Compression Methods

Compressors have numerous forms, the exact configuration being based on the application. For comparison, the different types of compressors can be subdivided into two broad groups based on compression mode. There are two basic modes: intermittent and continuous. The intermittent mode of compression is cyclic in nature, in that a specific quantity of gas is ingested by the compressor, acted upon, and discharged, before the cycle is repeated. The continuous compression mode is one in which the gas is moved into the compressor, is acted upon, moved through the compressor, and discharged without interruption of the flow at any point in the process.

Compressors using the intermittent compression mode are referred to as positive displacement compressors, of which there are two distinct types: reciprocating and rotary. Continuous-mode compressors are also characterized by two fundamental types: dynamic and ejector.

This chapter will give a brief overview of each of the different compressors commonly used in the process industries. Subsequent chapters will then cover each of the mechanical types in depth. (The ejector, which does not use mechanical action, will not be covered in detail.) Figure 1 -!

Types Compressor Chart
Figure 1-t. Chart of compressor types.
Reciprocating Pump Performance Graphs
Figure 1-3. General performance curve for axial flow, centrifugal, and positive displacement.

Intermittent Mode Compressors

Reciprocating Compressors

The reciprocating compressor is probably the best known and the most widely used of all compressors. It consists of a mechanical arrangement in which reciprocating motion is transmitted to a piston which is free to move in a cylinder. The displacing action of the piston, together with the inlet valve or valves, causes a quantity of gas to enter the cylinder where it is in turn compressed and discharged. Action of the discharge valve or valves prevents the backflow of gas into the compressor from the discharge line during the next intake cycle. When the compression takes place on one side of the piston only, the compressor is said to be single-acting. The compressor is double-acting when compression takes place on each side of the piston. Configurations consist of a single cylinder or multiple cylinders on a frame. When a single cylinder is used or when multiple cylinders on a common frame are connected in parallel, the arrangement is referred to as a single-stage compressor. When multiple cylinders on a common frame are connected in series, usually through a cooler, the arrangement is referred to as a multistage compressor. Figures 1-4 and 1-5 are typical reciprocating compressor arrangements, beginning with the single-stage and ending with a more complex multistage.

Figure 1-4. A three-stage single-acting reciprocating compressor. (Courtesy of Ingersoll Rand)

Reciprocating Cylinder Method

Figure 1-4. A three-stage single-acting reciprocating compressor. (Courtesy of Ingersoll Rand)

Figure 1-5. Cutaway of the frame end of a targe multistage reciprocating compressor. (Courtesy of Dresser-Rand)

The reciprocating compressor is generally in the lower flow end of the compressor spectrum. Inlet flows range from less than 100 to approximately 10,000 cfm per cylinder. It is particularly well-suited for high-pressure service. One of the highest pressure applications is at a discharge pressure of 40,000 psi. Above approximately a 1.5-to-i pressure ratio, the reciprocating compressor is one of the most efficient of all the compressors.

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  • chelsea
    How to size a compressor graph?
    8 years ago

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