Can someone 'splain me POA valve vs original?

[davepl]

My very rough understanding of AC systems is that when the pressure on the high side drops below a threshold, the sensor closes a circuit that latches the clutch on the compressor.

When the pressure comes up high enough again, the compressor kicks out, or something like that.

Is that correct? If not, how does it work?

And what's the difference between that setup (which I think is original) vs. a POA valve, and how does that work?

Thanks in advance for the tutorial!

[tim71]

If you go to Original Air Group website, they do a nice job of explaining the AC system with a POA valve. I am converting my ac over to 134a and they recalibrated my POA valve. There is no recycling of the compressor in our ac systems, when the ac is on the compressor is always in use.

[Bigdav160]

Since an orifice tube system is a fixed opening, it is common to have a low pressure switch that cuts the compressor off if the pressure (temp) drops too low that would cause the evaporator to freeze. It's a "cycling system.

Since an thermostatic expansion valve has a self-adjusting opening, it is common to have a high pressure switch to prevent compressor damage from high pressure if the expansion valve sticks/blocked closed. It's typically not a cycling system. The POA's job was to keep the evaporator's temp above freezing.

[tdangle]

This is something I really don't understand. The POA opens at a specific pressure allowing flow back to the compressor. If R134 runs at a higher pressure compared to R12 why is the POA valve actually adjusted to a lower pressure. I must be missing something here.

[clay68c10]

134 runs higher on the pressure side, and lower on the suction side, relative to r12. It's just not as efficient at transferring heat as 12.

All the late 60s and early 70s gm ac systems are POA. Cycling clutch came out later.

[Bigdav160]

Originally Posted by tdangle

This is something I really don't understand. The POA opens at a specific pressure allowing flow back to the compressor. If R134 runs at a higher pressure compared to R12 why is the POA valve actually adjusted to a lower pressure. I must be missing something here.

30 psi with R12 is 32 degrees.

With R134a, 30 psi would be 35 degrees.

Lowering the pressure allows the core to get colder.