Vacuum Pumps and Vacuum Generators Selection Guide     Vacuum Pumps and Vacuum Generators Selection Guide     Vacuum Pumps and Vacuum Generators Selection Guide

Image Credit: Clark Solutions | Gast Group | Agilent Technologies, Inc.

 

Vacuum pumps and vacuum generators provide sub-atmospheric pressure for a variety of industrial and scientific applications. Vacuum pump technologies are used for industrial gripping and chucking, laboratory degassing, and purification in the fields of chemical and semiconductor processing. Vacuum handling systems are used in diverse industries such as pharmaceutical, food processing, and agricultural applications.

 

Background and Operation

 

A vacuum is a vessel or space in which gas pressure is much less than atmospheric pressure. Pressure is created by the force of moving gas molecules colliding on the walls of a vessel. In essence, then, vacuum in a container is generated by removing gas molecules (matter) from the vessel, lowering the frequency of collisions and thus lowering the pressure. The quality of a vacuum refers to how close the pressure is to a perfect vacuum (zero pressure). Vacuum quality is defined by ranges of pressures which are standardized based on either a "Traditional" approach or the American Vacuum Society (AVS).

 

Traditional Approach

Pressure Range

Vacuum Designation

Torr

Rough

< 760, >1

Medium

< 1, >10-3

High

< 10-3, >10-8

Ultra High

< 10-8

Traditional designation for vacuum quality ranges.

 

AVS Approach

Pressure Range

Pressure Range

Vacuum Designation

Torr

Pascal

Low

750 - 25

< 105, >3.3x103

Medium

25 - 7.5x10-4

< 3.3x103, >10-1

High

7.5x10-4 - 7.5x10-7

< 10-1, >10-4

Very High

7.5x10-7 - 7.5x10-10

< 10-4, >10-7

Ultra High

7.5x10-10 - 7.5x10-13

< 10-7, >10-10

Extreme Ultra High

>7.5x10-13

< 10-10

AVS designation for vacuum quality ranges.

 

Vacuum pumps and vacuum generators create vacuums by physically removing gas molecules from a vessel or space. There are two main principles used to create these vacuums: gas-transfer and gas-capture.

 

Gas-Transfer

 

Vacuum pumps that operate on the gas-transfer principle create vacuums by moving gases through the pump. These pumps can be used continuously without the need for regeneration. All gas-transfer pumps are either positive displacement or kinetic (momentum transfer). Positive displacement (gas-displacement) pumps displace gas from sealed areas to the atmosphere to a downstream pump stage. Kinetic pumps displace gas by accelerating it in the pumping direction, either mechanically (mechanical vacuum pumps) or via an adjacent vapor stream (venturi jet vacuum pumps).

 

Gas-Capture

 

Vacuum pumps that operate on the gas-capture or gas-binding principle create vacuums by physically capturing the gas, holding it in a frozen state. These pumps must be periodically regenerated because they become saturated with captured material over time.

 

This diagram provides an overview of vacuum pump classification.

Vacuum Pump and Vacuum Generator Selection Guide

Vacuum pump classification. Image Credit: Pfeiffer Vacuum

 

Types of Vacuum Pumps

 

As shown in the previous chart, there is an extensive list of vacuum pump types. In general, however, there are three main categories of vacuum pumps: mechanical, high vacuum, and venturi jet. General selection between these types will depend on the application and the vacuum needs of the user.

 

Mechanical Vacuum Pumps

 

Mechanical vacuum pumps generate vacuums using mechanical devices such as pistons, diaphragms, impellers, or blowers. These pumps are typically associated with low vacuums (higher pressures) and are often used as backing pumps (described below) for high vacuum pumps. Mechanical vacuum pump types in the GlobalSpec SpecSearch Database include:

  • Axial Blower
  • Centrifugal Blower
  • Circumferential Piston
  • Claw
  • Diaphragm
  • Gear Pump
  • Linear
  • Liquid Ring
  • Lobed Rotor (Roots)
  • Reciprocating Piston
  • Rocking Piston
  • Rotary Piston
  • Rotary Screw
  • Scroll

High Vacuum Pumps

 

High vacuum pumps are those capable of producing higher quality (lower pressure) vacuums, typically in the high (< 10-3, >10-8 Torr) or ultra-high (< 10-8 Torr) range. They operate by acting on the mean free path of gas molecules using thermal, sorption and mechanical processes such as diffusion, molecular drag, cryosorption, gettering, and high speed turbomolecular rotors. High vacuum pumps almost always require a backing pump in order to generate an initial low vacuum from atmospheric pressure. High vacuum pump types in the GlobalSpec SpecSearch Database include:

  • Turbomolecular
  • Molecular Drag
  • Diffusion / Vapor
  • Ion
  • Cryogenic / Cryosorption
  • Getter / TSP / NEG Sorption

Venturi Jet Pumps

 

Venturi vacuum generators create vacuum by acting on the viscous properties of the gas or fluid being evacuated using the venturi chamber construction and the properties of a liquid or gas flowing through a tube or pipe. Venturi or fluid jet vacuum generators rely on a flow of compressed air, gas, or liquid as the "motive" fluid to pull or create a vacuum at a desired port. They contain no moving parts and require no other power than the compressed fluid. Venturi jet types in the GlobalSpec SpecSearch Database include:

  • Venturi Air Jet
  • Steam Ejector
  • Liquid Eductor / Ejector 

This video provides an animation of the operating principle of a jet ejector vacuum pump.

 

Steam ejector pump operation. Video Credit: NASH pumps

 

Backing Vacuum Pumps

 

Integral "backing" vacuum pumps, also called roughing pumps, are those specifically designed to support high vacuum pumps. In a vacuum system, they are usually connected to the primary pump's exhaust. They are used initially to pump the chamber through the primary pump (roughing) from atmospheric or near-atmospheric pressure to a pressure low enough for the primary pump fro operate. At this point, the backing pump provides a support pumping (backing) role.

 

These pumps are commonly mechanical pumps, but not exclusively. In a vacuum system, the backing pump is typically connected to a high-vacuum pump with a foreline manifold. It is important to consider the manifold pressure when dealing with diffusion-type high-vacuum pumps. If it is too high or too low, the system risks backstreaming contamination into the processing chamber or the high vacuum pump. Different types of high vacuum pumps will have different requirements for their associated roughing/backing pumps.

 

Specifications

 

Vacuum pumps and vacuum generators are described using a number of important specifications relating to performance, lubrication, mounting, and the number of stages.

 

Performance

  • Ultimate operating vacuum or ultimate pressure is the lowest pressure which the vacuum pump can generate (within a set time) referenced to atmospheric or absolute zero pressure. The rated ultimate pressure is a theoretical value often given based on certain assumptions that are not realistic under normal operating conditions (e.g. ignoring the pressure of condensable gases like water vapor). Base pressure is sometimes synonymous with ultimate pressure, but may be given as a separate value; as a separate rating, it defines the vacuum obtained under specific conditions underlined in standard ISO 21360-1. Base pressure is usually higher than the ultimate pressure because it must be achieved within a set period of time.
  • Pumping speed is the volumetric rate at which gas is transported across a plane, 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 throughput of the gas load (described below), and P is the inlet pressure of the gas . A pump's maximum achievable pumping speed 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.

 

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.

 

As shown in the diagram below, the relationship between pressure and pumping speed is positive for low vacuum backing-type pumps and negative for high-vacuum pumps. When selecting a backing pump for a high-vacuum system, comparison of performance curves for the pump types is important to ensure flow and pressure requirements are met.

 

Vacuum Pumps and Vacuum Generators Selection Guide

Image Credit: Vacuum Technology: Calculations in Chemistry by A. Morris

 

 

 

Stages

The stage of the vacuum pump defines how many sequential sections, chambers, or pumps are packaged together in the unit. For many applications, using a multi-stage vacuum pump may be more efficient than using multiple separate pumps in series.

  • Single stage pumps move gas molecules directly from the evacuated chamber into the atmosphere.
  • Two stage pumps evacuate gas molecules in two stages for lower absolute pressure.
  • Three stage pumps use three separate chambers in succession.
  • Pumps with four or more stages are employed in applications requiring extremely high vacuums.

Lubrication

  • Oil-free or oil-less pumps (sometimes called "dry" 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 of 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 screw, scroll, diaphragm, 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. Beyond the hassles of used oil disposal and/or cleanup, oil-sealed pumps are extremely reliable and require little maintenance outside oil changes. 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 to consider for these pumps:
    • Splash lubrication involves physically splashing oil onto the components from an oil bath.
    • Oil-flooded lubrication is sometimes used in rotary vane and other mechanical pump styles. This type of lubrication involves a heavy application of oil for moving parts. These pumps should not be used in conjunction with fluids which may react with oil or where oil contamination is unacceptable.
    • Positive pressure lubrication processes maintain oil pressure to ensure the highest level of lubrication.

There are a number of differences between oil-less and oil-sealed pumps which may help to determine what type is needed. Some of these differences are listed in the table below.

 

 

Oil-Sealed Pumps

Oil-Free Pumps

Capital cost

Low

High

Oil loss

Can be high at > 1 mbar

Very low

System contamination

Backstream at < 0.1 mbar

Very low

Add-on costs

Oil return/filtration

Not necessary

Aggressive process

Not suitable

Resistant

Purge

Sometimes

Almost always

 Wet and dry pump comparisons. Table Credit: Edwards Limited

 

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

 

Vacuum pumps may 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.
  • 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.
  • 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

 

Vacuum pumps and vacuum generators 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

 

Vacuum pumps and vacuum generators may incorporate additional features. The GlobalSpec SpecSearch Database contains a number of product features.

  • 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.

Applications

 

Vacuum pumps and vacuum generators are used in a wide variety of applications. Some specific examples include thin film coating, welding reactive alloys, leak detection, freeze drying, space simulation, sterilization, solvent recovery, fume and foul gas removal, and vacuum gripping and chucking.

 

References

 

Classification of vacuum pumps - Pfeiffer Vacuum

 

How to Match Pumping Speed to Gas Load - The Vacuum Lab (pdf)

 

Knowing Your Effective Pumping Speed - The Vacuum Lab (pdf)

 

Mechanical Vacuum Pumps - Edwards Limited (pdf)

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