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29" 30"
1/25,400
of an inch
So what’s
a micron?
DEEP VACUUM
Its Principle and Application
With deep vacuum, we are sure of our results before we
leave the job. No more waiting to see if we get a call back to
determine the results of our work. Deep vacuum is the only
method we can use to tell us, for sure, that a system is
thoroughly dry and free of noncondensables and leaks.
when provided, have 1/4" male flare ports which only
have a 3/16" orifice.
We also know that the only way to get more flow
through a given orifice is by increasing the pressure
across that orifice. But does a pump create pressure
that increases the flow? No. We tend to forget two basic
principles. A vacuum pump creates a void toward which
the system pressure flows. The second point is that as
pressure decreases in the system during evacuation,
flow decreases. Therefore, it’s impossible for us to
increase pressure or flow through our gauge ports with
a larger pump.
Pumps in the 1-1/2 to 10 CFM class are adequate
to handle 99% of our work. As a rule of thumb, the CFM
rating squared equals the maximum system tonnage. A
7 CFM pump is rated for 49 tons; 3 CFM pump is rated
for 9 tons. They are all that should be purchased for
service and installation. In many cases, depending on
the system line sizes of large tonnage systems, it is
better to put two or more of the small, easily handled
pumps at different locations. This will overcome some of
the pressure drop problems and actually be faster than
a single large pump.
Pump Construction
Rotary vane deep vacuum pumps are readily
available and are best suited for our work. Piston type
pumps, because of the clearance necessary between
piston and head, are incapable of producing a deep
vacuum or at best are very inefficient. Many single stage
compressors, similar to a hermetic compressor will not
evacuate a system into a micron range, the last inch of
pressure on the compound gauge, nor will it condense
any moisture vapor in the system.
Two stage pumps (2 pumps in series) have the best
record in our business because they are capable of
producing consistently lower pressures and are
much more efficient when removing moisture
vapor. The pump should be equipped with a
blankoff valve which allows us to perform the
isolation test (pressure rise) which is required in
deep vacuum procedures.
The gas ballast feature should be on all
pumps for refrigeration. At the beginning of
evacuation, water vapor is quickly removed and if
a system is laden with moisture, can very quickly
contaminate the oil. Through the gas ballast, a
fine metering valve connected to the second
stage of the pump, a small amount of relatively
Measuring Evacuation– Microns Or Inches?
A micron is a measurement of pressure starting
from a perfect vacuum (no pressure) expressed in linear
increments. One inch equals 25,400 microns. It should
be noted at this point that when we discuss vacuum in
terms of microns, we are referring to total absolute
pressure as opposed to gauge pressure. Besides using
a more accurate unit of measure (you can’t read frac-
tions on a bourdon tube type gauge),
we are also starting from the same
measuring point (theoretical perfect
vacuum).
The bourdon tube type gauge, you
will also remember, uses atmospheric
pressure as its reference point, which
is constantly changing during the day.
The weather forecaster always in-
cludes this reading, barometric pres-
sure, along with the temperature.
When an area is covered by a HIGH, it
translates into high barometric pres-
sure and vice versa for a LOW.
Pumps And How To Select Them
Deep vacuum pumps are the first item to come to
mind when we think of vacuum tools. Unfortunately the
first mistake is usually made in the selection of these
pumps with reasoning that goes like this— “The larger
the pump I get, the faster I can do the job.” Pump
capacity has very little to do with evacuation time in
refrigeration systems, as is easily seen when we
examine the following.
The refrigeration system itself is constructed of
several feet of small diameter tubing with return bends
and metering devices to offer restriction during evacua-
tion. Compound this with the fact that service valves,
(9 pages)
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