Diffusion vacuum pumps stand as the cornerstone of high-vacuum technology, with oil diffusion pumps being the most prevalent. These remarkable devices serve as secondary pumps, relying on mechanical pumps as their precursors. Additionally, at the heart of a diffusion pump’s functionality lies the utilization of high-speed vapor to expel gas molecules. This process disintegrates them at the base of the exhaust.
The Birth of Diffusion Vacuum Pumps
The concept of diffusion pumps was pioneered by Wolfgang Gaede in 1915, initially employing mercury vapor. In the realm of high vacuum, where gas molecules move independently, diffusion pumps offer a superior grasp of gas dynamics compared to traditional fluid-based systems. Gaede aptly named these pumps “diffusion pumps” based on his observation that gas does not diffuse as much as it is entrained by the exhaust stream.
However, a more precise description of their operation might classify them as jet pumps, considering the crucial role diffusion plays in their function, setting them apart from other high-vacuum pumps. Contemporary textbooks categorize diffusion vacuum pumps as momentum transfer pumps, and their applications span both industrial and research domains. Most modern diffusion pumps employ silicon oil as the working medium, an innovation credited to Cecile Leberich in 1928.
The Operating Principle of Diffusion Pumps
Within the pump chamber, a liquid working medium (contained in the oil pot) is subjected to heating by an electric heater (furnace), causing it to transition into vapor form. Subsequently, this vapor is forcefully ejected through a nozzle installed within the pump core, situated inside the chamber. This process creates a high-velocity, low-density vapor jet. Gas to be evacuated enters this vapor jet through diffusion, where it is then carried away by subsequent vapor jets and eventually expelled by the primary pump.
The working vapor reaches the water-cooled pump wall, where it condenses back into a liquid, trickling back into the oil pot. The cycle repeats, forming the standard evacuation process of the diffusion pump. Within the extensive family of vacuum pumps, diffusion pumps find their place as indispensable tools for achieving high vacuum.
Advantages and Limitations
Regarding advantages: The simplicity of their design, absence of moving parts, minimal noise, ease of use, straightforward maintenance, and lower manufacturing costs per unit of pumping speed make diffusion pumps versatile and widely applicable.
However, limitations persist: Challenges such as oil return, high energy consumption, and suboptimal thermal efficiency persist. Addressing these issues has been an ongoing quest for improvement within the diffusion pump domain.
Categories of Diffusion Pumps
- Oil Diffusion Pumps: Oil diffusion pumps operate by utilizing a low vapor pressure oil. Their primary objective is to achieve vacuum levels (low pressures) beyond what positive displacement pumps alone can accomplish. While their primary application lies in the high vacuum range (reaching down to 10^-9 millibars), modern diffusion pumps can achieve pressures as low as 10^-10 millibars when combined with contemporary fluids and accessories.
- Steam Ejector Pumps: Steam ejector pumps are a type of dry pump widely employed in diffusion vacuum distillation and refrigeration. These pumps are designed with the premise that vapor must be removed from the steam vacuum chamber. Steam ejector pumps can feature single or multiple stages without condensers between stages.
- Compressed Air Ejector Pumps: Among diffusion vacuum pumps, multi-stage compressed air-driven ejector pumps enjoy popularity. They are extensively used in various applications, including suction lines and vacuum gripping systems worldwide.