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What is a Diffusion Pump?

Paul Scott
Paul Scott

A diffusion pump is a device designed to pump gases with a high speed vapor jet as a transport medium. This process takes place as a result of the fact that gases resist diffusion in the vapor stream and are more inclined to move along with it. Diffusion pumps are characterized by very low operating pressures or, more accurately, high vacuum values which can reach 10-10 mbar. The vapor used to transport the gas is typically the product of boiling specialized oils such as silicone oil in the pump's heater section. When directed through the pump's nozzle assembly, the vapor accelerates considerably and draws the gas with it through out through the pump's discharge point.

The characteristic of gases that underpins the operation of the diffusion pump is their inclination not to diffuse or mix with a vapor stream but rather to be moved along with it. This can be clearly seen where steam exits from a window or flue. As its does so, it moves a column of air along with it, thereby causing a circulation of air in the space. Although rather modest in extent, this is a good example of the diffusion pump concept at work. The main difference between the workings of the pump and the chilly breeze in the bathroom, however, is the velocity at which the vapor moves. For a diffusion pump to reach its full potential, the vapor 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 lived and easy to maintain. The main sections on a typical example consists of a heater section where the vapor is produced, a pumped gas inlet, a series of nozzles, and an outlet. Secondary elements consist of cooling coils and oil return ducts. The vapor material is heated to the boiling point in the heater section with the resultant 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 recondenses to a liquid. The accelerated gas then exits the pump's inner section where it returns to atmospheric pressure and is discharged. The recondensed oil then flows back 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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