Machinery's Handbook, 31st Edition
2824 VACUUM PUMPS AND GENERATORS Coarse vacuum pumps used for rapid evacuation are commonly rotary vane-type posi tive displacement pumps. These pumps are usually flooded with oil, which lubricates, cools, and seals the pump. The oil is discharged with the gas. To achieve deep vacuum, vane pumps in series are sometimes used, as are screw pumps. Where the presence of oil is objectionable, multi-stage oil-free pumps are employed, but are not as efficient as oil sealed pumps. Turbomolecular pumps in general use rotors to accelerate gas molecules, similar to cen trifugal pumps. Drag pumps are an example of this type of vacuum pump. Diffuser, or jet pumps are the most commonly used type of vacuum pump for high vac uum applications. In these pumps, a high velocity jet of fluid is used to transfer momentum to gas in a channel. This type of pump can sustain a vacuum level up to 1 torr, but must be used in combination with an evacuation pump. Multi-stage diffuser pumps are available. Cryogenic pumps cool gases until they condense into a liquid or solid. In practice, a cooled pump is used, rather than cooling an entire reservoir or chamber. Helium is often used as the refrigerant, and cooling to temperatures of 10 Kelvin are possible. Gettering pumps use specially treated surfaces, called “gettering” surfaces. In vacuum, these surfaces are capable of holding greater amounts of gas than would occupy a section of free space of the same cross sectional area. Ion pumps use electron bombardment to ionize inert gases that would not normally be affected by gettering surfaces. Selection Factors: The type of pump or generator required by an application depends on whether the system must provide rapid evacuation, long term maintenance of vacuum, high suction flow rate, and/or deep vacuum. Diffuser pumps cannot be exposed to gas at atmospheric pressure once they are at operating temperature. To prevent the risk of dam age, a system of valves must be engineered to prevent this condition. Filters should always be used with vacuum generators to protect them from contamination. Sizing Vacuum Pumps and Generators: Vacuum pump performance is rated in terms of throughput, pumping speed, and vacuum pressure capability. They are rated for either intermittent or continuous duty. Throughput is the product of volumetric flow rate and pressure. Pumping speed has units of volume/time and is the mean volume flow through the cross section of the inlet port of a vacuum pump. Determination of the pumping speed is described in ISO 21360-1. Vacuum pump performance curves are plots of pumping speed as a function of inlet pressure. The lowest pressure that can be achieved by a vacuum pump can be calculated using the following equation: p min p 0 e S eff t – V ------------- Q o = where p min is the deepest vacuum achievable, p o is the initial pressure, t is time, S eff is the effective pumping speed, and V is the evacuation volume. Q o , Q D , and Q K are outgassing, diffusion, and permeation flow rates. The time required to reach a certain pressure depends on the pumping speed, the volume being evacuated, and the initial pressure. It can be calculated using the following equation. Effective pumping speed changes as the pressure decreases, so the following equations are best applied piecemeal using pumping speed values at increments as the pressure varies. t V S eff ---- ln S eff ---- Q K S eff ---- + + + Q D S eff ---- p 0 p 1 --- = where t is pumpdown time, V is volume, S eff is effective pumping speed, p 0 is initial pres sure (absolute), and p 1 is final pressure. Minimum achievable pressure is affected by ambient air pressure according to the fol lowing relationship:
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