Just Better Deep Vacuum Principles and Applications User Manual

Deep vacuum, Its principle and application

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29" 30"


of an inch

So what’s

a micron?


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


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,