Liquid-ring compressors

In contrast to other displacement compressors, liquid ring machines have a fluid piston . This is subject to a constant motion and deformation depending on effective forces. Potential differences are rapidly equalised by the motion of the fluid inside the piston. If the ring liquid plunges into the blade area, then the pressure of the enclosed medium is increased. 

API Std 681, Liquid Ring Vacuum Pumps and Compressors, 1st Edition, February 1996. 

The most used mechanical vacuum pumps or compressors are reciprocating, liquid-ring, rotary-vane, rotary blower, rotary piston, and diaphragm. 

Figure 12-103A. Functional operational schematic of Nash liquid ring compressor. (Used by permission Bui. 474-D, p. 3. Nash Engineering Co.)...

The liquid-ring or liquid-piston compressor is shown in Figure 36.8. It has a rotor with multiple forward-turned blades that rotate about a central cone that contains inlet and discharge ports. Liquid is trapped between adjacent blades, which drive the liquid around the inside of an elliptical casing. As the rotor turns, the liquid face moves in and out of this space due to the casing shape, creating a liquid piston. Porting in the central cone is built-in and fixed and there are no valves. 

Jets have been partially replaced by liquid ring-seal pumps. These are really positive-displacement compressors. The gas is squeezed between the vanes of the compressor s rotor and a pool of liquid in the compressor s case. Liquid ring-seal pumps are not interesting. They have no character. They are not as complex as steam jets. Anyway, I will discuss positive displacement compressors in later chapters. 

Figure 7.19. Some rotary positive displacement compressors, (a) A two-lobe blower, (b) Performance of a two-lobe blower (Rools-Connersville Co.), (c) A screw pump with one power and two idle rotors (Kristal and Annett, 1940). (d) Performance of 3.5" screw pump handling oils at 1150 rpm against 325 psig (Kristal and Annett, 1940). (e) Principle of the liquid ring seal compressor (Nash Engineering Co.). (0 A sliding vane blower (Beaeh-Russ Co.).

Because of a number of problems—e.g., weakness of the perforated plates, too low an ozone concentration, too large a volume of air, and costly maintenance of the compressor and other equipment because of corrosive action of ozone— this method has been modified in later installations. The perforated plates have been eliminated, and the air is diffused through plastic diffusers. Stainless steel rotary, liquid ring compressors are used. In some cases, even the Otto method of emulseurs and selfcontact columns is employed. 

Rotary compressors have efficiencies of 70%, except liquid-liner t5q)es (e.g., liquid-ring), which have 50%. 

Noncondensable gas-removal system, to remove and compress the noncondensable gases. A typical system uses two stages of compression. The first stage is a steam jet ejector. The second stage is another steam jet ejector, a liquid ring vacuum pump, or a centrifugal compressor. 

The vacuum at the top of the flash column is often produced by a sequence of three elevated steam-jet eductors with intermediate and final surface condensers to remove the steam. A liquid-ring compressor can be substituted for one or more eductors to conserve steam. Mechanical vacuum pumps are seldom used because of the corrosive nature of the off-gas. The noncondensible sour compressed gas and condensate are led away through a water seal for safety in case of steam system failure. The seal pot is equipped to skim off condensed oil continuously. 

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