Dynamic Compressors

In dynamic compressors, energy is transferred from a moving set of blades to the gas. The energy takes the form of velocity and pressure in the rotating element, with further pressure conversion taking place in the stationary elements. Because of the dynamic nature of these compressors, the density and molecular weight have an influence on the amount of pressure the compressor can generate. The dynamic compressors are further subdivided into three categories, based primarily on the direction of flow through the machine. These are radial, axial, and mixed flow.

The radial-flow, or centrifugal compressor is a widely used compressor and is probably second only to the reciprocating compressor in usage in the process industries. A typical multistage centrifugal compressor can be seen in Figure 1-11. The compressor uses an impeller consisting of

Pinionie Compressors
Figure 1-11. Radiat-flow horizontally split multistage centrifugal compressor. (Courtesy of Nuovo Pignone)

radial or backward-leaning blades and a front and rear shroud. The front shroud is optionally rotating or stationary depending on the specific design. As the impeller rotates, gas is moved between the rotating blades from the area near the shaft and radially outward to discharge into a stationary section, called a diffuser. Energy is transferred to the gas while it is traveling through the impeller. Part of the energy converts to pressure along the blade path while the balance remains as velocity at the impeller tip where it is slowed in the diffuser and converted to pressure. The fraction of the pressure conversion taking place in the impeller is a function of the backward leaning of the blades. The more radial the blade, the less pressure conversion in the impeller and the more conversion taking place in the diffuser. Centrifugal compressors are quite often built in a multistage configuration, where multiple impellers are installed in one frame and operate in series.

Centrifugal compressors range in volumetric size from approximately 1,000 to 150,000 cfm. In single-wheel configuration, pressures vary considerably. A common low pressure compressor may only be capable of 10 to 12 psi discharge pressure, in higher-head models, pressure ratios of 3 are available, which on air is a 30-psi discharge pressure when the inlet is at atmospheric conditions.

Another feature of the centrifugal is its ability to admit or extract flow to or from the main flow stream, at relatively close pressure intervals, by means of strategically located nozzles. These flows are referred to as side-

streams. Pressures of the multistage machine are quite varied, and difficult to generalize because of the many factors that control pressure. Centrifugals are in service at relatively high pressures up to 10,000 psi either as a booster or as the result of multiple compressors operating in series.

Axial compressors are large-volume compressors that are characterized by the axial direction of the flow passing through the machine. The energy from the rotor is transferred to the gas by blading (see Figure 1-12). Typically, the rotor consists of multiple rows of unshrouded blades. Before and after each rotor row is a stationary (stator) row. For example, a gas particle passing through the machine alternately moves through a stationary row, then a rotor row, then another stationary row, until it completes the total gas path. A pair of rotating and stationary blade rows define a stage. One common arrangement has the energy transfer arranged to provide 50% of the pressure rise in the rotating row and the other 50% in the stationary row. This design is referred to as 50% reaction.

Axial compressors are smaller and are significandy more efficient than centrifugal compressors when a comparison is made at an equivalent flow rating. The exacting blade design, while maintaining structural integrity, renders this an expensive piece of equipment when compared to centrifugals. But it is generally justified with an overall evaluation that includes the energy cost.

Figure 1-12. Axial-flow compressor. (Courtesy of Demag Oelaval Turbomachinery Corp.)

The volume range of the axial starts at approximately 70,000 cfm. One of the largest sizes built is 1,000,000 cfm, with the common upper range at 300,000 cfm. The axial compressor, because of a low-pressure rise per stage, is exclusively manufactured as a multistage machine. The pressure for a process air compressor can go as high as 60 psi. Axial compressors are an integral part of large gas turbines where the pressure ratios normally are much higher. In gas turbine service, discharge pressures up to 250 psi are used.

The mixed-flow compressor is a relatively uncommon form, and is being mentioned here in the interest of completeness. At first glance, the mixed-flow compressor very much resembles the radial-flow compressor. A bladed impeller is used, but the flow path is angular in direction to the rotor; that is, it has both radial and axial components (see Figure 1 -13). Because the stage spacing is wide, the compressor is used almost exclusively as a single-stage machine. The energy transfer is the same as was described for the radial-flow compressor.

Centrifugal Impeller 60° mixed-flow Impeller 45° mixed-flow impeller

Figure 1-13. Comparison of radial- and mixed-flow compressor impellers.

Centrifugal Impeller 60° mixed-flow Impeller 45° mixed-flow impeller

Figure 1-13. Comparison of radial- and mixed-flow compressor impellers.

The compressor size is flexible and covers the centrifugal compressor flow range, generally favoring the higher flow rates. The head per stage is lower than available in the centrifugal. The compressor finds itself in the marketplace because of the unique head-capacity characteristic, which can be illustrated by its application in pipeline booster service. In this situation the pressure ratio needed is not high, and as a result the head required is low. However, because of the high inlet pressure of the gas, a relatively high pressure rise is taken across the machine. Thus, there is a real need for a more rugged and less expensive alternative to the axial compressor.

Introduction

This chapter presents some basic thermodynamic relationships that apply to all compressors. Equations that apply to a particular type of compressor will be covered in the chapter addressing that compressor. In most cases, the derivations will not be presented, as these are available in the literature. The references given are one possible source for additional background information.

The equations are presented in their primitive form to keep them more universal. Consistent units must be used, as appropriate, at the time of application. The example problems will include conversion values for the units presented. The symbol g will be used for the universal gravity constant to maintain open form to the units.

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