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Compressor Thermodynamics

By Joe Honeywell

 

Introduction

Thermodynamics is a broad subject with specialized branches for every engineering discipline. It establishes the basis for fluid dynamics, heat transfer, mass balance and much more. It also presents the principles used in designing equipment and countless processes utilized in almost every industry. Whether you are and experienced engineer or just beginning the field of thermodynamics, it is an important subject and requires a thorough understanding.

There are many types of compressors that are utilized in a wide variety of applications and operating conditions. All of them have one thing in common – compressor thermodynamics. Advances in gas processing technology, the mixture of various types of gases handled by treatment facilities, and the specialized equipment needed to condition the gas, have brought about changes in compressors used in plants. Modern compressors are more efficiency and have lower operating costs than their predecessors. The reliability of compressors has increased due primarily to improved manufacturing processes, metallurgy and component design. One thing that has remained constant, however, is the thermodynamics that describes the performance of a compressor. The general knowledge needed by engineers to predict how compressors will perform with different gases and under various operating conditions is linked to the specialized branch of compressor thermodynamics.

There are many types and sizes of compressors. Some types are designed for a particular gas or applications. Other compressor types can be applied to a broad range of applications and gases. The gas processing industry is constantly evolving and so too is the compression equipment used to handle the wide range of applications. The vastly different physical properties of gases, specialized compressor equipment and process simulation programs can leave engineers uncertain about the thermodynamics. Specialized compression machines require special thermodynamic considerations, and gas mixtures do not always behave as the individual constituents. Regardless of the application, compressor complexity or type of process fluid, a basic understanding of thermodynamic principles is necessary. To help streamline the broad subject of thermodynamics, this review will focus on those principles that apply primarily to all compressors and the fluids they handle.

This article is divided into six sections that relate to compressor thermodynamics.

Throughout this article many equations are given that mathematically describe the various topics within thermodynamics. Many illustrations and tables support the topics discussed. Also, numerous example problems are included that explain how to apply the equations and reference material to solve practical applications. Finally, three appendices are included at the end of this article. Each contains information used to solve the example problems. The reader may find this information helpful in solving many of the compressor thermodynamic problems engineers face each day.

This article is intended to aid engineers in their understanding of compressor thermodynamics. It examines basic terminology and performance parameters, and the procedure used to specify and evaluate compressor performance. It does not develop thermodynamic relationships, evaluate calculated results or help determine if the outcome is reasonable. It does help engineers gather the necessary physical data and operating parameters to solve common gas compression problems. The reader is assumed to have a basic understanding of thermodynamic principles and a general understanding of compressor performance. This article will also help the engineer who turns to process simulators to solve engineering problems without knowing how the simulator arrives at the results. This article provides the equations and procedures for checking a simulator’s result with confidence.

 

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