Glossary
Principles of operation
Rotary vane pumps
The rotary vane pump's hollow body has a rotating cylinder mounted off-axis. In the rotor, two diametrically opposed, radially directed vanes force contact with the pump body. Since the rotor is positioned off-axis, its motion causes the volume between the vanes and the body to vary during each half turn. The gas inlet port is so positioned that the volume behind the last vane to pass increases, allowing gas to expand into it until the next vane passes.
As the volume exposed to the inlet increases, the volume trapped between the vane and the exhaust port decreases. In a single stage pump, the exhaust port has a valve connected to the atmosphere. In a two stage pump, the first stage's exhaust connects directly to the second stage's inlet. Gas exits the pump by bubbling up through the pump's oil reservoir.
Rotary vane pumps generally have lower vacuum ratings than piston pumps so are best suited to rough vacuum applications. However, Multi-stage rotary vane systems can provide an economical alternative medium to high vacuum tasks, depending on the application.
Advantages include high flow capacities, low starting and running torque, vibration-free operation, low running noise, and continuous airflow.
Liquid ring pumps
Gas entering via the suction port is conveyed into the impeller casing and trapped in the space between two impeller blades. As the impeller rotates - eccentrically to the liquid ring and casing - the volume between the blades increases creating vacuum. As the cycle progresses towards the discharge port, the volume decreases as the liquid ring creates compression. This compression continues until the gas is discharged through the discharge port.
A small amount of liquid is discharged with the gas and it is necessary to supply make-up continuously. This make-up liquid also maintains the liquid ring and absorbs the heat energy of compression.
Liquid ring pumps are suitable for continuous operation, and are tolerant of entrained liquids and solids. Safe, and offering low noise operations, they use a simple design which makes them very robust.
Reciprocating piston pumps
With reciprocating piston pumps (often referred to simply as piston pumps), the reciprocating motion of a piston within a cylinder alternately draws gas into the cylinder through the inlet valve and then compresses the gas. The gas is discharged through a separate outlet valve.
There are a number of ways of moving the piston back and forth (one of the most innovative of which involves using an electro-magnet working against a return-spring, significantly reducing the number of moving parts), but the most common arrangement is to use a rotating crankshaft, connected to the piston via a connecting rod.
Reciprocating piston pumps are able to generate relatively high levels of vacuum under a range of different operating conditions. But there are disadvantages in that capacity is somewhat limited, making reciprocating piston pumps best suited to pulling relatively small volumes of air through a high vacuum range. Also, noise levels and vibration are typically higher than for other technologies.
Diaphragm pumps
The diaphragm pump is essentially a modification to the reciprocating piston pump. A diaphragm flexing back and forth in a closed chamber compresses the gas, with the flexing generated by the motion of a connecting rod under the diaphragm. The design removes the need for a sliding seal between the moving parts, and only a short stroke is required to generate vacuum levels similar to those produced by a significantly larger reciprocating piston pump.
Rocking piston pumps
Rocking piston pumps (also called swing piston pumps or Wob-L pumps) combine the vacuum capabilities of reciprocating piston pumps with the compact size and light weight of diaphragm pumps, and operate using a combination of these two alternative technologies. The design mounts a piston rigidly on top of an eccentrically mounted connecting rod. The piston is surrounded by a flexible cup. The cup functions as a seal and as a guide member for the rod. It expands as the piston travels upwards, thus maintaining contact with the cylinder walls and compensating for the rocking motion.
Rocking piston pumps are available in single stage or two stage units for higher vacuum capability. They also have the advantage of being able to supply vacuum and pressure simultaneously. Disadvantages include a limited airflow.
Rotary screw pumps
With rotary screw pumps, two meshing rotors with helical contours trap air as the screws turn in opposite directions. This action creates chambers of decreasing volume behind and increasing volume in front of the rotor chambers.
Tending to be larger than alternative positive displacement technologies due to the size of the gears and rotors, rotary screw pumps provide vacuum capabilities similar to those of piston pumps with the advantage of being nearly pulse-free. A key disadvantage is that they tend to run hot, making them unsuitable for thermally sensitive applications.
Rotary lobe pumps
Rotary lobe pumps employ a pair of mating lobed impellers that rotate in opposite directions, trapping air and removing it from the system. Based on the Roots blower principle, these pumps bridge the gap between the conventional positive displacement vacuum pumps and the various centrifugal exhauster designs.
Rotary claw vacuum pumps
Non-contacting claws trap a volume of air at the inlet and convey it to the exhaust where it is compressed and discharged. Like the rotary lobe, the rotary claw is a dry running, positive displacement design, but each claw rotor has a unique profile so that, as they counter-rotate, separate expansion and compression chambers are created.
Rotary claw pumps are available in two-stage and three-stage designs. Two stage units produce vacuum using gas compression alone. A three-stage unit uses a combination of mechanical and gas compression to produce a higher vacuum, but at the expense of a greater discharge temperature.
Rotary claw designs are larger than either rotary lobe or rotary screw pumps, and tend to be slightly noisier.
Centrifugal and regenerative exhausters
Typically operating at high speeds and attaining moderate vacuum levels, these offer a complementary solution to rotary lobe pumps. The exhauster consists of a series of impellers on a common shaft. The housing is constructed so that air is directed sequentially through each impeller from the inlet to the outlet in a quasi-staging effect.
As the impeller rotates, centrifugal force moves the gas from the blade root to its tip. Leaving the blade tip, the gas flows around the housing contour and back down to the root of a succeeding blade, where the flow pattern is repeated.
Ideal for central vacuum systems, exhausters maintain a constant vacuum as the volume of air changes with the number of operators. The end result is not a particularly high vacuum, but flow capacity is very high. Multistage versions produce higher vacuum levels, but at lower flow rates. The physical size of the impeller determines the air flow range.
Venturi vacuum generators
With venturi vacuum generators, high-pressure compressed air is forced through an orifice or restricting nozzle into an expansion chamber. As the air velocity increases, a negative pressure is produced. Venturi vacuum systems are selected because they have no moving parts and use no electricity. They are also safe, easy to installation, and compact. However, venturi vacuum generators are a very inefficient use of compressed air (approximately 4% efficiency) and should only be specified for intermittent uses.
