A liquid-ring compressor (Figure 1) is a rotating positive-displacement device that is very similar to a rotary vane compressor; it differs in having vanes that are an integral part of the rotor that churn a rotating ring of liquid to form the compression chamber seal. Both types of compressors are inherently low-friction designs, with the rotor being the only moving part. Sliding friction often is limited to the shaft seals, although other frictions might exist. Many points presented in this article actually apply to both types.
An induction motor typically powers a liquid-ring compressor. The motor rotates a vaned impeller located within a cylindrical casing to compress gas. Liquid (often water) is fed into the compressor and, by centrifugal acceleration, forms a moving cylindrical ring against the inside wall of the casing. This liquid ring creates a series of seals in the space between the impeller vanes, forming compression chambers. The eccentricity between the impeller’s axis of rotation and the casing geometric axis results in a cyclic variation of the volume enclosed by the vanes and the ring. Gas is drawn into the compressor via an inlet port in the end of the casing; the gas is trapped in the compression chambers created by the impeller vanes and the liquid ring. The reduction in volume of the gas caused by the impeller rotation compresses the gas, which exits through the discharge port in the end of the casing.
Design and Operation
Liquid-ring compressors employ a rotor centrally positioned in an oval-shaped casing. The rotor does not touch the casing, which contains a precise amount of liquid. During rotation, centrifugal force causes the liquid to form a ring that follows the shape of the casing’s inside wall. Because of that oval shape, the volumes between the rotor’s blades differ. At two points, the liquid completely fills the volumes. In between these two points, the liquid recedes in the beginning and up to half way, thus creating suction; it then advances, creating pressure. The gas enters the casing through the inlet port, where suction is generated, and leaves, after compression, through the outlet port. A dedicated line supplies the sealant liquid, which flows continuously. This liquid absorbs the heat of compression. It usually leaves the casing together with the compressed gas, and is split off in a discharge separator tank.
The operation of rotary vane compressor is slightly different: the centrifugal force acting on the blades due to rotor rotation pushes the blades radially outward to continuously contact the cylinder bore and form closed pockets or cells filled with gas when open to the inlet port during the (nearly) half of revolution on the inlet side of the cylinder. During the second half of revolution, a cell moves toward the discharge port while its volume decreases due to the progressively diminishing space between the cylinder and rotor. Thus, a blade is subjected to a pressure differential between its preceding cell at a higher pressure and the following cell at a lower pressure. This pressure differential can cause the blade to rub against the outer corner of the rotor slot as it slides in and out of the slot.
Cylindrical gas distributors and relatively large inlet-gas passages can reduce friction losses to the minimum. An overhung arrangement often is preferred for these small compressors because it requires only one shaft seal and bearing housings are externally accessible.
Liquid-ring compressors usually come fitted with rolling-element bearings. Unfortunately, the compressor’s small size often rules out more-reliable bearings such as hydrodynamic ones. So, it’s essential to select rolling-elements bearings that promise reasonable reliability and performance. As a very rough guide, choose rolling elements that have an 8-y theoretical operating life or more; a basic life (L10) exceeding 60,000 hours or more is preferred. The bearing life calculation should consider all possible loads — for instance, all potential dynamic loads, high loads due to compressor degradation and fouling, and all other non-ideal cases. Review of many failed rolling-element bearings has shown they were subjected to actual loads much greater than the theoretical (assumed) ones; such higher actual loads can reduce the bearing life to 5–10% or less of the expected life.