Fundamentals of vacuum

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TECHNOLOGIES > VACUUM TECHNOLOGY
Fundamentals of Vacuum
For a deeper look at vacuum systems, read "Putting vacuum to
work," "Squeeze energy savings from pneumatic
systems,"  "Handling vacuum design," and
"Designing with vacuum and suction cups."
Jan 01, 2012
Page 1 of 15Fundamentals of Vacuum
6/7/2019https://www.hydraulicspneumatics.com/print/2254

Evacuating air from a closed volume develops a pressure differential between
the volume and the surrounding atmosphere. If this closed volume is bound
by the surface of a vacuum cup and a workpiece, atmospheric pressure will
press the two objects together. The amount of holding force depends on the
surface area shared by the two objects and the vacuum level. In an industrial
vacuum system, a vacuum pump or generator removes air from a system to
create a pressure differential.
Because it is virtually impossible to remove all the air molecules from a
container, a perfect vacuum cannot be achieved. Of course, as more air is
removed, the pressure differential increases, and the potential vacuum force
becomes greater.
Figure 1. Atmospheric pressure force determines height of mercury column
in simple barometer.
The vacuum level is determined by the pressure differential between the
evacuated volume and the surrounding atmosphere. Several units of measure
can be used. Most refer to the height of a column of mercury — usually

inches of mercury (in.-Hg) or millimeters of mercury (mm-Hg). The
common metric unit for vacuum measurement is the millibar, or mbar.
Other pressure units sometimes used to express vacuum include the
interrelated units of atmospheres, torr, and microns. One standard
atmosphere equals 14.7 psi (29.92 in.-Hg). Any fraction of an atmosphere is
a partial vacuum and equates with negative gauge pressure. A torr is defined
as 1/760 of an atmosphere and can also be thought of as 1 mm-Hg, where
760 mm-Hg equals 29.92 in.-Hg. Even smaller is the micron, defined as
0.001 torr. However, these units are used most often when dealing with
near-perfect vacuums, usually under laboratory conditions, and seldom in
fluid power applications.
Atmospheric pressure is measured with a barometer. A barometer consists of
an evacuated vertical tube with its top end closed and its bottom end resting
in a container of mercury that is open to the atmosphere, Figure 1. The
pressure exerted by the atmosphere acts on the exposed surface of the liquid
to force mercury up into the tube. Sea level atmospheric pressure will
support a mercury column generally not more than 29.92-in. high. Thus, the
standard for atmospheric pressure at sea level is 29.92 in.-Hg, which
translates to an absolute pressure of 14.69 psia.
The two basic reference points in all these measurements are standard
atmospheric pressure and a perfect vacuum. At atmospheric pressure, the
value 0 in.-Hg is equivalent to 14.7 psia. At the opposite reference point, 0
psia, — a perfect vacuum (if it could be attained) — would have a value equal
to the other extreme of its range, 29.92 in.-Hg. However, calculating work
forces or changes in volume in vacuum systems requires conversions to
negative gauge pressure (psig) or absolute pressure (psia).
Atmospheric pressure is assigned the value of zero on the dials of most
pressure gauges. Vacuum measurements must, therefore, be less than zero.

Negative gage pressure generally is defined as the difference between a given
system vacuum and atmospheric pressure.
Vacuum measurement
Figure 2. U-tube manometer, filled with mercury, measures vacuum as a
difference between vacuum source and atmospheric pressure.
Several types of gauges measure vacuum level. A Bourdon tube-type gauge is
compact and the most widely used device for monitoring vacuum system
operation and performance. Measurement is based on the deformation of a
curved elastic Bourdon tube when vacuum is applied to the gauge's port.
With the proper linkage, compound Bourdon tube gauges indicate both
vacuum and positive pressure.
An electronic counterpart to the vacuum gauge is the transducer. Vacuum or
pressure deflects an elastic metal diaphragm. This deflection varies electrical
characteristics of interconnected circuitry to produce an electronic signal
that represents the vacuum level.

Figure 3. Absolute pressure gauge measures vacuum as the difference in
mercury level in its two legs.
A U-tube manometer, Figure 2, indicates the difference between two
pressures. In its simplest form, a manometer is a transparent U-tube half-
filled with mercury. With both ends of the tube exposed to atmospheric
pressure, the mercury level in each leg is the same. Applying a vacuum to one
leg causes the mercury to rise in that leg and to fall in the other. The
difference in height between the two levels indicates the vacuum level.
Manometers can measure vacuum directly to 29.25 in.-Hg.
An absolute pressure gauge shows the pressure above a theoretical perfect
vacuum. It has the same U-shape as the manometer, but one leg of the
absolute pressure gauge is sealed, Figure 3. Mercury fills this sealed leg when
the gauge is at rest. Applying vacuum to the unsealed leg lowers the mercury
level in the sealed leg. The vacuum level is measured with a sliding scale
placed with its zero point at the mercury level in the unsealed leg. Thus, this
gauge compensates for changes in atmospheric pressure.
Industrial vacuum systems
Vacuums fall into three ranges:

• rough (or coarse), up to 28 in.-Hg
• middle (or fine), up to one micron,
• high, greater than one micron.
Almost all industrial vacuum systems are rough. In fact, most lifting and
workholding applications operate at vacuum levels of only 12 to 18-in.-Hg.
This is because it generally is more economical to increase the lifting or
holding force by increasing the contact area between the workpiece and
vacuum cup than it is to pull a higher vacuum and use the same contact area.
Middle vacuum is used for process applications such as molecular
distillation, freeze drying, degassing, and coating operations. High vacuums
are used in laboratory instruments, such as electron microscopes, mass
spectrometers, and particle accelerators.
A sampling of the multitude of standard components for assembling a

vacuum system: single- and multi-stage vacuum generators, valves, switches,
suction cups, etc.
A typical vacuum system consists of a vacuum source, delivery lines, fittings,
and various control valves, switches, filters, and protective devices. Leakage
prevention is especially important with vacuum systems because even very
small leaks can greatly diminish performance and efficiency. If plastic tubing
is used — as is often the case — be sure it is designed for vacuum service.
Otherwise, the walls of the tubing could collapse under a vacuum and block
flow. Also, vacuum lines should be as short and narrow as is practical to limit
the volume of air that must be evacuated.
An important design consideration for workholding applications is to use the
vacuum pump only to achieve the vacuum level required. Once the workpiece
is in contact with the vacuum cup and the required vacuum achieved, de-
energizing a normally closed valve will hold the vacuum indefinitely -
provided no leakage occurs. Holding a vacuum in this manner consumes no
energy and avoids having to operate the vacuum pump continuously.
Companies also offer proprietary devices, such as vacuum cups with integral
valves and valves that terminate flow from a cup that exhibits excessive
leakage. This valve is designed to avoid false-alarm shutoff when holding
porous workpieces (such as cardboard), yet prevent a leak at one vacuum cup
from reducing vacuum at an adjacent cup.
Vacuum pump selection
The first major step in selecting the right vacuum pump is to compare
application vacuum requirements with the maximum vacuum ratings of
commercial pumps. At low levels, there is a wide choice of pumps. But as

vacuum level increases, the choice narrows, sometimes to the point where
only one type of pump may be available.
To calculate a system's vacuum needs, consider all work devices to be driven.
The working vacuum of the devices can be determined by calculations based
on handbook formulas, theoretical data, catalog information, performance
curves, or tests made with prototype systems. Once you know the vacuum
required, you can begin looking for pumps that can accommodate
application requirements.
The maximum vacuum rating for a pump is commonly expressed for either
continuous or intermittent duty cycles, and can be obtained from pump
manufacturers. Because the maximum theoretical vacuum at sea level is
29.92 in.-Hg, actual pump capabilities are based on and compared to this
theoretical value. Depending on pump design, the vacuum limit ranges from
28 to 29.5 in.-Hg or about 93% or 98% of the maximum theoretical value.
For some pump types, the maximum vacuum rating will be based on this
practical upper limit. For others, where heat dissipation is a problem, the
maximum vacuum rating might also take into account allowable temperature
rise.
Mechanical vacuum pumps
A conventional vacuum pump may be thought of as a compressor that
operates with its intake below atmospheric pressure and the discharge at
atmospheric pressure. Compressors and vacuum pumps have identical
pumping mechanisms. The vacuum pump is simply piped to withdraw air
from a closed container and exhaust to atmosphere, which is just the
opposite of what a compressor does. Although the machines have many
similarities, two significant differences between compression and vacuum
pumping actions must be considered in system design. The maximum

change in pressure produced by a vacuum pump is limited; it can never be
higher than atmospheric pressure. Plus, as vacuum increases, the volume of
air passing through the pump drops continuously. Therefore, the pump itself
finally must absorb virtually all heat generated.
Mechanical vacuum pumps generally are categorized as either positive
displacement or non-positive displacement (dynamic). Positive-
displacement pumps draw a relatively constant volume of air despite any
variation in the vacuum level and can pull a relatively high vacuum. The
principle types of positive-displacement pumps include: reciprocating and
rocking piston, rotary vane, diaphragm, lobed rotor, and rotary screw
designs.
Non-positive-displacement pumps use kinetic energy changes to move air
out of a closed system. They provide very high flow rates, but cannot achieve
high vacuum. Major non-positive-displacement pumps are multi-stage
centrifugal, axial flow units, and regenerative (or peripheral) blowers. Of
these, only the blower is an economical choice for stand-alone or dedicated
vacuum systems.
Temperature considerations are very important when selecting a mechanical
vacuum pump because high external or internal heat can greatly affect pump
performance and service life. Internal pump temperature is important
because as vacuum level increases, less air is present to carry away the heat
generated, so the pump must absorb more of the heat. Heavy-duty pumps
with cooling systems are often required for high vacuum applications. But
light-duty pumps can operate at maximum vacuum for short periods of time
if there is an adequate cool-off period between cycles. The pump experiences
a total temperature rise as a result of all the heat sources acting on it -
internally generated heat plus heat from internal leakage, compression,
friction, and external ambient temperature.

Venturi-type vacuum pumps
Many machines that require vacuum also use compressed air. And if vacuum
is required only intermittently, the compressed air that already is available
can be used to generate vacuum through a device called a vacuum generator,
also known as a vacuum ejector or vacuum pump. Furthermore, the
compressed air also can be used in combination with a vacuum cup by
producing a puff of air to hasten release of the workpiece.
Figure 4. Venturi-type vacuum generator produces vacuum from stream of
compressed air. Most recent designs pull vacuum to 27 in.-Hg from a source
of compressed air of less than 50 psig.
Vacuum generators operate based the venturi principle, Figure 4. Filtered,
non-lubricated compressed air enters through inlet A. A diffuser orifice
(nozzle), B, causes the air stream to increase in velocity, thereby lowering its
pressure, which creates a vacuum in channel C. The air stream exhausts to
atmosphere through muffler D.
Vacuum generators offer several advantages. They are compact and
lightweight, so they often can be mounted at or near the point of use. They
are inexpensive, and because they have no moving parts, do not require the
maintenance associated with mechanical vacuum pumps. They do not need
an electrical power source because they generate vacuum by tapping into an
existing compressed air system. However, if retrofitted into a machine,
capacity of the existing pneumatic system may have to be increased. Heat

generation, which often is the limiting factor with mechanical vacuum
pumps, is of little concern with vacuum generators.
Mechanical pumps most often are specified to provide a machine with
vacuum on a continuous basis. But many of these machines actually use
vacuum only intermittently at many different locations. In cases like this,
vacuum generators can provide a practical alternative by supplying vacuum
intermittently at each source rather than continuously for the entire
machine.
Vacuum generators are controlled simply by initiating or terminating
compressed air flow to the nozzle. Vacuum generators have been used for
decades, but relatively recent improvements have led to nozzle designs that
provide higher operating efficiencies.
Another development using venturis is the multi-stage vacuum generators.
In this configuration, two or more vacuum generators are piped in series to
produce greater vacuum flow without using more compressed air.
Essentially, the exhaust from the first nozzle (which determines the
maximum attainable vacuum level) serves as input for a second stage.
Exhaust from the second stage then serves as input for a third stage. This
means that a multi-stage generator evacuates a given volume faster than a
single-stage generator does, but they both will eventually pull the same
vacuum level.
Selecting a vacuum generator depends on the lifting force required and the
volume of air that must be evacuated. Lifting force depends on the vacuum
level the generator can pull — which, in turn, depends on the air pressure
supplied — and the effective area of the vacuum cup. In most applications it
is important that a generator be able to pull the required vacuum in as short
a time as possible to minimize air consumption.

How long to reach maximum vacuum?
When choosing among several vacuum pumps, an important factor may be
how long it takes each pump to reach the needed vacuum.
In general, a small capacity pump and a large capacity pump with equal
maximum vacuum capabilities will both produce the same vacuum. The
smaller pump simply takes longer. How much longer depends on the
capacity of the pump and the size of the system. But simply dividing system
volume by open pump capacity won't produce the proper answer.
During pump-down, the higher a vacuum becomes, the fewer air molecules
remain in the closed volume. Therefore, fewer molecules can be removed by
each pump stroke. As a result, there is a logarithmic relationship when
approaching a perfect vacuum. The time required to pump a system down to
a certain vacuum level can be approximated using this formula:
t = (V×n) ÷ 4q,
where:
t is time, min
V is system volume, ft
q is flow capacity, cfm, and
n is a constant for the application.
For exact applications, n can be determined by using a natural logarithm. For
most purposes, the following will suffice:
n = 1 for vacuum to 15 in.-Hg
n = 2 for vacuum >15 but ≤ 22.5 in.-Hg., and
n = 3 for vacuum ≥ 22.5 and up to 26 in.-Hg.
One additional complication: pump capacity in the equation is not the open
capacity (capacity at atmospheric pressure) usually cataloged by
3

manufacturers. Instead, it represents the average capacity of the pump as
system pressure drops to the final vacuum level. This value is not readily
available but can be approximated from manufacturers' pump performance
curves. These curves plot pump capacity at various vacuum levels.
To mesh these curves with the equation, simply substitute values in the
equation using pump capacity readings from the curve at various vacuum
levels at 5-in.-Hg increments, up to the desired level. Then total these times.
Finally, note that this pump-down time is based on all system components
operating at optimum levels. A 25% additional time allowance is
recommended to compensate for system inefficiencies and leakage.
Vacuum at high altitudes
Atmospheric pressure determines the maximum vacuum force that can be
achieved. And standard atmospheric pressure at sea level is 29.92 in.-Hg.
But what happens at locations a mile above sea level? The maximum vacuum
that can be achieved in locations above sea level will be less than 29.92-
in.-Hg. The force will be limited by the ambient atmospheric pressure.
Vacuum pumps have maximum vacuum ratings based on sea level
conditions and must be re-rated for operation at higher elevations.
First, determine the local atmospheric pressure. A rule of thumb is that for
every 1000 ft. of altitude above sea level, atmospheric pressure drops by 1
in.-Hg. Using rounded-off figures, for a city at an elevation of 5,000 ft, the
atmospheric pressure is about 25 in.-Hg.
To adjust a pump rating, think of that rating as a percentage of atmospheric
pressure at sea level. If a pump is rated for 25 in.-Hg, it can achieve 83.4%

(25 29.92) of a sea level perfect vacuum. At a 5000-ft elevation, that same
pump can achieve 83.4% of 25 in.-Hg - or a vacuum of 20.85 in.-Hg.
Pressure vs. vacuum
Percent
vacuum
Inches of
mercury
(in.-Hg)
Pressure
10 3.0 -1.47 psi -0.10 bar
15 4.5 -2.21 psi -0.15 bar
20 6.0 -2.94 psi -0.20 bar
25 7.5 -3.68 psi -0.25 bar
30 9.0 -4.41 psi -0.30 bar
35 10.5 -5.15 psi -0.35 bar
40 12.0 -5.88 psi -0.40 bar
45 13.5 -6.62 psi -0.45 bar
50 15.0 -7.35 psi -0.50 bar
55 16.5 -8.09 psi -0.55 bar
60 18.0 -8.82 psi -0.60 bar
65 19.5 -9.56 psi -0.65 bar
70 21.0 -10.29 psi-0.70 bar
75 22.5 -11.03 psi -0.75 bar
80 24.0 -11.76 psi -0.80 bar
85 25.5 -12.50 psi-0.85 bar
90 27.0 -13.23 psi-0.90 bar
95 28.5 -13.97 psi-0.95 bar
100 30.0 -14.70 psi -1.01 bar
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Fundamentals of vacuum

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