Mechanical pumps single-stage

Fig. 6.3. Comparison of the pumping speed of a typical two-stage mechanical puinp with a single-stage diffusion pump.

Aside from the two types of rotary pumps mentioned, you will often see the terms single-stage and double-stage mentioned in the context of pumps. A two-stage pump (also called a compound pump) simply refers to a mechanical pump that has one or two pumps connected in line together. The exhaust of the first one... 

The basic principle of a diffusion pump can be explained with a simple single-stage mercury diffusion pump (see Fig. 7.21). On the system side of the pump (at about 10 2 to 10 3 torr, or better), gas molecules wander around, limited by their mean free path and collisions with other molecules. The lowest section of this diffusion pump is an electric heater that brings the diffusion pump liquid up to its vapor pressure temperature. The vapors of the diffusion pump liquid are vented up a central chimney where, at the top, they are expelled out of vapor jets at supersonic speeds (up to 1000 ft/sec). Below these jets is a constant rain of the pumping fluid (mercury or low vapor-pressure oil) on the gases within the vacuum system. Using momentum transfer/ gas molecules are physically knocked to the bottom of the pump, where they are trapped by the vapor jets from above. Finally, they are collected in a sufficient quantity to be drawn out by the auxiliary (mechanical) pump. 

Thus, using the appropriate system factor for each pump, the actual pump-down time for the single-stage mechanical pump is ta = 130(1.5) = 195 min. For the compound mechanical pump, ta = 114(1.25) = 142.5 min. For the combination mechanical booster pump, ta = 109(1.35) = 147 min. 

Figure 1. Vacuum line and transfer system for the determination of single stage fractionation factors and composition of the phases (1) storage hulb for purified NO (2) manometer (3) silica gel tube for further purification of NO (4) gas burette containing mercury covered with a layer of a-bromonaphthalene (5) manometer (6) equilibrium vessel (7) connection for mechanical pump (8) connection for sample tube (9) stainless steel Helicoid gage (10) connection to mercury diffusion pump...

Hgure 12.9 is a flowsheet for a system with two stages. Mechanical pumps can achieve die ratios required in chlor-alkali plants in a single stage. 

In many cases, multistage vacuum pumps consume less energy than single-stage pumps while doing the same job. Power requirements may be reduced as much as 50%. Rotary blowers are often called mechanical booster pumps because they are commonly used as a first stage in a multistage system. 

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