An Introduction To Vaccum Pump Types , Selection And Application

An Introduction To Vaccum Pump Types , Selection And Application

Introduction
Vacuum pump - A device for creating, improving and/or maintaining a vacuum. Two basically distinct categories may be considered: gas transfer pumps and entrapment or capture pumps

A vacuum is a space entirely devoid of matter “absolute vacuum”, when the air pressure in a space lies below atmospheric pressure. In physics, a vacuum is defined as “a state of emptiness that can be achieved by experiment” – in other words, nothing. This definition refers to the state of a space entirely devoid of matter (sometimes also referred to as an “absolute vacuum”). In practice, however, this state cannot be achieved. Therefore, talk instead about a vacuum when the air pressure in a space is lower than the atmospheric pressure or when the density of air molecules is reduced.

Range of applications of a vacuum pump

The major application of liquid ring vacuum pumps and compressors is in the handling of wet gas and vapours that will condense partially during compression. Because compression is near isothermal, these machines are specifically suitable for the handling of explosive or polymerising gases or vapours.

The vacuum ranges below are classified per physical attributes and technical requirements.

FIG 1

1atm = 1.013 bar = 760torr

Vacuum Pumps Basic Operation
A vacuum pump converts the mechanical input energy of a rotating shaft into pneumatic energy by evacuating the air contained within a system. The internal pressure level thusbecomes lower than that of the outside atmosphere. The amount of energy produced de-pends on the volume evacuated and the pressure difference produced. Mechanical vacuum pumps use the same pumping mechanism as air compressors,except that the unit is installed so that air is drawn from a closed volume and exhausted to the atmosphere. A major difference between a vacuum pump and other types of pumps isthat the pressure driving the air into the pump is below atmospheric and becomes vanish-ingly small at higher vacuum levels. Other differences between air compressors andvacuum pumps are:

         • The maximum pressure difference produced by pump action can never be higher than 29.92                in. Hg (14.7 psi), since this represents a perfect vacuum.

         • The mass of air drawn into the pump on each suction stroke, and hence the absolute pressure                change, decreases as the vacuum level increases

         • At high vacuum levels, there is significantly less air passing through the pump. Therefore,                   virtually all the heat generated by pump operation will have to be absorbed and dissipated by               the pump structure itself.

FIG 2

.
Vacuum Pump Types

Vacuum systems are placed into the following broad-based grouping of pressure ranges:
            -  Rough/Low Vacuum: > Atmosphere to 1 mbar 
            - Medium Vacuum: 1 mbar  to 10⁻³ mbar 
            - High Vacuum: 10⁻³ mbar  to 10⁻⁷ mbar 
            - Ultra-High Vacuum: 10⁻⁷ mbar  to 10⁻¹¹ mbar 
            - Extreme High Vacuum: < 10⁻¹¹ mbar 

FIG 3

Vacuum Pump Types According To Application System

Different types of pumps for these vacuum ranges can then be divided into Primary (Backing) Pumps,  Booster Pumps and secondary (High Vacuum) Pumps: High, very high and ultra-high vacuum pressure ranges 

FIG 4

1- Primary (Backing) Pumps:

Rough and low vacuum pressure ranges.
2- Booster Pumps:

Rough and low vacuum pressure ranges.
3- Secondary (High Vacuum) Pumps:

High, very high and ultra-high vacuum pressure ranges.

Vacuum Pump Types According To Pressure Range

1- Gas Transfer Pumps 

Transfer Pumps transfer gas molecules by either momentum exchange (kinetic action) or positive displacement. The same number of gas molecules are discharged from the pump as enter it and the gas is slightly above atmospheric pressure when expelled. The compression ratio is the ratio of the exhaust pressure (outlet) to the lowest pressure obtained (inlet).

FIG 5

A- Positive Displacement Vacuum Pumps

Vacuum pumps fall into the same categories as air compressors do. That is, they are either positive displacement or nonpositive displacement machines. A positive displacement pump draws a relatively constant volume of air despite variations in the vacuum levels

As with air compressors, the principle types of positive displacement vacuum pumps are the piston, diaphragm, rocking piston, rotary vane, lobed rotor, and rotary screw designs.

FIG 6

 B- Kinetic Transfer Pumps

Kinetic transfer pumps use high speed blades or introduced vapor to direct gas towards the outlet, working on the principle of momentum transfer. These types of pump can achieve high compression ratios at low pressures but typically don’t have sealed volumes.

FIG 7

3- Entrapment Pumps

Pumps which capture gas molecules on surfaces within the vacuum system are unsurprisingly known as, Capture or Entrapment Pumps. These pumps operate at lower flow rates than vacuum pumps such as transfer pumps, however, they can provide extremely high vacuum, down to 10-12Torr. Capture pumps operate using cryogenic condensation, ionic reaction, or chemical reaction and have no moving parts, therefore creating oil-free vacuum.

Those Entrapment Pumps that work using chemical reactions, perform more effectively as they are usually placed inside the container where vacuum is required. Air molecules create a thin film which is removed as the pumps operation cause a chemical reaction to the internal surfaces of the pump. Entrapment pumps are used along with positive displacement vacuum pumps and momentum transfer vacuum pumps to create ultra-high vacuum.

FIG 8

Vacuum Pump Types According To Labrication

1- Oil-Less (dry)

OiI-less pumps are almost essential when production processes cannot tolerate any oil vapor carry over into the exhaust air. They also can be justified on the basis of avoiding the cost and time of regularly refilling the oil reservoirs. This is particularly important when the pumps are to be mounted in inaccessible locations.

Modern piston pumps have rings of filled Teflon, which provide hundreds of hours of duty, depending on ambient temperature and air cleanliness. Diaphragm and rocking piston pumps are designed to be oil-less.

FIG 9

2- Oil-Lubricated (wet)

The oil-lubricated types have distinct advantages if proper maintenance is provided. They can usually provide about 20 percent higher vacuums because the lubricant acts as a sealant between moving parts. And they usually last about 50 percent longer than oil-less units in normal service because of their cooler operation. They also are less subject to corrosion from condensed water vapor.

FIG 10

Vacuum Pump Types According To Application

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.

1- 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  include:
          Axial Blower
          Centrifugal Blower
          Circumferential Piston
          Claw
          Diaphragm
          Gear Pump
          Linear
          Liquid Ring
          Lobed Rotor (Roots)
          Reciprocating Piston
          Regenerative Blower
          Rocking Piston
          Rotary Piston
          Rotary Vane
          Rotary Screw
          Scroll

2- High Vacuum Pumps

High vacuum pumps are those capable of producing higher quality (lower pressure) vacuums, typically in the high (< 10⁻³, >10⁻⁸Torr) 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  include:
           Turbomolecular
           Molecular Drag
           Diffusion / Vapor
           Ion
           Cryogenic / Cryosorption
           Getter / TSP / NEG Sorption


3- 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  include:
             Venturi Air Jet
             Steam Ejector
             Liquid Eductor / Ejector

            

4- 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 highvacuum 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.

Vacuum Pump Types According To 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.

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

Rotary Vane Pump (Mechanical, Wet, Positive Displacement,1st or 2st)

In the rotary vane pump, the gas enters the inlet port and is trapped by an eccentrically mounted rotor which compresses the gas and transfers it to the exhaust valve (Fig 11. ). The valve is spring-loaded and allows the gas to discharge when atmospheric pressure is exceeded. Oil is used to seal and cool the vanes. The pressure achievable with a rotary pump is determined by the number of stages used and their tolerances. A two-stage design can provide a pressure of 1×10-³ mbar.

FIG 11

The rotary vane design offers significant advantages: compactness; larger flow capacities

for a given size; lower cost (about 50 percent less for a given displacement and vacuum level); lower starting and running torques; and quiet, smooth, vibration free, continuous air evacuation without a receiver tank.

Liquid Ring Pump (mechanical,Wet, Positive Displacement , 1st or multistage)
Liquid ring vacuum pumps are similar to a rotary vane pump, with the difference being that the vanes are an integral part of the rotor and churn a rotating ring of liquid to form the compression-chamber seal. They are an inherently low-friction design, with the rotor being the only moving part. Sliding friction is limited to the shaft seals. Liquid-ring pumps are typically powered by an induction motor.

Liquid-ring systems can be either single or multistage.

FIG 12

Diaphram Pump (mechanical , Dry, Positive Displacement , 1st or 2 st)

The diaphragm unit creates vacuum by flexing of a diaphragm inside a closed chamber. Small diaphragm pumps are built in both one- and two-stageversions. The single stage design provides vacuums up to 24 in. Hg, while the two stage unit is rated for 29 in. Hg.

FIG 13

Scroll Pump (mechanical,Dry, Positive Displacement ,)
The scroll pump (Fig.14 ) uses two scrolls that do not rotate, but where the inner one orbits and traps a volume of gas and compresses it in an ever decreasing volume; compressing it until it reaches a minimum volume and maximum pressure at the spirals’ center, where the outlet is located. A spiral polymer (PTFE) tip seal provides axial sealing between the two scrolls without the use of a lubricant in the swept gas stream. A typical ultimate pressure of 1 x 10⁻² mbar can be achieved. It has a pumping speed range of 5.0 to 46 m³/h (3.0 to 27 ft³/min).

FIG 14

Reciprocating Piston Pumps(Mechanical , Dry , Positive Displacement)

The primary advantage of the piston design is that it can generate relatively high vacuums from 27 to 28.5 in. Hg-and do so continuously under all kinds of operating conditions. The major disadvantages are somewhat limited capacities and high noise levels, accompanied by vibrations that may be transmitted to the base structure. In general, the reciprocating piston design is best suited to pulling relatively small volumes of air through a high vacuum range.

FIG 15

Rotary screw pumps (mechanical , Dry, Positive Displacement, booster, 1st)
A screw vacuum pump consists of two parallel, screw-shaped rotors, one with a right-hand thread and
the other with a left-hand thread. Both screws turn in the compressor housing without friction and at
very tight clearances.
They are synchronized via a precision gear. The compression housing and the special shape of the screws form the compression chambers. Due to the opposite rotation of both screws the chamber connected with the suction port is enlarged and the gas is transported into the compression chamber. Then the chamber moves axially from the suction side to the pressure side (arrow).
In variable pitch models, the gas is compressed at each pitch change and cooled before the next pitch
change, resulting in greater efficiency. On the pressure side the chamber is moved against the axial
housing wall and the volume is reduced until the front surface of the screw opens the pressure channel and the pre-compressed gas is discharged through the pressure connection. Cooling is achieved using a water cooled outer chamber. For some pump sizes additional cooling gas can be introduced into the pump.

FIG 16

Rocking Piston vaccum Pumps (mechanical, Dry , Positive Displacement,1st or 2st)
This design combines the light weight and compact size of the diaphragm unit with the vacuum capabilities of reciprocating piston units. Vacuums to 27.5 in. Hg are available with a single stage; two-stage units can provide vacuums to 29 in. Hg. Air flows, however, are limited, with the largest model available today (a twin cylinder model) offering only 2.7 cfm.

FIG 17

Rotary Piston vaccum pumps(mechanical , dry , positive displacment ,)
A rugged type of vacuum producing device is the Rotary Piston Vacuum Pump. Its piston is attached to a cam that is mounted eccentrically to the main bore of the vacuum pump cylinder.
At the start of the cycle, the volume between the piston and the cylinder increases as the shaft rotates the piston cam assembly. Gas is drawn in through a channel in the piston, until volume is at its maximum. At that point, the pocket becomes sealed from the inlet as the inlet channel in the piston closes off. Lubricating oil helps seal the clearances.
The shaft then further rotates the piston and cam assembly, in a way that compresses sealed off gas against the pump cylinder and the discharge valve. The discharge valve opens when the gas pressure is slightly above atmospheric. The gas and lubricating oil is then forced out and cycle repeats itself.

FIG 18

Regenerative Blowers (mechanical keintic, dry , nonpositive displacement , 1st)

Regenerative blowers are low-pressure, high-volume blowers that generate centrifugal airflow. Regenerative blowers are capable of both pressure (or compression) and vacuum (or suction) service. Some products are geared toward either pressure or vacuum service, while others are suitable for both. Compression blowers are often configured for use as air supply units or compressors, while vacuum blowers are frequently used as vacuum pumps or fume exhausters.

The construction and operation principles of a regenerative blower gives the product its name. All regenerative blowers consist of an impeller that spins within a housing compartment. The housing contains both an inboard (or intake) channel and an outboard channel; for this reason regenerative blowers are also known as side channel blowers. When the impeller spins past the intake port, air is drawn in and trapped between the impeller blades. As the impeller continues to spin, the air is pushed both inward and outward through both channels, and this process continues until the impeller stops rotating. It is this regenerative process that allows the blower to function as both a pressure blower and a vacuum blower. While the air trapped between two impeller blades represents only a small pressure increase, the sum total of the blades, from intake to outlet, is capable of powerful continuous operating pressures.

For the purpose of this selection guide, the blue spheres represent air molecules traveling through the blower. A regenerative blower may feature an integral OEM controller or control panel with multiple adjustment controls in addition to a simple regulator knob. Additionally, vacuum blowers may or may not include a vacuum gauge to provide a pressure readout.

FIG 19

Lobed Rotor (Roots) Vacuum Pumps ((Mechanical ,Dry, Positive Displacement, booster , 1st)

The Roots pump (Fig. 20) is primarily used as a vacuum booster and is designed to remove large volumes of gas. Two lobes mesh without touching and counter-rotate to continuously transfer the gas in one direction through the pump. It boosts the performance of a primary/backing pump, increasing the pumping speed by approximately 7:1 and improves ultimate pressure by approximately 10:1. Roots pumps can have two or more lobes. A typical ultimate pressure of < 10⁻³ Torr can be achieved (in combination with primary pumps). It can achieve pumping speeds in the order of 100,000 m³ /h (58,860 ft ³/min).

FIG 20

Gear Vacuum Pumps (mechanical , wet , positive displacement, 1st or 2st)
A gear pump is a type of positive displacement (PD) pump. Gear pumps use the actions of rotating cogs or gears to transfer fluids. The rotating gears develop a liquid seal with the pump casing and create a vacuum at the pump inlet. Fluid, drawn into the pump, is enclosed within the cavities of the rotating gears and transferred to the discharge. A gear pump delivers a smooth pulse-free flow proportional to the rotational speed of its gears

There are two basic designs of gear pump: internal and external (Figure 21 ). An internal gear pump has two interlocking gears of different sizes with one rotating inside the other. An external gear pump consists of two identical, interlocking gears supported by separate shafts. Generally, one gear is driven by a motor and this drives the other gear (the idler). In some cases, both shafts may be driven by motors. The shafts are supported by bearings on each side of the casing.

FIG 21

Linear Vacuum pump (mechanical , dry , positive displacement, 1 st)
The Linear-motor-driven Free Piston System (Pat.) has an internal piston inside the cylinder which is driven by an electro-magnet and spring system controlled by the alternating input current cycle. The piston thus forms a single combined structure of two usually different devices; motor and pump. The system is quiet and vibration free, and offers the advantages of easy maintenance and a long operating life.

FIG 22

Claw Pump (mechanical Dry, Positive Displacement)

The claw pump (Fig.23 ) features two counter-rotating claws and operates similarly to the Roots pump, except that the gas is transferred axially, rather than top-to-bottom. It is frequently used in combination with a Roots pump, which is a Roots-claw primary pump combination in which there are a series of Roots and claw stages on a common shaft. It is designed for harsh industrial environments and provides a high flow rate. A typical ultimate pressure of 1 x 10⁻³ mbar can be achieved. It has a pumping speed range of 100 to 800 m³ /h (59 to 472 ft³ /min).

FIG 23

Axial Vacuum Blower (mechanical , dry , nonpositive displacement ,kientic )

The fluid enters and exits along the same direction parallel to the rotating shaft. The fluid is not accelerated but instead “lifted” by the action of the impeller. Axial flow pumps (Fig.24 ) operate at much lower pressures and higher flow rates than radial flow pumps. Some-times axial flow pumps are used to transfer solids at high rates, more specifically in non-dirty applications 

FIG 24

Centrifugal Blowers (mechanical , dry , nonpositive displacement , 1 st,multi stage)
Centrifugal Blowers are the perfect solution for vacuum or compressed air applications. Centrifugal blowers provide dry, clean, oil-free air and require low energy consumption.

Air enters the center of a spinning impeller and is divided between the impeller’s vanes. As the impeller turns, it accelerates the air outwards using centrifugal force. This high-velocity air is then diffused and slowed down in the surrounding blower housing to create pressure.

FIG 25

Turbomolecular Pumps ( High Vacuum Pumps Dry, Kinetic  , scondary )

Turbomolecular pumps (Fig.26 ) work by transferring kinetic energy to gas molecules using high speed rotating, angled blades that propel the gas at high speeds: the blade tip speed is typically 250 -300 m/s (670 miles/hr.) By transferring momentum from the rotating blades to the gas, they provide a greater probability of molecules moving towards the outlet. They provide low pressures and have low transfer rates. A typical

ultimate pressure of less than 7.5 x 10⁻¹¹ Torr can be achieved. It has a pumping speed range of 50 – 5000 l/s. The bladed pumping stages are often combined with drag stages that enable turbomolecular pumps to exhaust to higher pressures (> 1 Torr).

FIG 26

Vapor Diffusion Pumps (Wet, High Vacuum Pumps, Kinetic , scondary )

Vapor diffusion pumps (Fig.27 ) transfer kinetic energy to gas molecules using a high velocity heated oil stream that “drags” the gas from the inlet to the outlet, providing a reduced pressure at the inlet. These pumps feature an older technology, largely superseded by dry turbomolecular pumps. They have no moving parts and provide high reliability at a low cost. A typical ultimate pressure of less than 7.5 x 10⁻¹¹ Torr can be achieved. It has a pumping speed range of 10 – 50,000 l/s.

FIG 27

Sputter Ion Pumps (High Vacuum Pumps , Dry, Entrapment)

The sputter ion pump (Fig. 28) traps gases using the principles of gettering (whereby chemically active

materials combine with gases to remove them) and ionization (gas molecules are made electrically conductive and captured). A high magnetic field combined with a high voltage (4 to 7kV), creates a cloud of electronspositive ions (plasma) which are deposited onto a titanium cathode and sometimes a secondary additional cathode composed of tantalum. The cathode captures the gases, resulting in a getter film. This phenomenon is referred to as sputtering. The cathode must be periodically replaced. These pumps have no moving parts, are low maintenance, and can achieve a pressure as low as 7.5 x 10⁻¹² Torr. They have a maximum pumping of 1000 l/s.

FIG 28

Cryopump Vacuum pump (High Vacuum Pumps,Dry, Entrapment)

The Cryopump (Fig.29 ) operates by capturing and storing gases and vapors, rather than transferring them through the pump. They use cryogenic technology to freeze or trap the gas to a very cold surface (cryocondensation or cryosorption) at 10°K to 20°K (minus 260°C). These pumps are very effective but have limited gas storage capacity. Collected gases/vapours must periodically be removed from the pump by heating the surface and pumping it away through another vacuum pump (known as regeneration). Cryopumps require a refrigeration compressor to cool the surfaces. These pumps can achieve a pressure of 7.5 x 10⁻¹⁰ Torr and have a pumping speed range of 1200 to 4200 l/s.

FIG 29

molecular drag vacuum pump (High Vacuum Pumps Dry, Kinetic )

A molecular drag vacuum pump configured for pumping a gas stream from an inlet to an outlet, the pump including a high-speed spinning disk or rotor disposed within a housing. A passageway is formed inside the housing adjacent the disk, and gas comes in contact with surfaces of the spinning disk in successive stages, conformable wipers being disposed adjacent the spinning disk to direct the gas stream to the successive stages. The disk can be powered by an integrated motor, comprising permanent magnets associated with the disk and cooperating coils associated with the housing. The wipers can include parallel ridges on a contacting face to facilitate creation of a conformable fit with the rotor. Seal rings may be disposed against the disk between gas passageways to reduce leakage there between, and the pump may include regenerative pumping pockets to help prevent backflow. The housing may have a modular configuration to allow two or more pump modules to be connected and operate in series. Successive stages may be independently or commonly powered, and may counter-rotate.

FIG 30

Getter Vacuum Pumps (High Vacuum Pumps , Dry, Entrapment)

The titanium-sublimation pump (TSP) is usually used together with the sputter ion pump, for

electron microscopes UHV-EM and AES. On the other hand, the non-evaporable getter (NEG) pump has not yet been adopted in the evacuation system of the JEOL electron microscopes, which may be due to the high temperatures of the pump during operation and degassing. Recently, the performance of NEG has been improved in the direction of lowering the temperatures of the pump during operation and degassing

FIG 31

Vacuum ejectors (wet ,Entrapment, keintic)

These have been considered as work horses and have been the most widely used vacuum producers over the last few decades. This works on the principle of converting the energy of Motive fluid (which may be the same as or different from process fluid) into velocity (kinetic energy) as it flows through a relatively small converging – diverging nozzle. This lowered pressure of the motive fluid creates suction in a mixing chamber, into which the process fluid is drawn. The process fluid thus mixes and becomes entrained in the motive fluid stream. This mixed fluid then passes through a converging – diverging diffuser, where the velocity is converted back to kinetic energy. Hence, the resultant pressure is higher than the suction pressure of the ejectors

Depending on the degree of vacuum required, Ejectors usually come in a series configuration with various stages attached (called a multi stage unit). Usually the backing work is done by Water Ejectors/Steam Ejectors and front end stages are known as Steam Boosters. Barometric leg (Direct contact type) condensers are installed in the intermediate stages.

FIG 32

1- Single-stage ejector: The design principle for a single-stage ejector includes a jet nozzle and only one receiver nozzle. After exiting the receiver nozzle, the exhaust air is generally discharged via a silencer or directly into the atmosphere.
2- Multi-stage ejector: This design principle also includes a jet nozzle. Behind the first receiver nozzle there are additional nozzle stages, each of which has a bigger nozzle diameter in proportion to the falling air pressure. The drawn-in air from the first chamber, combined with the compressed air from the jet nozzle, is thus used as a propulsion jet for the other chambers.
Features:
• Low maintenance and low wear-resistant because there are no moving parts
• Low initial costs
• Compact design, smallest possible dimensions
• Suitable for pulsed applications
• Fast reacting
• Small line lengths between vacuum generation and application
• Easy to install, can assume any mounting position
• Low weight
• Multiple functions possible in a single device
• Supply port 4 - 6 bar optimal

Vacuum equipment can be roughly divided into "Steam Ejectors" and "Vacuum Pumps", Three major factors should be considered in the type selection stage for vacuum devices. These factors are operating requirements (i.e., suction pressure), suction gas properties and cost. As a general procedure for type selection, the flow chart shown in Figure 33  can be used

FIG 33

Selection of Process Vacuum Pumps

The first step in evaluating alternatives is to eliminate from consideration pumps or pumping systems that can't meet process requirements. This involves a consideration of 

      (1) required suction pressure and capacity,  

      (2) reliability and maintenance. Following elimination of those pumping systems that can't meet               process requirements, the most economical system can be determined by considering 

      (3) purchase and installation costs and 

      (4) operating costs. Final selection is subject to constraints imposed by 

      (5) environmental considerations.

The most important parameters affecting final selection of the vacuum pump comes down to the suction pressure (P2) and the throughput the pump must handle (V2). The suction pressure is calculated by subtracting the losses in the suction line from the system operating point back to the vacuum pump. Line Losses include losses across sections of manifold, bends, and losses across filters, KOP's, scrubbers and precondensers.

When calculating the load to the vacuum pump, it's important to first examine the process. What are the primary sources of vapor or gas load to the pump. Is evacuation time a major concern. The checklist below will provide a basis for calculating capacity requirements for a vacuum pump.

Sources of Vapor-Gas Load
1) Air Leakage
2) Vapors of Saturation
3) Evaporated Vapors
4) Evacuation of Process Equipment
5) Decomposition
6) Reaction Products
7) Sparge Gas
8) Stripped Gas out of Process Material
9) Dissolved Gas
10) Purge gas from instrument lines

After the process design has been completed pump selection can be prepared through the following questions:

1)   Starting pressure
2)   Operating Pressure
3)   Discharge Pressure
4)   Gas Temperature
5)  Temperature and type of available cooling water
6)  Temp and type of sealing liquid in the case of a liquid ring
7)  Non-Condensable Pump Load
8)  Condensable Pump Load

Total Recovery System Compnents 

FIG 34

Sizing Vacuum Pumps 

Pumping speed, or capacity, is measured in terms of gas volume drawn in a length of time. Cubic Feet per Minute (cfm) or Cubic Meters per Hour (m3 /h) are the two standard measurements. The number corresponds to the volume of the suction chamber multiplied by the pump revolutions in the time unit and by a correction coefficient which is usually 0.85.

Conductance 

 for a pipe with a constant gas flow of Q, the quantity of conductance, C, can be expressed by:

Where 

P2 and P1 are the pressures at pipe sections 2 and 1 respectively. From a physical standpoint, conductance equates a resistance and expresses the ease which the gas flow Q passes along the pipe itself.  

Conductance is a function of two factors: circuit configuration and pressure. Conductance is relatively high when pump-down begins, and decreases progressively. Its values are particularly low for vacuum levels better than 1 mbar. For a typical straight tube with round sections, having a length (L) much larger than the diameter (D) conductance is calculated by means of the following formulae:

Viscous or Laminar Flow: When P x D > 5x10⁻¹ torr x cm: 

Molecular flow: When P x D < 1x10⁻²  torr x cm:

It is understood that conductance under molecular flow is independent of the pressure P, not included in the second formula. So, the Viscous or Laminar flow is used most of the time when talking about conductance. 

Actual Speed 

The actual speed at a specific point of the circuit A never matches the nominal pumping speed Sp due to the constrictions and pressure fluctuations in the circuit itself. The actual speed looks more like this:

where 

Sp is the pumping speed of the pump 

C  is the conductance of the evacuation pipe.

Se is Effective pumping speed

Pumping-down Characteristics

Ascertain that the desired high-vacuum pressure in the chamber is achieved within a specified evacuation time, using the following volume-evacuation equation. When a chamber of volume V (in litter L) is evacuated by a pumping speed S (in L/s):

Steady-State Evacuation

The pressure P at an equilibrium state is expressed by the equation

a general list of available capacities and operating pressure ranges for the most often used process vacuum pumps and systems.

Steam Ejectors, single stage: 10-1,000,000 cfm (50mm)
Steam Ejectors, two-stage: 0-1,000,000 cfm (4mm)
Steam Ejectors, three-stage: 10-1,000,000 cfm (800 microns)

Liquid Ring Vacuum Pumps, single stage: 3-18,000 cfm (25mm)
Liquid Ring Vacuum Pumps, two stage: 3-6,000 cfm (25mm)

Rotary Piston Pumps - single stage: 3-800 cfm (5 microns)
Rotary Piston Pumps - two stage: 3-800 cfm (.001 microns)

Rotary Vane Pumps - oil sealed once-thru two stage: 100-600 cfm (.5mm)
Rotary Vane Pumps - oil sealed recirculation two stage: 3-150 cfm (.001 microns)

Rotary lobe blowers - single stage: 30-30,000 cfm (400mm)
Rotary lobe blowers - two stage: 30-30,000cfm (60mm)

Dry Screw - single stage: 60-600 cfm (.0075mm)

Dry Claw - single stage: 30-350 cfm (75mm)