Multi-stage steam jet vacuum pumps

Mode of operation

Multi-stage jet vacuum pumps using steam as motive medium are constructed for suction pressures of up to 0.01 mbar and for suction capacities of up to 2 million m³/h. There is no limit to the variety of applications for jet vacuum pumps within a defined suction capacity and suction quantity. It is even possible to combine them with mechanical vacuum pumps. 

The compression ratio of a single stage steam jet vacuum pump is limited (1:10, max. 1:20). Therefore, for low suction pressures, several jet vacuum pumps are arranged in series. A condenser is arranged between two jet pumps in order to condense the motive steam as far as possible. In this way, the volume of the complete gas mixture and thus the energy requirement of the next stage is reduced.  

To compress process gases from a pressure of 0.3 mbar to a condenser pressure of 56 mbar (i.e. a compression ratio of 56 : 0.3 % 187), two jet stages are just sufficient, each handling a compression ratio of approx. 14. For a suction pressure of 0.1 mbar, the pressure gradient is 56 : 0.1 % 560, and therefore, three jet pumps must be arranged in series, each handling a compression ratio of 8.25 per stage. The maximum compression ratio for a steam jet vacuum pump depends on the suction pressure and the pressure of the available motive steam.

The condensers used are water-cooled mixing condensers or surface condensers, in some individual cases, even air-cooled condensers are used. 

Steam jet vacuum pumps with mixing condensers are used where the extracted medium is allowed to get in contact with the cooling water and where the recovery of condensate is not required.

If, however, the contact with cooling water is not allowed, such as for example, if there is ammonia and chalky water, or if condensed product or motive steam condensate has to be recovered, surface condensers have to be used instead of mixing condensers.

For steam jet pumps for suction pressure of less than 6 mbar the head and the mixing nozzle are heated, depending on the installation position. In this way, the formation of ice inside the pump is avoided which otherwise could lead to trouble. 

If steam jet vacuum pumps are required for the extraction of steams which contain any constituents with a high melting point (e.g. caprolactam, oligomers, low polymers in poly-condensation plants, etc.), heat jacketing is recommended even for higher suction pressure levels. For processes involving fluids with very high melting points, the ejectors are heated by means of vaporous or liquid diphyl, high temperature resistant oils or any other heat carrier fluid.

Steam jet vacuum pumps are mainly operated with water steam. Water steam is easily available in industry and proved well as motive fluid for jet pumps. In special cases in which the product condensate must not be diluted by or mixed with water steam condensate, steam jet pumps are operated on product steam. 

Steam jet vacuum pumps can be manufactured of different materials, mainly:

• For jet pumps: cast iron, steel, stainless steel
• For condensers: steel, rubberized or enameled steel, stainless steel, bronze or other special alloys.

For applications in which metallic materials are not resistant, steam jet vacuum pumps in porcelain, graphite and glass are used.

The purpose of cleaning the condensate is not an important factor for this application. There are no limitation on materials of selection (pending on process application).

Mixing condensers are constructed with internal distributions and / or distribution nozzles. The distribution nozzles are in most case insensible against fouling.

The mixed condensate (cooling media and condensate) shall be drained barometrical. In case the contact between the cooling media and process vapor is not allowed, a surface condensation with tube / shell heat exchangers is recommended.

Type dpo1

There are no limitation on materials of selection (pending on process application). Surface condensers are constructed with internal tubes, where in most of the cases the cooling media is on tube side. Variations are possible (process on tube side). The condensate shall be drained barometrical.

Type pgv1