 'chart' as noted in post 59, P&T. The other tool you can use is the 'rule of thumb' in post 76 Rule of Thumb. So, for example, if you know you should have about 39 oz in the system, its 90f outside, you only got 24 oz in and you are seeing 300 psi instead of 216-238psi, you got some cold out of the vent, the evap outlet pipe is sweating, your low is 30-40 psi.... you either got air in the system or condenser function issue. To determine which you verify if the condenser function is 'normal' (aka a stock or better system and the fans are running)... all of which is to help guide Ducman through his question in the OP.
2) Before I blame a TEV (exp valve) for abnormal high side pressures or excessive low side pressures, I review the evacuation procedure and weight of refrigerant put in. Again using the P&T chart or rule of thumb I can more easily determine if we have a "mechanically" failed TEV or a TEV that has moisture frozen in it mimicking the same. And, if anyone wants to know 'how the heck can you have moisture in a system that was pulled down to -30inHg ?" It's rather simple. You do it quickly such as here:
Hence is why it is necessary to pull several 'vacuums' with refrigerant or nitrogen 'purges' in between to raise the pressure again to change the phase of the 'water' from a solid back to liquid.
3) Drier sight glasses- with R12 and mineral refrigerant oil it was common to see bubbles telling you there is air in the system. With R134a refrigerant and PAG or Ester the refrigerant/oil flow tends to be cloudy or milky white, you can't really see bubbles often. So, back again, to the P&T chart, rule of thumb and count on the weight of refrigerant you put in vs. what the bogey should be.
I'm just going on our working experience here. With respect to why so many have problems with R134a? Experience tells me they are learning the basics (how to use a gauge, pump, and what a P&T relationship is). And, Ducman took some good notes and documented his data. Bravo!
I had R134a in all of my sharks (1978, 1984, 1989), all running stock condensers and fans. Every shark blew ice cold. Lovely cruising machines.
Its all dogmatic to what you experience.
Last edited by griffiths; 10-03-2015 at 11:36 PM. Reason: Dogmatic
10-01-2015, 01:20 AM
#79
Chronic Tool Dropper
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Got your email, and will let you share your typical pressures and temps. I focus a lot on the theoretical, and can usually nail the predictions under controlled static conditions. Nobody else drives in those conditions though.
The car pretty much spits ice cubes on demand after clearing anyheat-soak, and managed that in Los Angeles regular-driver duty for years. I did the system evacuation overnight in April, when L.A. temps only dropped into the sixties. No issues converting water liquid into water solid apparently. I do have a slight compressor seal leak that shows up when the car sites for a long time, but it's not worth a reseal quite yet. Otherwise it's all perfect as far as system performance. It was cycling the outside air door at 100º+ ambient coming up through the central valley last summer, with the temp slider set at 70º and cruising at 75. Not bad for a black car with no tint.