The modern trend in compressor industry is an extension of the use of multi-shaft centrifugal compressors. Multi-shaft compressors have a number of advantages over single-shaft. The design of such compressors gives opportunity to use an axial inlet for all stages and select the optimum rotational speed for each pair of impellers, which, along with the cooling of the gas after each stage, makes possible to achieve high levels of efficiency. The design of high-efficiency centrifugal compressor stages can be performed on the basis of highly effective stage elements. Such elements are: impellers with spatial blades, vaned and channel diffusers with given velocity distribution. In this paper, impellers with axial-radial blades are considered. The blade profile is determined by the specified pressure distribution along the blade. Such design improves the structure of the gas flow in the interblade channels of the impeller, which leads to an increase in its efficiency. Characteristics of loss coefficients from attack angles for impellers were obtained experimentally. Vaned and channel diffusers, the characteristics of which are given in this article, are designed with the given velocity distribution along the vane. Compared to the classic type of diffuser, such diffusers have lower losses and a wider range of economical operation. For diffusers as well as for impellers, characteristics of loss coefficients from attack angles were obtained. High efficient impellers and diffusers and obtained gas-dynamic characteristics were used in the design of a multi-shaft compressor unit for the production of liquefied natural gas. The initial pressure of the unit is 3bar. The obtained characteristics of loss coefficients from attack angles for the considered impellers and diffusers make it possible to calculate the gas-dynamic characteristics of high-efficient centrifugal compressors stages. The high-efficient centrifugal compressors stages can be designed using high-efficient elements, such as: impeller with spatial blades and vaned diffuser with given velocity distribution.
Gas-dynamic characteristics of the compressor make it possible to evaluate its energy and economic properties, to predict the values of capacity, the generated gas pressure and the power consumption during the compressor operation. For more in-depth consideration of the compressor, it is desirable to have the characteristics of its individual stages. The element-by-element analysis of the characteristics of each stage makes it possible to improve the coordination of the operation of the individual elements with each other and thereby improve the gas-dynamic characteristics of the compressor. The loss factor and the static pressure recovery factor can be used as the values characterizing the properties of the individual elements of the stage. Coefficients are suitable for evaluating the energy properties of any element of the stage. To assess the effect of the element in question on the economy of the stage, it is necessary to establish what proportion of the work required for compression is the "loss" of energy in a given element, i.e. find the reduction in efficiency stage due to dissipation of energy into heat in this element. Calculation of performance of the centrifugal compressor is performed from the inlet to the outlet using the equations of state, of process, of continuity and conservation of energy. The initial data are geometric parameters of the compressor, the composition and the initial parameters of compressed gas, the rotational speed of the rotor. The basis of the elementwise calculation of gas-dynamic characteristics is the gas-dynamic characteristics of the stage elements. The calculation can be performed using the characteristics of the stage elements taken from the own bank of experimental data or using the generalized characteristics of the stage elements. To obtain generalized characteristics of the impeller, blade and no-blade diffusers, reverse guide vanes, experimental data were used, published in the works of Galerkin, Den, Rees, Seleznev and others, as well as experimental data obtained by the author. The generalized characteristics are obtained in the form of analytical dependences of the loss coefficients on the angles of attack or flow angles by approximation of experimental data. These dependences were used to analyze the gas-dynamic characteristics of a centrifugal compressor, which made it possible to develop recommendations for their improvement.
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