The pump utilized to create a vacuum inside a closed container is called a vacuum pump. Absolute vacuum cannot be created, hence the degree of vacuum comes into picture when we discuss a vacuum pump. Any vacuum pump has three main features.
Exhaust Pressure
Degree of Vacuum
Pumping Speed.
Exhaust Pressure of a Vacuum Pump
Any vacuum pump has an outlet and an inlet. The gaseous substance or air of the vessel to be vacuum enters in the pump through the inlet and gets out from the pump to the atmosphere through the outlet of the pump. The process is controlled by a valve mechanism. Exhaust pressure is the pressure measured at the outlet of the pump. This pressure can be equal to or lower than the atmospheric pressure. Different vacuum pumps are rated for different exhaust pressure. Normally, pumps for creating high vacuum have low exhaust pressure. For very creating a very high vacuum, very low exhaust pressure of the pump is required such as 10 – 4 or 10 -7 Torr. For creating a very high vacuum backing pump is used along with the main vacuum pump. Here, the vacuum pressure or exhaust pressure of the vacuum pump is first maintained by the backing pump then the main vacuum pump is run. The vacuum pressure created by a backing pump is called forced vacuum or blacking pressure.
Degree of Vacuum of a Vacuum Pump
The minimum pressure can be created by a vacuum pump is called the degree of vacuum of that pump. Theoretically, it is not impossible to make any vessel absolutely vacuum but practically it is not possible to create a vacuum below 10-13 Torr. This is because, when the difference between the pressure of the outside atmosphere and inside of the vessel is being increased, the effectiveness of the motor and other movable parts of the pump gets decreased proportionately.
Pumping Speed of a Vacuum Pump
It is defined as the rate at which a pump can suck the volume of the air or gaseous substance at the instantaneous pressure inside the vessel. The pumping speed is measured in the suction of air or gaseous volume per second.
Types of Vacuum Pump
There are many types of vacuum pump available
- Rotary Oil Vacuum Pump
- Molecular Vacuum Pump
- Diffusion Vacuum Pump
Both Oil Vacuum Pump and Mercury Vacuum Pump can be of two types, Piston type and Rotary type.
Rotary Oil Pump
Wolfgang Max Paul Gaede a German Physicist invented rotary oil pump. This pump is of two types
- Rotary Vane Oil Pump
- Stationary Vane Oil Pump
The principle of these two types of oil pump is similar. There is one heavy cylindrical shaft forms the rotor which eccentrically rotates in a hollow cylinder. This hollow cylinder is referred to as the stator of the pump. During rotation of the rotor the air or gas enters in the stator get compressed and pushed toward the outlet. There is a non – return valve fitted at the outlet which blocks the return of the exhausted air or gas. The entire system is immersed in oil. As this system is oil immersed, the pump does not require periodic external lubrication, leakage of the gas or air from the internal chamber of the pump is quite impossible and the oil itself serves as a coolant of the pump.
Rotary Vane Oil Pump
Above animated figure shows a Rotary Vane Oil Vacuum Pump. Here, vanes are rotating along with the rotor.
Stationary Vane Oil Pump
Above animated figure shows a Stationary Vane Oil Vacuum Pump. Here, the vane is not rotating rather it is moving up and down according to the positions of the eccentrically rotating rotor.
Diffusion Vacuum Pump
This pump works on the principle of interdiffusion of gaseous medium. It shucks the air or gaseous medium of a vessel by interdiffusion of other gas or vapor. The concept was first developed by Gaede in the year of 1815 and later it was further developed by Langmuir. In the diffusion process, the gaseous substances flow from higher concentration zone to lower concentration zone. That means gases flow from a high-pressure zone to a low-pressure zone. Here, the vapor of either oil or mercury is allowed to release through a nozzle to reduce its pressure. The low-pressure zone is connected to the vessel where vacuum to be created with an inlet pipe. The gaseous molecules from the vessel diffuse and get away with the flow of mercury or oil vapor. Then the vapor gets condensed by an water forced cooling system and the air or gaseous substances get removed from the pump through the outlet.
Molecular Vacuum Pump
The layer of a gaseous substance in contact with a solid moving surface gets a motion along the direction of movement of the surface because of the viscous property of the gas. On the basis of this principle of viscosity, the molecular pump had been developed. Here, air or gas enters through the inlet pipe to the very thin slot of the stator where the air or gas comes in contact with the rotor of the pump. As the rotor rotates, the rotor surface moves very fast and due to viscosity, the layer of air or gas in contact with the rotor surface gets a motion and comes to the outlet and gets removed from the pump. 
