Understand
that vacuum is really the absence of air and positive pressure. There are
various levels of vacuum, but when it comes to auto A/C systems, it’s
really about not having any air in the system.
When
we talk about not having any air in the A/C system, we mean none at all!
Whenever you discharge the A/C system, regardless of whether it’s been
open or not, you must pull a complete deep vacuum before recharging the system.
Even removing a valve core would allow small amounts of air into the system,
and those small amounts of air can create problems with A/C cooling performance
and cause increased high side pressures.
The
problem with air is that it’s a non-condensable gas. Above that, air
holds moisture. That adds another problem.
With
respect to just having air in the system, understand that because it’s
a non-condensable, it will not circulate throughout the system. Typically,
it will sit at the top of the A/C condenser, taking up part of the cooling
capacity of the condenser. When you limit the cooling capacity of the A/C
system, you automatically have increasing temperature of the refrigerant
(can’t cool as well, right?). That leads to increased pressures of the
A/C system. Remember, refrigerant has a pressure/temperature relationship.
When we increase the temperature, we automatically increase the pressure.
Those increased pressures will cause a variety of problems ranging from louder
than normal compressor operation because of excess strain to premature compressor
failures.
Other
problems related to air is the fact that air contains moisture. Yes, automotive
A/C systems include driers to help trap and contain moisture, but also understand
that every drier or accumulator can only hold a specific amount of moisture.
When the drier becomes contaminated and can no long hold moisture, that moisture
is allowed to circulate and mix with refrigerant. When that happens, a chemical
reaction takes place that causes the formation of harmful acids. Those acids
corrode auto A/C components like condensers and evaporators. In addition
to the premature failure of those components, the corrosion itself becomes
yet another contaminant in the A/C system. That corrosion will travel through
the system and start to restrict refrigerant flow at various components like
the thermostatic expansion valve or the orifice tube. Now remember, typically
it takes less that 1/10th of a teaspoon of debris to cause a restriction
that could quickly lead to a catastrophic compressor failure.
Now,
with respect to pulling just a quick vacuum (to maybe 26 or 27 inHg), understand
that you are still not doing anything to help the A/C system. Measuring vacuum
in inches is not very accurate. In order to better understand vacuum, you
must understand the measurement of vacuum in microns. A Micron is just another
scale of measurement, but it is far more accurate. (When measuring in microns,
the lower the number, the better the vacuum). One Micron is one millionth
of a millimeter. Therefore, 0 microns is a perfect vacuum. When auto A/C
service manuals refer to pulling deep vacuum, they typically mean a level
of vacuum that is under 500 microns. Water does not start to boil until a
level of vacuum under 1,500 microns is reached.
Understanding
that, you should know that pulling vacuum to 26 or 27 inHg will do absolutely
no good for the A/C system. In fact, when you look at vacuum gauge readings
even closer, you will see why.
28.9
inHg of vacuum is equal to about 25,000 microns. That’s not even enough
to boil water!
29.9
inHg of vacuum is equal to about 100 microns. Now that’s deep vacuum!
The
problem is that when watching vacuum on your typical compound low pressure
gauge, you really can not tell the difference between 28.9 and 29.9 inches
of vacuum!
All
of those details aside, it’s important to understand that whenever servicing
any auto A/C system, it is very important that you pull deep vacuum for at
least 1/2 hour before adding refrigerant. Regardless if whether you’ve
opened the system or not, you must pull deep vacuum. |
This
was probably one of the best books on recharging air conditioning that I
have seen...
