CALIFORNIA POLYTECHNIC STATE UNIVERSITY Mechanical Engineering Department ME 347 Prof. Shollenberger Fall 2018 Homework #7 Reading Assignment: Chapter 10.1-10.3 Problems: 10.10, 10.28, 10.37, 10.45 (NOTES: inlet pressure reading is given as gage pressure, assume negligible major and minor losses for suction side), 10.50, 10.51 Answers: 10.10 77.3˚, 11.7 kW, 34.2 m 10.28 1.66, 761 hp, H = 276 ft - (2.18 x 10 min /ft ) Q , 102 ft, 87%, 170 hp 10.37 (a) flow rate scales poorly, head scales well, and power scales poorly with diameter, (b) all scale well with operating speed 10.45 82.2 kPa (or 8.38 m), 80.7 kPa (or 8.23 m) 10.50 2,710 gpm, L /D = 26,900 10.51 4,570 gpm, L /D = 9,970 -5 2 5 2 e e Objectives: 1. Explain the differences between positive displacement and dynamic pumps. For dynamic pumps, explain the differences between axial, radial and mixed flow. Explain the differences between fans, blowers, and compressors. 2. Draw a velocity triangle at the inlet and outlet of the rotor of a turbomachine and apply the angular momentum equation to determine the ideal torque, power output, and head of a turbomachine. 3. Calculate the hydraulic (fluid) power, mechanical power, and efficiency of an actual turbomachine and plot performance curves. 4. Explain the difference between a pump curve and a system curve. Be able to determine the operating point and best efficiency point and use it for pump selection. 5. Be able to explain how the system curve changes for changes in major losses, minor losses, and elevation. Be able to explain how the pump curve changes for multiple pumps operated in parallel or in series. 6. Calculate the dimensionless specific speed and the dimensional U.S. Customary specific speed for both pumps and turbines. Be able to use specific speed for pump selection. 7. Be able to use scaling laws to predict pump or fan performance at different rotational speeds and for different size impellers. 8. Define net positive suction head (NPSH) and explain difference between NPSH and NPSH . Be able to determine if a particular pump will experience cavitation for a given system. R A
