Diffusion pumps use high-velocity steam to transport gases, creating high vacuum values. They have no moving parts, are easy to maintain, and use silicone oil or mercury as vapor materials.
A diffusion pump is a device designed to pump gases with a high-velocity jet of steam as a means of transport. This process occurs as a result of the gases resisting diffusion in the vapor stream and are more inclined to move with it. Diffusion pumps are characterized by very low operating pressures or, more precisely, by high vacuum values which can reach 10-10 mbar. The steam used to carry the gas is typically the product of boiling specialty oils such as silicone oil in the heater section of the pump. When directed through the pump nozzle assembly, the vapor accelerates dramatically and carries the gas with it through the pump discharge point.
The underlying characteristic of gases for diffusion pump operation is their propensity not to diffuse or mix with a vapor stream but rather to be moved with it. This can clearly be seen where the steam exits through a window or flue. As it does so, it moves a column of air with it, thus causing a circulation of air in the space. While quite modest in size, this is a good example of the diffusion pump concept at work. The main difference between running the pump and the freezing breeze in the bathroom, however, is the speed at which the steam moves. For a diffusion pump to reach its full potential, the steam used as a transport medium is accelerated to extremely high speeds.
Diffusion pumps are very simple devices with no moving parts; this makes them particularly long-lasting and easy to maintain. The main sections of a typical example consist of a heater section where the steam is produced, a pumped gas inlet, a set of nozzles and an outlet. Secondary elements consist of cooling coils and oil return lines. The vapor material is heated to boiling point in the heater section with the resulting vapor being directed into the nozzle section. There the vapor accelerates before passing the pumped gas inlet where it draws the gas into the outlet channel.
In the outlet channel, the vapor is cooled and condensed back into a liquid. The accelerated gas then exits the internal section of the pump where it returns to atmospheric pressure and is vented. The recondensed oil then returns to the heater section. The vapor materials in a diffusion pump are typically silicone-based oils, although mercury is often used in sensitive laboratory applications where oil contamination is undesirable.
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