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:

1. They impart energy to the gas being compressed by way of an input shaft moving a single or multiple rotating element.

2. They perform the compression in an intermittent mode.

3. They do not use inlet and discharge valves.

The helica! 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 1-6 shows a cutaway view of a helical-lobe compressor. The spiral-lobe version is the more limited of the two and is used only in the lower pressure applications. Therefore, only the helical-lobe compressor will be covered in depth in this book (see Chapter 4).

The helical-lobe compressor is further divided into a dry and 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 media to keep 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 the "screw compressor" or the "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. On

Figure 1-6. Cutaway of an oil-free helical-lobe rotary compressor. (Courtesy of A-C Compressor Corporation

supercharged or multistage applications, pressures of 250 psi are attainable. The spiral-lobe version is limited to 10,000 cfm flow and about !5 psi discharge pressure.

The straight-lobe compressor is similar to the heiical-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; rather, it is compressed by the backflow from the discharge port. Four cycles of compression take place in the period of one shaft rotation on the two-lobe version. The operating cycle of the straight-lobe rotary compressor is shown in Figure 1-7.

Figure 1-7. Operating cycle of a straight-lobe rotary compressor. (Modified, courtesy of Ingersoll-Rand)

Volume range of the straight-lobe compressor is 5 to 30,000 cfm. Pressure ranges are very limited with the maximum single-stage rating at 15 psi. In a few applications, the compressors are used in two-stage form where the discharge pressure is extended to 20 psi.

The sliding-vane compressor uses a single rotating element (see Figure 1-8), The rotor is mounted eccentric to the center of the cylinder portion of the casing and is slotted and fitted with vanes. The vanes are free to

Figure 1-7. Operating cycle of a straight-lobe rotary compressor. (Modified, courtesy of Ingersoll-Rand)

Figure 1-8. Cross section of a sliding vane compressor. (Courtesy ofA-C Compressor Corporation)

move in and out within the slots as the rotor revolves. Gas is trapped between a pair of vanes as the vanes cross the inlet port. Gas is moved and compressed circumferentially as the vane pair moves toward the discharge port. The port locations control the pressure ratio. (This compressor must have an external source of lubrication for the vanes.)

The sliding-vane compressor is widely used as a vacuum pump as well as a compressor, with the largest volume approximately 6,000 cfm. The lower end of the volume range is 50 cfm. A single-stage compressor with atmospheric inlet pressure is limited to a 50 psi discharge pressure. In booster service, the smaller units can be used to approximately 400 psi.

The liquid piston compressor, or liquid ring pump as it is more commonly called, uses a single rotor and can be seen in Figure 1-9. The rotor consists of a set of forward-curved vanes. The inner area of the rotor contains sealed openings, which in turn rotate about a stationary hollow inner core. The inner core contains the inlet and discharge ports. The rotor turns in an eccentric cylinder of either a single- or double-lobe design. Liquid is carried at the tips of the vanes and moves in and out as the rotor turns, forming a liquid piston. The port openings are so located as to allow gas to enter when the liquid piston is moving away from center. The port is then closed as rotation progresses and compression takes place, with the discharge port coming open as the liquid piston approaches the innermost part of the travel. As with some of the other rotary com-

Figure 1-10. Cross section of an ejector. {Courtesy of Graham Manufacturing Co., Inc.)

The ejector is operated directly by a motive gas or vapor source. Air and steam are probably the two most common of the motive gases. The ejector uses a nozzle to accelerate the motive gas into the suction chamber where the gas to be compressed is admitted at right angles to the motive gas direction. In the suction chamber, also referred to as the mixing chamber, the suction gas is entrained by the motive fluid. The mixture moves into a diffuser where the high velocity gas is gradually decelerated and increased in pressure.

The ejector is widely used as a vacuum pump, where it is staged when required to achieve deeper vacuum levels. If the motive fluid pressure is sufficiently high, the ejector can compress gas to a slightly positive pressure. Ejectors are used both as subsonic and supersonic devices. The design must incorporate the appropriate nozzle and diffuser compatible with the gas velocity. The ejector is one of the few compressors immune to liquid carryover in the suction gas.

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