What is a liquid ring compressor

Liquid ring vacuum pumps and liquid ring compressors Technology and application

Transcript

1 Liquid ring vacuum pumps and liquid ring compressors Technology and application STERLING FLUID SYSTEMS GROUP

2 Liquid ring vacuum pumps and liquid ring compressors Technology and application

3 Pump technology Vacuum technology System technology Service Delivery program Pump technology Standardized chemical pumps Standard water pumps Side channel pumps Heat transfer pumps Sectional casing pumps Shaft sealless pumps Digested sludge mixers System technology Vacuum and compressors Systems Membrane technology Vacuum technology Liquid ring vacuum pumps Liquid ring compressors Gas ejectors Dry running vacuum pumps Service standard program Large pumps

4 Contents of vacuum pumps and compressors ... 4 Liquid ring vacuum pumps and liquid ring compressors ... 6 Properties ... 6 Mode of operation and types of construction ... 7 Gas and liquid conduction Working areas Operating behavior Operating modes Drive Control of the suction volume flow Construction versions Emissions Combination of liquid ring vacuum pumps with gas jet vacuum pumps Accessories Acceptance rules Use Literature Appendix Vapor pressure of water Frequently used materials and material combinations for liquid ring gas pumps ... 70

5 4 Vacuum pumps and compressors Compressors Vacuum pumps and compressors are work machines for compressing gases and vapors. They are always used wherever procedural tasks have to be carried out that would otherwise be uneconomical, unsafe or impossible. The vacuum pumps and compressors compress the gases or gas-vapor mixtures produced in the process from the "suction pressure" to the "compression pressure". In the case of vacuum pumps, the suction pressure is lower and in the case of compressors the compression pressure is higher than atmospheric pressure. According to DIN, vacuum is divided into different pressure ranges (specifications in mbar): Vacuum ranges Rough vacuum: up to 1 Fine vacuum: 1 up to high vacuum: up to ultra-high vacuum:

6 vacuum pumps and compressors 5 several vacuum pumps of various types can be combined. Since e.g. As a rule, high vacuum pumps cannot deliver against the atmosphere, they require low vacuum pumps as backing pumps. There are various types of vacuum pumps and compressors. Figure 2 contains an overview: The following criteria can be regarded as decisive for the selection of the design: Combination of several pumps, the working area with regard to the suction and compression pressure, the suction volume flow at the required pressures, the requirements from the type and properties of the ones to be compressed Gases and vapors, the requirements with regard to the environment and operating conditions, the available equipment, the safety requirements, the economic efficiency. Fig. 2: Types of vacuum pumps and compressors

7 6 Liquid ring vacuum pumps and liquid ring compressors Properties of liquid ring as energy carrier Liquid ring vacuum pumps and liquid ring compressors are compressors in which a liquid ring formed from the operating fluid serves as an energy carrier for compressing gases and vapors. The contact of the pumped medium with the operating fluid causes interactions between these media. The liquid ring vacuum pumps and liquid ring compressors thus occupy a special position among the various types of compressor. They have a wealth of properties that cannot be achieved in their type and variety by any other design: Large number of properties Liquid ring vacuum pumps and liquid ring compressors are able to compress almost all gases and vapors. They have no parts sliding on each other and the materials can be adapted to the operating conditions. The compression process is almost isothermal. The pumps offer the highest possible safety when compressing ignitable substances as well as toxic and carcinogenic media. If the pumped medium contains condensable components, the suction volume flow can increase. Single and double shaft seals and magnetic couplings

8 Mode of operation and design 7 ensure low leakage rates. Operation is associated with low noise emissions and low-vibration running. A high level of operational reliability is achieved with little maintenance. The conveyance of liquid in the gas flow is possible. Mode of operation and designs In this book, the term "liquid ring gas pump" is used when the explanations refer to liquid ring vacuum pumps and liquid ring compressors. The contact of the pumped medium with the operating fluid causes thermodynamic effects and enables gas-liquid reactions. When compressing dry gases or vapors that do not condense in the suction chamber or during the compression process, a liquid ring gas pump works as a positive displacement compressor. The pumped medium is sucked in by enlarging the space and compressed by reducing the space. If gas-vapor mixtures are conveyed, the vapor portion of which condenses after entering the suction chamber or during compression, a liquid ring gas pump works as a positive displacement compressor with a condensation effect. If media are conveyed that are absorbed by the operating fluid or that react with the operating fluid, a liquid ring gas pump acts as a displacement compressor and absorption machine. Large and medium-sized liquid ring gas pumps are generally designed in the single-acting design. Figure 3 shows the functional principle in a perspective view: Liquid ring gas pump Single-acting design

9 8 Liquid ring vacuum pumps and compressors Fig. 3: The functional principle of a liquid ring gas pump in perspective. An impeller (2) is arranged in a circular housing (1) eccentrically to the housing axis. If the housing is partially filled with liquid and the impeller is set in rotation, a liquid ring (3) running concentrically to the housing axis is formed as a result of the centrifugal force. This arrangement has the effect that the inner contour of the liquid ring touches the impeller at points (20) and (21) and the liquid exits and re-enters the impeller cells like a piston during one revolution of the impeller. There is thus an increase in space in the area of ​​the exiting liquid ring, and the delivery medium is sucked in via the suction opening (5) in the control body (4) which is connected to the suction nozzle. In the area of ​​the entering liquid ring, the space is reduced and the pumped medium is compressed. After compression, it is pushed out via the in the control body (6)

10 Mode of operation and design 9 Existing pressure opening (7) that is connected to the pressure port. The double-acting design is used for special applications, especially for compressors for high differential pressures. Here, the housing has a shape that enables the liquid ring to be fed twice out of the impeller cells and then in again during one revolution of the impeller and thus to fill and empty the impeller cells twice with gas. Figure 4 shows sections through a single-acting liquid ring gas pump, Figure 5 shows sections through a double-acting liquid ring gas pump. The changing depth of immersion of the liquid ring in the impeller, which is required for the function of a liquid ring gas pump, can also be achieved with an impeller arranged concentrically in the housing and channel-like spaces with changing depths to the side of this. Double-acting design Fig. 4: Longitudinal and cross-section of a single-stage, single-acting liquid ring gas pump 1 housing 2 impeller 3 liquid ring 4 control element 5 suction opening 6 control element 7 pressure opening 8 shaft

11 10 Liquid ring vacuum pumps and compressors Fig. 5: Longitudinal and cross-section of a single-stage, double-acting liquid ring compressor 1 Housing 2 Impeller 3 Liquid ring 4 Suction control element 5 Suction opening 6 Pressure control element 7 Pressure opening 8 Shaft Such pumps are suitable for conveying low suction volume flows and can also be used for pumping liquids . Figure 6 shows a longitudinal section through this design known as a side channel pump. Fig. 6: Longitudinal section of a two-stage liquid ring gas pump based on the side channel principle 1 Housing 2 Impeller 3 Suction control body 4 Pressure control body 5 Shaft In Figures 3 to 6, the suction and pressure openings are arranged in flat control bodies (so-called control disks) which are located to the side of the impeller. The gas is supplied and discharged through the end faces of the impeller. Deviating from this, there are designs in which the suction and pressure openings are located in control bodies that extend into the impeller hub.

12 Mode of operation and design 11 The control bodies have a conical or cylindrical shape. The gas is supplied and discharged through the hub of the impeller. If the impeller has a relatively small width, an arrangement of the suction and pressure openings on one side is sufficient for filling and emptying the impeller cells. The openings are then either arranged together in a control body or the suction opening is located in one and the pressure opening in the opposite control body, as shown in FIGS. 3 and 5. Number of suction and pressure openings With relatively wide impellers, the gas is supplied and discharged via openings arranged on both sides. Figure 4 shows this version. The impeller cells of pump stages for low suction pressures and thus for gases with a low density, on the other hand, can be sufficiently filled with gas via a suction opening even with a relatively large impeller width. The suction openings extend over almost the entire angle at which the liquid ring emerges from the impeller cells. The pressure openings are in the area of ​​the liquid ring entering the impeller cells. Their angular extensions and shapes depend on the quotient of the compression and suction pressure for which the pump stage is intended. It is taken into account that the stage must work economically both with the required pressure and with lower pressure conditions. With the beginning of the pressure opening, there is a "built-in pressure ratio" in each stage. The beginning of the pressure opening would therefore theoretically have to be different for each pressure ratio. In practice, however, such a machine can be operated in a relatively wide range of pressure ratios. Position of the suction and pressure openings

13 12 Liquid ring vacuum pumps and compressors Large pressure range Fig. 7: Longitudinal section of a two-stage, single-acting liquid ring vacuum pump Liquid ring vacuum pumps must be able to work economically in a very large pressure range or at least to pass through it. If, for example, during an evacuation process, the suction pressure initially corresponds to atmospheric pressure and is then reduced to 33 mbar, the pressure ratio increases from 1 to approx. 30. If the stage pressure ratio of liquid ring vacuum pumps rises above a value of approx. 7, which is the case with vacuum pumps for suction pressures less than 150 mbar, there are two options: Either the pressure openings are provided with valve constructions that automatically adapt the start of the pressure opening to the Allow pressure ratio, or the liquid ring vacuum pumps are designed in two stages and the total pressure ratio is thus divided into two stages. Figure 7 is a longitudinal section of a two-stage liquid ring vacuum pump. Ball and plate valves are used as valve constructions, the latter in the form of rigid or flexible plates. The sealing elements of the valve are moved in front of and behind the control body due to the different pressures. The mode of operation of the ball valves illustrates the

14 Gas and liquid routing 13 Figure 8: If the pressure in the impeller cells is higher than in the pressure chamber, the ball detaches from the valve seat and the gas flows out of the cells into the pressure chamber. If the pressure in the pressure chamber is higher than in the impeller cells, the ball is pressed onto the valve seat and prevents gas and liquid from flowing back into the impeller cells. Fig. 8: How the ball valves work. Gas and liquid conduction With the gas, part of the liquid forming the ring is expelled from the pressure openings. Operating fluid must therefore be continuously fed to a liquid ring gas pump. The operating fluid is supplied via the operating fluid connection provided for this purpose. Normally, the required operating fluid pressure is the same as or slightly lower than the pressure in the pressure port of the pump. Supply of operating fluid

15 14 Liquid ring vacuum pumps and compressors Fig. 9: Liquid ring vacuum pump with separator and heat exchanger Reuse of the liquid After the gas and liquid mixture has left the pressure port of the pump, the liquid can be separated from the gas in a liquid separator and the liquid after it has been cooled or after it has been mixed with sufficient cool liquid, it can be used again as operating liquid. Cooling is necessary because most of the heat generated during compression and condensation processes is absorbed by the liquid. Only if it can be economically justified will the liquid not be reused. If the separation of gas and liquid is not important, the liquid separator can be dispensed with. By suitably designing the liquid separator, the moisture content of the gas emerging from the separator is kept low.

16 Gas and liquid routing 15 Figure 10 shows the gas and liquid routing in a liquid ring gas pump and the liquid separator. The "combined liquid operation" operating mode is shown. Part of the liquid separated from the gas in the liquid separator is reused as operating liquid in this operating mode. Deviating from the shape shown in Figure 10, the liquid separator can have a different shape and can also be mounted on the pressure port or serve as a base plate for the vacuum pump or the compressor and the drive motor. Fig. 10: Gas and liquid routing Types of liquid separators Working areas Liquid ring gas pumps are built for suction volume flows from approx. 1 m³ / h to more than m³ / h. The suction pressures that

17 16 Liquid ring vacuum pumps and compressors The influence on suction pressures that vacuum pumps can achieve are determined in particular by the physical properties of the operating fluid. If water with a temperature of 15 C is used, suction pressures of up to 33 mbar are possible with relatively good suction volume flows. It should be noted that the compression pressure is equal to or greater than atmospheric pressure. Lower suction pressures can be achieved through combinations with gas jet vacuum pumps, steam jet vacuum pumps and Roots vacuum pumps. Liquid ring compressors work at atmospheric suction pressure, depending on the design, up to absolute compression pressures of around 9 bar. Figure 11 shows the working areas of the liquid ring gas pumps. Fig. 11: Working areas of the liquid ring gas pumps

18 Operating behavior 17 Operating behavior The transfer of energy from the liquid ring to the pumped medium as well as the contact between the pumped medium and the operating liquid determine the properties and the operating behavior of the liquid ring gas pumps. Intake volume flow The theoretically possible intake volume flow V theor for a pump that works as a positive displacement compressor is equal to the product of the vane cell volume V cells available for the gas during one vane revolution and the speed n of the vane: V theor = 60 V cells n (1) Theoretical suction volume flow V theor is given in m³ / h, V cells in m³ and n in 1 / min. In practice, however, the suction volume flow is lower, since the impeller cells are not completely emptied of the gas and flow losses occur in the suction and pressure openings as well as gap losses. If the sucked in gas is dry, it will tend to saturate itself with steam from the operating fluid after entering the pump. The saturation process causes the temperature of the gas and the liquid to drop because heat is required for the formation of vapor. The space available for the gas in the impeller cells is reduced by the proportion that is taken up by the steam. The Dalton gives the connection between the spatial proportions of gas and steam

19 18 Liquid ring vacuum pumps and compressors Law (2), which states that in the case of mixtures of gaseous substances the individual volumes relate to one another like their partial pressures: Dalton's law VG p G p 1 - p DVD = p D = p D (2) where V stands for volume and p for pressure with the indices G for gas and D for steam; p 1 denotes the suction pressure. The specifications of the intake volume flow V L in lists and catalogs are usually based on dry air with a temperature of 20 C as the pumped medium and water with a temperature of 15 C (20 C for compressors) as the operating fluid. If the dry air sucked in has a temperature other than 20 C, the quotient of the absolute temperatures of air and process water as well as the steam content of the air-steam mixture in the impeller cells changes. The result is a change in the intake volume flow. The same applies if the operating water temperature deviates from the stated values. Assuming that the temperature t 1 of the conveyed air has changed to the temperature t 3 of the process water by the time it enters the impeller cells and that Dalton's law is still applicable for air-water vapor mixtures and for this case, the result would be theoretically calculate the intake volume flow V 1 for air temperatures other than 20 C or for process water temperatures other than 15 C using equation (3): t 1 20 p 1 - p DV 1 = VL +1 (3) tp 1 17.04

20 Operating behavior 19 The number 17.04 stands for the vapor pressure (in mbar) of the water at a temperature of 15 C. In practice, the change in the intake volume flow does not fully follow these principles, as they only apply to ideal gases for the formation of steam only a limited time and a limited surface are available and pump-specific conditions prevent this. Empirical equations were determined from measured values ​​in order to take into account the practical dependence of the intake volume flow on the vapor pressure of the operating fluid [1]. The intake volume flow V 1 for operating fluid temperatures other than 15 C is calculated as follows: The vapor pressure influences the intake volume flow V 1 = VL λ I (4) For single-stage vacuum pumps and compressors, p 1 (0.27 ln p 1-0.0783 ) - 1.05 p D λ Iein = (5) p 1 (0.27 ln p 1-0.0783) - 1.05 17.04 and for two-stage vacuum pumps p 1 (0.35 ln p 1-0, 1) - p D λ Izwei = (6) p 1 (0.35 ln p 1-0.1) - 17.04 p 1 p DV 1 VL = suction pressure in mbar = vapor pressure of the operating liquid in mbar = suction volume flow in m³ / h = suction volume flow when conveying dry air (20 C) with water (15 C) as the operating fluid in m³ / h

21 20 Liquid ring vacuum pumps and compressors Equations (5) and (6) give mean values. They apply to water as the operating fluid, an air temperature t 1 of 20 C and values ​​for p D between 17 and 123 mbar and for p 1 between 33 and 1013 mbar. Cavitation Fig. 12: Influence of the process water temperature on the intake volume flow If the proportion of steam in the pumped medium is too high, cavitation can occur in the pump and damage components, especially the impellers and control bodies. It is known from observations that a certain standard volume flow of air or another non-condensable gas must be present in the conveying medium in order to avoid cavitation. This current is independent of the process water temperature, but depends on the pump size and its speed. Figure 12 graphically shows calculation values ​​that were determined using equations (5) and (6). The line referred to as the cavitation limit is a guideline value for vacuum pumps that are used when conveying dry

22 Operating behavior 21 air can achieve a suction pressure of 33 mbar without cavitation. To prevent cavitation damage, liquid ring vacuum pumps are equipped with cavitation protection. This involves the targeted return or addition of non-condensable media into the impeller cells via an existing connection. As a rule, this connection is connected to the gas space of the liquid separator. If the pumped medium consists of dry gas or vapor that cannot be condensed in the suction chamber, then, apart from its solubility in the operating fluid, the suction volume flow is influenced by the temperature of the pumped medium. An influencing variable was determined from measured values ​​with the aid of which the intake volume flow when conveying dry air with a temperature of t 1 can be determined from the intake volume flow when conveying air with a temperature of 20 C [1]. Cavitation protection High gas temperature increases the intake volume flow The intake volume flow when conveying dry air with a temperature of t 1 and water with a temperature of t 3 is then calculated using the following equation: V 1 = VL λ I λ III (7) where λ III = 1 + 0.66 (t 1-20) t (8) In Figure 13, the dependence of the factor λ III on the air temperature is for various

23 22 Liquid ring vacuum pumps and compressors Fig. 13: Influence of the temperature of the pumped medium on the suction volume flow when conveying dry gas Process water temperatures shown graphically. The temperature of the medium must not exceed certain maximum values. Depending on the size, the maximum permissible temperature for liquid ring gas pumps that are mass-produced is between 60 and 200 C. The permissible value is also influenced by the pumped medium. Condensing vapors increase the suction volume flow If the pumped medium contains vapors that condense in the pump, the suction volume flow is greater than when conveying dry gases, if the condensation takes place in the suction chamber or in the impeller cells connected to the suction chamber. The size of the increase depends on the suction pressure, the gas / vapor mixture temperature, the operating fluid temperature and a design size.

24 23 From measured values, an empirical equation could be determined with which the increase in the suction volume flow when pumping air saturated with water vapor and water as the operating fluid compared to the suction volume flow when pumping dry air at a temperature of 20 C can be calculated [1] . Empirical equation from measured values ​​V 1 = VL λ I λ II (9) where [0.75 p 1 (ln p 1-0.2877)] E λ II = (10) [0.75 p 1 (ln p 1 0.2877)] E - 0.75 p S hp D 0.0369 E = 0.82 + 0.791 (11) d 17.04 dhp 1 p D p SV 1 VL = impeller diameter in m = impeller width from a suction opening is applied, in m = suction pressure in mbar = vapor pressure of the operating liquid in mbar = vapor pressure of the water at the temperature of the air-water vapor mixture in mbar = suction volume flow in m3 / h = suction volume flow when conveying dry air (20 C) with water ( 15 C) as operating fluid in m3 / h. Equation (9) applies to air / water vapor mixtures and values ​​for p D and p S between

25 24 Liquid ring vacuum pumps and compressors 17 and 123 mbar and for p1 between 33 and 1013 mbar. Figure 14 shows the dependence of the factor λ II on the above-mentioned influencing variables for the construction size h / d = 0.43.

26 Operating behavior 25 With regard to the cavitation limit, it also applies here that a certain standard volume flow of air or another, non-condensable gas must be present in the pumped medium. The most commonly used operating fluid is water. In process engineering, however, liquids often have to be used whose chemical and physical properties are very different from those of water. The liquid can be adapted to the respective process. It should be noted that most of the vapors contained in the pumped medium condense in the pump and are expelled from the pressure port as condensate together with the gas-liquid mixture. In this case, it is advisable to choose an operating fluid that also occurs as condensate. Different operating fluids If the physical properties of the operating fluid differ from those of water, this affects the intake volume flow, the power requirement, the operating fluid flow and the temperature of the gas-liquid mixture on the pressure side. Since the vapor pressure is determined by the type of liquid and increases with increasing temperature, the suction volume flow is dependent on the type of liquid, especially at low suction pressures. The suction volume flow is theoretically zero when the suction pressure is equal to the vapor pressure of the operating fluid, which thus represents the lower physical limit of the suction pressure that can be achieved. The power P is required for isothermal compression of a gas from suction pressure p 1 to compression pressure p 2 is proportional to Fig. 14 (opposite): Influence of the temperature of the pumped medium on the suction volume flow when pumping a saturated air / water vapor mixture

27 26 Liquid ring vacuum pumps and compressors suction pressure, the suction volume flow V 1 and the natural logarithm of the compression ratio: p 2 p 1 V 1 ln p 1 P is = (12) 3, p 1 = suction pressure in mbar p 2 = compression pressure in mbar P is = Isothermal compression capacity in kw V 1 = intake volume flow in m³ / h Since the compression capacity is provided by the energy of the rotating liquid ring, at least one energy equivalent to the isothermal compression capacity must be present in it. To assess the energy present in the liquid ring, it can be assumed that this changes proportionally with the volume of the rotating liquid, its density and the square of the rotational speed of the liquid ring. At a certain speed of the impeller, the density of the operating fluid is decisive for the energy contained in the fluid ring and thus also for the possible compression performance. Furthermore, the inner limitation of the liquid ring on the suction as well as on the pressure side is influenced. The result is a dependency of the suction volume flow and the power requirement on the density of the operating fluid. Isothermal compression performance Law of similarity enables conversions With the help of the law of similars it is possible to determine the intake volume flow and the power requirement when the density of the operating fluid changes

28 Operating behavior 27 to determine the existing operating values. If the operating fluid consists of immiscible components, stratification in the fluid ring is possible if the densities of the mixed components are different. The intake volume flow then depends on whether the lower-boiling portion has the lower or the higher density. The influence of the viscosity of the operating fluid on the intake volume flow is usually small. The improvement in the sealing effect in the gaps between the impeller and the control element, which results in a higher viscosity, is lost because a lower operating fluid flow than normal enters the pump. At a temperature of 20 C, water has a specific heat capacity of 4.183 kJ / kg K. The liquids used in process engineering often have a lower specific heat capacity. The temperatures of the liquid ring and the gas-liquid mixture emerging from the pressure port are then higher. The heat of evaporation is the decisive physical variable for the heat flow that is released during the condensation of vapor components of the pumped medium and predominantly passes into the liquid ring, as well as for the heat required for saturation when transporting dry gases and is mainly extracted from the liquid ring. Influence of viscosity Specific heat capacity Heat of evaporation The information in lists and catalogs relates to the compression of air. Liquid ring gas pumps are, however, suitable for pumping almost all gaseous and vaporous media and their mixtures. Direct impact on the

29 28 Liquid ring vacuum pumps and compressors Suction volume flow have the following properties and conditions of the pumped medium as well as effects: Influences of the pumped medium on the suction volume flow Temperature Saturation state Condensation in the suction chamber Density (technically effective only with hydrogen and helium as well as mixtures with these gases) Solubility in the operating fluid Conveying of Liquid (this does not mean the operating liquid) Reaction with the operating liquid Have an indirect influence: Condensation during compression Specific heat capacity Evaporation heat Power requirement The power requirement of the liquid ring gas pumps is negligibly dependent on the vapor pressure of the operating liquid. The viscosity of the operating fluid has an impact on the frictional power loss and thus on the power requirement. If the power requirement PL with water as the operating fluid is known, the power requirement P can be calculated using equation (13) with an average impeller peripheral speed and a kinematic viscosity of the operating fluid of up to 20 mm² / s: P = PL υ 0.05 (13) speed of the impeller Characteristic maps The speed of the impeller determines the suction volume flow and the power requirement of the pump. Figures 15 and 16 are representations of the suction volume flow

30 Operating behavior 29 Fig. 15: Intake volume flow and power requirement of a liquid ring vacuum pump at different speeds Conveying medium: air 20 C; Operating fluid water 15 C; Compression pressure 1013 mbar Fig. 16: Intake volume flow and power requirement of a liquid ring compressor at different speeds Conveying medium: air 20 C; Operating fluid water 20 C; Suction pressure 1013 mbar

31 30 Liquid ring vacuum pumps and compressors of the power requirement as a function of the speed n for a vacuum pump or a compressor. Compression pressure and suction pressure deviations from atmospheric pressure With liquid ring vacuum pumps, the compression pressure and with liquid ring compressors the suction pressure can deviate considerably from the atmospheric pressure normally present and thus result in a change in the suction volume flow and the power requirement. If the change in pressure is associated with an increase in the mechanical load on components, their permissibility must be checked. Figures 17 and 18 are representations of the suction volume flow and the power requirement at constant speed and water as the operating fluid for a single-stage vacuum pump at different compression pressures and for a single-stage compressor at different suction pressures. Temperature at the outlet Generation of heat In compressors, the entire power used to compress the gaseous substances is converted into heat. If vapors are conveyed that condense in the suction chamber or during compression, then condensation heat is created which has to be dissipated. If heat-generating reactions take place inside the pump between the pumped medium and the operating fluid, the heat of the reaction must also be dissipated. When liquid is conveyed, a heat flow is released due to the change in temperature. Heat exchange, which has to be taken into account, only takes place over the surface of the pump in the event of exceptional temperature differences.

32 Operating behavior 31 Fig. 17: Intake volume flow and power requirement of a liquid ring vacuum pump at different compression pressures Conveying medium: air 20 C; Operating fluid water 15 C Fig. 18: Intake volume flow and power requirement of a liquid ring compressor at different intake pressures Conveying medium: air 20 C; Operating fluid water 20 C

33 32 Liquid ring vacuum pumps and compressors The heat is transferred to the liquid ring In liquid ring gas pumps, a large part of the heat generated is transferred to the liquid ring and is carried away with the gas-liquid mixture. The following heat flows occur: Compression power and friction loss: Q v = P 3600 (14) PQV = power requirement of the pump in kw = heat flow in kj / h Heat exchange between the pumped medium and the operating fluid: QG = m G cp (t 1 - t 2) (15) c P = specific heat capacity of the pumped medium in kj / kg K m G = mass flow of the gas in kg / ht 1 = temperature at the suction nozzle in C t 2 = temperature at the discharge nozzle in C = heat flow in kj / h QG heat of condensation: QK = r (m D1 m D2) (16) m D1 = steam mass flow that condenses, in kg / hm D2 = steam mass flow that leaves the pressure port, in kg / hr = heat of evaporation in kj / kg QK = heat flow in kj / h remaining the cooling of the vapor to the condensation temperature, the cooling of the condensate to the liquid temperature

34 Operating behavior 33 and any heat of reaction that may be present are not taken into account, the heat flow Q transferred into the liquid ring is calculated according to equation (17): Q = QV + QG + QK (17) Heat flow If there is an equilibrium between the masses entering and exiting the pump - and heat flows orwhose equivalents exist: Q = B ρ 3 c (t 2 - t 3) (18) Q = heat flow in kj / h B = operating fluid flow in m3 / h ρ 3 = density of the operating fluid in kg / m3 c = specific heat capacity of the operating fluid in kj / kg K t 2 = temperature at the pressure port in C t 3 = temperature at the operating fluid connection in C This results in a temperature increase in the liquid gas pump of: Q t = t 2 - t 3 = (19) B ρ 3 c temperature increase To calculate the heat flows QG and QK, the temperature t 2 at the pressure port of the pump and the vapor pressure of the vaporous substances at this temperature must be known. The temperature t 2 can therefore not be calculated directly from equation (19). In practice, this temperature is determined with the help of an iterative calculation. For this, a relationship between temperature and vapor pressure in the form of an equation is required [2].

35 34 Liquid ring vacuum pumps and compressors Figure 19 shows as an example the temperature difference t = t 2 - t 3 as a function of the suction pressure for a liquid ring vacuum pump when conveying dry air saturated with water vapor at t 1 = 20 C. Fig. 19: Temperature difference t = t 2 - t 3 for a liquid ring vacuum pump when pumping dry air and air saturated with water vapor Law of similarity The law of similars states that if the vacuum pumps or compressors are geometrically similar, the same pressure ratio p 2 / p 1 and With the same k-value, the compression process is similar, and thus the wheel utilization and the isothermal efficiency are the same. The wheel utilization is understood to mean the volumetric efficiency for an impeller: Wheel utilization V 1 1 λ R = (20) 60 π d² b n 4 d b n = impeller diameter in m = impeller width in m = speed of the impeller in 1 / min

36 Operating behavior 35 The isothermal efficiency is the quotient of the isothermal compression output and the power requirement of the pump. Equation (21) gives: p 2 p 1 V 1 ln p 1 η is = (21) 3, P isothermal efficiency p 1 p 2 V 1 P = suction pressure in mbar = compression pressure in mbar = suction volume flow in m3 / h = Power requirement in kw The k-value is the quotient of the isothermal compression power and the power of the rotating liquid ring and thus a relative measure of how far the energy of the liquid ring is consumed for the compression power: p 1 k = 10 2 ρ 3 (22) 2 u² k-value p 1 p 3 u = suction pressure in mbar = density of the operating liquid in kg / m3 = circumferential speed of the impeller in m / s In equation (22) the suction volume flow is reduced against the volume flow of the liquid ring and the natural logarithm from the Pressure ratio p 2 / p 1 set equal to 1. Operating modes The operating mode here means the operating fluid flow outside the pump.

37 36 Liquid ring vacuum pumps and compressors Figures 20 to 22 show single pumps and liquid separators as floor-standing separators. The illustrations also apply accordingly to other types of liquid separator and when several pumps are connected to one separator. There are three different operating modes, which are briefly explained below. Combined liquid operation The combined liquid operation is the usual operating mode under normal circumstances. The fresh liquid flow is limited to what is necessary to dissipate the heat. The fresh liquid requirement is low.The structure is shown in Figure 20: The operating liquid B consists of a mixture of fresh liquid F and circulating liquid U. The fresh liquid is taken from a liquid network (e.g. the water pipe), the circulating liquid from the liquid separator. Before entering the operating fluid connection, the two streams U and F are mixed to form stream B. The required fresh liquid flow F is calculated according to the following equation: Fresh liquid flow t 2 - t 3 F = B (23) t 2 -t 4 F = fresh liquid flow in m³ / h B = operating liquid flow in m³ / ht 2 = temperature at the pressure port in C t 3 = Temperature of the operating liquid in C t 4 = temperature of the fresh liquid in C The discharge liquid A is the sum of the following flows: Fresh liquid flow F, the vapor components of the pumped medium condensing in the pump

38 Operating modes 37 and any liquid components of the flow rate that may be present. If the compression pressure is higher than atmospheric pressure, the drainage liquid must be discharged via a liquid drain, or control devices on the liquid separator are required to keep the liquid level in the required range. Circulating liquid operation Fig. 20: Combined liquid operation A Drain liquid B Operating liquid F Fresh liquid MI Conveying medium suction side M II Conveying medium pressure side PG Liquid ring vacuum pump XBp Liquid separator U Circulating liquid h F Shut-off valve i F Regulating valve l B Operating liquid line l F Fresh liquid line m U Liquid standpipe t Thermometer t MI check valve u B Operating fluid connection uc cavitation protection ue drainage ul connection for venting valve u MI suction line connection u MII pressure line connection u ml connection for drain valve u se dirt drainage u U circulating fluid connection The circulating fluid operation is used with corrosive, sewage-damaging or health-damaging pumping media as well as for recovering the condensate.

39 38 Liquid ring vacuum pumps and compressors Fig. 21: Circulating liquid operation A Discharge liquid B Operating liquid K Cooling liquid MI Conveying medium, suction side M II Conveying medium, pressure side PF Liquid pump PG Liquid ring vacuum pump XBp Liquid separator b K Heat exchanger h K Shut-off valve IK Regulating valve IU Regulating valve l B Operating liquid line with Manifold B cooling liquid U liquid standpipe t thermometer t MI check valve u A liquid drain u B operating liquid connection uc cavitation protection ue drainage ul connection for ventilation valve u MI suction line connection u MII pressure line connection u ml connection for drain valve u se dirt drain u U circulating fluid connection Figure 21 shows the structure. In this operating mode, the liquid separated from the pumped medium in the liquid separator is reused as operating liquid. The liquid in the pump unit is therefore repeatedly circulated. To cool the liquid heated during the compression process, a heat exchanger must be connected to the circulating liquid line during continuous operation. The flow resistance of the heat exchanger must be low if there is no liquid pump in the circulating liquid line.

40 Operating modes 39 The heat flow to be dissipated through the heat exchanger is calculated according to equation (17). A liquid pump must be provided in the circulating liquid line to increase the pressure if the operating liquid has a high viscosity (> 2 mm² / s) or if the liquid ring gas pump is operated with a low pressure difference between the compression and suction pressure. Components condensing in the liquid ring gas pump and liquid components of the pumped medium emerge from the liquid separator as drainage liquid. If the compression pressure is higher than atmospheric pressure, the drainage liquid must be discharged via a liquid drain, or control devices are required on the separator to keep the liquid level in the required range. Pump to increase pressure Checking the liquid level If the exhaust gas from the separator (pumped medium on the pressure side M II) contains a higher vapor mass flow than the pumped medium MI entering the pump, the difference in the flows must be replenished to prevent the liquid level in the unit from dropping to to prevent inadmissible height. It must be avoided that gas enters the circulating liquid line. Fresh liquid operation The fresh liquid operation is used if the reuse of the operating liquid as such is not important. All of the operating fluid required to maintain operation is fed into a fluid network, e.g. taken from the water pipe.

41 40 Liquid ring vacuum pumps and compressors Fig. 22: Fresh liquid operation A Discharge liquid B Operating liquid F Fresh liquid MI Conveying medium suction side M II Conveying medium pressure side PG Liquid ring vacuum pump XBp Liquid separator h F Shut-off valve i F Regulating valve IB Operating liquid line IF Fresh liquid line MI B Pressure gauge check valve m U Liquid level pipe t u A liquid drain u B operating liquid connection uc cavitation protection ue drainage ul connection for ventilation valve u MI suction line connection u MII pressure line connection u ml connection for drain valve u se dirt drain The structure is shown in Figure 22. The drainage liquid A is the sum of the following flows: Fresh liquid flow F, in which Pump condensing vapor and any liquid components of the pumped medium. If the compression pressure is higher than atmospheric pressure, the drainage liquid must be drained off via a liquid drain, or control devices on the liquid separator are required to keep the liquid level in the required range. The liquid separator can be dispensed with if the pumped medium

42 drive 41 and the liquid do not have to be discharged separately. Drive motor drive Liquid ring gas pumps are normally driven by three-phase electric motors, the degree of protection of which is adapted to the operating conditions. High-voltage motors are used for high outputs. Other prime movers, such as internal combustion engines and steam turbines, can also be used. Direct drive with electric motor Torque transmission Liquid ring gas pumps have a relatively uniform torque. They therefore do not place any exceptionally high demands on the torque transmission. For small and medium-sized pumps, the direct drive from the motor via a flexible coupling dominates. In the case of large units that run at a relatively low speed, a four-pole electric motor is usually used and a gear is connected between the motor and the pump, which rotates the motor speed to the pump speed. Gear drives, V-belt drives and flat belt drives are used as the gearbox. Belt drive is possible Belt drives exert a load acting as a radial force on the shaft and its bearings, which is dependent on the power transmitted, the speed and the diameter of the belt pulley as well as the pretensioning force of the belt. Therefore, and in order not to impair the service life of the belts, a minimum diameter must be specified for the pulley.

43 42 Liquid ring vacuum pumps and compressors Fig. 23: Pumping station with a Roots vacuum pump and a liquid ring vacuum pump Since a single-acting stage of a liquid ring gas pump exerts a radial force on its shaft, the direction in which the belt is pulled is important. Belt pull can increase the bearing load and thereby decrease the shaft deflection, or it can decrease the bearing load and increase the shaft deflection. Inertial moment and load moment The moment of inertia and the load moment of the vacuum pumps and compressors are relatively low. The moment of inertia is a machine-specific quantity. The load torque at start-up depends not only on the size of the pump but also on the liquid level in the pump, the type of liquid and the pressures in the suction and discharge ports that are reached during the start-up process.

44 Regulation of the suction volume flow 43 Start-up Direct activation is generally preferred. Motors with star-delta starting can be used if a particularly low starting current is required. However, it must be checked whether the torque of the motor running in star connection is large enough. By using an automatic soft starter, the current surge that occurs when an electric motor is switched on can be avoided and the drive starts up with an almost constant torque. Soft starters are also used as a starting aid for driving large liquid ring gas pumps that are equipped with magnetic couplings. The gentle start-up prevents the magnetic coupling from "tearing off". Direct activation is preferred. Switching frequency The permissible switching frequency depends more on the drive than on the pump. Electric motors heat up a lot during the start-up process due to high starting currents. Clutches are heavily stressed by the acceleration of the rotating pumps and any gear parts that may be present. As a rule, a switching frequency of 15 switchings per hour is considered permissible. In special cases, the motor and clutch manufacturers should be consulted. When starting up large pumps, the load on the power grid must also be taken into account. High switching frequency Regulation of the intake volume flow Rising energy costs force the operators of vacuum pumps and compressors to consider profitability with the aim of reducing operating costs to a minimum.

45 44 Liquid ring vacuum pumps and compressors If the volume flow is not regulated, the suction pressure is automatically set at which the process volume flow and the suction volume flow of the liquid ring gas pump are the same. This can result in excessive energy and cooling water consumption. Figure 24 shows how the suction pressure of a liquid ring vacuum pump changes when the process volume flow changes. Fig. 24: Change in suction pressure when the process volume flow changes. Energy saving In the following, the usual types of control for liquid ring gas pumps are explained, with which the suction volume flow can be adapted to the operating conditions. Attention is also drawn to energy-saving measures. Speed ​​control With speed control, the intake volume flow can be adapted to the operational requirements and energy can be saved. When selecting the speed, however, limit values ​​must be taken into account.

46 Regulation of the suction volume flow 45 The minimum speed results from the impeller peripheral speed required for the compression output. It should be noted that the maximum compression output of a vacuum pump is at a suction pressure of approx. 400 mbar when the compression pressure is 1013 mbar (see Figure 15). The maximum speed is determined by the load capacity of the rotating components, in particular the shaft and the impeller. Minimum speed Maximum speed A change in speed not only changes the compression output but also changes the power loss. On average, with liquid ring vacuum pumps there is an overlinear dependence of the power requirement on the impeller peripheral speed and thus on the speed. In order to save energy, the lowest possible speed should be aimed for. The range in which the intake volume flow can be regulated via the speed is in the order of magnitude between 50 and 100 percent of the maximum intake volume flow. In practice, however, the control range is usually much smaller because the existing liquid ring gas pumps do not optimally suit the respective operating cases. Control range Control of the operating liquid temperature The costs for fresh or cooling liquid are a significant part of the operating costs. The selection of the suitable operating mode and the creation of the greatest possible difference between the fresh liquid temperature and the cooling liquid temperature in connection with a regulation of the fresh or cooling liquid flow

47 46 Liquid ring vacuum pumps and compressors make it possible to limit operating costs to what is absolutely necessary. Automatic adjustment of the fresh liquid flow It must not be disregarded that the suction volume flow of a liquid ring vacuum pump is also dependent on the vapor pressure of the operating liquid and thus on its temperature. A one-time adjustment of the fresh liquid flow does not meet the high demands on the savings effect; an automatic adjustment of this current must take place. In practice, thermostatic control valves have proven themselves. In the case of frequently changing suction pressures, a regulation that is dependent on the suction pressure and therefore more complex is the better solution. Saving liquid by reducing the operating liquid flow is not recommended because the suction volume flow decreases in an uncontrolled manner and impermissible vibrations of the liquid ring gas pump can occur. Parallel connection of liquid ring gas pumps Increased availability By dividing the process volume flow over several pumps, an extensive adaptation to the operating requirements is achieved. In addition to the control effect, the availability of the system is also increased, because if a pump fails, it can continue to work with a reduced volume flow. Gas circulation control (bypass control) Gas circulation control is the most frequently used type of control.It can be used to set the process volume flow from zero to the maximum value. There is no energy saving associated with this type of control.

48 Control of the suction volume flow 47 Fig. 25: Gas circulation control (bypass control) Figure 25 shows a gas circulation control: The gas that is pumped too much by the liquid ring gas pump is installed between the gas outlet line of the separator and the pump suction line and equipped with a control element returned to the suction line. This gas circulation line must under no circumstances be laid between the pressure line that carries the gas-liquid mixture and the suction line of the pump, because then a gas-liquid mixture of undefined composition would flow into the suction nozzle. If it is possible to mix the conveyed gas with air in liquid ring vacuum pumps, the gas conveyance can be reduced by adding air to the pump suction nozzle. Construction versions There are various construction versions for liquid ring gas pumps, which differ in terms of the number of pump stages, shaft bearings, shaft seals, the materials of the components and the static test pressures.

49 48 Liquid ring vacuum pumps and compressors Number of stages Decisive for the number of stages required in a pump are the possible compression capacity of the stages as well as their operating behavior and economic efficiency. Liquid ring vacuum pumps are designed in one or two stages. Liquid ring compressors for absolute compression pressures up to about 3.5 bar usually have one or two stages. Higher compression pressures are achieved with two or more stages. Shaft bearing Fig. 26: Longitudinal section of a liquid ring vacuum pump in motor mount design A shaft usually has two radial bearings. These are either arranged on both shaft ends or both bearings are on one side. Figures 4 to 7 can be viewed as examples of the two-sided storage. Figures 26 and 27 show designs where both bearings are on one side of the pump. The arrangement according to Figure 26 is used as a bearing block or motor support pump and the construction according to Figure 27 as an engine block pump

50 Construction versions 49 Fig. 27: Longitudinal section of a liquid ring vacuum pump in engine block construction. Motor block pumps use the motor shaft and its bearings as a pump shaft and as a pump bearing. Shaft seals The type of seal is selected according to the required tightness. Compared to other compressor types, liquid ring gas pumps have the advantage that liquid is available at the shaft seals and this can be used as a barrier or flushing liquid, for the lubrication of sliding partners and for dissipating frictional heat. Usually single and double mechanical seals as well as single and double stuffing boxes are used for liquid ring gas pumps. Adaptation to the operating case Construction designs without shaft seals If liquid ring gas pumps are used to convey toxic, carcinogenic and malodorous gases and vapors or to compress radioactive media, extraordinarily high tightness requirements must be met, which can only be achieved with construction without shaft seals. The drive takes place via a magnetic coupling. High tightness requirements

51 50 Liquid ring vacuum pumps and compressors Fig. 28: Principle of a magnetic coupling or a canned motor. Both versions have the can and a shaft guided in plain bearings as design features. Magnetic couplings essentially consist of two magnet carriers, one of which is firmly connected to the shaft of the pump and the other is firmly connected to the shaft of the drive. The can is located between the two magnet carriers. This thin-walled tube is tightly attached to the pump housing on one side and closed on the other side (it is therefore also known as a containment can). Fig. 28 illustrates the principle. Construction without shaft seals Fig. 30: (opposite) liquid ring vacuum pumps with Roots pumps The magnetic materials used are rare earth cobalt and rare earth iron alloys, the outstanding properties of which are high energy density and high magnetic stability. Eddy current losses occur in metallic cans, which cause the can to heat up. How much heat is lost depends on the installed magnet mass and the speed of the clutch. Operating fluid flows around the inner magnet carrier (rotor) to dissipate the heat. Figure 29 shows the longitudinal section of a two-stage liquid ring vacuum pump with magnetic coupling. Two radial bearings and one axial bearing guide the shaft. The bearings will

52 construction versions 51 supplied with operating fluid for lubrication and heat dissipation. In difficult start-up conditions, e.g. If the operating fluid has a high density, an automatic soft starter is used, which prevents the magnet from tearing off during the starting process. It is recommended to use the Fig. 29: Longitudinal section of a two-stage liquid ring vacuum pump with magnetic coupling

53 52 Liquid ring vacuum pumps and compressors Fig. 31: Longitudinal section of a liquid ring compressor with a canned motor for extremely high tightness requirements Switch off the drive of a magnetic coupling immediately after it has broken off in order to prevent damage to the components. Figure 31 shows a liquid ring compressor with a canned motor for extremely high tightness requirements. Materials of the components Liquid ring gas pumps are manufactured in different material combinations in order to guarantee an adaptation to the operational requirements (see appendix). The materials of the accessories, such as liquid separators, pipelines and pipeline parts, are adapted to the respective basic designs. Static test pressures When pumping ignitable substances (gases or vapors with explosive

54 Emissions 53 Properties), pressure shock-resistant constructions are used, which are characterized in particular by a high static test pressure. Emissions Sound A working pump generates vibrations and some of these vibrations are emitted as sound. Sound sources are the moving solid, liquid and gaseous parts and substances. From the sound sources, the sound is conducted as gas sound and as structure-borne sound to the housing or to the walls of the pump, and from there as air-borne sound and as structure-borne sound to the environment. The speed of the liquid and gaseous substances flowing in the pump depends on the speed of the impeller and the suction or Compression pressure of the pump dependent. These variables therefore indirectly influence the emitted sound power. There is a relationship between sound power and sound pressure via the size of the sound-emitting surface. Sound sources Sound power and sound pressure National and international standards on sound measurements (e.g. DIN EN and ISO 1996) contain regulations on measurement techniques and measuring devices. Figure 32 contains measured values ​​of the sound pressure levels from two liquid ring vacuum pumps of different sizes. The peaks of the curves are at a frequency that can be traced back to the product of the speed and the number of blades (number of blades on the impeller). The designation NR stands for "Noise Rating". Low sound pressure level

55 54 Liquid ring vacuum pumps and compressors Figure 32: Measured values ​​of the sound pressure level of liquid ring vacuum pumps The operating data of these pumps and the A-weighted sound pressure and sound power levels are listed in Table 1. Tab. 1: Operating data and A-weighted sound pressure and sound power level Pump size I II Suction pressure mbar Compression pressure mbar Power requirement kw 9.5 64 Speed ​​rpm Sound pressure level db (a) Sound power level db (a) Vibrations The size and permissibility of vibrations are assessed according to the VDI guideline This document contains information on measuring devices, the installation of the object to be tested and

56 Emissions 55 Assessment of vibration severity. The vibrations generated by liquid ring gas pumps result from imbalances in the rotating parts and from the sudden changes in pressure in the impeller cells and the pressure chambers when an impeller cell sweeps over the pressure opening. The points at which vibration energy is transmitted to other parts are preferably selected as measuring points. These are the bearings, the feet and the connection flanges of the machines. The VDI guideline 2056 contains a division of the test objects into groups. They differ in that the permissibility of vibrations increases with the size of the machine. Figure 33 shows the measured values ​​of a medium-sized liquid ring vacuum pump, the vibrations are low. Fig. 33: Measured values ​​of the vibration speed of a liquid ring vacuum pump