Wärmehaushalt von Schraubenspindel-Vakuumpumpen

Abstract

For dry running displacement vacuum pumps the abolition of cooling, sealing and lubricating fluids causes the problem of an increased thermal load. The thermodynamics of this process differ fundamentally from the thermodynamics of an air compressor. The former are characterised by a - usually - unadjusted pressure ratio, the oscillation of the working fluid and the periodic mixing process of the working gas (that has already been expelled) and the gas in the working chamber. This process results in adiabatic outlet temperatures that are distinctly higher than the outlet temperatures of an adjusted isentropic process. Lower suction pressure results in a discrepancy between the adiabatic model and reality, which is the reason why a calculation of the diabatic process is necessary. In support of a isothermal model the dependence of the working behaviour on both the speed and the gap height of the screw vacuum pump can be demonstrated, but it does not allow conclusions to be drawn about the thermal load of the gas and the pump. This work describes additional modules of the simulation program Kasim that has been developed as a solution to this problem. The lack of Nußelt equations for rotating systems in vacuum results in the application of heat transfer models of the overpressure range in order to calculate the heat transfer of those systems. Pressure-dependent substance properties have been based on gas cinematic models. Apart from these further models were investigated in order to determine the flow velocity and state variables of the working gas at the heat transferring areas.Initially, the simulation program was applied to a cold model machine which consisted of a srew vacuum pump with constant pitch. Compared with the steady state machine, the cold machine had the advantage of known boundary conditions. The large differences between simulation and experiment cannot be explained either by the sensitivity of the heat transfer models or the property models, or by the polytropic exponents of the expansion flow in the gap. Measurement technique appears to be the most likely explanation.That is the reason why a further investigation was carried out on the steady state machine. This included also body temperatures and an energy balance. In order to find out about body temperatures, a module was developed which allowed the surface elements of the finite element mesh of the pump to be allocated to the capacities and connections of the thermodynamic simulation system. After the boundary conditions had been defined, this assignment formed the basis for the calculation of the heat fluxes, based on a homogenous, isothermal machine whose temperature was to be re-determined by the thermal FE-calculations of its parts. This allowed a calculation of the steady state heat balance to be carried out iteratively.The verification was successful. The calculated heat flows of the machine parts were acceptably close to the experimental energy balance. Moreover, both the body temperatures and the maximum gas temperatures during the working process displayed a high degree of concordance.The present study proves that both the adaptation of heat transfer models of the overpressure range in the rotating system of a vacuum pump and the implementation of gas cinematic property models results in a good representation of the working behaviour. At the same time, it offers the possibility of further improvements. Moreover, it opens up the perspective of a completely closed simulation of rotating displacement vacuum pumps.