Vacuum components

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Vacuum is pressure that is lower than atmospheric —14.7 psia at sea level. In a vacuum system, the differencebetween atmospheric and vacuum pressure creates the abilityto lift, hold, move and generally perform work.

There are two types of vacuum applications: closed, or nonporous; and open, or porous. In a closed system, removing air progressively decreases the air density within the sealed, confined space and creates a vacuum. In an open system, a vacuum unit must remove more gas molecules than are able to leak back into the system.

Vacuum is typically divided into three areas of application, depending on the level of vacuum required. Lowlevel vacuum applications are typically those requiring high flows and low force. These systems are primarily serviced by blowers. Screen printing on cloth is one application that falls into this range.

The majority of industrial vacuum falls within the range of 6 to 29.5 in.-Hg. Application examples include pick-and-place and thermoforming.

Scientific or process applications encompass the deepest levels — approaching a near-perfect 29.92 in.-Hg. Flow in this range is minimal. Examples of applications are ion implantation and space simulation.

The vacuum generators that evacuate air and create the required low pressure come in an extensive array of types, sizes, designs and efficiencies to suit widely ranging applications. Two basic types are electric-motor-driven vacuum pumps and vacuum ejectors.

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Vacuum pumps. Mechanical vacuum pumps generally fall into one of two different types: positive-displacement and dynamic/kinetic. Displacement vacuum pumps essentially operate as compressors with the intake below atmospheric pressure and the output at atmospheric pressure. They draw in a fairly constant volume of air, which is mechanically shut off, expanded, and then ejected. The main feature of vacuum pumps of this type is that they can achieve a high vacuum with low flow rates. Types include reciprocating piston, rotary vane, diaphragm and rotary screw. They are often suited for precision industrial applications.

Kinetic vacuum pumps cause gas particles to flow in the delivery direction by applying additional force during evacuation. Rotary blowers, for example, operate according to the impulse principle: a rotating impeller transfers kinetic energy by impacting air molecules. In operation, air is drawn in and compressed on the suction side by the impeller blades.

These vacuum pumps generate a relatively low vacuum, but at high flow rates (high suction capacity). They are usually suited for handling extremely porous materials, such as clamping cardboard boxes.

Among the advantages, typical positive-displacement industrial pumps generate up to about 98% vacuum — beyond the capability of ejectors. And blowers can offer high suction rates well beyond 1,000 m 3 /hr. However, electromechanical vacuum pumps tend to operate continuously with vacuum requirements regulated by valves. And compared to ejectors, they are larger, heavier, and usually cost more.

Vacuum ejectors. Vacuum ejectors basically generate vacuum using pneumatically driven nozzles without moving parts. They produce high vacuum at relatively low flow rates.

A classic ejector consists of a jet nozzle (also called a Laval or venturi nozzle) and, depending on the design, at least one receiver nozzle. Compressed air enters the ejector and a narrowing of the jet nozzle accelerates the flowing air to up to five times the speed of sound. The ejector has a short gap between the jet-nozzle exit and the entry to the receiver nozzle. Here, expanded compressed air from the jet nozzle creates a suction effect at the gap which, in turn, creates a vacuum at the output vacuum port.

Vacuum ejectors come in two basic versions, single and multi-stage. A single-stage ejector includes a jet nozzle and one receiver nozzle; a multi-stage ejector has a jet nozzle and several nozzle stages, each of which has a larger diameter in proportion to the falling air pressure. Air drawn in from the first chamber, combined with compressed air from the jet nozzle, is thus used as a propulsion jet for the other chambers. In both versions, air exiting the receiver nozzle generally discharges via a silencer or directly to the atmosphere.

Among their benefits, vacuum ejectors are compact, lightweight, and relatively inexpensive and they respond quickly, with fast start and stop times. They resist wear, can mount in any position, experience no heat build-up in operation, and consume energy only as needed — as they switch off when no vacuum is needed. On the downside, vacuum ejectors only generate pressures to about 85% vacuum, and do not produce extremely high suction rates.

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