Vacuum Pumps

Chapter 1: What is a Vacuum Pump?

A vacuum pump is a piece of equipment capable of generating a partial vacuum or a low-pressure space by drawing gas molecules out of a sealed chamber. A vacuum is a relative state at which the chamber pressure has a lower pressure than the ambient atmosphere or adjacent systems. Note that this is different from absolute vacuum in which the pressure is at 0 Pa absolute and is completely devoid of gas molecules. There are different degrees of vacuum that can be created. It can range from being a low vacuum with an absolute pressure range of 1 to 0.03 bars to an extremely high vacuum with a pressure of a billionth of a Pascal. Low and medium vacuum are commonly seen in industrial systems such as vacuum grippers, vacuum cleaners, incandescent bulbs, painting, sandblasting, vacuum furnaces, and negative pressure ventilation. Higher vacuum systems are used for laboratory applications such as particle reactors and accelerators.

There are two main categories of generating partial vacuum. One is by gas transfer or gas feeding and the other is through entrapment. Gas transfer types of vacuum pumps work by mechanically removing gases through positive displacement or momentum transfer. Positive displacement vacuum pumps have chambers that alternately expand and contract with check or non-return valves to draw and eject flow. Momentum transfer pumps work by accelerating gases creating a low-pressure region in its wake. Entrapment vacuum pumps, on the other hand, capture gas molecules by various principles such as condensation, sublimation, adsorption, ionization, and so on.

Chapter 2: Vacuum Ranges

Vacuum ranges or regimes are classifications of the quality of vacuum characterized by the measurement of the absolute pressure of the system. The absolute pressure represents the amount of remaining matter inside the system, mostly composed of gas molecules such as nitrogen, oxygen, water vapor, and trace gases such as neon, helium, and hydrogen.

Different vacuum ranges require different pumping techniques. Low and medium vacuum ranges can be achieved by positive displacement vacuum pumps. These are suited for most industrial systems. Achieving high and ultra-high vacuum ranges for special applications such as surface analytic techniques, microscopy, and nanolithography are achieved by both momentum transfer and entrapment pumps.

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Chapter 3: Types of Vacuum Pumps and Their Operating Principles

The two main classifications of vacuum pumping principles are gas transfer and entrapment. Gas transfer is further divided into positive displacement and momentum transfer. To further grasp the concepts of vacuum pumps, it is best to understand the three types of flow: viscous, transitional, and molecular. Viscous or continuous flow occurs at high pressures to medium vacuum. In this type of flow, the gas is dense enough for gas molecules to collide with each other. The mean free path or the average distance traveled by a gas molecule is less than the dimensions of the chamber. When a higher vacuum is reached, the gas molecules tend to collide on the walls of the chamber more than other gas molecules. Transitional flow occurs when the viscous flow starts to change into molecular flow. Molecular flow is characterized by the random movement of gases where their mean free path is much longer than the dimensions of the chamber.

Fluids flowing under viscous flow can be pumped mechanically by positive displacement pumps. However, molecular flow will be reached when the gas cannot be evacuated by pressure difference. At this point, another pumping system, either momentum transfer or entrapment, is used. Most high vacuum systems have two pumps in tandem. Positive displacement pumps alone are not sufficient at higher vacuum. Momentum transfer pumps will stall if the system when operated at viscous flow. Entrapment pumps will be frequently regenerated or exhausted when there is too much gas to be captured particularly at viscous flow.

Positive Displacement Vacuum Pumps

Positive displacement vacuum pumps operate by expanding and contracting a sealed chamber where the flow of fluid is controlled by one-way valves. The vacuum generation process starts by expanding a sealed chamber generating a vacuum. This vacuum draws the fluid into the chamber through an intake valve. Upon reaching the maximum expansion, the intake valve closes while the exhaust opens. The fluid is ejected out of the chamber as it compresses or contracts. The cycle repeats several times per second itself creating a pulsating flow.

Like ordinary pumps, positive displacement vacuum pumps are classified according to the motion and the design of the chamber. There are two main categories: reciprocating and rotary.

• Reciprocating Vacuum Pumps: These types of pumps have chambers that expand and contract through the reciprocating or repetitive back-and-forth motion. The range of motion of the moving component is called a stroke. Reciprocating pumps have two one-way ports or valves, one for the inlet and the other for the exhaust. The alternate opening and closing of these valves allow the build-up of vacuum and ejection of the fluid. The three main types of reciprocating vacuum pumps are piston, plunger, and diaphragm vacuum pumps.
  • Reciprocating Piston Vacuum Pump: This type of pump generates vacuum and compression through the movement of the piston sealed against a cylinder. The piston is connected to the crankshaft via a connecting rod. As the crankshaft rotates, the piston is pushed back-and-forth within the cylinder. The pistons are commonly made of cast iron, bronze, or steel.
    
    

    

    
    
  • Plunger Vacuum Pump: This type of pump operates the same way as a reciprocating piston pump. The piston or plunger of this pump is a long, solid cylinder typically made of hard-coated ceramic. The long profile of the plunger allows the high-pressure seal to be stationary relative to the cylinder, in contrast with piston pumps where the seal is attached to the piston. This enables the use of more complex sealing systems. Plunger vacuum pumps are more suited for more demanding conditions than piston vacuum pumps.
    
    

    

    
    
  • Diaphragm Vacuum Pump: Diaphragm vacuum pumps use a deformable metallic or elastomer membrane permanently joined into the chamber creating a hermetic seal. Piston vacuum pumps have the advantage when it comes to reliability and power, while diaphragm vacuum pumps are mostly suited for ejecting hazardous or corrosive substances.
    
    

    

    
    

Reciprocating vacuum pumps can also be classified according to the number of the chambers mainly to address the problem of pulsating flow. A pulsating flow is an undesirable characteristic of reciprocating pumps where the flow is delivered in short bursts. Adding more pistons and cylinders will create a more constant flow. This brought the development of reciprocating pumps with multiple piston-cylinder assemblies known as multiplex pumps.

In terms of the achieved cycle phase per stroke, reciprocating pumps are categorized as either single- or double-acting. Single-acting pumps create only either vacuum or compression in a single stroke. In this configuration, the piston or diaphragm is coupled to only one chamber where only one side engages the fluid. A double-acting pump, by contrast, creates both vacuum and compression in a single stroke. A common configuration is a twin piston-cylinder or twin diaphragm assembly which is actuated by a single drive rod. Other designs can feature a single piston or diaphragm serving two chambers. Double-acting pumps are more commonly used due to better efficiency, higher flow rate, and less pulsating flow.

• Rotary Vacuum Pumps: As the name suggests, these types of vacuum pumps create low-pressure regions through the rotation of the moving components against the pump housing. The mating surfaces between the rotor and the housing have very small clearances coated with self-lubricating or low friction materials such as graphite, PTFE (polytetrafluoroethylene), or PEEK (polyether ether ketone). The small clearance prevents the fluid from leaking to the low-pressure side. In comparison with reciprocating types, rotary vacuum pumps have lower pulsating delivery making the flow more continuous. However, they are not suited to handle fluids contaminated by abrasive media since it can erode the small clearances between the rotor and the housing. Rotary vacuum pumps are classified according to the design of the rotor.
  • Rotary Vane Vacuum Pump: Rotary vane vacuum pumps are the most common type of positive displacement vacuum pump. This pump has vanes inserted radially into a circular rotor. The rotor is eccentrically installed relative to the stator housing. This eccentricity is known as the stroke of the pump. The individual chambers separated by the vanes progressively become smaller as it approaches the discharge. The vanes are allowed to move radially which press against the housing mainly through the centrifugal force as the rotor rotates. A spring energizes the vanes or holds the vanes in place when the rotor is not in motion.
    
    

    

    
    
  • Liquid Ring Vacuum Pump: Liquid ring pumps operate similarly to rotary vane pumps but have different vane construction. As mentioned earlier, rotary vane pumps have vanes that are free to move radially. The vanes of a liquid ring vacuum pump, however, are fixed to the rotor. When the rotor rotates, the liquid is drawn into the chamber by the expansion of cavities due to the eccentricity of the rotor. Centrifugal forces hold the liquid against the housing forming a ring of liquid, hence the name.
    
    

    

    
    
  • Rotary Piston Vacuum Pump: A rotary piston vacuum pump has an eccentric wheel as the rotor which is attached to a slide valve. Rotary piston valves can be considered as two-stroke, double-acting pumps with two separate compression chambers. As the wheel rotates during the intake stroke at the first chamber, the slide valve opens allowing the entry of the fluid. Opposite this chamber is another that undergoes an exhaust stroke. This second chamber has an exhaust valve where the compressed fluid is ejected. Like the rotary vane, the compression chamber is created by mating the rotor, in this case, the eccentric wheel, against the pump housing. This chamber progressively becomes smaller at the end of the exhaust stroke.
    
    

    

    
    
  • Screw Vacuum Pump: A rotary screw pump is one of the earliest developed positive displacement pumps which was known as the Archimedes‘ screw. In its simplest form, this pump consists of a single screw inside a hollow cylinder. Modern designs incorporate double or triple screws meshing against each other. As the fluid enters the pump, it becomes trapped in the cavities between the threads of the screw and the housing. The pressure is developed by the rotation of the screw ejecting the fluid on the other side. It is suitable in handling single and multiphase fluids and has higher tolerance in handling fluids with abrasive contaminants.
    
    

    

    
    
  • Gear Vacuum Pump: This type of reciprocating pump has rotors in the form of two meshing gears where one gear drives the other. Gear pumps can be either external or internal. An external gear pump has two mating external gears. External gear pumps operate by creating an expanded cavity as the teeth come out of mesh when rotating towards the inlet. The fluid is drawn into this cavity due to the vacuum generated. As the gears rotate, fluid is trapped between the teeth and the pump housing. The fluid is ejected to the other side of the chamber. On the other hand, internal gear pumps have rotors composed of a driven external gear and an internal gear. Pumping is achieved the same way as external gear pumps where the fluid is drawn from the expanding cavity as the gear teeth come out of mesh.
    
    

    

    
    
  • Lobe (Roots) Vacuum Pump: Lobe vacuum pumps deliver fluid the same way as gear pumps. But instead of having mating gear teeth, lobe pumps have two or more meshing lobes. These two-lobe rotors are both driven and timed with one another. This allows the rotors to rotate with no contact. Thus, high rotating speeds are possible with less wear to the rotor. This feature of timed rotation is available in gear pumps as well. Moreover, the lobes also allow a continuous fluid seal contact across the surfaces of the lobes. Gear pumps have a fluid seal that jumps discontinuously from teeth to teeth.
    
    

    

    
    
  • Scroll Vacuum Pump: This type of pump is composed of two co-wound spirals or scrolls with one acting as the rotor while the other as a stator. The rotor does not rotate but moves eccentrically relative to the other. Scroll pump works by drawing fluid from the periphery of the scrolls. The fluid trapped between the scrolls is transported towards the center where the volume is progressively decreased.
    
    

    

    
    

Momentum Transfer Pumps

Momentum transfer pumps operate by inducing the movement of gas or liquid molecules through kinetic energy transfer. This happens at the molecular flow, in contrast to the viscous or continuous flow occurring in positive displacement pumps. The uniform velocity distribution of the molecules is altered continuously to a preferred direction by the fast-moving surfaces hitting them. These surfaces are not only limited to impeller surfaces, but to other liquids as well. An example is a diffusion pump where high-speed jets of motive fluid impart momentum to the gases to be drawn from the inlet. Momentum transfer pumps are suited for creating a high vacuum. However, to create a molecular flow, low pressure must exist throughout the system. The exhaust cannot be directly released to the atmosphere or at pressures where backstreaming can occur. To solve this problem, a backing pump is installed in tandem with the vacuum pump. The backing pump can be a positive displacement pump that operates at a lower vacuum level which can directly discharge to the atmosphere.

• Turbomolecular Vacuum Pump: A turbomolecular vacuum pump has multiple stages of rotating and stationary turbine blades. The rotating blades are angled in such a way that it transfers sufficient momentum to the gas molecules moving them axially towards the subsequent stages until it reaches the exhaust. The stator has angled blades as well and ensures the correct direction of the gas. Since the mass of the gas is very small, the rotors must rotate at very high speeds. Friction heat build-up and rotor deflection limit the design of turbomolecular pumps.
  
  

  

  
  
• Diffusion Vacuum Pump: As mentioned earlier, a diffusion pump works by using a motive fluid used to transfer momentum to the gas molecules. The motive fluid is usually oil or steam. The general design of an oil diffusion pump involves a heater to heat the oil and is ejected to nozzles on top of the boiler or vaporizing chamber. The vaporized oil leaves the nozzles at supersonic speeds which collects randomly flowing gases drawn from the low-pressure chamber. Cooling coils are present to condense the vaporized oil which then returns to the boiler. The collected gas molecules continue to flow towards the exhaust. Steam or hydrocarbon gas ejectors work similarly. But these types do not need a boiler since the steam or motive fluid is already vaporized and has sufficient speed.
  
  

  

  
  

Entrapment Vacuum Pumps

Entrapment vacuum pumps employ multiple physical and chemical phenomena to capture gas molecules. The working principle is different from each type. Common to almost all entrapment pumps is their ability to operate at high vacuum regimes without any oil contamination. Entrapment vacuum pumps do not rely on rotors or other moving parts. The downside, however, is that it cannot operate continuously since it needs to be regenerated once the surface or material capturing the gases is full. Moreover, they cannot remove lighter gases such as hydrogen, helium, and neon. Below are some of the common entrapment vacuum pumps.

• Cryogenic Vacuum Pump: This type of vacuum pump works by cooling the gas down to its condensation or freezing point. It captures gases such as nitrogen and oxygen below 20 K at the high vacuum regime. For capturing lighter gases such as helium and hydrogen, they must be cooled down to 8 to 10 K. The typical design of a cryogenic pump is a two-stage cooler. The first stage is for removing water vapor and oil by cooling at around 70 to 80 K. The second stage is for removing gases that cool at around 10 to 20 K. At this stage, an adsorbent such as activated charcoal is integrated to capture the cooled gases.
  
  

  

  
  
• Sorption Vacuum Pump: This type of pump uses adsorbents such as activated charcoal, zeolite, or other types of molecular sieves for capturing gas molecules. It is usually paired with cryogenic pumps for condensing the gases or lowering the gas vapor pressure.
  
  

  

  
  
• Sputter Ion Vacuum Pump: Sputter ion vacuum pump, also known as ion getter pump or ion pump, operates by ionizing the incoming gases by an anode. The ionized gas then binds to a cathode or a getter, typically made of titanium. The binding may be through chemical or physical means depending on the type of gas present. As the ionized gas impacts the cathode, some atoms or electrons of the cathode are ejected from the surface, known as sputtering.
  
  

  

  
  
• Titanium Sublimation Vacuum Pump: In this type of vacuum pump, an electric current is periodically introduced through a titanium filament. This heats the titanium and directly vaporizes it inside a chamber. The gases flowing through or present in the chamber is captured by the vaporized titanium by bonding while in transit or upon developing a film on the chamber wall. Once the titanium film is consumed, the remaining titanium filament is again vaporized to create another layer.
  
  

  

  
  

Chapter 4: Wet and Dry Vacuum Pumps

Aside from the pumping principles, vacuum pumps can be categorized according to the type of lubrication and sealing system. Vacuum pumps can be wet or dry lubricated. Choosing between the two mainly affects other performance factors such as wear resistance, pumping speed, fluid contamination, and so on.

• Wet Vacuum Pumps: In contrast with dry vacuum pumps, wet vacuum pumps are generally cheaper. The main lubricating fluid used is oil. Different types of oil are used depending on the application. Oil is supplied to the pump by an auxiliary lubricating system that performs additional functions such as heat rejection and contaminant filtration. The downside of using wet systems is the contact of oil with the fluid. An oil separator is used downstream to separate the oil from the gas.
• Dry Vacuum Pumps: These types of pumps do not have a lubrication system. Rather, these pumps use dry or solid lubricants such as graphite, PTFE, and molybdenum disulfide. These materials have a low coefficient of friction and prevent the coated mating surfaces to be in metal-to-metal contact. Water cooling jackets lined along the pump housing are used to remove heat from the system. The main advantage of dry systems is the contaminant-free pumping. Moreover, they require less maintenance than wet vacuum pumps.

Chapter 5: Vacuum Ejectors

A vacuum ejector is a type of vacuum pump that operates using the Venturi effect. Inside the vacuum ejector is a venturi which is often called a jet nozzle or orifice. The venturi has a cross-section that narrows into a smaller area and then gradually expands to the cross-section of the discharge pipe. Connected immediately after the venturi is another stream of fluid. This is the vacuum connection.

The Venturi effect is based on Bernoulli's principle. This physical concept is basically the law of conservation of energy applied to fluids. It states the inverse relationship of kinetic energy and pressure: when a fluid‘s velocity increases, its pressure decreases, and vice versa.

A motive fluid is used to create the Venturi effect. The motive fluid initially has a higher pressure which is converted into velocity as it passes the venturi. The high-speed jet of the motive fluid has a lower pressure which creates a vacuum in the surrounding regions. Vacuum is produced to any connection made on the low-pressure regions.

A vacuum ejector has two main advantages.

• It has no moving parts: There are no reciprocating or rotating parts inside a vacuum ejector, unlike other vacuum pumps. It is only composed of a static venturi tube. This construction increases the reliability of the device and requires little to no maintenance.
• It uses waste energy from high-pressure sources: Vacuum ejectors are used when there is a steady source of high-pressure fluid such as steam, water, and effluent gas. Typically, these high-pressure lines are discharged into the atmosphere which wastes its potential energy. High-pressure fluid is used as the motive fluid that drives the vacuum ejector.

On the other hand, the disadvantage of using a vacuum ejector is the inevitable mixing of the motive fluid and the fluid from the vacuum connection. If the intention is to recover the fluid drawn from the evacuated chamber, special separation techniques must be performed.

Vacuum ejectors are usually used in applications for drawing liquids such as water and steam where mixing the motive and vacuum streams poses no negative issues. They are commonly seen in power plants, petroleum and petrochemical plants, and water treatment facilities.

Conclusion:

• A vacuum pump is a piece of equipment capable of generating a partial vacuum or a low-pressure space by drawing gas molecules out of a sealed chamber.
• A vacuum is a relative state at which the chamber pressure has a lower pressure than the ambient atmosphere or adjacent systems.
• Vacuum ranges or regimes are classifications of the quality of vacuum characterized by the measurement of the absolute pressure of the system.
• The two main classifications of vacuum pumping principles are gas transfer and entrapment. Gas transfer is further divided into positive displacement and momentum transfer.
• Vacuum pumps can also be categorized according to the type of lubrication and sealing system. Vacuum pumps can be wet or dry lubricated.

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