3 - Compressor and Turbine Performance Characteristics

Abstract

This chapter examines the overall performance characteristics of compressors and turbines, which are together classified under the broad term “turbomachinery.” Pumps and compressors are used to produce pressure, whereas turbines produce power. These machines have some common characteristics. The main element is a rotor with blades or vanes, and the path of the fluid in the rotor may be axial, radial, or a combination of both. To understand the flow in turbomachines, an understanding of the basic relationships of pressure, temperature, and type of flow must be acquired. Ideal flow in turbomachines exists when there is no transfer of heat between the gas and its surroundings, and the entropy of the gas remains unchanged. This type of flow is characterized as a reversible adiabatic flow. The work in a compressor or turbine under ideal conditions occurs at constant entropy. Polytropic efficiency is another concept of efficiency often used in compressor evaluation, and is often referred to as small stage or infinitesimal stage efficiency. Turbomachines can be compared with each other by dimensional analysis, in which variables representing a physical situation are reduced into dimensionless groups. These groups can then be used to compare performance of various types of machines with each other. Dimensional analysis is used to compare data from various types of machines, select various types of units based on maximum efficiency and pressure head required, and predict a prototype's performance from tests conducted on a smaller scale model or at lower speeds.