The working principle of a diffusion pump depends on the interdiffusion phenomenon between two different gasses. Gaede had developed this type of pump in the year 1815. After that Langmuir had developed the practical and commercial version of this diffusion pump.
Whatever may be the type of a diffusion pump but the basic principle is the interdiffusion between two gasses. During the diffusion process, the gas of high concentration diffuses to the gas of low concentration. That means a gas always tries to flow from the space of higher partial pressure to the space of lower partial pressure. But it is obvious that the interdiffusion does not depend upon the total pressure of the system.
Basic Construction of a Diffusion Pump
The figure below shows a simple diffusion pump. Warren had developed this model of the pump. Sometimes we also call a diffusion pump as a diffusion condensation pump. From the name diffusion condensation pump it is obvious that there must be an arrangement of liquid in the pumping system.
For better understanding the working principle of a diffusion pump, let us mark the different sections of the pump with different numerical numbers. 1 is a glass or metal conical pot. There is one bend tube marked with 2, fitted on the top of the cone. After bending, the cross-section of this tube suddenly increases. Let us mark this bigger cross-sectional portion of the bent tube with numeric 3.
The cross-section of the lower portion of the tube has become again reduced and ultimately connected to the conical pot.
In general, asbestos fibers cover the portion of the tube marked with numeric 2. The asbestos fibers act as the heat insulator of this tube. There is one inlet at the upper portion of section 3. Also, there is one outlet from the lower portion of the same section 3. The circulating water surrounds the tube of section 3 serves as the water-cooling system.
Working Principle of Diffusion Pump
Now we connect the inlet to the vessel in which we have to create a vacuum by this diffusion pump. Although, we generally do not connect the inlet directly to the vessel. Instead, we use a liquid air trap in between the inlet of the pump and the vessel. We will discuss later the purpose of this liquid air trap. Now, you just remember that there should be a liquid air trap in the inlet pipeline.
Now we heat up the mercury inside the conical jar with the help of an external heater. As a result, heat vaporizes the mercury. This mercury vapor then goes up through the tube, marked as 2. Then it ultimately reaches the wider cross-sectional portion of the tube of the section marked as 3. Here, the vapor gets suddenly a wider space to expand. The expanded vapor traps the air molecules coming through the inlet. The trapping of air molecules is due to the diffusion process. The sudden expansion of the mercury vapor reduces the pressure of the system.
After that, due to the water cooling system, the temperature of the mercury vapor reduces in section 3. The mercury vapor comes down to the lower portion of the tube of section 3 and also during its journey it gets cooled down because of the water cooling. At the lower portion of the tube of section 3, the mercury vapor gets condensed to liquid mercury.
Therefore, it is collected in the form of liquid mercury to the conical jar through the pipe of section 4. Since the mercury vapor becomes liquid in the lower portion of section 3 the air molecules which have been trapped due to diffusion with the mercury vapor gets separated and come out through the outlet of the pump. We generally use an auxiliary pump in addition to that system to collect the exhausted air from the diffusion pump.
Purpose of Liquid Air Trap in the Diffusion Pump
During the operation of the pump, there may be a chance of entering mercury vapor to the vessel in which we are making the vacuum through this diffusion pump. The liquid air trap connected in this inlet traps the mercury vapor molecules. So, the chance of entering mercury vapor to the vessel gets minimized. Although in our figure we have not shown the liquid air trap. But we should remember that there should be a liquid air trap in between the inlet point and the vessel.
Purpose of Liquid Air Trap in the Auxiliary Pump
Generally, an auxiliary pump connected to the outlet sucks the exhausting air. That we have already told during our discussion.
Advantages of using Mercury as the Fluid of the Pump
There is a number of advantages of using mercury as the diffusion pump fluid.
- The vapor pressure of the mercury is quite low. At the same time, the atomic weight is quite high. It reduces the chance of reverse diffusion in the system.
- The vaporization and liquefaction are quite easy for mercury.
- In normal temperature, mercury does not react with air.
- Due to higher atomic weight, the momentum of the mercury molecule is quite high, so easily they can trap and carry the air molecule downwards in the diffusion pump.
- Mercury is an easily available metal.
Oil Diffusion Pump
There is another form of diffusion pump where we use oil as a liquid instead of mercury. The working principle is the same. So, we have not discussed the oil diffusion pump separately in this article.
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