Mechanical Vacuum Pumps Selection GuideMechanical Vacuum Pumps Selection GuideMechanical Vacuum Pumps Guide

Image Credit: Techno, Inc. | Thomas Division - Gardner Denver | Vacuubrand GMBH + CO KG

 

Mechanical pumps generate vacuum by acting on the viscous properties of the gas or fluid being evacuated using mechanical devices such as pistons, claws, scrolling spirals, and diaphragms.

 

Types of Mechanical Vacuum Pumps

Mechanical vacuum pump types are grouped by the technology or mechanism used. The GlobalSpec SpecSearch Database includes a large number of different types of mechanical vacuum pumps.

 

Centrifugal Pumps

Centrifugal pumps operate on the kinetic/dynamic principle, using velocity and momentum to move fluids. Centrifugal pumps use fans and impellers which generate fluid velocity through centrifugal force. The pumping speed of these pumps will vary depending on the properties of the fluid being pumped. For more information on centrifugal pumps, visit the Centrifugal Pumps Selection Guide.

  • Axial blower - Axial blowers are multiple-blade rotors that impart axial velocity which is converted to pressure. Axial blowers generally have very high flow rates. For related information on axial flow pumps, visit the Axial Flow Pumps Selection Guide.
  • Mechanical Vacuum Pumps Selection GuideCentrifugal blower - Centrifugal blowers are high-speed impellers that produces radial air flow. Velocity is converted to pressure energy. Evacuated air flows radially outward against blower wall, as opposed to axially through tube axial blowers.
  • Regenerative blower - Regenerative blowers are similar to centrifugal blowers. They have airflow chambers designed to generate higher pressure. Air flows radially outward along blade to housing wall, whereupon it flows down to the root of the next blade gap and repeats the cycle. The airflow thus creates several "stages" of compression.

Regenerative blower.

Image Credit: Gast Group

 

Positive Displacement Pumps

Most mechanical vacuum pumps operate on the positive displacement principle, in which fluid is pushed mechanically through a fixed volume chamber. Positive displacement vacuum pumps have the same pumping speed (described below) for all gases. For more information on positive displacement pumps, visit the Positive Displacement Pumps Selection Guide.

  • Claw - Claw or rotary claw pumps use precision-designed intermeshing hooked claws with very tight tolerances and clearances for highly efficient internal compression. They are typically noncontact, minimizing, or eliminate friction and wear. Claw pumps have ultimate pressures of just below 10-1 Torr. They are used in harsh industrial environments, specifically semiconductor processing and where water vapor content is high.
  • Mechanical Vacuum Pumps Selection GuideDiaphragm - Diaphragm pumps are dry positive displacement pumps which generate vacuum via the oscillation of a flexible diaphragm. This oscillation is driven by a reciprocating or eccentric "rocking" rod with no sliding seals or parts. This design isolates the pumped fluid from any contact with the pumping mechanism, providing a clean vacuum valued for analytical applications. It also makes these pumps particularly well-suited as backing pumps for high vacuum turbomolecular pumps. Diaphragm pumps achieve lower ultimate vacuums (1 to 10 Torr), but frequently offer quiet, economical performance. They often have two stages in series to generate lower vacuum, or in parallel to produce higher pumping speeds. Typical applications include simple vacuum filtration, thin film evaporation, distillation, sample movement, and gel drying.

Diaphragm Vacuum Pump. Image Credit: Thomas Division: Gardner Denver.

  • Mechanical Vacuum Pumps Selection GuideLinear - Linear pump technology uses the stroke of sliding piston compression. It is generated with electromagnetic oscillation, and is clean and quiet.
  • Liquid ring - Eccentrically mounted rotors and vanes utilize liquid ring on the inside of chambers to compress air. The liquid also absorbs compression heat and entrained particles in the air.
  • Lobed rotor (roots) - In a lobed rotor or roots pump, two counter-rotating lobes move synchronously without touching to continuously move gas through the pump. When backed by a rotary vane or piston pump, roots pumps can achieve ultimate pressures of 10-4 Torr. Roots (rotary lobe) action is excellent for moving huge quantities of gas at pressures between 0.01–10 Torr. These pumps are favored in high throughput process applications such as diffusion bonding, distillation towers, and IC fabrication lines.

Roots pumping unit. Image Credit: Vacuubrand GMBH + CO KG 

  • Piston pumps are traditional positive displacement pumps which use one or more pistons to displace air inside a cylinder to move fluid and generate vacuum. Dry piston pumps have higher pumping speeds than diaphragm pumps, and are used where a clean vacuum is required while operating near ultimate vacuum. They are well suited as backing pumps for turbomolecular pumps, as well as for analytical applications and leak detectors. They are not suitable for pumping corrosive or abrasive media. For more information on piston pumps for all fluid flow applications, visit the Piston Pumps and Plunger Pumps Selection Guide.
    • Circumferential piston - Circumferential pistons have multiple-rotor configurations, each with two "wings". The wings trap air against the outside wall during rotation and compress it in their rotary stroke toward the outlet. The wings of the two rotors alternate in compression strokes.
    • Reciprocating piston - These pumps use pistons which move up and down in a cylinder, similar in design to a 2- or 4-stroke internal combustion engine. They produce high ultimate vacuums up to 2x10-2 Torr. They are commonly used for roughing load locks in MBE and UHV processing systems.
    • Rocking piston - Rocking pistons are similar to reciprocating pistons, but are not articulated. These pistons "rock" with eccentric motion of drive.
    • Rotary piston - Rotary pistons or cams rotate eccentrically and evacuate air through a check valve on the "compression" or evacuation stroke, while air from the vacuum application fills in behind it. They are used extensively for backing large roots pumps and/or diffusion pumps attached to production-sized vacuum furnaces.
  • Rotary vane - Rotary vane pumps are oil-sealed rotary displacement pumps. They have eccentrically mounted rotors and sliding vanes that compress air in diminishing volume compartments. They can be employed universally throughout the entire low and medium vacuum ranges. Single-stage rough rotary vane pumps have ultimate pressures around 10-2 Torr while two-stage versions have pressures up to 10-3 Torr. Rough vane pumps are used primarily as backing pumps for roots or high-vacuum pumps.  Coarse vane pumps are used in freeze drying, vacuum filtering, vacuum impregnation, materials handling, meat packing, and "house" vacuum systems.
  • Mechanical Vacuum Pump Selection GuideRotary screw - Rotary screws have two or more spindles with intermeshing screws that rotate oppositely, creating axially progressing "chambers" that move the air from suction to discharge. Larger pumps often have double suction, where air is let in from both ends to a central discharge port. Rotary screw pumps produce ultimate pressures in the 10-3 Torr range. They do not use valves, are pulse-free, and are lighter weight than reciprocating piston pumps. The construction materials are chosen to fit the harsh environments and aggressive gases and particulates found in semiconductor etching and CVD processes. They are also used for backing dry, high vacuum transfer pumps or initial pumpdown for capture pumps.

Screw Pump

Image Credit: Houston Vacuum Pump & Compressor

  • Scroll - In scroll technology, air is moved as intake air is compressed between the surfaces of mating involute spirals. One spiral moves to progressively push the evacuated air out to the exhaust. Scroll pumps achieve ultimate pressures in the 10-2 Torr range. They are used in clean, dry processes and as dry backing pumps for high vacuum pumps. They should not be used outside an ambient temperature range of 5–40ºC.

A summary of the general specifications of different types of mechanical pumps is given in the following table.

 

Pump Technology

Ultimate Vacuum

Pumping Speed

 

Torr

ft3/min (cfm)

Claw

1

50 - 250

Diaphragm

1-10

<10

Piston

2x10-2

6 - 32

Roots (rotary lobe)

10-4 (when backed)

147 - 9424

Rotary vane

10-2

1 - 650

Rotary screw

10-3

30 - 318

Scroll

10-2

12 - 25

Mechanical vacuum pump spec comparison. (Data compiled from Lesker Company and other sources.

 

Specifications

Performance

  • Ultimate (maximum) operating vacuum or ultimate pressure is the lowest pressure which the mechanical vacuum pump can generate (typically within a set time). Buyers should note the conditions or assumptions used to obtain this value, since manufacturers may provide this rating using assumptions that are not realistic under normal operating conditions (e.g. ignoring the pressure of condensable gases like water vapor). Vacuum ratings which follow the ISO 21360-1 standard adhere to standard methods for measuring vacuum-pump performance.
  • Pumping speed or vacuum flow is the volumetric rate at which gas is emptied from the vacuum chamber, typically given in ft3/min (cfm), m3/s, L/min, or gal/min (gpm). It is defined mathematically by the equation:

S = Q/P

 

where S is pumping speed, Q is the throughput of the gas load (described below), and P is the partial pressure of the gas at or near the pump inlet. A pump's maximum achievable pumping speed (over its entire pressure range) is always referred to as its rated pumping speed, and pumping speeds listed by manufacturers are typically referenced to STP (standard temperature and pressure). Pumping speed needs to be matched according to the needs of the application, which are dependent on the system's chamber volume, desorption, and process gas loads. Keep in mind that the speed of the pump itself is seldom the actual pumping speed in the system's chamber. In blowers and non-positive displacement vacuum pumps, speed also varies with the type of gas material.

 

Selection Tip: Pumping speed is generally defined under the same ideal conditions as the ultimate pressure (minimum volume, right at the pump inlet, lowest possible outgassing rate, etc.). Care should be taken on these details when considering these specifications for pump performance assessment.

  • Throughput or gas load is the quantity of gas (i.e. the volume of gas at a known pressure) that passes a plane in a known time. In SI units, throughput is often given in Pa-m3/s. It defines the energy required to transport the gas molecules across a plane in the system or chamber. At a specified temperature, throughput is proportional to the mass flow rate of the pump. When discussing a system leak or backstreaming, throughput can also refer to the volume leak rate multiplied by the pressure at the vacuum side of the leak. This leak rate throughput can be compared to the throughput of the pump.

The pumping speed vs. pressure curve shows how the pressure generated by a vacuum pump varies with pumping speed. It describes the pump's performance throughout its probable application range, allowing users to assess the pump's capability at specific operating conditions. A typical low vacuum pump may have a performance curve like this one:

 

Performance curve for a rocking piston type dry vacuum pump. Image Credit: ULVAC KIKO Inc.

 

  • Motor power is a reference value used to characterize vacuum pump size. Different power sources for vacuum pumps are described in the next section.

Power Source

Vacuum pumps may be designed to be powered by one specific source, or may be capable of running based on a number of different sources.

  • Compressed air or compressed fluid systems derive their power from nonelectric media such as motive fluid. Compressed air is most common, but hydraulic or other power sources are possible.
  • DC power includes battery and power supply inputs.
  • Single-phase AC motor voltages include 115V/60Hz and 230V/50Hz.
  • AC three-phase such as 208V or 337V are used in many industrial vacuum pumps.
  • Pumps may be powered by combustion engines which typically run on gasoline or diesel.
  • Magnetic drive pumps typically use a rotating magnetic field to drive the pump impeller. This can allow for seal-less construction which eliminates the chance of oil contamination of the gas or fluid stream in the pump.

Lubrication

Mechanical vacuum pumps may be either dry (oil-less) or oil-sealed based on the means of lubrication.

  • Dry or oil-less pumps use permanently sealed bearings or other isolation technology to eliminate oil in the fluid train. Often dry pumps use oil-lubricated bearings, so they are not always truly oil-free. However, they do minimize the potential for oil contamination within the system because they do not rely on oil for sealing. Dry pumps are more tolerant of particulates and vapors than other types. Most blower, screw, scroll, diaphragm, claw, and piston pumps are dry vacuum pumps.
  • Oil-sealed pumps (also known as "wet" pumps) use oil to lubricate and seal bearings and parts. These types of pumps may leak small amounts of oil into the flow chamber, and should not be used if oil contamination presents a problem. Most rotary vane and rotary piston pumps are oil-sealed.

There are a variety of different types of lubricants available; properties to consider in selection include lubricity, chemical stability, viscosity, and material compatibility. There are also a number of lubrication styles for oil-sealed pumps:

  •  
    • Splash lubrication styles splash oil onto components from an oil bath.
    • Oil-flooded lubrication, used in some mechanical pump styles, involves the heavy application of oil for moving parts. Oil contamination is likely in pumps incorporating this lubrication style.
    • Positive pressure styles maintain oil pressure for the highest level of lubrication.

Mounting

Based on construction and weight, vacuum pumps can be mounted in a variety of different ways. The mounting style needed will depend on the pump's location and the application.

  • Benchtop mounting is used for pumps small enough to mount or be placed on a bench or table. These models may also be portable.
  • Carts or other portable mounts include wheels, casters, or easily movable frames for frequent relocation of the pump.
  • Larger vacuum pumps may rest on the floor or on a skid.
  • Permanent installation is used with very large systems or stations that are installed permanently in one place.

Configuration

Mechanical vacuum pumps can be purchased in a number of different configurations depending on the needs of the client.

  • Individual vacuum pumps are typically for insertion into or used with a larger system or process. Mechanical pumps, which are typically low vacuum backing pumps, are often purchased as individual units and incorporated into larger high vacuum systems.
  • Vacuum pumping units consist of two or more pumps or stages using different technology, which are coupled or stacked together to increase capacity or take advantage of the features of each type. The most typical combination is a high vacuum pump supported by a mechanical backing pump.
  • Vacuum systems can include multiple pumps and associated piping, valves, controls, receivers, etc. These pumps can also indicate centralized vacuum sources for manufacturing or automation cells or plants.

Material Compatibility

Mechanical vacuum pumps and their respective lubricants need to be compatible with the fluids they move or capture to prevent wear or corrosion and ensure safe and clean operation. The type and consistency of gas being handled also has an effect on the pump's performance, since most pumps will pump different gases at different speeds. For example, a cryopump will have a much higher water vapor pumping speed than a turbomolecular pump, even though both pumps might have exactly the same nitrogen speed.

 

Features

Mechanical vacuum pumps may incorporate a number of additional features. The GlobalSpec SpecSearch Database contains a number of additional features.

  • Filters and separators - used to protect vacuum pumps against wear and corrosion. They separate or filter out particles or vapors from a fluid stream that may otherwise damage parts of the pump.
  • OEM controller or control panels - used to adjust controls and provide additional functions above that of a simple regulator knob.
  • Valve sequencing control - used to control the state of valves in a vacuum pump system, often from a central panel or location.
  • Vacuum gauge - provides a dial, numeric, or other type of readout of the pressure at points in the vacuum system.
  • Integral trap - used to prevent backstreaming and resultant contamination from roughing pumps in high vacuum or sanitary applications. There are various trap types or technologies designed for the prevention of chamber entry by oil or water vapor, hydrocarbons, etc.
  • Gas ballast - allows atmospheric air into the compression chamber to minimize condensate in the oil and prevent corrosion.
  • Magnetic bearing - used to lift the pump rotor through magnetic levitation to eliminate contact and contact friction between the two surfaces.

References

 

Rotary Screw Compressor Selection - Houston Vacuum Pump & Compressor

 

Vacuum Pumps - Kurt J. Lesker Company

Read user Insights about Mechanical Vacuum Pumps and Systems

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