Back to Basics Selecting a Centrifugal Compressor James M. Sorokes End users must specify certain performance requirements Dresser-Rand 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. entrifugal compressors, also called radial com- provides valuable information on impellers, and explains pressors, are critical equipment in a wide variety basic centrifugal compressor sizing. (Reference 1 provides Cof applications in the chemical process industries information on other types of compressors, such as positive- (CPI). As their name suggests, their primary purpose is to displacement, axial, and others.) compress a fluid (a gas or gas/liquid mixture) into a smaller volume while simultaneously increasing the pressure and Turbocompressors temperature of the fluid. In other words, compressors accept Centrifugal compressors are members of a class of a mass of gas at some initial pressure and temperature technologies called dynamic compressors, or turbocompres- and raise that gas to a higher pressure and temperature sors. Axial compressors are also part of this class of turbo­ (Figure 1). At the higher discharge pressure and tempera- machines. Axial and centrifugal compressors draw their ture, the gas density is also higher, so the mass of gas occu- names from the primary direction in which the flow moves pies a smaller volume — i.e., the gas is compressed. within the compressor. Axial compressors (Figure 2) handle Of the numerous technologies that can achieve com- much higher flowrates than centrifugal compressors, but pression, this article focuses on centrifugal compressors. generate lower pressure ratios. Modern centrifugal compres- It explores the various types of centrifugal compressors, sors accommodate lower flowrates than axial compressors but are capable of generating much higher pressure ratios. Inlet Pressure (P1) Discharge In turbocompressors, the increase in pressure and reduc- Temperature (T1) Pressure (P2) tion in volume is accomplished by adding kinetic energy Volumetric Flow (Q1) Temperature (T2) to the fluid stream (i.e., adding velocity pressure) and then Mass Flow (m) Volumetric Flow (Q2) Mass Flow (m) 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 Discharge vs. Inlet increase is needed for the process. As a result, compressors P2>P1 can have one or as many as 10 (or more) impellers. Compressor T2>T1 Q2<Q1 The conversion of the velocity pressure to static pressure m=Constant occurs in downstream stationary components, such as dif- fusers, return channels, and/or volutes. The type of station- p Figure 1. Compressors accept a mass of fluid at an initial pressure and ary component(s) in compressors depends on the style of temperature, and raise it to a higher pressure and temperature, thereby centrifugal compressor being considered. The role of each of compressing its volume. these components is discussed in this article. 44 www.aiche.org/cep June 2013 CEP Copyright © 2013 American Institute of Chemical Engineers (AIChE) and Dresser-Rand Inlet u Figure 2. In an axial Intake Stator Casing Guide Vane Discharge compressor, flow moves in Spring Link Discharge an axial direction (from left to Stator Vane Drive Ring Guide Vane right in this diagram), rather Assembly than in a circular direction as Tie Bolt in centrifugal compressors. Plain Bearing Body and Cap Stub Shaft A simple analogy To help understand the concepts of velocity Discharge Seal Tie Bolt and dynamic pressure, Labyrinth Rotor Blade think about a fan that Rotor Package Labyrinth Ring you might have in your Package Disc home or office. If you Ring Discharge Volute Assembly place your hand in front of the fan, you can feel the kinetic energy that the fan blades centrifugal compressors have multiple impellers stacked have added to the air. If you place your hand behind the together and connected by flow passages. fan, you can feel movement of the air as it is being drawn Although centrifugal compressors are sometimes called into the fan. The suction is caused by a reduction in static radial compressors, most of the flow exiting a centrifugal pressure due to the acceleration of the air by the fan blades, impeller does not travel in a radial direction. Rather, the flow thereby drawing more air into the fan. travels to a large extent in a tangential direction. Now imagine that you arrange several fans in a row This motion is characteristic of a rotating disk. Consider inside an enclosure to ensure that all of the flow goes in one the direction that wood dust travels when you are using a disk direction. Imagine how much force you would feel coming sander, or that sparks fly when you are using a grinding wheel out of the last fan in the stack after each fan accelerates the (Figure 4). Similarly, the fluid that passes through a centrifu- air (i.e., adds more kinetic energy). That is the basic concept gal impeller is flung out along a path that has both a radial behind a compressor — a series of rotating blades adding velocity component and a tangential velocity component. energy to the gas. Now suppose that the flow changes direction as it passes Impellers — the heart of the centrifugal compressor through the rotating blades so that it exits the blades travel- The most critical components in any centrifugal com- ing radially outward rather than in an axial direction (Fig­- pressor, regardless of style, are the impellers. If the impel- ure 3). That is the fundamental difference between the axial lers do not provide a high efficiency and good overall flow compressor’s rotor and the centrifugal (or radial) compres- range, it is impossible for the compressor to achieve a sor’s impeller — the axial rotor discharges flow in the axial direction while the centrifugal impeller discharges in a Radial radial direction. Exit Flow Component The impeller adds kinetic energy to the fluid in the same Tangential way the blades of a household fan do, although the cen- Component 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 Exit Flow Inlet Axial Centrifugal p Figure 4. Flow exits a centrifugal impeller in the direction of rotation, p Figure 3. Flow exits an axial rotor (left) in an axial direction, while flow just as sparks fly from a grinding wheel. The red arrows indicate the from a centrifugal impeller (right) exits radially. direction of rotation. Copyright © 2013 American Institute of Chemical Engineers (AIChE) and Dresser-Rand CEP June 2013 www.aiche.org/cep 45 Back to Basics high efficiency and good flow range. sure ratios of 3:1 or less, whereas unshrouded impellers can Impellers are the only rotating aerodynamic components reach pressure ratios of 10:1 or higher. in a centrifugal compressor. They provide 100% of the However, unshrouded impellers tend to be less efficient kinetic energy that is added to the gas and can be responsible because of the high losses associated with the tip leakage for up to 70% of the static pressure rise in a stage. flow (i.e., the flow that leaks over the rotating blades). Tip They are also the most efficient component in a stage. leakage does not occur in a covered impeller. Well­designed impellers can achieve efficiencies in excess The selection of blade style depends on many factors; of 96%, that is, only 4% of the energy expended is lost. The from an aerodynamic perspective, the most important is the losses in the stationary hardware reduce the overall stage impeller flow coefficient. The flow coefficient, ϕ, relates an efficiency from the baseline established by the impeller. impeller’s volumetric flow capacity, Q, operating speed, N, Therefore, if the performance of the impellers is poor, the and exit diameter, D2: overall compressor performance can only be worse. Q Centrifugal compressor impellers can be categorized ϕ = 3 ()1 ND× as shrouded or unshrouded (Figure 5), and their blades as 2 two-dimensional or three-dimensional (Figure 6). The type Low­flow­coefficient impellers are characterized by of impeller chosen for a particular application depends on long, narrow passages, while high­flow­coefficient impel- many considerations, such as required operating speed, lers have much wider passages to accommodate the higher desired pressure ratio, desired efficiency, and equipment flowrates. cost. The compressor rotor shown in Figure 7 contains impel- The absence of a cover allows unshrouded impellers to lers with a wide range of flowcoefficients. The impeller with operate at higher rotational, or tip, speeds. The pressure ratio the highest flow coefficient is located at the right end of the generated by an impeller is proportional to the square of the rotor. The remaining impellers are progressively narrower, operating speed. Therefore, open (unshrouded) impellers with increasing fluid pressure and decreasing volumetric are capable of generating much higher pressure ratios than flowrate. The impeller with the lowest flow coefficient (at shrouded impellers. Most shrouded impellers generate pres- the left, closest to the center of the machine) is much nar- p Figure 6. Impeller blades can be either two-dimensional (top) or three- dimensional (bottom). The 2D blades in the top image have a circular arc p Figure 5. Impellers can be either shrouded (top) or open (bottom). shape, whereas the blades in the lower image have a complex 3D shape. 46 www.aiche.org/cep June 2013 CEP Copyright © 2013 American Institute of Chemical Engineers (AIChE) and Dresser-Rand Medium-Flow- High-Flow-Coefficient require the use of impellers with a low flow Low-Flow-Coefficient Impeller Coefficient Impeller Impeller coefficient when the process flowrate is small relative to the operating speed or impel- ler diameter (see Eq.