Basics of Vacuum

                              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.

                              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.

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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.

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 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.

                              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.

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