December 26, 2019
3 MIN READ
50 years ago, if you needed to achieve pressure lower than 1 x 10-3 mbar, the diffusion pump was always the first choice. It was the only widely available and reasonably priced option for high vacuum pumping. When the turbomolecular pump became commercially viable in the 1970s, they began to replace diffusion pumps as cleaner and more efficient high vacuum pumps, although only slowly, because of the hefty price premium over diffusion pumps. The advance of the semiconductor industry changed that in the 1990s, creating demand for thousands of turbo pumps which were chosen due to the very strict cleanliness requirements of chip making processes. With such high demand and the simultaneous advance in manufacturing, turbo pumps became more and more cost effective. Now there are very few applications where diffusion pumps are the best choice in modern factories.
Diffusion pumps can have an inlet diameter as large as one meter. When you operate in pressures with molecular flow, inlet diameter is almost the only criteria that affects total pumping speed. Thus, the much larger inlet flanges available in diffusion pumps make them the pump of choice for very high pumping speed requirements. Generally, turbo pumps are limited to an inlet flange size of 250mm (10 inches). Because area increases with the square of the radius, this means that a 1-meter diameter has a flow area that is 16 times larger than with 250mm diameter. Let’s compare that. A 250mm diameter turbo pump can have a pumping speed of up to 2800 l/s. A 1000mm inlet diffusion pump has a pumping speed of 50000 l/s. 2800 l/s x 16 = 44800 l/s. Pretty close. This means you would need 16 to 20 turbo pumps to provide the same speed as a single 1000mm inlet diffusion pump, and that is cost prohibitive
In applications like investment casting, ceramic coating, and any process with sticky condensates, the vapors that reach the high vacuum pump can cause severe durability issues with a turbomolecular pump. In these cases, diffusion pumps are often the only high vacuum pump that will work reliably.
There are two major variants of diffusion pumps: the standard diffusion pump and the oil vapor booster pump. Both diffusion pumps operate in a similar fashion. Discover how they work in more detail in our blog post, Basic Diffusion Pump Operating Principles.
The essential criteria for choosing an oil vapor booster pump over a standard diffusion pump is when you want to operate with chamber pressures between 1x10-3 mbar (1 micron) and 2x10-2mbar (20 microns). In this pressure range, the oil vapor booster pump outperforms every other vacuum pump on earth by a wide margin.
Related: You may also want to check out these blog posts to help make the right choice: Factors to Consider When Choosing Vacuum Pumps and How Do I Know Which Vacuum Pump Is Right For Me?.
Still unsure? Every business, project and process is unique. If you're still unsure whether a diffusion pump is the best choice for your requirements, or if you've got a bespoke pump in mind, click the button below and start a conversation with the Leybold team. We'd be happy to help!
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