Compression Cycle

The sliding vane compressor consists of a single rotor mounted eccentrically in a cylinder slightly larger than the rotor. The rotor has a series of radial slots that hold a set of vanes. The vanes are free to move radially within the rotor slots. They maintain contact with the cylinder wall by centrifugal force generated as the rotor turns.

The space between a pair of vanes and the rotor and the cylinder wall form crescent shaped cells (see Figure 1-8). As the rotor turns and a pair of vanes approach the inlet, gas begins to fill the cell. The rotation and subsequent filling continue until the suction port edge has been passed by both vanes. Simultaneously, the vanes have passed their maximum extension and begin to be pushed back into the rotor by the eccentricity of the cylinder wall. As the space becomes smaller, the gas is compressed. The compression continues until the leading vane crosses the edge of the discharge port, at which time the compressed gas is released into the discharge line.

The port location must be matched to the pressure ratio dictated by the application for efficient compression to take place. Figure 4-22 is an indicator diagram of a compression cycle. If the port has been optimized for a ratio of P2/Pi, the compression line is a smooth curve, as can be seen as compression proceeds from point 1 to 2. Note point 2 is at the same pressure level as P2. If the external pressure is higher than the pres sure for which the port was cut, so that P0/Pi > P2/Pi, then, when the port opens at point 2, discharge gas will return to the compressor from the line and must again be expelled from the compressor. This energy waste is depicted by the shaded area to the left of the line o-2. Conversely, if the external pressure ratio is lower than the pressure ratio for which the port was cut, where Py/Pi < then the gas will be overcompressed to point 2 and when the port opens, it will expand to point u. The lost energy is represented by the shaded area to the right of the line u-2.

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