"Steve G" <NospamforSteve@Steve-Garner.com> wrote in message
news:LRWxd.557009$nl.118459@pd7tw3no
> "Abeness" <news@nada.x> wrote in message
> news:VOidnRkII9T3glXcRVn-2A@rcn.net...
>> Philip wrote:
>>> In fact, A/C systems start cycling the compressor at no less than
>>> 38 degrees F. SO... when the air coming into the evaporator
>>> reaches this low temperature, the compressor is cycled or shut
>>> OFF if the air temperature entering the evaporator continues to
>>> drop. Otherwise the evaporator would become a block of ice,
>>> which would prevent any air from reaching the heater core (in
>>> nearly all cases).
>>
>> Now THIS makes perfect sense. (I'd have to look up the temp at
>> which refrigerants go liquid to confirm or deny Steve G's belief.)
>> I'm almost certain that the A/C on my old 85 Pontiac would cycle
>> at any temp,
>> though. I'll check up with the Honda soon.
>>
>> Anyway, I doubt it would be that difficult (in the original
>> design) to construct an airflow system that would shunt warmer air
>> around the evap unit when the temp dropped in winter, in order to
>> preserve the dehumidifying characteristics of an A/C-based system.
>
> You would also have to direct warm air to the entire low pressure
> side of the ac system to prevent the condensing of refrigerant gas
> back into liquid. I live in a climate of 10 months of winter and 2
> months poor sledding, have been a licensed mechanic since about '72
> and have never seen an ac comp cycle in ambient temps colder than
> somewhere around that 40 degree mark. and remember from my days in
> tech school that is the reason.

Question for you: Water condensation *on* the evaporator surface freezes at
32°F. At what temperature does low pressure refrigerant gas condense to a
solid liquid? Keep in mind that it is common to see frost all over much of
the exiting line from the evaporator leading to the compressor inlet while
the A/C system is functioning normally. Clue: it's lower than 32°F.

> Don't forget when researching boiling point of refrigerant to
> factor in the pressure. Adding pressure also raises the boiling
> point. As I'm typing this I'm also recalling that there are
> cautions that when servicing/charging the system to not allow low
> side pressure to drop too low for fear of damaging the compressor.
snip
>Steve

Not entirely accurate. The compressor's inlet side can drop into a slight
vacuum when a suction throttling valve is used on the outlet side of the
evaporator instead of an expansion valve to regulate refrigerant flow into
the evaporator. In any case, the low side pressure safety switch (if/when
there is one installed) will prevent such a condition from occurring on
systems equipped with only an expansion valve. Excessive high side pressure
is FAR more damaging to compressors which is why a high pressure safety
switch is present on all but the oldest automotive systems.
--

- Philip