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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 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.