AC- subcooling

[T77911S]

anyone ever check sub-cooling on their AC system.

I was just curious what it would or should run so I checked my BMW (TXV) and it was 20 degrees SC.
low side was a tad low- 30 at 1500rpm
r134
hi ide was 240
air temp- hot as a mother, 92 or so
the AC blows really cold.

thought it might help knowing what it should be when I charge my 930.

[DRACO A5OG]

Assuming BMW is R134a, it seems a bit low on fluid.

Are you using r12 or r134a in the 930?

[T77911S]

134 in bmw.
yea, just curious if anyone has checked subcooling.

[rick-l]

What are you measuring? Would super heat would be the low side temperature above the temperature of saturated r134? Sub cool the temperature of the high side below PT of r134?

The TXV is a feedback device that controls the super heat at 10 to 20 degrees as best it can.

[billybek]

Sub cooling on the liquid line is a measure that can change dramatically with operating conditions.
Basic explanation.
If you have a very high load on the system (high air temperature on the inlet side of the evaporator) the refrigerant will be boiled off and the suction superheat will be higher than under ideal conditons.
Due to the high superheat, the TXV (metering device) will open more fully.
This will result in less sub cooling on the liquid line before the metering device.
Very low load conditions will result in the metering device closing resulting in more subcooling as the refrigerant is held back in the liquid lines and condenser resulting in more subcooling.
To be honest, I am not sure that it is a measurement that auto mechanics pay much attention to.

[T77911S]

sub cooling is done at the condenser and is not dependent on the evap load.
its the only way to really tell if a TXV system is charged properly because the TXV is controlling the superheat to a fixed amount.
but, you have to know what the subcooling amount is.
20 degrees on my BMW seemed high with the pressures looking like the system is under charged,

my plan is to do some testing on my 930 when I charge it, part of the reason I want to keep it stock and perhaps put r134 in it and take subcoolling measurements to see exactly how much needs to go back in, but I need a subcooling temp or something to base it on,

I am crossing HVAC knowledge with auto AC. auto AC has to work under a wider range of conditions than HVAC so I am not sure if the subcooling will work.

[rick-l]

I see. So if you take the temperature of the tube at the inlet to the TXV on a very hot day and have 20 degrees sub cooling you know you have liquid going into the TXV.

Interested in the results in a 911.

[T77911S]

yes
you know, I did check it at the condenser, probably why it is hi.
I was really expecting around 12-15 degrees.
I will check it again after the dryer.