selecting a centrifugal compressor

44 http://doc.guandang.net/cep June 2013 CEP Back to Basics

C entrifugal compressors, also called radial com-pressors, are critical equipment in a wide variety of applications in the chemical process industries (CPI). As their name suggests, their primary purpose is to compress a fluid (a gas or gas/liquid mixture) into a smaller volume while simultaneously increasing the pressure and temperature of the fluid. In other words, compressors accept a mass of gas at some initial pressure and temperature and raise that gas to a higher pressure and temperature (Figure 1). At the higher discharge pressure and tempera-ture, the gas density is also higher, so the mass of gas occu-pies a smaller volume — i.e., the gas is compressed. Of the numerous technologies that can achieve com-pression, this article focuses on centrifugal compressors. It explores the various types of centrifugal compressors,

provides valuable information on impellers, and explains basic centrifugal compressor sizing. (Reference 1 provides information on other types of compressors, such as positive-displacement, axial, and others.) Turbocompressors Centrifugal compressors are members of a class of technologies called dynamic compressors, or turbocompres-sors. Axial compressors are also part of this class of turbo­machines. Axial and centrifugal compressors draw their names from the primary direction in which the flow moves within the compressor. Axial compressors (Figure 2) handle much higher flowrates than centrifugal compressors, but generate lower pressure ratios. Modern centrifugal compres-sors accommodate lower flowrates than axial compressors but are capable of generating much higher pressure ratios. In turbocompressors, the increase in pressure and reduc-tion in volume is accomplished by adding kinetic energy to the fluid stream (i.e., adding velocity pressure) and then converting that kinetic energy into potential energy in the form of static pressure. In centrifugal compressors, kinetic energy is added by impellers. The number of impellers in a compressor depends on how large a compression or pressure increase is needed for the process. As a result, compressors can have one or as many as 10 (or more) impellers. The conversion of the velocity pressure to static pressure occurs in downstream stationary components, such as dif-fusers, return channels, and/or volutes. The type of station -ary component(s) in compressors depends on the style of centrifugal compressor being considered. The role of each of these components is discussed in this article. End users must specify certain performance requirements when requesting a quote for a new centrifugal

compressor. Understand your process, as well as the

advantages and disadvantages of each centrifugal

compressor configuration, in order to choose the optimal

centrifugal compressor for your application.

James M. Sorokes

Dresser-Rand Selecting a

Centrifugal Compressor

Inlet

Pressure (P1)

Temperature (T1)

Volumetric Flow (Q1)

Mass Flow (m)Discharge Pressure (P2)Temperature (T2)

Volumetric Flow (Q2)

Discharge vs. Inlet P2>P1

T2>T1

Q2<Q1

m=Constant

p Figure 1. Compressors accept a mass of fluid at an initial pressure and temperature, and raise it to a higher pressure and temperature, thereby compressing its volume.Copyright © 2013 American Institute of Chemical Engineers (AIChE) and Dresser-Rand

A simple analogy

To help understand the concepts of velocity and dynamic pressure, think about a fan that you might have in your home or office. If you

place your hand in front

of the fan, you can feel the kinetic energy that the fan blades have added to the air. If you place your hand behind the fan, you can feel movement of the air as it is being drawn into the fan. The suction is caused by a reduction in static pressure due to the acceleration of the air by the fan blades, thereby drawing more air into the fan.

Now imagine that you arrange several fans in a row inside an enclosure to ensure that all of the flow goes in one direction. Imagine how much force you would feel coming out of the last fan in the stack after each fan accelerates the air (i.e., adds more kinetic energy). That is the basic concept behind a compressor — a series of rotating blades adding energy to the gas.

Now suppose that the flow changes direction as it passes through the rotating blades so that it exits the blades travel-ing radially outward rather than in an axial direction (Fig ­ure 3). That is the fundamental difference between the axial compressor’s rotor and the centrifugal (or radial) compres-sor’s impeller — the axial rotor discharges flow in the axial direction while the centrifugal impeller discharges in a radial direction.

The impeller adds kinetic energy to the fluid in the same way the blades of a household fan do, although the cen-trifugal impeller adds more energy to the fluid than can be added with a typical fan blade. Thus, it is possible to achieve much higher pressures with centrifugal impellers. Multistage centrifugal compressors have multiple impellers stacked together and connected by flow passages.

Although centrifugal compressors are sometimes called radial compressors, most of the flow exiting a centrifugal impeller does not travel in a radial direction. Rather, the flow travels to a large extent in a tangential direction.

This motion is characteristic of a rotating disk. Consider the direction that wood dust travels when you are using a disk sander, or that sparks fly when you are using a grinding wheel (Figure 4). Similarly, the fluid that passes through a centrifu-gal impeller is flung out along a path that has both a radial velocity component and a tangential velocity component. Impellers — the heart of the centrifugal compressor The most critical components in any centrifuga …… 此处隐藏：25449字，全部文档内容请下载后查看。喜欢就下载吧 ……