There are two major types of vane compressors, single-vane (rotary) and multivane. A rotary compressor simply consists of a bladed, eccentric rotor in a cavity. As the rotor turns, the blades extend and retract, sealing off the cavity into segments of varying size. The gas enters the intake port where the segments are large, is compressed as the cavities are reduced, and is discharged where the segments are small. These compressors are commonly used in domestic refrigerators, freezers, and air conditioners. The possible maximum compression ratios achieved are on the order of 7:1. Small systems and some ammonia systems also employ compressors of this type. In multistage systems in which each stage has a low compression ratio, vane compressors can be used as boosters. Figure 3.8 shows the cutaway view of a rotary vane compressor. These compressors have some basic advantages:
• Simple, compact design: Sturdy construction with few moving parts, easy to access and maintain, easy to replace parts, very reliable and durable.
• Single-stage compression: The nature of the design produces sufficient compression in a single stage, resulting in a very high compression ratio during cycle, as well as better energy efficiency, reduced risk of fault and reduced maintenance requirements.
• Direct axial coupling to the motor: Direct coupling is possible because the high compression ratio permits low rotation speeds, eliminating the need for transmission or gears. Fewer parts mean lower energy dissipation and simplified maintenance.
• Low rotation speeds: Lower speeds reduce vibration, thus diminishing noise and wear, lowering temperature and eliminating the need for foundations.
• Low cycle temperature: Lower temperatures reduce wear, oil consumption and leakage caused by distension of parts. Less energy is needed for cooling, and the purity of delivered air is enhanced.
• Low need for maintenance: With fewer parts suffering little wear, single-stage rotary vane units offer cleaner and more reliable operation, significantly reducing maintenance needs.