. . .
I found this excellent write-up in an April 2010 EPA rule analysis:
"126.96.36.199 Compressor Shaft Seal
A major source of refrigerant leakage in automotive A/C systems is the
compressor shaft seal. This seal is needed to prevent pressurized
refrigerant gasses from escaping the compressor housing. As the load
on the A/C system increases, so does the pressure, and the leakage
past the seal increases as well. In addition, with a belt-driven A/C
compressor, a side load is placed on the compressor shaft by the belt,
which can cause the shaft to deflect slightly. The compressor shaft
seal must have adequate flexibility to compensate for this deflection,
or movement, of the compressor shaft to ensure that the high-pressure
refrigerant does not leak past the seal lip and into the atmosphere.
When a compressor is static (not running), not only are the system
pressures lower, the only side load on the compressor shaft is that
from tension on the belt, and leakage past the compressor shaft is at
a minimum. However, when the compressor is running, the system
pressure is higher and the side load on the compressor shaft is higher
(i.e. the side load is proportional to the power required to turn the
compressor shaft) - both of which can increase refrigerant leakage
past the compressor shaft seal. It is estimated that the rate of
refrigerant leakage when a compressor is running can be 20 times that
of a static condition.14 Due to the higher leakage rate under running
conditions, SAE J2727 assigns a higher level of impact to the
compressor shaft seal. In the example shown in the August 2008 version
of the J2727 document, the compressor is responsible for 58% of the
system refrigerant leakage, and of that 58%, over half of that leakage
is due to the shaft seal alone . . ." pp. 75, "Final Rulemaking to
Establish Light-Duty Vehicle Greenhouse Gas Emission Standards and
Corporate Average Fuel Economy Standards Regulatory Impact Analysis",
EPA-420-R-10-009, April 2010.
All of our traditional, belt-and-clutch, air conditioner systems have
needed refrigerant added after about 8-10 years. We've also had a
failed electric clutch with one car too. In contrast, both have been
eliminated with the electric compressor.
. . .
One of the interesting aspects is how few Prius air conditioner
failures occurred after the electric compressor was introduced in
2004. I have seen at least one ordinary air conditioner failure with
the NHW11 that still uses the pulley, belt, electric clutch