DIFFUSION PUMP WORKING PRINCIPLE
Diffusion pumps are probably the most commonly used mechanisms for creating a high vacuum in industrial vacuum processing. It is also commonly used in mass spectrometry, analytical instrumentation, research and development, and nanotechnology. Since there are no moving mechanical parts, diffusion pumps are extremely reliable and operates practically without noise or vibration. For the same reason, diffusion pumps are relatively low cost to purchase, operate, and maintain.
It is also highly effective in producing vacuums 10−10 to 10−2 mbar even in poor conditions where reactive gases or gases with excess particles are present. A diffusion pump is a stainless steel chamber that vary in size based on the application. Generally speaking, the interior of diffusion pumps regardless of size is the same and consist of three varying sized, cone-shaped pressure jets stacked vertically. The lowest stacked cone is the largest and decreases in size as you move upwards representing a shape of an upwards pointing arrow. The very bottom of the chamber is a heater where silicone-based diffusion pump oil is heated until it reaches gaseous state, usually between 180 – 270°C. The excited gas travels upward and exits through the pressure jets that are pointed at an downward angle. The downward shooting vapor travels at an incredible 750 miles per hour sometimes breaking the sound barrier, one mach. As the gas travels toward the walls of the pump chamber, it traps air molecules along the way through “diffusion”. As the walls of the pump chamber are usually water-cooled, as the gas reaches the chamber walls, it immediately returns to liquid state releasing the trapped air molecules at a lower position and at increased pressure creating the vacuum. The oil drips back to the bottom of the pump chamber where it is heated again.
To recap, the top of the chamber is where the vacuum begins and where the air molecules are pulled into the diffusion pump and moves downwards at increased pressures. At the bottom of the pump is a nozzle where the vacuumed air exits the pump. As the diffusion pump itself cannot maintain an outlet pressure, an additional forepump is required to maintain an outlet pressure of approximately 0.1 mbar. While diffusion pumps are probably the most cost-effective method of creating a high-vacuum, there are limitations that make it not suitable for some applications. For instance, back-streaming can occur where the gaseous pump oil backs into and contaminates the vacuum environment. Therefore, the use of a diffusion pump may not be suitable for applications where a clean vacuum environment is required such as where highly sensitive analytic equipment is used. As alternative high vacuum pumps are much more expensive, diffusion pumps are sometimes still used with added cold traps and baffles to prevent back-streaming at the cost of slightly lower pumping abilities.
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A simple diagram showing how the diffusion pump actually works.