Working principle of the liquid ring pump in a vacuum distillation tower

The water ring vacuum pump (abbreviated as water ring pump) is a type of rough vacuum pump; the ultimate pressure it can achieve is 2.66 to 9.31 kPa for single-stage pumps ； For two-stage pumps, it is 0.133 to 0.665 kPa. Water ring pumps can also be used as compressors; they are low-pressure compressors with a pressure range of (1 to 2) X105 Pa gauge pressure (under specific conditions). Water ring pumps are widely used in many processes across various industries such as petroleum, chemicals, machinery, mining, light industry, papermaking, power generation, metallurgy, pharmaceuticals, and food processing, as well as in municipal and agricultural sectors. Applications include vacuum filtration, vacuum feeding, vacuum degassing, vacuum evaporation, vacuum concentration, and vacuum rehumidification. Since the gas compression process carried out by water ring pumps is isothermal, they can be used to extract flammable and explosive gases. Additionally, they can handle gases that contain dust or moisture; as a result, their use is increasing steadily. As shown in the diagram, this is a schematic illustration of the working principle of a water ring pump. A water ring pump consists of an impeller, a pump body, suction and discharge disks, a water ring formed by water on the inner wall of the pump, a suction port, a discharge port, and an auxiliary discharge valve. The impeller is mounted eccentrically within the pump body; as the impeller rotates in the direction indicated, the water that enters the pump body is thrown outward by the impeller. Due to centrifugal force, this water forms a closed ring of uniform thickness that resembles the shape of the pump chamber. The upper inner surface of the water ring is in exact tangency with the impeller hub (as shown in section I-I), while the lower inner surface of the water ring is in contact with the tips of the blades (in fact, the blades extend to a certain depth within the water ring). At this point, a crescent-shaped space is formed between the impeller hub and the water ring, and this space is further divided by the impeller into several small chambers, one for each blade. If 0° at the upper part of the impeller is taken as the starting point, then when the impeller has rotated 180°, the volume of the small chamber gradually increases (from section I-I to section II-II); the pressure continues to decrease, and the chamber is in communication with the suction port on the suction and exhaust plates. When the pressure inside the small chamber falls below the pressure inside the container to be evacuated, according to the principle of gas pressure equilibrium, the gas to be evacuated is continuously drawn into the small chamber – this is the suction process. When inhalation is complete, it is isolated from the inhalation port; from section II-II to section III-III, the volume of the small chamber gradually decreases while the pressure keeps rising. This is the compression phase. When the compressed gas reaches the exhaust pressure in advance, exhaust occurs through the auxiliary exhaust valve ahead of time. From section III-III to I-I, the volume of the small chamber connected to the exhaust port further decreases, causing the pressure to rise further. When the pressure of the gas exceeds the exhaust pressure, the compressed gas is discharged through the exhaust port. During the continuous operation of the pump, the processes of suction, compression, and exhaust occur repeatedly, thereby achieving continuous gas extraction. In water ring pumps, an auxiliary exhaust valve is a special type of valve; typically, a rubber ball valve is used for this purpose. Its function is to eliminate the problems of over-compression and under-compression that occur during the operation of the pump. Both of these phenomena can cause excessive power consumption. Since a water ring pump does not have a direct exhaust valve and the exhaust pressure remains constant, its compression ratio is determined by the ending position of the intake port and the starting position of the exhaust port. However, these two positions are fixed, which means the pump cannot adapt to changes in suction pressure. To solve this problem, a rubber ball valve is generally installed below the exhaust port, so that when the exhaust pressure is reached too early in the pump chamber, the ball valve opens automatically to allow gas to escape, thereby eliminating over-compression. When designing water ring pumps, the compression ratio is generally determined based on the lowest suction pressure, which in turn helps to determine the starting position of the exhaust port; this approach prevents under-compression.

The answer above is already quite clear; it would be more intuitive if pictures could be included. In modern chemical manufacturing, especially in the field of fine chemicals, liquid ring pumps are widely used to create vacuum, and they perform very well in such applications. My experience is as follows: 1. The working fluid of a liquid ring pump should be inert with respect to the medium, and no chemical reactions should occur ； 2. If the working fluid can dissolve the medium, it should be replaced frequently, recycled, or discarded ； 3. Generally, a liquid ring pump can achieve a pressure of around -0.08 MPa at most; if a higher vacuum level is required, other vacuum devices must be connected in series.

The water ring vacuum pump (abbreviated as water ring pump) is a type of rough vacuum pump; the ultimate pressure it can achieve is 2.66 to 9.31 kPa for single-stage pumps ； For two-stage pumps, it is 0.133 to 0.665 kPa. I don’t know where the person on the 2nd floor found that information, but I’m sure it’s incorrect. It’s acceptable for the working fluid to absorb trace amounts of precipitates, as long as this is done in line with the actual manufacturing process.

A space is formed between the sealing fluid and the impeller; this space is largest at the inlet of the vacuum pump and smallest at the outlet, which allows the incoming gas to be compressed and delivered to the outlet for discharge. Such vacuum pumps can also achieve high levels of vacuum (I have seen absolute pressures of 80 mbar), but the sealing fluid must have a high boiling point, and the temperature must not be too high. In short, the sealing fluid is a key factor in the operation of liquid ring pumps. Additionally, liquid ring pumps use less water than steam jet pumps.

A water ring pump should be capable of drawing the pressure in the equipment down to absolute zero; 80 mbar shouldn’t be a problem. Our facility can achieve a high vacuum level of 0.0931 Mpa. If a higher vacuum is required, a device such as a steam jet pump can be used in front of it. As for the working fluid, it certainly must not react with the medium, and of course it must not dissolve in the working fluid either

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