|In a sorption pump, molecules are held on the zeolite surface by physical adsorption. The number of molecules that can be held on an adsorbent is dependent on the temperature of both gas and surface, the chemical nature of gas and surface, the microscopic roughness of the surface, and the incident flux of molecules. The key is to have equilibrium conditions such that practical amounts of gas can be captured at the desired pressures. It follows that a large surface area at low temperatures will have the capability of adsorbing large volumes of gas. By providing large surface areas, practical amounts of nitrogen can be pumped. The key elements of a sorption pump include an aluminum body, an array of fins that remove heat from the zeolite, and a pressure relief mechanism. All MDC sorption pumps are fitted with an elastomer stopper that automatically releases positive pressure. When a sorption pump is saturated with air and allowed to warm up to room temperature, very high pressures can be generated. The elastomer stopper is a key safety element of MDC sorption pumps and operation of this stopper should never be obstructed or disabled.
The adsorbent used is a Type 5A synthetic zeolite molecular sieve material. Zeolite is a highly porous material with a surface to volume ratio of about 800 square meters per cubic centimeter. It is supplied in pellets of about 1.58mm in diameter with molecular sized cavities that are linked by 5 Angstrom size pores. These pores are large enough to trap nitrogen, oxygen, and argon molecules, the main constituents of air. Zeolite also has a very high affinity for water vapor.Water vapor accumulated through repeated pump cycles of a chamber filled with ambient air will eventually saturate the sieve material, reducing and eventually eliminating its capacity for adsorbing nitrogen and oxygen. To remove the accumulated water and regenerate the adsorbent material, the pump must then be baked to 250°C or higher. Under normal operating conditions, the sieve
material can be recycled indefinitely.
Sorption pumps do not include the optional bakeout heater. Heaters must be purchased separately. During pump operation, do not run the heater while it’s immersed in liquid nitrogen.
Noble gases such as neon and helium are pumped poorly by sorption pumps. If, for instance, neon is pumped together with air, its capacity will be less because the neon will be replaced by the active air gases, starting at pressures below 7.5 Torr. For this reason , sorption pumps are quite often staged. When two pumps are staged, one pump is used to achieve a pressure of 7.5 Torr and is then valved off. The second pump is then opened and the pressure is further reduced. By this method, 99% of the air is removed with the first pump, and noble gases are also swept into this pump and cannot backstream into the system when pressure is further reduced. Staged or multiple pump assemblies are fitted with both Bourdon and Thermocouple vacuum gauge tubes for monitoring vacuum levels. These manifolds are supplied with three manual UHV angle valves which allow the isolation of each pump and the manifold from the main vacuum system.
Pump operation is simple and fast. To begin pumping, add liquid nitrogen to the dewars. No electrical power is required and there are no moving parts and no vibration. A single sorption pump can evacuate a 100 liter chamber from atmosphere to 10-2 Torr in approximately 10 minutes. Each sorption pump has a capacity of 60,000 Torr-liters. Multiple pump systems are commonly used as they are faster and more efficient. Double and triple pump systems are mounted or connected to the chamber via a vacuum manifold. Single pumps, on the other hand, mount directly to a chamber. Pumpdown begins as soon as the adsorbent material in the pump is chilled with liquid nitrogen.