People might feel that vacuum generation comprises only plugging the pump, waiting for it to start up, and then dropping to the desired level. However, the reality is far from it and involves considering several other processes.
The vacuum pumps are employed to remove gas molecules or air from a sealed space to create a vacuum. The level of the vacuum can be maintained by employing the process of maintaining the gas at a certain pressure.
So, it becomes essential to choose the correct vacuum pump for you. It involves a critical understanding of the process conditions of different vacuum pumps and the operating range for each with the advantages and drawbacks involved with it.
In this article, we have elaborated on all the vacuum pump types to provide a clear understanding of each. This will help you determine your vacuum pump choice for what you are looking for.
What is a Vacuum Pump?
So, first, we need to understand what one means by a vacuum pump. It is a machine used to eliminate the air or gas molecules from a sealed area and thus, creating a relative vacuum within the given capacity. In 1650, Otto Van Guericke assembled the first-ever vacuum pump which was predated by the suction pump. In the modern day, industrial vacuum pumps are used to maintain, improve and create the vacuum.
Which Operating Principle is Used by Vacuum Pumps?
The fundamental functioning principle of any industrial vacuum pump is the same regardless of the individual technology. The air and gas molecules are released from the enclosed space using vacuum pumps. Then, when the pressure in the chamber is low, it becomes problematic to remove the remaining gas molecules.
So, it is better to go for a vacuum pump system that can function in a wide pressure range, starting from 1 to 10-6 Torr or 1.3 to 13.3 mBar of pressure. In the case of scientific or research applications, the pressure range needs to extend to 10-9 Torr. There are multiple kinds of pumps used to cover their pressure range and work in series in a given time.
What is the Classification of the Vacuum Pumps?
The vacuum pumps are categorised separately as gas transfer pumps and capture pumps or gas-binding pumps. Among these, the gas-displacement vacuum pumps can work without any limitations, while the gas-binding pumps have restrained gas absorption capability. Therefore, the capacity is generated through certain processes at different intervals.
Now, the gas displacement pumps or the gas transfer pumps are either kinetic pumps or positive displacement pumps. The kinetic pumps remove the gas from the sealed area by guiding it in the pumping direction, with the help of a mechanical drive technology or via a direct vapour stream condensed at the end of the pump system.
The positive displacement pumps remove gas from the enclosed area and guide it downstream of the pumped stage or to the atmosphere. Now, the gas-binding pumps either condense the gas at a certain temperature or bind the gas to a special active substrate via gettering. Chemisorption is technically performed by the getter pumps that generate pure getter surface constantly through sputtering or sublimation or vapourization.
What are the Types of Vacuum Pumps?
There are several kinds of vacuum pumps, and understanding each type based on their functioning and their operating principles will give you a clearer idea about which one you should go for. In the following section, we have listed the vacuum pumps and discussed the advantages and drawbacks of each. You need to go through all these types and understand how they operate and which applications they cater to or the applications they are most suited for.
1. Positive Displacement Pumps
These pumps function by contracting and expanding a sealed area where single-way valves coordinate the fluid flow. The generation of the vacuum process is by expanding the sealed chamber. Through the intake valve, the vacuum extracts the liquid into the sealed area. When nearing the full expansion, the exhaust opens and the input valve closes. When the chamber contracts or compresses, the liquid is displaced out of it.
Like all the usual pumps, the positive displacement pumps are categorised according to the structure of a chamber and the motion. The two major sub-types are reciprocating vacuum pumps and rotary vacuum pumps.
2. Reciprocating Vacuum Pump
The reciprocating vacuum pump has different compartments that contract and expand through the repetitive and reciprocating to and fro motion. One complete movement of the chamber is known as a stroke. These pumps feature 2 one-way valves or ports, 1 for the exhaust and another for an inlet. The alternating motion of closing and opening valves enables the fluid’s release and the vacuum’s formation. Again, the mentioned pumps are divided into three main types elaborated below.
- Reciprocating Piston Pump: In this pump, compression and vacuum is generated by moving a piston that is sealed in a cylinder. Through a connecting rod, a piston is attached to a crankshaft. So, when the crankshaft spins, the piston moves to and fro in the cylinder. Steel, bronze or cast iron is generally used to make the piston.
- Plunger Vacuum Pump: The functioning of this pump is also similar to the above-mentioned pump. The plunger or pump piston is one solid and long cylinder, usually constructed of hard ceramic. The plunger’s long profile helps the seal to be stationary as opposed to the piston pumps where the seal needs to be invariably attached. This allows these pumps to work with more complex mechanisms. It also makes it suitable for highly challenging conditions.
- Diaphragm Vacuum Pump: These pumps feature an elastomer or a metallic membrane joined to the chamber making a sealed seal permanently. When power and reliability are concerned, the piston pumps take the lead. The diaphragm vacuum pumps are more suited to dissipate corrosive and hazardous substances.
3. Rotary Vacuum Pump
True to their name, rotary vacuum pumps generate low-pressure areas by the cycle of moving parts against the pumping system. The surface between the housing and rotor have undersized clearances overlaid with low friction or self-lubricating materials like PEEK or polyether ether ketone, PTFE(polytetrafluoroethylene), or graphite.
The miniature clearances prevent any leakage of fluids to the regions of low pressure. In contrast to the reciprocating types, these pumps enable low pulsating delivery and continuous flow. But, these pumps are not ideal for the abrasive media-contaminated fluids as the clearances can erode between the housing and rotor. These pumps are categorised based on their designs.
- Rotary Vane Vacuum Pump: These are among the most familiar kinds of positive displacement pumps. They feature vanes radially inserted into the circular rotor. This is installed eccentrically to the housing of the stator. The pump stroke is determined by eccentricity. The separate chambers reduce gradually as they near the discharge. These vanes move radially to push against the housing and create a centrifugal force for the rotor to rotate. A spring secures the vanes in one place while the rotor does not move or energise the vanes when it is.
- Liquid Ring Vacuum Pump: The operation of this is similar to the rotary vane but they have a distinct vane construction. In this type, the vanes are attached to the rotor in contrast to the rotary vanes that are allowed to move freely. Owing to the eccentricity of the rotor, the development of the cavities leads the fluid to the chamber when the rotor is in motion. The liquid forms a ring due to the centrifugal forces which derive the name of the pump.
- Rotary Piston Vacuum Pump: This pump features a strange wheel connected to the side valve. These rotational piston valves are considered double-acting, two-stroke pumps with 2 individual compression chambers. During the intake stroke, the wheel rotates in the first chamber, allowing the liquid to be admitted. The other chamber opposite the first has an exhaust valve to eject the compressed liquid. As the rotational vane pump, the chambers in this pump also decrease gradually in size as they near the stroke.
- Screw Vacuum Pump: This is among the earliest positive displacement pumps and was earlier popular by the name of Archimedes’ screw. In a simple structure, this pump houses one screw in the cylinder. Modern variations feature two to three screws meshing against one another. Once the liquid penetrates the pump and gets trapped between the housing and the screws’ thread. The cycle of the screws develops pressure and the liquid is ejected from the other end. It is appropriate for handling two different kinds of liquids and has high tolerance against abrasive pollutants.
- Gear Vacuum Pump: They have 2 meshing gears-like rotors where one is chosen to drive the other. The pumps can either be internal or external. The external pumps create a developed cavity by letting the teeth out of the mesh while it is performing rotary action to the inlet. The liquid is then let into the cavity when the vacuum is generated for you. When the gears spin, the liquid is trapped in the space between the housing and the teeth. The liquid is let go on the opposite flank of a chamber. Alternatively, the internal gear pumps feature rotors made of internal gear and drive external gear. Pumping is achieved through the same process as the external gear pumps where the liquid is removed from the elongated cavity as the teeth of the gear are let out of the pump.
- Lobe Vacuum Pump: They are often called ‘Roots’ after the invention by the Roots brothers and are used for gas and air. The liquid is moved the same as the gear pumps, but they have lobe pumps with rotors featuring 2 or more lobes. These rotors are powered by gears that do not make any contact with the rotors, enabling high rotating speed with less damage to the rotors. The lobe pumps enable constant fluid seal contact through all the lobe’s surface.
- Scroll Vacuum Pump: These vacuum pumps are made of double co-wound scrolls or spirals with an acting as stator and the other as rotor. In this, the second one is not rotary but moves eccentrically concerning the other. The scroll pump functions by drawing liquid from the scrolls’ periphery. The fluid trapped between the two scrolls is then transported to the centre where the magnitude is decreased progressively.
4. Momentum Transfer Pump
These vacuum pumps function by generating the direction of fluid or gas molecules via transfer of kinetic energy. On the contrary of the continuous or vicious flow, molecular flow takes place in the momentum transfer pumps. The molecules’ uniform velocity distribution is changed continuously to the desired path by the fast-moving textures hitting them.
The surfaces are not limited to the impeller walls but also extend to several fluids. These pumps are ideal for curating a high vacuum. The only problem is that the exhaust does not directly release the molecules into the atmosphere. So, to solve this issue, an extra backend pump is set up in the system.
5. Turbomolecular Pump
The turbomolecular pumps have several stages of stationary and rotating turbine blades. These rotatory blades are designed to transmit enough momentum to the air or gas molecules moving towards the following stages axially till reaching the exhaust.
These stator features angled blades and makes sure of the right direction of gas. Rotors need to rotate at great speeds as the gas mass is relatively small. The rotor deflection and heat build-up due to friction restrict the turbomolecular pump designs.
6. Diffusion Pump
They function by making use of motive liquid used to transfer the momentum to all the molecules of gas. Steam or oil is usually used as the motive liquid. The usual design of the oil pump among these features one heater to warm the oil and then released to the nozzles at the top of the vapourising chambers or boilers.
The vapourised oil exits the nozzles at great speeds which captures the gasses flowing from the chamber with low-pressure . Then, the featured cooling coils condense the vapourised oil and return it at the boiler. Then, the gas molecules that are collected then flow to the exhaust. Hydrocarbon gas or seam ejectors also work the same. It has sufficient speed which enables the motive fluid or steam to vapourize and eliminates the need for a boiler.
7. Entrapment Pumps
These vacuum pumps involve several chemicals and physical phenomena to seize the gas or air molecules. The different types are segregated based on their working principle. The USP of these vacuum pumps is to work at increased vacuum controls without any contamination of the oil. They do not depend on any moving components like the rotors.
The drawback is that they cannot function continuously as the system needs to regenerate once the material or surface capturing the gases are full. Also, they cannot work on lighter gases like neon, helium and hydrogen. We have elaborated on the most familiar entrapment pumps below.
8. Cryogenic Pumps
These pumps function by cooling gas molecules down to the freezing or condensation point. At a elevated vacuum regime, they capture oxygen, nitrogen and similar gases at a temperature of 20K. It is further cooled to 8K to 10K for lighter gases like hydrogen and helium.
The design of the cryogenic pump features a 2-stage cooler. At around 70K to 80K, it removes the water and oil in the foremost stage. In the next stage, it cools down to 10K to 20K to remove the gases. In the second set, an activated charcoal-like adsorbent is used for cooling gases.
9. Sorption Pumps
This vacuum pump incorporates adsorbents like zeolite, activated charcoal or other molecular sieves to capture the gas molecules. This is generally coupled with cryogenic pumps to lower the gas vapour pressure or condense the gasses.
10. Sputter Ion Pumps
Also referred to as the ion pump or the ion getter pump, it operates by ionising the incoming gases by the anode. Then, the ionized gas attaches to a getter or a cathode, generally constructed of titanium. The binding might be through physical or chemical means relying on the kind of gas molecules present. Some of the electrons or atoms at the cathode are also ejected as the ionised gas impacts the cathode. This process is called as sputtering.
11. Titanium Sublimation Pumps
For this vacuum pump, an electric charge is introduced periodically through the titanium filament. This enables the titanium to heat and vapourises inside the chamber. The gases present or flowing through a chamber are enclosed by bonding with a vapourised titanium while in the middle or by developing one film on the wall of the chamber. After the consumption of the titanium filament, the remaining part is vapourised again to create one more layer.
12. Centrifugal Pumps
These pumps use momentum and velocity to move the liquids using impellers and fans to generate liquid momentum. The operational principle is generated through the forced vortex concept, which implies that when an external force revolves the mass, the pressure is increased.
The growing and mounting pressure causes the liquid to be transmitted. They are employed for large-scale liquids with greater flow rates in which the flow rates can be adjusted easily. The main component of the centrifugal pump acts as the impeller, which accelerates the fluid movement and gets attached to the shaft, which transmits the torque through the sleeve of the shaft.
Final Word
Vacuum pumps are essential components of vacuum furnaces. The quality and the conduct are attained through the operation of these systems and how they work. We have tried to clear all your doubts and questions about the vacuum pumps giving a clear idea about each kind. If you still have any queries, do mention them in the comments section below.
