Reduction of the suction pressure of a liquid ring vacuum pump with a supersonic gas ejector

Transcription

1 MATEC Web of Conferences 68, 000 (08) XXI. AEaNMiFMaE-08 htts://doi.org/0.05/matecconf/ Reduction of the suction ressure of a liquid ring vacuum um with a suersonic gas ejector Róbert Olšiak,*, Zoltán Fuszko, and Zoltán Csuka Slovak University of Technology, Faculty of M.E., Institute of energy machinery, Námestie slobody 7, 8 Bratislava, Slovakia Abstract. A suersonic gas ejector in conjunction with a liquid ring vacuum um is used for creating and maintaining vacuum in a chamber for technological uroses. In this aer the authors submit an overview about the roblematics of suction ressure reduction with a suersonic gas ejector used as a re-stage of a liquid ring vacuum um. This system has also the function of a cavitation rotection due to the higher ressure resent at the suction throat of the vacuum um. A art of this aer is devoted to the governing equations used at the definition of the flow through an ejector. The exerimental studies are then carried out in or own laboratory for verification uroses. Introduction The vacuum ums are technological devices used to create and maintain vacuum for technological uroses. In ractice the vacuum means the state of diluted gas, which absolute ressure is lower than the atmosheric. Liquid ring vacuum ums can be used at extraction of condensated vaor and gas mixtures. In this case a art of the umed mixture is condensating in form of vaor inside the vacuum um, while the condensate is drained with the working fluid. At the other vacuum um tyes this henomenon is undesirable due to the resent lubricating grease. The relatively low efficiency and the high minimal suction ressure can be considered as a disadvantage. This minimal ressure is limited by the used working fluid inside the vacuum um. Is roblematic to determine this correlation, because the arameters of the liquid ring vacuum um are bounded not only with the tye of used fluid (density, viscosity) but with the change of these roerties in deendence on the temerature and ressure also. The barrier of the minimal suction ressure, at which the vacuum um can be used, is deending on the fluid roerties, construction tye, number of stages etc. This ressure is according to the following authors stated as: Bannwarth 5 kpa, Faragallah 5 kpa, Szabó 4-4 kpa. In the ractice is a common requirement to reach a suction ressure, which is much lower than the listed values. This is not ossible due to the occurrence of cavitation inside the vacuum um. This can be artially solved by lowering the ressure ratio, so using a * Corresonding author: robert.olsiak@stuba.sk The Authors, ublished by EDP Sciences. This is an oen access article distributed under the terms of the Creative Commons Attribution License 4.0 (htt://creativecommons.org/licenses/by/4.0/).

2 MATEC Web of Conferences 68, 000 (08) XXI. AEaNMiFMaE-08 htts://doi.org/0.05/matecconf/ multistage liquid ring vacuum um, however even this solution is unaccetable from the viewoint of long-term run. The suction ressure can be lowered using a gas ejector as a re-stage of the vacuum um. This arrangement can reduce the ressure u to /5 of the initial ressure. The otions of the suction ressure reduction The ejector as a technological device is due to the simle construction and oerational reliability are widely used at different alications. It is basically a device, in which the rimary medium entrains the secondary driven medium, accelerates it to suersonic seed at reduced ressure. The ressure measured at the ejector inlet is decisive to achieve the design ressure in the considered ressure chamber. From the viewoint of ejector design, there are some limiting arameters (Figure ): ressure measured at the ejector outlet, mass flow of the mixture m, which are corresonding to the occurrence of cavitation inside the vacuum um. The values of the rimary (driving) fluid and m are used at the design of the rimary suersonic nozzle. In case that the ejector is correctly designed, it will extend the working range of the liquid ring vacuum um as seen on the figure below. Fig.. The construction of a suersonic ejector (left) and the erformance characteristics of a liquid ring vacuum um with and without a suersonic ejector re-stage. From this simle analysis results that for the correct design of a suersonic ejector is necessary to know the following arameters: The required minimal suction ressure at the chamber throat. The required mixture mass flow m at the chamber throat. The minimal suction ressure of the vacuum source. It is obvious that the ressure and mass flow m are the chosen arameters of the designed ejector, the suction ressure and mass flow m can be secified exerimentally, which is the most commonly used method, or with theoretical calculation according to the geometrical dimensions of the device, oerational data and roerties of the working fluid. The governing equations The mostly used ejector design method is based on a -dimensional analysis of comressible gas flow. This method was first time roosed by Keenan (94) and Neumann (950) and it uses the following assumtions: The kinetic energy on the inlet of both fluids is negligible. The two working fluids are considered as ideal gases. At the end of the mixing chamber are both of the fluid uniformly mixed, without occurring shocks inside the mixing chamber.

3 MATEC Web of Conferences 68, 000 (08) XXI. AEaNMiFMaE-08 htts://doi.org/0.05/matecconf/ Based on these assumtions the equations describing the character of the flow from the inlets to the outlet can be given as follows []: -the momentum equation -the continuity equation A Q v A Qv A Qv () Q Q Q () -the energy equation for adiabatic flow v v v m h m h m h () The required comression ratio (CR) of an ejector determines the ratio of the static ressures on the ejector exit (identic to the ressure measured at the inlet throat of the vacuum um) to the ressure measured at the secondary suction throat of the ejector. The entrainment ratio (ER) [] exresses the relationshi between the desired mass flow rates through the secondary and rimary inlets of the ejector. CR, m ER m where: f M RT 4 Design method A T A T fb, M f, M m, 0 M M A The motion of the working fluid is given by the ressure difference between the atmosheric air at the rimary inlet and the minimal suction ressure given by the vacuum um. Due to the small molecular weight of the used working fluid (atmosheric air), the value of the kinetic energy of the rimary fluid deends on the outlet seed of the suersonic nozzle. For this reason is the nozzle considered as the most imortant art of an ejector. The mostly used nozzle geometry due to his simle design is the conical, with constant wall angle ( -8 ). Since the outlet velocity field has a radial comonent also, it is necessary to introduce a correction factor, which takes into account the ratio of the ideal momentum of the conical nozzle, with the axial comonent of the velocity. Parabolic de Laval nozzles take into account the resence of Prandtl-Mayer exansion. The shae of the nozzle is obtained either by an emirical method, or by the method of characteristics (MoC) (Figure ). The Prandtl-Meyer formula (6) is used to determine the Mach number at a given oint of the flow field followed by the Mach angle calculation (7). arcsin M M arctan M arctan M a (4) (5) (6) (7)

4 MATEC Web of Conferences 68, 000 (08) XXI. AEaNMiFMaE-08 htts://doi.org/0.05/matecconf/ Fig.. The divergent ortion of a arabolic de Laval nozzle with characteristic lines and distribution of the observed arameters (left), the velocity comonents with the angles used by the method of characteristics. These equations alongside with the equations of quasi one-dimensional-isentroic flow of comressible fluids in ducts was an automated software develoed []. The flowchart of the calculation can be seen on the following figure. Fig.. Flowchart of the automated calculation rocess. The high seed mixture of the rimary air and secondary working fluid is entering the mixing chamber. The -D design model uses the constant-area mixing model, for which is the suersonic nozzle exit located directly on the cross section lane of the mixing chamber inlet (Figure 4). Fig. 4. Constant-area-mixing chamber. With use of the ER alongside with equation () and the known flow rate at the diffuser outlet, the velocity at the exit of the mixing chamber can be obtained: v mch, ex mixing m v m v m m (8) 4

5 MATEC Web of Conferences 68, 000 (08) XXI. AEaNMiFMaE-08 htts://doi.org/0.05/matecconf/ The exerimental setu The exerimental station consists of main arts: a two-stage suersonic ejector and a liquid ring vacuum um. The ejector stages are connected in series, so the outlet of the first stage is connected to the secondary suction of the second stage. The two-stage ejector is then added as a re-stage to the vacuum um as shown on the following figure. Fig. 5. The exerimental setu (installed sensors and meters: m-flow rate, T-temerature, -ressure). The used liquid-ring vacuum um tye SZO is designed to extract air, chemically active and non-active vaors in metallurgical, chemical and food industry. The um consists of a stationary housing in which is an eccentrically rotating imeller laced. The working liquid is fed through a searate circuit to the um, which by the centrifugal force creates a cylindrical ring inside the casing which acts as a liquid iston. The gradually reducing volumes of the individual interblade chambers are comressing the umed air, which reorts to the discharge ort of the um. The working fluid is along with the umed gas entering the searator, where the extraction of heated fluid and condensate occurs. As the first ste the digitization of the signals from the installed sensors was made [4]. Since the signal in the digital form has an electrical value, it was necessary to convert them to the values of the actual measured arameters. This ste was done in the LABView environment with the known ranges of measuring sensors. The exeriment has been made in hases: at first was the vacuum um itself measured, than the suersonic ejector was added to comare the characteristics of the both setus [5]. The exeriment begins by running the um of the external circuit to constantly deliver water to the um. As the housing of the vacuum um fills u to the desired level, the oerating seed was set with the frequency converter. The shut-off valve located on the motive fluid inlet of both stages is fully oened. After the stabilization of the monitored values, the first oint of the Q- diagram was recorded. By gradually closing the valve on the secondary inlet ort was the suction ressure reduced. At each sto was the actual oint data recorded. The measured Q- values were then imorted to MS Excel, aroximated, than the characteristics were lotted as shown on the following figure. 5

6 MATEC Web of Conferences 68, 000 (08) XXI. AEaNMiFMaE-08 htts://doi.org/0.05/matecconf/ Fig. 6. Comarison of erformance characteristics of the liquid ring vacuum um with and without the suersonic ejector at 400 RPM. 6 Evaluation of the obtained exerimental results At the analysis of the rojected ejector erformance characteristics should be remembered, that the erformance is not given by the flow rate of air entering the suction throat of the vacuum um. In case of an ejector used as a re-stage, the flow rate of air measured at the secondary suction ort (inlet no. from Figure ) can be considered as effective erformance. This amount of air is exhausted from the vacuumed chamber. Also, the effective suction ressure is referred analogically to the secondary suction ort. From the comarison of the erformance characteristics is evident, that the added ejector had a ositive imact on the erformance of the whole vacuum system. At lower suction ressures (arox. under 0 kpa abs.) is a significant erformance increase resent. The added ejector also extended the working range of the vacuum um, so the vacuum system can be oerated in a wider range of suction ressures in cavitation-free regime. References. Ch. Liao, Gas ejector modeling (dissertation thesis, Texas, 008). P. Vician, M. Palacka, M. Holubčík, Effect of ext. tem. on erformance of bin dryer, AIP Conference Proceedings, 889, (06). M. Puškár, T. Brestovič, N. Jasminská, Num. sim. and ex. analysis of acoustic wave influences on brake mean eff. ressure in thrust-ejector inlet ie of combustion engine, Int. Journal of Vehicle Design, 67, 6-76 (05) 4. J. Jablonská, M. Mahdal, M. Kozubková, Sectral an. of ressure, noise and vibration velocity measurement in cavitation, The Journal of Institute of Measurement Science of Slovak Academy of Sciences, 7, (07) 5. S. Daneshmand, C. Aghanajafi, A. Bahrami, Analytical and exerimental methods of design for suersonic two-stage ejectors, Int. Journal of Aerosace and Mech. Eng.,, (009) 6