Mašiny i Ustanovki: Proektirovanie, Razrabotka i Èkspluataciâ (Jun 2016)
Calculating Characteristics of the Screws with Constant And Variable Step
Abstract
This work is devoted to creating a technique for calculating power characteristics of the screws with constant and variable step for the centrifugal pumps. The technique feature is that the reverse currents, which are observed in screws working at low flow, are numerically taken into account. The paper presents a diagram of the stream in the screw with flow to the network Q=0, and the static pressure of the screw in this mode is computed according to reverse current parameters. Maximum flow of screw is determined from the known formulas. When calculating the power characteristics and computing the overall efficiency of the screw, for the first time a volumetric efficiency of the screw is introduced. It is defined as a ratio between the flow into the network and the sum of the reverse current flows and a flow into the network. This approach allowed us to determine the efficiency of the screw over the entire range of flows.A comparison of experimental characteristics of the constant step screw with those of calculated by the proposed technique shows their good agreement.The technique is also used in calculating characteristics of the variable step screws. The variable step screw is considered as a screw consisting of two screws with a smooth transition of the blades from the inlet to the outlet. Screws in which the step at the inlet is less than that of at the outlet as well as screws with the step at the inlet being more than that of at the outlet were investigated. It is shown that a pressure of the screw with zero step and the value of the reverse currents depend only on the parameters of the input section of the screw, and the maximum flow, if the step at the inlet is more than the step at the outlet, is determined by the parameters of the output part of the screw. Otherwise, the maximum flow is determined a little bit differently.The paper compares experimental characteristics with characteristics calculated by the technique for variable step screws, thereby demonstrating good compliance.Results of the research allowed us to draw the following conclusions.1. Taking into account the reverse currents enabled us to refine the calculation of power characteristics of the constant and variable step screws over the entire range of flows.2. A program developed on the basis of the proposed technique enables us to calculate screw characteristics from their geometrical parameters and frequency of rotation. This is very convenient when performing the design calculations of the flow part of the screwcentrifugal pump.3. If the screw-set pump is intended for use in the continuous flow mode, it is advisable to set either constant step screws S1=S2 or variable step screw S1<S2.4. If the pump is supposed to be used in a wide range of flows, it is advisable to install the screw with variable step S1>S2.
