Steam Jet Vacuum Pumps

The basic principle of the jet pump is that the liquid or gas jet exits the motive nozzle at a high velocity and low pressure and entrains and accelerates the surrounding liquid, gas or solid medium. The result of this action is the mixing of the driving and the entrained material at a mean velocity. In a second nozzle, this velocity of the mixture is reduced and the pressure increases to an outlet pressure that is higher than the suction pressure. 

Vacuum Technology in the Chemical Industry, First Edition. Edited by Wolfgang Jorisch. 

A Motive flow B Suction flow C Mixture flow 

The conversion of this principle to a practical purpose requires a simple apparatus, which normally consists of only three main parts the motive nozzle (1), the diffuser (2) and the head (3). There are three external connections to be designated the motive medium connection (A), the suction nozzle (B) and the pressure nozzle (C). 

Steam jet pumps are frequently used In the chemical industry for vacuum generation. The main reasons for this are their high operational reliability, their resistance to corrosion, their tolerance of fouling as well as their suitability for very large volume flows. However, when jet pumps are operated with steam as the motive medium, there will inevitably be waste water generated because the motive steam must be condensed. 

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Ketterer, S. G. and C. G. Blatchley, Steam Jet Vacuum Pumps, Schutte and Koerting Co. 

Figure 4.2 First stage of a steam Jet vacuum pump for a steel degassing plant.

It can be seen that the end point of the expansion (point 2 ) is in the area of wet steam. Thus small droplets of condensate will form in the motive jet. In the motive nozzle, combined with the decrease of pressure, the temperature will also decrease. In the wet steam region the steam is cooled to the boiling point temperature corresponding to the pressure. When the motive steam expands to a pressure lower than 6mbar, the corresponding temperature is below 0°C, thus ice will form. Steam jet vacuum pumps for such applications are often heated at the mixing nozzle and sometimes also at the motive nozzle, to prevent the ice crystals from adhering to the internal wall. This would cause a constriction of the cross-sectional area and adversely effect the flow. 

Figure 4.13 shows the setup of the described four-stage steam jet vacuum pump with two inter-condensers and one after-condenser for a first stage suction pressure of 1 mbar. 

Figure 4.13 Four-stage steam jet vacuum pump with mixing condensers.

Surface condensers are used instead of mixing condensers, if the suction flow contains components that are not permitted to be mixed into the cooling water. A schematic representation of a four-stage steam jet vacuum pump with surface... 

Figure 4.15 shows an example for a multi-stage steam jet vacuum pump with a material combination of porcelain and graphite. These materials are suitable if aggressive components are in the suction flow. 

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