Basics of Vacuum

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Basics of Vacuum

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Like compressed air, vacuum puts the atmosphere to work. But unlike compressed air, vacuum uses the

surrounding atmosphere to create the work force.

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.

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 in.-Hg or 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).

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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

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.

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


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coating operations. High vacuums are used in laboratory instruments, such as electron microscopes, mass

spectrometers, and particle accelerators.

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.

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


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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.

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.


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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.

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 = vn4q,

where:

t is time, min

v is system volume, ft3

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.


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One further complication: pump capacity in the equation is not the open capacity (capacity at atmospheric

pressure) usually cataloged by 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


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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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