Figure 1-1 Chart of compressor types.

Figure 1-2 Typical application ranges of compressor types.

Figure 1-3 General performance curve for axial-flow, centrifugal, and positive displacement compressors.

Reciprocating Piston 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 that 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 from the discharge line into the compressor 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 illustrate typical reciprocating compressor arrangements, beginning with the single-stage and ending with a more complex multistage.

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 an approximately 1.5-to-1 pressure ratio, the reciprocating compressor is one of the most efficient of all the compressor types.

Reciprocating Diaphragm Compressors

In the reciprocating compressor family, there is a piece of equipment that is not as well known as the piston type reciprocating compressor. This compressor is the diaphragm compressor—used primarily in lower flow applications—but is capable of high pressure services. It consists of a crankcase assembly very much like the piston compressor, which in fact it is identical to, including a piston. The piston moves oil rather than gas, using the oil to move a diaphragm pack that in turn moves the gas. By the nature of the head construction, the diaphragm does not have any leakage to the atmosphere. Also, there is no leakage toward the inside to contaminate the process gas. Because relatively little power is used, most units are belt-driven by a motor. A single-stage diaphragm compressor is shown in Figure 1-6. The two-stage compressor can be arranged in various configurations similar to the piston compressor, that is, in an L arrangement or an opposed arrangement. Figure 1-7 is an example of an opposed arranged diaphragm compressor.

Rotary Compressors

The rotary compressor portion of the positive displacement family is made up of several compressor configurations. The features these compressors have in common are:

Helical and spiral-lobe compressors are generally similar and use two intermeshing helical or spiral lobes to compress gas between the lobes and the rotor chamber of the casing. The compression cycle begins as the open part of the spiral form of the rotors passes over the inlet port and traps a quantity of gas. The gas is moved axially along the rotor to the discharge port where the gas is discharged into the discharge nozzle of the casing. The volume of the trapped gas is decreased as it moves toward the outlet, with the relative port location controlling the pressure ratio. Figure 1-8 shows a cutaway of an oil-free helical-lobe compressor. The spiral-lobe version is the more limited of the two and is used only in lower pressure applications. Therefore, only the helical-lobe compressor will be covered in depth in this book (see Chapter 4).

Helical-lobe compressors are in either a dry or a flooded form. The dry form uses timing gears to hold a prescribed timing to the relative motion of the rotors; the flooded form uses a liquid medium to prevent the rotors from touching. The helical-lobe compressor is the most sophisticated and versatile of the rotary compressor group and operates at the highest rotor tip Mach number of any of the compressors in the rotary family. This compressor is usually referred to as a “screw compressor” or an “SRM compressor.”

The application range of the helical-lobe compressor is unique in that it bridges the application gap between the centrifugal compressor and the reciprocating compressor. The capacity range for the dry configuration is approximately 500 to 35,000 cfm. Discharge pressure is limited to 45 psi in single-stage configuration with atmospheric suction pressure. In supercharged or multistage applications, pressures of 250 psi are attainable. The spiral-lobe version is limited to 10,000 cfm flow and about 15 psi discharge pressure.

The straight-lobe compressor is similar to the helical-lobe machine but is much less sophisticated. As the name implies, it has two untwisted or straight-lobe rotors that intermesh as they rotate. Normally, each rotor pair has a two-lobe rotor configuration, although a three-lobe version is available. All versions of the straight-lobe compressor use timing gears to phase the rotors. Gas is trapped in the open area of the lobes as the lobe pair crosses the inlet port. There is no compression as gas is moved to the discharge port;...