Centrifugal pump characteristic curve will be determined by the experimental Q, H, N, η and other data plotted a set of curves. This figure is provided by the manufacturer of the pump for use by the department pumping and operation reference. Different types of pumps have different characteristic curves, but all have the following three curves: (1) HQ line shows the relationship between head and flow; (2) NQ line shows the relationship between pump shaft power and flow; (3) η-Q line Pump efficiency and flow relationship; (4) The pump characteristic curves are measured at a certain speed, so note the speed n value characteristic curve. Centrifugal pump characteristic curve of the highest point of efficiency is called the design point, the pump at the corresponding pressure head and the flow of work under the most economical. Centrifugal pump nameplate performance parameters that are the most efficient point of the operating parameters. Centrifugal pump performance curve can be used as a basis for selection of the pump. After confirming the type of pump, select the pump according to the flow rate and pressure head. Example 2-2 Determining the main performance parameters of a centrifugal pump with fresh water. The measured flow rate in the experiment was 10m3 / h, the pressure gauge reading at the outlet of the pump was 0.17MPa (gauge pressure), the reading of the vacuum gauge at the entrance was -0.021Mpa, the shaft power was 1.07KW, the motor speed was 2900r / min, The vertical distance between the pressure gauge and the pressure gauge is 0.2m. Try to calculate this lift and efficiency at the experiment point. The main pump performance parameters include speed n, flow Q, head H, shaft power N and efficiency. Directly measured parameters for the speed n = 2900r / min flow Q = 10m3 / h = 0.00278m3 / s shaft power N = 1.07KW need to be calculated with head H and efficiency. Calculated by the head H, which is known: So Second, the main factors affecting centrifugal pump performance 1 Physical properties of liquids on the characteristics of the curve provided by the manufacturer to use water as a working medium measured when the other liquids , To consider the impact of liquid density and viscosity. (1) Viscosity When the viscosity of the liquid transported is greater than the viscosity of the water under the experimental conditions, the energy loss in the pump body increases, the pump flow rate, head pressure decreases, the efficiency decreases, and the shaft power increases. (2) Density Centrifugal pump volume flow and pressure head has nothing to do with the liquid density, the power increases with increasing density. 2 Centrifugal pump speed on the characteristic curve When the liquid viscosity is not large, the efficiency of the pump constant, the pump flow, head pressure, shaft power and speed can be approximated by the proportion of law, that is: Q1, H1, N1 Centrifugal pump speed n1 flow, head and power. Q2, H2, N2 centrifugal pump speed n2 when the flow, lift and power. The above set of equations is called the law of proportion. When the speed changes less than 20%, the efficiency can be considered unchanged, with the workers to calculate the error is not. If you select several points on the characteristic curve of speed n1, calculate the corresponding data of the rotational speed n2 by the law of proportionality, and plot the result on the coordinate paper, you can get the characteristic curve when the rotational speed is n2. Impeller diameter on the characteristic curve When the pump speed is constant, its head, flow and impeller diameter. The following is the law of cutting. Where: Q1, H1, N1 centrifugal pump speed at D1 flow, head and power. Q2, H2, N2 centrifugal pump speed D2 flow, lift and power.
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