From small laboratory facilities to the largest production plants – in every industry there are processes where vacuum plays a central role. Vacuum can be generated by means of mechanical pumps or ejector systems or in combination of both (hybrid systems).
Figure 1. 3-stage ejector vacuum system downstream of the 1st stage (pre-stage, heated) with surface condensation in the chemical industry.
Suction capacity: 3462 kg/h from 26.7 mbar abs.
Modern vacuum plants require relatively little energy and can be designed and operated in an environmentally friendly way. Testing, research and development together with continual improvement also contribute to enhanced operation.
The range of vacuum required for the above applications begins with atmospheric pressure and usually ends at 10
-5 bara.
Fundamental Facts About Ejector Vacuum Pumps
In order to use ejector vacuum pumps successfully in process technology, their particular characteristics must be harmonized with the particular operating conditions and requirements. Ejectors are compressing devices without moving parts.
The enthalpy of the motive medium is converted in the motive nozzle into kinetic energy, which conveys the gas mixture and performs the compression work inside the throat.
The velocity of the motive jet stream leaving the motive nozzle is several times the speed of sound. Due to the flexibility of the ejector construction, huge volumes under vacuum can easily be handled.
Figure 2. Working principle of a jet pump and the pressure differences over the flow path
1 head
2 motive nozzle
3 inlet cone
4 neck= mixing nozzle
5 outlet cone
p
1 motive steam pressure
p
0 suction pressure
p discharge pressure
p
s pressure at sonic velocity
Δp
v shock wave
M
1 motive steam flow
M
0 suction flow
M mixing steam flow
Construction and Operation can be understood in terms of three main distinctive processes:
•Flashing of the motive medium in the motive nozzle and the formation of a directional motive jet stream,
•Mixing of the motive jet stream thus formed with the medium to be drawn off (air, gas and vapor),
•Transforming the velocity of the mixture into pressure in the neck and outlet cone.
Multistage Ejector Vacuum Pumps
Since a single ejector can handle efficiently a limited compression ratio, several jet pumps must be arranged one after the other if very low suction pressures are required. A condenser is arranged between two ejectors 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. These sorts of jet vacuum pumps are built for suction pressures of approx. 0.01 mbar.
For the most effective use of energy, the motive medium and the condensable compounds shall condense between two stages. The condensation pressure depends on the temperature of the cooling medium and characteristics of the motive medium.
For example, in order 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 ejector stages are 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, 3 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.
With suction pressures below 6 mbar, the saturated steam temperature reaches 0°C with the risk of ice building. This has a negative effect on the performance therefore. In order to prevent this, the ejectors in this pressure range shall be equipped with heating jackets with steam.
The condensers can be either water-cooled mixing condensers or surface condensers, and in certain cases, air-cooled condensers are also used. Surface condensers are usually preferred as intermediate condensers to avoid any contamination of the cooling water with the suction medium.
With surface condensers, the condensation take place in the tubes or around the tubes. They can be constructed with fixed or removable tube bundles
Figure 3. Two-stage steam jet liquid ring vacuum pump with closed circulation operating medium and integrated stand-by LRVP for the chemical industry Suction capacity: 4.5 air/benzyl alcohol from 2 mbar
Hybrid Vacuum Systems
Liquid ring vacuum pumps (LRVP) combined with ejector vacuum pumps are particularly suitable for situations where no barometric arrangement of condensate leg is available.
A hybrid vacuum systems consists generally of 1 to 3 ejectors, a surface condenser and a liquid ring vacuum pump. Hybrid vacuum systems can be built for any required suction pressures of approx. 0.01 mbar.
Liquid ring vacuum pumps are standardized machines. The project engineer will design the ejector and thus the intermediate stage pressure in such a way that the liquid ring vacuum pump optimally meets the requirements with regard to the consumption of the entire hybrid system.
Hybrid vacuum systems have a very efficient energy consumption.
Ejector Vacuum Systems for Highly Corrosive Applications
Processes in the chemical industry are often highly corrosive and need materials, which are strongly resistant. In the past, porcelain was the material of choice but it has a high risk of damage and cannot be worked accurately, which directly affects the consumption data of an ejector.
In cooperation with SGL Carbon, a major producer of carbon-based products, GEA has developed a new ejector body made of DIABON® graphite.
Graphite has a higher mechanical resistance and the internal dimensions can be worked accurately according to the performance requirements of the customer. Due to very high standardization, this new product can be offered to a very competitive price.
Figure 4. Steam jet vacuum pump with
ejectors and block condenser made of graphite
Ejector Vacuum Systems Operated by Product Vapor
Ejector vacuum pumps are generally operated with steam. Steam is mostly available in the chemical industry and has proven to be a good motive medium.
There are applications in which the use of steam as motive medium is not permitted for process reasons in order to avoid contamination of the product.
In this case, it is possible to use compatible product compound as motive medium.
The condensate of the motive vapor can either be returned to the process or evaporated and re-used as motive medium. Product vapor operated jet vacuum pumps do not differ in their function from steam operated jet vacuum pumps and offer the same advantages.
Figure 5. Product vapor operated vacuum unit in a polyester
production plant
Fields of Applications
Ejector vacuum systems are used to create and maintain the vacuum in many applications in the in the chemical, petrochemical and food industries i. e, in evaporators, dryers, distillation and rectification plants, in the processes of freeze drying, poly-condensation, deaeration, degassing and deodorizing.
Short Summary of the Advantages of Ejector Vacuum Pumps
• Simple construction, no moving parts.
• Operationally safe.
• Little wear and tear and minimum maintenance.
• Resistant to corrosion if appropriate material is chosen.
• Can be supplied in all materials used in the equipment, Even Graphite, Hastelloy and Titanium.
• Can be used for suction flows of 10 m
3/h to 2,000,000 m
3/h.
• Suitable for vacuum of approx… 0.01 mbar (abs.).
• Driven by water vapor or product vapor; with over pressure or with vacuum steam.
• Can be combined with mechanical vacuum pumps (hybrid vacuum systems).
Dr. Alberto Riatti
Product Manager Vacuum Systems
GEA Germany
İlker Damar
Sales Manager
Industrial Applications
GEA Türkiye
