Just Better Deep Vacuum Principles and Applications User Manual
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.
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
So what’s
a micron?
29" 30"
1/25,400
of an inch
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 evacuation. Compound this with the fact that service valves,
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 includes this reading, barometric pressure, along with the temperature.
When an area is covered by a HIGH, it
translates into high barometric pressure and vice versa for a LOW.
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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
dry ambient air is admitted to help prevent the moisture
vapor from condensing in the oil.
So far, we have defined our pump requirement as
follows: 2-stage, rotary vane; blankoff valve; gas ballast
valve; 1-1/2 to 10 CFM. A system is evacuated to
between 300 and 400 microns so obviously these
pumps should be able to produce vacuum in the low
micron range with a safety factor of at least 25 microns
total absolute. Thus, the pump should be able to achieve
vacuum readings of at least 25 microns total absolute.
We should also look for light weight and rugged construction because we all know the vacuum pump will be
at our side as we climb those ladders to the roof top.
Finally, when checking out pumps, look at safety.
Belt driven units should never be used without belt
guards—if you don’t give a darn about your own fingers,
etc., give children and others exposed a chance.
Hospitals and court rooms around the world are full
because of this negligence.
The Electronic Vacuum Gauge
Coupled with good procedures which we will get
into later, the electronic gauge tells
1
0
us positively that we have a
noncondensable and a leak free
ATM
Last Inch
Of Pressure
0
1
0
2
0
system. In general these gauges are
heat sensing devices, in that the
sensing element which is mechani-
3
0
cally connected to the system being
evacuated generates heat. The rate
at which this heat is carried off
The last inch of
pressure, as
indicated on the
compound
gauge is 25,400
microns.
changes as the surrounding gases
and vapors are removed. Thus, the
output of the sensing element (either
thermocouple or thermistor)
changes as the heat dissipation rate
changes. This change in output is
indicated on a meter which is
calibrated in microns of mercury.
Evacuation is complete when a system holds at 500
microns. The compound gauge only indicates that a
vacuum is being produced. The vacuum gauge on the
other hand, is the only tool for accurately reading that
low pressure.
Vacuum Gauge
Selection And Accuracy
The most important feature of all is range. If the
micron gauge only indicates from 50 to 1,000 microns,
The electronic vacuum gauge
is the least purchased deep
vacuum tool. Yet, without
this instrument you might
just as well forget about
deep vacuum
altogether.
Battery
Battery/Electeric
you will not be able to determine whether you are
pumping against a leak or against moisture. Look for an
instrument that reads from 50 microns to at least
9,000 microns.
A digital display with easily read numbers gives you
instant and continuous readout, whereas a gauge with
color-coded lights, displays the reading “within a range”
of microns. You have a “wait” period to see whether the
system is going up or down in microns.
Portable micron gauges typically operate from
battery power and should have a low battery sensor.
Some models have AC adaptor capability so you won’t
run out of power on the job.
Another feature to look for is a sturdy case to
protect the instrument. Finally, when you buy instruments of this type, remember that you are really only
buying answers, and the instrument should give you
these answers quickly and accurately. You get paid for
adjusting refrigeration systems, not your tools.
As already noted, we are talking about accuracy
when we talk about micron type gauges. Gauge accuracy is affected by two factors. Extreme temperatures
especially with exposure to the summer sun on a hot
roof top or pavement and sensor contamination.
The vacuum sensor is factory calibrated on air. If
refrigerant gas or oil is drawn into the vacuum sensor of
a remote reading unit or unit connected to the pump
during the system evacuation, the gas will cause an
erroneous reading. Any oil entering into the vacuum
sensor via hose will also affect gauge accuracy. Improper shut down of pump after evacuation and loss of
power will suck back oil and contaminate the hose and
micron gauge.
A hose used for charging or testing will contain
droplets of system oil spurted into the hose when the
schrader valve is opened. If this same hose is used on
the hookup to the gauge, oil will collect in the gauge
sensor. This can be prevented by using a dedicated
hose, preferably O-Ring type, for evacuation.
Evacuate Through The Gauge Manifold
Evacuate through the gauge manifold if, and only if,
it is O-ring sealed, piston construction. Other types leak
under vacuum. Next look at the design of the center
port. In order to handle the full capacity of both the high
and low side, the center intake should have double size
flow path throughout its length. All J/B 1/4" manifolds
have this feature. We suggest the fitting be replaced with
a 3/8mf x 1/8mp. You will now have a fulll flow
3/8" to the vacuum pump. You also have the
option of using the M4-Series manifold which is
designed to evacuate, charge or test a system
without disconnecting hoses and features
a 3/8" fitting.
Leak-Proof Hook-up
Deep vacuum has it own unique properties
which requires leak-proof design not only in the
manifold but in all components. The only connecting lines that are absolutely vacuum tight are
soft copper tubing or flexible metal hose. Charg-
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