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When Is a Diffusion Pump the Right Choice?

By Gene Ligman
Making choices

50 years ago, if you needed to achieve a 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, more efficient high vacuum pumps, though 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.

There are essentially two reasons to choose a diffusion pump over other high vacuum pumps:

  • Huge pumping speed. Diffusion pumps can have an inlet diameter as large as one meter. When you operate in pressures where you have 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 check 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.
  • Dirty pumping environment. 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 of these diffusion pumps operate in 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? Chat to the experts.

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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Tags: High Vacuum

About Gene Ligman

Gene Ligman

Gene Ligman is an engineer with a passion for vacuum applications and equipment. He started his engineering career over 30 years ago in the nuclear power industry where he was first introduced to the utility of vacuum in steam systems and some basic vacuum generating equipment. In the mid 1990’s, he joined Edwards Vacuum where his knowledge of vacuum applications and equipment expanded exponentially.

Having a degree in mechanical engineering with a focus on vapor physics has propelled him to become one of the primary resources in applications where phase change creates complexities above and beyond the normal complexities of vacuum applications. Now with Leybold USA, Gene is a Sales Development Manager for Leybold’s largest and most industrial vacuum generating equipment. He trains the US organization in key insights about how the right vacuum equipment can radically improve the productivity and profitability of vacuum using factories. These insights, along with his passion for vapor physics makes him a leading authority in vacuum system design for cannabis processing facilities.

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