Machine Design

Vacuum pumps

In principle, industrial vacuum pumps are merely compressors run with the inlet attached to the vacuum system and the outlet open to exhaust. In smaller sizes, compressors and vacuum pumps are often identical machines. However, in the large sizes that might power a plant-wide vacuum system, the machines differ in minor ways that are intended to enhance efficiency for one application or the other. Manufacturers strongly advise that the same machine not be used for both vacuum and compression at the same time. The heavy loads will damage it.

Three criteria control pump selection: degree of vacuum produced, rate of air removal, and power requirement. However, applications such as filtration may subject the unit to the ingestion of foreign material.

The first pump performance criterion is the vacuum it produces. Manufacturers provide a maximum vacuum rating expressed as absolute pressure in mm Hg, or vacuum in in. Hg. Larger units are usually rated only for continuous duty, but smaller units may have a higher vacuum rating for intermittent duty. In smaller units, temperature-rise considerations limit the vacuum that can be produced.

Continuous and intermittent vacuum ratings are determined for standard atmospheric pressure: 29.92-in. Hg. Lower ambient pressures reduce the vacuum that can be produced. The rating is determined from:

Va = ( Vo * Pa ) / 29.92

where Va = adjusted vacuum rating, in. Hg; Vo = original vacuum rating at standard conditions, in. Hg; and Pa = anticipated atmospheric pressure at the application site, in. Hg.

Rate of air removal is the second criterion. Vacuum pumps are flow rated according to the volume of air exhausted with no pressure differential across the pump. Manufacturers provide curves showing free air delivery at rated speed for vacuum levels ranging from 0-in. Hg (so-called "open capacity") to maximum vacuum rating. Some manufacturers also provide curves of capacity at different speeds for a given vacuum.

The last pump criterion is power requirement. Compared with air compressors, vacuum pumps require relatively little power. At low flows, vacuum (or pressure differential) is high; at high flows, vacuum is low. Therefore, power, which is proportional to flow and pressure differential, is generally low.

Power output of the pump can be found from pressure-flow curves provided by manufacturers. Input power and speed requirements are also shown in the data. Overall pump efficiency (including both volumetric and mechanical efficiency) can be evaluated by combining this data. This is done by dividing the free-air capacity of the pump at the required vacuum level by drive power required at that condition. The result is proportional to the product of gage vacuum and air-flow rate and is representative of efficiency.

All three performance criteria -- vacuum, flow and power -- can be affected by pump temperature. At higher vacuum levels, little air flows through the pump, so little heat is transferred to the air. Much of the heat generated by friction must be dissipated by the pump. This heat gradually raises pump temperature and can drastically reduce service life. Temperature excursions are especially important to intermittent-duty pump, which can overheat if on time greatly exceeds off time.

Vacuum pumps are classified as either positive or nonpositive displacement. A positive-displacement pump creates vacuum by isolating and compressing a distinct, constant volume of air. The compressed air is vented out one port, and a vacuum is created at the other port where the air is drawn in. This generates relatively high vacuum, but little flow.

A nonpositive-displacement pump, on the other hand, uses rotating impeller blades to accelerate air and create a vacuum at the inlet port. While nonpositive-displacement pumps cannot produce high levels of vacuum, they provide high flow rates.

Principal types of positive-displacement vacuum pumps include piston, diaphragm, rocking-piston, rotary-vane, lobed-rotor, rotary-screw, and liquid-ring designs.

Reciprocating-piston pumps generate relatively high vacuums -- from 27 to more than 29 in. Hg -- under a variety of operating conditions. Typical pumps of this type have one or more pistons linked to a rotating crankshaft. The alternating piston action moves air past check valves in the cylinder head to create a vacuum at the inlet port. Lubricated piston pumps are quieter, produce less vibration, have a higher capacity, and feature a much longer life than oilless designs, but they are also heavier and more expensive.

Diaphragm pumps offer the advantage of the fluid chamber being totally sealed from the pumping mechanisms. An eccentric connecting rod mechanically flexes a diaphragm inside the closed chamber to create a vacuum. This results in somewhat lower vacuum compared to that produced by a reciprocating piston. However, the diaphragm's lower compression ratio -- low flow, large diameter, and short stroke -- makes for quiet, economical, and reliable operation. The design is available in both one and two-stage versions. Single-stage pumps provide vacuum up to 25.5 in. Hg, while two-stage units are rated to 29 in. Hg.

Rocking-piston pumps combine the compact size and quiet, oilless operation of the diaphragm pump with the high-vacuum capabilities of the reciprocating-piston pump. Here, a piston is rigidly mounted (no wrist pin) on top of the diaphragm unit's eccentric connecting rod. An elastomeric cup skirts the piston and functions both as a seal -- equivalent to the rings on a piston compressor -- and as a guide member for the rod. The cup expands as the piston travels upward, thus maintaining contact with the cylinder walls and compensating for the rocking motion. The absence of a wrist pin is the key to the pump's light weight and compact size.

Single-stage rocking-piston pumps produce vacuum to 27.5 in. Hg; two-stage designs can generate 29 in. Hg or more of vacuum. Rocking-piston pumps are also relatively quiet, operating at sound levels as low as 50 dBA. A drawback to rocking-piston pumps is that they cannot generate a lot of airflow. Even the largest twin-cylinder models have flow rates of less than 10 cfm.

Rotary-vane pumps use a series of sliding, flat vanes rotating in a cylindrical case to generate vacuum. As an eccentrically mounted rotor turns, the vanes slide in and out, trapping a quantity of air and moving it from the inlet side of the pump to the outlet.

Rotary-vane pumps usually have lower vacuum ratings than piston pumps, in the 20 to 28 in. Hg range. However, there are a few exceptions. Some two-stage, oil-lubricated designs have vacuum capabilities up to 29.5 in. Hg. Pumps with recirculating oil systems reach still higher vacuums, in the less than 1-torr range. The pumps offer a number of advantages, including high flow capacities, low starting and running torque requirements, vibration-free operation, and continuous airflow. No valves restrict flow or require maintenance in the rotary design. The compact units are also quiet, generating as little as 45 dBA or sound.

Depending on the application and vacuum level required, an economical alternative to using a high-vacuum pump is two standard, staged rotary-vane pumps. Or, a high-volume, low-duty pump rated for continuous duty of 20 in. Hg sometimes can be operated at restricted airflow or "blanked-off" conditions for short periods of time to provide higher vacuums. As with other types of pumps available in both lubricated and oilless configurations, lubricated rotary-vane pumps are capable of slightly higher vacuum compared to oilless designs.

Liquid-ring pumps feature a multiblade impeller, mounted eccentrically in a cylindrical case that is partly filled with water. As the impeller rotates, liquid is thrown outward by centrifugal force to form a liquid ring concentric with the periphery of the casing. Due to the eccentric position of the impeller, the air space in the impeller cell expands during the first 180° of rotation, creating a vacuum. During the next 180° of rotation, the air space is reduced, discharging compressed air and water. In addition to being the compression medium, the liquid ring absorbs the heat of compression as well as any powder or liquid slugs entrained in the air.

Rotary-screw and lobed-rotor vacuum pumps are two other types of positive displacement pumps. Neither lubricated design is as widely used as rotary-vane and piston pumps, especially in smaller sizes. Due to the size of the gears and rotors, both designs lend themselves to larger installations.

A rotary-screw pump's vacuum capabilities are similar to those of piston pumps, with the added advantage of being nearly pulse-free. 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.

Lobed-rotor pumps bridge the gap between positive and nonpositive-displacement units. The pumps have a pair of mating lobed impellers that rotate in opposite directions, trapping air and withdrawing it from the system.

High-speed, multistaged centrifugal blowers and regenerative blowers are the major types of nonpositive-displacement pumps, generally operating at high speeds and attaining moderate vacuum levels.

Centrifugal blowers, for example, are an excellent choice where only intermittent use is required. To keep costs down, a short-life brush-type ac or dc motor powers these blowers, which are widely used in vacuum cleaners.

Regenerative blowers have many advantages because individual air molecules pass through many compression cycles with each revolution compared to the single compression per stage for multistaged centrifugal types. At first glance, regenerative blowers are similar to rotary-vane pumps, but have a special blade and housing configuration.

As the impeller rotates, centrifugal force moves the air molecules from the blade root to its tip. Leaving the blade tip, the air flows around the housing contour and back down to the root of a succeeding blade, where the flow pattern is repeated. This action provides a quasi-staging effect to increase pressure differential capability. The speed of the rotating impeller determines the degree of pressure change.

The end result is not a particularly high vacuum -- approximately 100-in. H2O in single-stage models. But flow capacity is very high, up to several hundred cfm. Multistage versions produce higher vacuum levels, but at lower flow rates.

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