Rotary Sliding Vane Compressor
A well-tried, mechanically simple design, Figures 17 and 18. The rotor and the vanes are the only moving parts. The rotor is mounted offset in the casing; as can be seen, the compression is obtained through this eccentricity. Air is admitted when the crescent shaped chamber is increasing in volume and delivered when the design pressure is reached. As in all other rotary positive displacement types, it has a built-in pressure ratio, dependent on the position of the delivery port.
The rotor is a simple cylinder, with longitudinal slots machined in it, and the vanes can be cut from plate material. These are available in three forms- lubricated, oil-free and oilflooded.
Lubricated types inject an oil mist into the incoming air stream. Alternatively oil can be fed from a reservoir through the shaft or directly into the cylinders. Vanes are made of steel, in which case a floating ring (see below) is required, or of a synthetic fibre material (newer types). The use of synthetic fibre is advantageous in that the lower mass means a smaller centrifugal force.
Oil-free types, suitable for process applications require self-lubricating vanes made from carbon.
Oil-flooded types are similar to screw compressors in that large quantities of oil are injected into the compression space, which then has to be separated out and cooled. As with screw compressors, the oil lubricates, cools and seals. The vanes are made of aluminium, cast-iron or a synthetic fibre material.
The rotational frequency has to be low, otherwise the centrifugal forces on the vanes would cause frictional failure, so a direct motor drive is customary. Wear of the surface of the vanes that contacts the stator can be a problem, but because they are free in their slots, such wear is self-compensatory. Wear on the side of the vanes or on their ends causes a fall in efficiency, and they should be replaced when such wear exceeds the manufacturer’ s recommendations.
Medium or large capacity vane compressors running at high speeds normally incorporate a method of avoiding contact between the vanes and casing wall in order to reduce wear and minimize frictional losses. One method is to fit restraining or floating rings over the vanes, the internal diameter of these rings being slightly less than the cylinder bore. The difference is calculated to preserve a minimum clearance between the vane tips and the casing walls under operating conditions, see Figure 19.
The rings actually rotate with the vanes but the peripheral speed of each vane tip varies with the degree of extension. The rings rotate at constant speed. There is some relative motion between vane and rings, but the contact between rotor and rings is essentially a rolling motion.