A cryopump or a "cryogenic pump" is a vacuum pump that traps gases and vapours by condensing them on a cold surface, but are only effective on some gases. The effectiveness depends on the freezing and boiling points of the gas relative to the cryopump's temperature. They are sometimes used to block particular contaminants, for example in front of a diffusion pump to trap backstreaming oil, or in front of a McLeod gauge to keep out water. In this function, they are called a cryotrap, waterpump or cold trap, even though the physical mechanism is the same as for a cryopump.
Cryotrapping can also refer to a somewhat different effect, where molecules will increase their residence time on a cold surface without actually freezing (supercooling). There is a delay between the molecule impinging on the surface and rebounding from it. Kinetic energy will have been lost as the molecules slow down. For example, hydrogen does not condense at 8 kelvins, but it can be cryotrapped. This effectively traps molecules for an extended period and thereby removes them from the vacuum environment just like cryopumping.
Early experiments into cryotrapping of gasses in activated charcoal were conducted as far back as .[1]
The first cryopumps mainly used liquid helium to cool the pump, either in a large liquid helium reservoir, or by continuous flow into the cryopump. However, over time most cryopumps were redesigned to use gaseous helium,[2] enabled by the invention of better cryocoolers. The key refrigeration technology was discovered in the s by two employees of the Massachusetts-based company Arthur D. Little Inc., William E. Gifford and Howard O. McMahon. This technology came to be known as the Gifford-McMahon cryocooler.[3][4][5][6] In the s, the Gifford-McMahon cryocooler was used to make a vacuum pump by Helix Technology Corporation and its subsidiary company Cryogenic Technology Inc. In , cryopumps began to be used in IBM's manufacturing of integrated circuits.[7] The use of cryopumps became common in semiconductor manufacturing worldwide, with expansions such as a cryogenics company founded jointly by Helix and ULVAC (jp:アルバック) in .
Cryopumps are commonly cooled by compressed helium, though they may also use dry ice, liquid nitrogen, or stand-alone versions may include a built-in cryocooler. Baffles are often attached to the cold head to expand the surface area available for condensation, but these also increase the radiative heat uptake of the cryopump. Over time, the surface eventually saturates with condensate and thus the pumping speed gradually drops to zero. It will hold the trapped gases as long as it remains cold, but it will not condense fresh gases from leaks or backstreaming until it is regenerated. Saturation happens very quickly in low vacuums, so cryopumps are usually only used in high or ultrahigh vacuum systems.
The cryopump provides fast, clean pumping of all gases in the 10−3 to 10−9 Torr range. The cryopump operates on the principle that gases can be condensed and held at extremely low vapor pressures, achieving high speeds and throughputs. The cold head consists of a two-stage cold head cylinder (part of the vacuum vessel) and a drive unit displacer assembly. These together produce closed-cycle refrigeration at temperatures that range from 60 to 80K for the first-stage cold station to 10 to 20K for the second-stage cold station, typically.
Some cryopumps have multiple stages at various low temperatures, with the outer stages shielding the coldest inner stages. The outer stages condense high boiling point gases such as water and oil, thus saving the surface area and refrigeration capacity of the inner stages for lower boiling point gases such as nitrogen.
As cooling temperatures decrease when using dry ice, liquid nitrogen, then compressed helium, lower molecular-weight gases can be trapped. Trapping nitrogen, helium, and hydrogen requires extremely low temperatures (~10K) and large surface area as described below. Even at this temperature, the lighter gases helium and hydrogen have very low trapping efficiency and are the predominant molecules in ultra-high vacuum systems.
Cryopumps are often combined with sorption pumps by coating the cold head with highly adsorbing materials such as activated charcoal or a zeolite. As the sorbent saturates, the effectiveness of a sorption pump decreases, but can be recharged by heating the zeolite material (preferably under conditions of low pressure) to outgas it. The breakdown temperature of the zeolite material's porous structure may limit the maximum temperature that it may be heated to for regeneration.
Sorption pumps are a type of cryopump that is often used as roughing pumps to reduce pressures from the range of atmospheric to on the order of 0.1 Pa (10−3 Torr), while lower pressures are achieved using a finishing pump (see vacuum).
Regeneration of a cryopump is the process of evaporating the trapped gases. During a regeneration cycle, the cryopump is warmed to room temperature or higher, allowing trapped gases to change from a solid state to a gaseous state and thereby be released from the cryopump through a pressure relief valve into the atmosphere.
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Most production equipment utilizing a cryopump have a means to isolate the cryopump from the vacuum chamber so regeneration takes place without exposing the vacuum system to released gasses such as water vapor. Water vapor is the hardest natural element to remove from vacuum chamber walls upon exposure to the atmosphere due to monolayer formation and hydrogen bonding. Adding heat to the dry nitrogen purge-gas will speed the warm-up and reduce the regeneration time.
When regeneration is complete, the cryopump will be roughed to 50μm (50 milliTorr or μmHg), isolated, and the rate-of-rise (ROR) will be monitored to test for complete regeneration. If the ROR exceeds 10μm/min the cryopump will require additional purge time.
Cryogenic Pumps are specialist machines that handle and transfer cryogenic fluids, which are substances that have been chilled to extremely low temperatures, often less than -150°C (-238°F). These pumps are essential in a variety of industries, including healthcare, aerospace, and energy, where precise handling and transfer of liquefied gases are required. Understanding cryogenic pump components and characteristics offers insight into their operation and applications.
Impeller or Rotor: The impeller or rotor, a revolving component responsible for providing the necessary force to transfer cryogenic fluids, is at the heart of a cryogenic pump. The impeller’s design and material are critical for withstanding severe temperatures while maintaining efficient fluid flow.
Casing: The casing, a protective housing that encloses the pump’s internal components, surrounds the impeller. The casing is built to endure low temperatures and high pressures, ensuring the cryogenic pump’s safe and efficient functioning.
Seals and Bearings: Cryogenic pumps use specially engineered seals and bearings for low-temperature operations. These components reduce friction, minimize fluid leakage, and protect the integrity of the pump’s internal components, extending the pump’s lifespan and performance.
Drive Mechanism: The driving mechanism, which is either an electric motor or a mechanical drive system, powers the spinning of the impeller, allowing fluid to flow through the pump. The effectiveness and dependability of the driving mechanism are critical for ensuring consistent fluid flow rates and operating performance.
Low-Temperature Design: Cryogenic Pumps are particularly engineered to perform efficiently at extremely low temperatures, guaranteeing the safe handling and transport of liquefied gases including nitrogen, oxygen, and helium. At cryogenic temperatures, the materials employed in their manufacture, such as stainless steel alloys and specialist coatings, resist brittleness and preserve structural integrity.
High Flow Rates: Cryogenic pumps are designed to offer high flow rates, allowing for the quick transfer and distribution of cryogenic fluids in a variety of industrial applications. Their innovative design and precise engineering ensure that cryogenic fluid handling procedures function optimally in terms of efficiency, productivity, and performance.
Safety Features: Recognizing the inherent hazards of working with cryogenic fluids, cryogenic pumps have a variety of safety measures such as pressure relief valves, leak detection systems, and emergency shutdown mechanisms. These safety measures reduce risks, prevent possible dangers, and guarantee the pump and associated systems operate safely.
Customization and Adaptability: Cryogenic pumps are available in a variety of layouts, sizes, and specifications to satisfy specific application needs. Manufacturers provide bespoke solutions that include specialist components, materials, and features that are adapted to specific industrial demands, operational conditions, and performance objectives.
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