The diffusion pump is not used at atmospheric pressure because it requires a pre-vacuum environment to function effectively. This is due to its design and operational principles, which rely on the vapor of a boiling fluid to capture air molecules and cannot operate efficiently at higher pressures.
Detailed Explanation:
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Operational Requirements of Diffusion Pumps: Diffusion pumps operate by using the vapor of a boiling fluid to entrain air molecules. This fluid vapor, typically oil, moves upwards and captures air molecules, which are then directed towards the bottom of the pump. The captured molecules are released when the fluid vapor cools, and the process repeats. This mechanism is highly effective in creating a high vacuum but requires a pre-existing lower pressure environment to start the process.
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Need for a Backing Pump: Diffusion pumps do not have pumping speed in the rough and fine vacuum range, necessitating the use of a backing pump, such as a rotary vane pump or a Roots pump. These backing pumps create the initial vacuum environment by reducing the pressure from atmospheric levels to a range where the diffusion pump can start operating. Without this initial vacuum, the diffusion pump's fluid vapor would not effectively entrain air molecules, rendering the pump ineffective.
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Pressure Levels and Efficiency: The efficiency of a diffusion pump is optimized at very low pressures, typically in the range of 8.0 x 10^-6 Torr. At atmospheric pressure, the density of air molecules is too high for the diffusion pump to effectively capture and remove them. This is why diffusion pumps are typically part of a multi-stage vacuum system, where the initial stages reduce the pressure to a level suitable for the diffusion pump to operate.
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Comparison with Other Pumps: In contrast to diffusion pumps, other types of pumps such as turbomolecular pumps (TMPs) can achieve the desired vacuum levels more quickly and without the need for cooling water and oil. This makes them more suitable for some applications, especially where rapid evacuation and cleanliness are critical.
In summary, the diffusion pump is not used at atmospheric pressure because it requires a pre-existing lower pressure environment to operate effectively. Its design and operational principles are optimized for high vacuum conditions, necessitating the use of a backing pump to achieve the necessary initial vacuum.
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