Jet pumps

Dipl.-Ing. Andrea Heuser

Jet pumps are equipment for conveying, compressing or mixing gases, vapours, liquids and solids, using a gaseous or liquid medium as the motive force.

The basic principle of the jet pump consists in the liquid or gas jet emitted at a high velocity by the motive nozzle entraining and accelerating the surrounding liquid, gas or solids. The result of this action is a mixture of the driving and the entrained (drawn in) substance, the velocity of which has to be reduced again in a second nozzle while the pressure is increased. Unlike conventional pumps, jet pumps work without moving parts.

Jet pumps are available in a variety of materials such as cast iron, steel, stainless steel, Hastelloy, titanium, porcelain, glass, plastics and graphite. They can be built in all sizes, with nominal connec- tion diameters from a few cm to more than 30 m for suction volume flow rates between 1 and 2,000,000 m³/h as well as suction pressures up to 10-2 mbar (Fig. 1).

Jet pumps that use steam as their motive medium convey at very high motive-jet velocities, normally several times the speed of sound, and are thus capable of handling the large volumes in the vacuum easily. As a single jet pump can overcome only a limited compression ratio, several jet pumps have to be arranged in series for low suction pressures. Such multistage steam-jet vacuum pumps are built for suction pressures up to 0.01 mbar. If possible, a condenser is arranged between two jet pumps in order to condense most of the motive steam and mini-mize the suction quantity for the following stage. Depending on the application, direct-contact condensers, surface condensers and in isolated cases also air-cooled condensers are used (Fig 2).

Water vapour is mainly used as the motive medium for jet pumps, because in most industrial plants it is produced in large quantities as an energy carrier and is thus easily available. During the condensation of the motive steam, however, waste water is occasionally produced. Although the waste water quantities are only small, and the steam-jet pump is often still the most economical vacuum pump despite the costs for waste water removal, this has a negative effect on the image of this type of pump.

As an alternative, product vapours are therefore increasingly used to operate jet pumps, for example solvent vapours, monochlorobenzene, toluene, butanediol, ethylene glycol or furane.

The motive vapour condensate is re-used as a motive medium after renewed evaporation. Due to the lower evaporation temperatures of the organic vapours, this mode of opera-tion is in many cases preferable energy-wise to that using steam.

Another important application of jet pumps that use product vapour as their motive medium is when water must be prevented under all circumstances from entering into the process. The design of jet pumps that use product vapour as their motive medium requires detailed studies in order to achieve the optimum efficiency at any given time. As regards construction and functioning, jet pumps that use product vapour as their motive medium are no different from jet-vacuum pumps that use steam and thus basically offer the same advantages:

• simple design,

• high level of operational reliability, little maintenance,

• long service life, little wear,

• corrosion-resistance thanks to the choice of materials,

• availability in all materials used for plant manufacturing,

• freedom from effluent due to condensate backfeed.

A multistage jet vacuum pump was tested in the large-scale plastics production process of a major German industrial firm as a means of creating a vacuum under laboratory conditions, as an alternative to mechanical vacuum pumps.

The jet vacuum pump consists of several ejectors, surface condensers, vapour generators and a liquid-ring backing pump (Fig. 3). In-plant-produced phenolic vapour was used as the motive fluid for the ejectors, since vapour containing phenol had to be exhausted in any case. All parts in contact with the phenol are fully jacketed.

The test results are excellent and prove that jet systems can be run on virtually all organic vapours, and integrated into the process extremely well even in difficult situations.

This company has also developed universal pump assemblies for the chemical industry. The most prominent features of these pumps are their compact dimensions and their twin design. Two assemblies, which are independent of one another, are installed in parallel and can be operated either simultaneously or separately. It is thus possible, for example, to carry out maintenance without having to shut down the entire plant.

Jet vacuum pumps made of porcelain/graphite are also used in the chemical industry to remove aggressive gases and vapours if stainless steels are not resistant.

The jet pumps are made of porcelain and the surface condenser of graphite. The surface condenser in this case takes the form of a compact block condenser. This design, with its reduced dimensions, requires little space and is easy to assemble in the field.

In crude-oil refineries jet vacuum pumps are combined with liquid-ring pumps for creating vacuums in crude-oil distillation columns. This combination of the two pump types ensures that the consumption of steam, water and electricity remains very low. The jet vacuum pumps are ideal for this application, as they are capable of conveying large gas quantities (between 104 and 106 m³/h); at the same time they are operate reliably and are not susceptible to malfunctions. High volumetric flow rates are also involved when vacuum is created for steel degassing (Fig. 4). In order to produce defined steel qualities, certain raw-steel treatment processes need to take place in a vacuum. The vacuum pumps used here have to perform two main tasks. Firstly, the process tank must be evacuated quickly to the required pressure. This means that the initial suction capacity of the jet vacuum pump must be very high (several thousand kg/h). Secondly, a low pressure must be maintained while simultaneously removing a large inert-gas flow (several hundred kg/h with a vacuum of approximately 0.6 mbar).

Furthermore, jet vacuum pumps are used for deodorizing edible oil, for polymerizing synthetics, for producing foils, for desalinating seawater, for dehydrating oils, for condi-tioning food, beverages and tobacco, for exhausting spinning baths as well as for many other applications.

GEA Wiegand

Fax: ++49/7243/705-330

Further information cpp-257