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Ebook Turbomachinery Design and theory: Part 2

(BQ) Part 2 book "Turbomachinery Design and theory" has contents: Centrifugal compressors and fans, axial flow compressors and fans, steam turbines, axial flow and radial flow gas turbines, cavitation in hydraulic machinery. | 4 Centrifugal Compressors and Fans 4.1 INTRODUCTION This chapter will be concerned with power absorbing turbomachines, used to handle compressible fluids. There are three types of turbomachines: fans, blowers, and compressors. A fan causes only a small rise in stagnation pressure of the flowing fluid. A fan consists of a rotating wheel (called the impeller), which is surrounded by a stationary member known as the housing. Energy is transmitted to the air by the power-driven wheel and a pressure difference is created, providing airflow. The air feed into a fan is called induced draft, while the air exhausted from a fan is called forced draft. In blowers, air is compressed in a series of successive stages and is often led through a diffuser located near the exit. The overall pressure rise may range from 1.5 to 2.5 atm with shaft speeds up to 30,000 rpm or more. 4.2 CENTRIFUGAL COMPRESSOR The compressor, which can be axial flow, centrifugal flow, or a combination of the two, produces the highly compressed air needed for efficient combustion. In turbocompressors or dynamic compressors, high pressure is achieved by imparting kinetic energy to the air in the impeller, and then this kinetic energy Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved 144 Chapter 4 converts into pressure in the diffuser. Velocities of airflow are quite high and the Mach number of the flow may approach unity at many points in the air stream. Compressibility effects may have to be taken into account at every stage of the compressor. Pressure ratios of 4:1 are typical in a single stage, and ratios of 6:1 are possible if materials such as titanium are used. There is renewed interest in the centrifugal stage, used in conjunction with one or more axial stages, for small turbofan and turboprop aircraft engines. The centrifugal compressor is not suitable when the pressure ratio requires the use of more than one stage in series because of aerodynamic problems. Nevertheless, two-stage .