Die Flüssigkeitsring-Vakuumpumpe als verfahrenstechnische Maschine

Abstract

Flussigkeitsring-Vakuumpumpen werden in der verfahrenstechnischen Industrie sehr vielfaltig eingesetzt. Hierbei stehen zunachst die Forderung von Gasen und Dampfen sowie die damit verbundene Herstellung einer Druckdifferenz im Vordergrund des Interesses. Selbstverstandlich findet in Flussigkeitsring-Vakuumpumpen aber auch ein Warme- und Stoffaustausch zwischen den zu fordernden Gasen und Dampfen auf der einen und der Betriebsflussigkeit auf der anderen Seite statt. Dieser Warme- und Stoffaustausch kann gezielt fur verfahrenstechnische Grundoperationen eingesetzt werden. So konnen in diesem Pumpentyp neben den Hauptaufgaben Fordern und Verdichten auch gezielt Warmeaustausch, Kondensation/Verdampfung, Absorption/Desorption sowie chemische Reaktionen durchgefuhrt werden. Liquid ring vacuum pumps have many and diversified applications in the process industry. Primarily they are used to transfer gases and vapours from a lower to a higher pressure level. Due to the specific principles of operation, a heat and mass transfer between the gas or vapour and the sealing liquid takes place also. This can be utilised to perform unit operations. Beside transportation and compression of gases the pump can perform the following operations: exchange of heat, condensation/evaporation, absorption/desorption as well as chemical reactions. Because of these features is the liquid ring vacuum pump a „process engine” and an interesting alternative to other process apparatus. Following the definition of H. Brauer (1985) is the objective function of a process engine achieved by a moving element, whereas the objective function of a process apparatus is achieved without any moving element. For nearly all possible combinations of sealant, gases and vapours reliable empirical principles to determine the suction volume and the power requirements of liquid ring vacuum pumps are at hand. Besides has the use of liquid ring vacuum pumps to perform unit operations a long successful tradition. However, the field of applicability can be enlarged if the feasibility of the engine for individual unit Operations can be predicted with the help of physical models. Due to the eccentrically arranged rotor a sickle shaped space is formed between rotor hub and liquid ring where suction, compression and discharge of an gas/vapour mixture can be realised if the suction and discharge slots are arranged in a suitable manner. As the seal liquid is in direct contact with the gases and vapours all reactions inside the pump develop in the direction of a thermodynamical equilibrium between the two phases. An important consequence of this feature is the limit of operation of liquid ring vacuum pumps: the suction pressure has to be above the vapour pressure of the seal liquid. Another consequence is the possibility to utilise this type of pumps for unit operations. The turbulent surface of the liquid ring which is formed by a dense hale of droplets is ideal for heat and mass transfer processes. Following an attempt of C. Pfleiderer, the azimuthale distribution of the static pressure and the contour of the liquid ring can be determined under considerably simplifying assumptions. One of these simplifications is that the condensation and evaporation in the pump is neglected. The further simplifications deal with the significant idealisation of the geometry. Taking back the simplifying assumptions step-by-step and including the thermodynamical circumstances it is possible to calculate the azimuthale distribution of the static pressure and the real contour of the liquid ring for the condensation and the evaporation. These attempts can be carried on to taking several components in the vapour-portion of the gas/vapour mixture into account and form the basis of the calculation of the absorption and desorption. In this case the assumption that the thermodynamical equilibrium is attained instantly is no longer valid.