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Chapter One

BASIC CONSIDERATIONS




1-1. Introduction and Classification of Fluid Machines
The word turbo was coined in 1828 by Claude Burdin (1790-1873) to describe the subject of an engineering competition for a water power source. It comes from Latin turbo, turbinis meaning a "whirling" or a "vortex" and by extension a child s top or a spindle.
Fluid machines may be broadly classified as either positive displacement or dynamic. In positive-displacement machines, energy transfer is accomplished by volume changes that occur due to movement of the boundary in which the fluid is confined. Fluid handling devices that direct the flow with blades or vanes attached to a rotating member are termed turbomachines. In contrast to positive-displacement machinery, there is no closed volume in turbomachine. All work interactions in a turbomachine result from dynamic effects of the rotor on the fluid stream.
A further distinction among types of turbomachines is based on the geometry of the flow path. In radial flow machines, the flow path is essentially radial; with significant changes in radius from inlet to outlet (such machines sometimes are called centrifugal machines). In axial flow machines, the flow path is nearly parallel to the machine centerline, and the radius of the flow path does not vary significantly. In mixed flow machines the flow path radius changes only moderately. Schematic diagrams of typical turbomachines are shown in Figures (1-1) through (1-3).
Machines that add energy to a fluid stream are called pumps when the flow is liquid or slurry, and fans, blowers, or compressors for gas or vapor-handling units, depending on pressure rise. Fans usually have small pressure rise (less than 1 inch of water) and blowers have moderate pressure rise (perhaps 1 inch of mercury); pumps and compressors may have very high pressure rises. Current industrial system operates at pressure up to 150000 psi (104 atmospheres).
The rotating element of a pump is frequently called the impeller; the impeller is contained within the pump housing or casing. The shaft that transfers mechanical energy to the impeller usually must penetrate the casing; a system of bearings and seals is required to complete the mechanical design of the units.
Three typical centrifugal machines are shown in Figure (1-1). Flow enters each machine nearly axially at small radius through the eye of the rotor, diagram (a) at radius r1 flow is turned and leaves through the impeller discharge at radius r2, where the width is b2. Flow leaving the impeller is collected in the scroll or volute, which gradually increases in area as it nears the outlet of the machine, diagram (b),