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t is important to understand what vacuum is and
how it is measured in order to correctly apply it in
the hvacr industry. The dictionary defines vacuum
as “a space with nothing in it.” In the real world
there is no such place because molecules of
various elements are everywhere. It is the density of
these molecules within an enclosed space that we
measure as pressure.
For our purposes, a vacuum is a space where the density
of molecules is less than that of the earth’s atmosphere.
In other words, a space with a vacuum has less
than atmospheric pressure.
The inventor of the mercury barometer, Evangelista
Torricelli, first measured atmospheric pressure in 1643. A
long glass tube with one sealed end was filled and then
inverted in a bath of mercury.
Torricelli found that at sea level the earth’s atmosphere
exerts enough force to support a column of mercury 760
millimeters (29.921 inches) high at a temperature of 32°
F. The resulting empty space at the top of the tube was a
(nearly) total vacuum.
The unit of measurement for vacuum was named for
the inventor, and one atmosphere of pressure is now
referred to as 760 Torr, a measurement of absolute pressure.
If it were possible to achieve a vacuum of zero Torr,
we would have a perfect vacuum or zero absolute pressure.
When vacuum is measured in inches of mercury it
must be remembered that atmospheric pressure (gauge
pressure) is the starting point and that the gauge readings
are negative numbers. Thus, -29.921 inches Hg is a perfect
vacuum. It is not practical to measure a deep vacuum (low
absolute pressure) in inches of mercury because the units
are so large; it is measured in microns. There are 1,000
microns in a millimeter (Torr) and 25,400 microns in 1 inch
of mercury, so the use of this unit makes the measurement
of deep vacuum much more precise.
Applying vacuum to hvacr
Refrigeration and air-conditioning equipment must properly
be evacuated prior to charging to prevent unwanted
molecules - primarily water vapor - from damaging the
system. Moisture in a system can cause freeze-ups in capillary
tubing and expansion valves. The natural tendency
of moisture to cause corrosion is multiplied when it reacts
with certain refrigerants and forms hydrochloric or
hydrofluoric acid.
Refrigeration oils - especially the newer POE and
PAG oils - absorb moisture, and the combination forms
what is commonly referred to as sludge. The most practical
method for removing these molecules is to connect a
high vacuum pump to the system and reduce the absolute
internal pressure (vacuum) to less than 1,000 microns.
Reducing the pressure lowers the boiling point of water
vapor so it can be removed from the system at temperatures
below 32° F.
If a system is wet, reaching this pressure may take
quite some time and require several changes of pump oil
due to water vapor contamination. After evacuating, the
system must be isolated and the pressure must be maintained
at less than 2,000 microns for a reasonable length
of time. This ensures that there are no leaks or significant
contamination present. Either of these conditions
may result in premature system failure and a warranty
service call.
Understanding th
5 сен 2024
