Ejector Vacuum Systems

Vacuum plays an important role in processes across every sector of industry, from small laboratories to large production facilities. Vacuum can be obtained through mechanical pumps, ejector systems, or a combination of both (hybrid systems).

Figure 1. A 3-stage ejector vacuum system used in the chemical industry at the system outlet of a surface condenser first stage (pre-stage, heated), Suction capacity: 3462 kg/h at 26.7 mbar abs.

Modern systems require relatively lower energy and are environmentally friendly in terms of design and operation. Through continuous testing and research and development activities, improved operation is achieved. The vacuum range required for the above applications starts with atmospheric pressure and typically ends at 10^-5 bar.

Basic Characteristics of Ejector Vacuum Pumps

To successfully use ejector vacuum pumps in process technology, it is necessary to match the different characteristics of the pumps with different operating conditions and requirements. Ejectors are compression devices with no moving parts. The enthalpy of the driving fluid is converted into kinetic energy through the driving nozzle, which advances the gas mixture through the throat and accomplishes compression. The velocity of the moving jet flow exiting the nozzle is several times the speed of sound. Due to the flexibility of the ejector's structure, very large volumes can be easily processed under vacuum.

Figure 2. Operating principle of a jet pump and pressure changes along the flow path

1 body 2 driving nozzle 3 inlet cone 4 diffuser = mixing nozzle 5 outlet cone p₁ driving steam pressure p₀ suction pressure p discharge pressure p_s pressure at sonic velocity Δp_v shock wave M₁ driving steam flow M₀ suction flow (vacuum point) M mixed steam flow

Construction and operation can be addressed through three distinct processes:

• Sudden passage of the driving fluid through the moving nozzle and formation of a directed jet flow, • Mixing of the driving jet flow (air, gas, and steam) created with the medium to be moved, • Conversion of the velocity of the mixture into pressure in the diffuser and outlet cone.

Multi-Stage Ejector Vacuum Pumps

Since a single ejector can efficiently handle only a limited compression ratio, when very low suction pressures are needed, several jet pumps must be positioned in series. A condenser is positioned between two ejectors to condense the driving steam as much as possible. In this way, the volume of the complete gas mixture and consequently the energy requirement of the next stage is reduced. This type of jet vacuum pump is designed for approximately 0.01 mbar suction pressure. For the most effective energy use, the driving fluid and condensable components are condensed between the two stages. The condensation pressure depends on the temperature of the cooling medium and the properties of the moving fluid. For example, to compress process gases from a pressure of 0.3 mbar to a condenser pressure of 56 mbar (a compression ratio of 56/0.3 ≅ 187), two ejector stages, each with approximately a compression ratio of 14, are sufficient. For a suction pressure of 0.1 mbar, the pressure gradient is 56/0.1 ≅ 560, and therefore, three spray pumps, each with a staged compression ratio of 8.25, must be arranged in series. The maximum compression ratio for a steam spray vacuum pump depends on the suction pressure and the pressure of the available driving steam. With suction pressures below 6 mbar, the saturated steam temperature reaches 0°C with the risk of ice formation. This has a negative effect on performance. To prevent this, ejectors in this pressure range must be equipped with steam-heated jackets. Condensers can be water-cooled contact condensers or surface condensers, and in some cases air-cooled condensers may also be used. Surface condensers are generally preferred as intermediate condensers to prevent contamination of the suction medium with cooling water. With surface condensers, condensation occurs inside or around the tubes. They can be manufactured with fixed or removable tube bundles.

Figure 3. For the chemical industry, a two-stage steam spray liquid ring vacuum pump with closed-circulation working medium and integrated stand-by LRVP. Suction capacity: 2 mbar at 4.5 air/benzyl alcohol

Hybrid Vacuum Systems

Liquid ring vacuum pumps (LRVP) combined with ejector vacuum pumps are particularly suitable for cases where there is no barometric regulation of the condensate leg. A hybrid vacuum system typically consists of 1 to 3 ejectors, a surface condenser, and a liquid ring vacuum pump. Hybrid vacuum systems can be manufactured for any desired suction pressure at approximately 0.01 mbar. Liquid ring vacuum pumps are standard machines. Design engineers can size the ejector, and thus the intermediate pressure, so that the liquid ring vacuum pump best meets the consumption needs of the entire hybrid system. The energy efficiency of hybrid vacuum systems is very high.

Ejector Vacuum Systems for Highly Corrosive Applications

Processes in the chemical industry are typically highly corrosive and require corrosion-resistant materials. In the past, porcelain materials were mostly preferred, but there is a high risk of damage and it cannot be machined with the required precision, which directly affects the consumption data of an ejector. In collaboration with SGL Carbon, an important producer of carbon-based products, GEA has developed a new ejector body made of GEA DIABON® graphite. Graphite has higher mechanical strength and the internal dimensions can be precisely engineered according to the customer's performance requirements. Due to very high standardization, this new product can be offered at a very competitive price.

Figure 4. Steam spray graphite vacuum pump with ejectors and block condenser

Ejector Vacuum Systems Operating with Product Vapor

Ejector vacuum pumps typically operate with steam. Steam is commonly available in the chemical industry and has proven to be an effective driving medium. There are processes where steam is not permitted to be used as the driving medium to prevent product contamination. In this case, it is possible to use a compatible product compound as the driving medium. The condensed driving vapor can be returned to the process or evaporated and reused as the driving medium. Jet vacuum pumps operating with product vapor do not differ in function from jet vacuum pumps operating with steam and offer the same advantages.

Figure 5. Vacuum unit operating with product vapor at a polyester production facility

Application Areas

Ejector vacuum systems are used to create and maintain vacuum in many applications in the chemical, petrochemical, and food industries. For example, they are used in evaporators, dryers, distillation and purification facilities, deodorization, freeze-drying operations, multi-stage condensation, air removal, and gas removal processes.

Brief Summary of the Advantages of Ejector Vacuum Pumps

• Simple design with no moving parts. • Reliable operation. • Low wear and abrasion, minimal maintenance requirements. • Corrosion-resistant design through appropriate material selection for the process. • Process-appropriate material alternatives (such as graphite, Hastelloy, and titanium) can be produced. • Can be used for suction flows between 10 m³/h and 2,000,000 m³/h. • Suitable for approximately 0.01 mbar vacuum. • High-pressure or vacuum steam, water vapor, or product vapor can be used as the driving medium. • Can be combined with mechanical vacuum pumps (hybrid vacuum systems). Dr. Alberto Riatti Head of Vacuum Systems GEA Germany       İlker Damar Sales Manager Industrial Applications GEA Turkey