Liquid Ring Vacuum Pump is a type of rotary positive displacement pump which functions by means of a spinning rotor/impeller that forms a ring of liquid around its cylindrical casing. The ring fluid is usually water or oil. The liquid ring makes a seal around the impeller which makes the gases or vapors draw into the pump casing through suction. The liquid ring is continuously replenished by circulation. As the gas or vapor is drawn into the pump, it's forced towards the discharge port by the impeller. At this point, the centrifugal force of the liquid ring compresses the gas or vapor, elevating its pressure. Finally, the compressed gas or vapor is discharged from the pump via the discharge port, and directed to a collector or condenser.
Working Principle of a liquid ring vacuum pump can be summarized by the following steps
As the impeller rotates counterclockwise (as seen from the inlet port), the first blade passes the inlet port and a chamber is formed between the blade and the casing wall. The chamber is filled with gas at atmospheric pressure and volume.
As the impeller continues to rotate, the chamber moves along the casing wall and its volume decreases as it approaches the discharge port. The gas is compressed by the liquid ring and its pressure increases.
When the chamber reaches the discharge port, the gas is expelled from the pump at a higher pressure and volume than at the inlet port. The liquid ring also loses some of its volume due to leakage and evaporation.
As the impeller rotates further, the chamber moves away from the discharge port and its volume increases as it approaches the inlet port again. The liquid ring is replenished by a fresh supply of liquid from an external source or a recirculation system (Cooling Tower).
The cycle repeats for each chamber as the impeller rotates.
A liquid ring vacuum pump can achieve a vacuum level of up to 33 mbar (absolute), which is equivalent to 97% vacuum. It can also handle wet or corrosive gases, as well as solid particles or liquids that may be entrained in the gas stream.
Liquid ring vacuum pumps are widely used in sugar refineries because of their high efficiency. Generally, water ring pumps are used as it is much more convenient to remove vapor from the vacuum pan section through condenser system. Vapor isn’t directly fed to the vacuum pump suction. It passes through the Knock-out Vessel and then enters to the vacuum pump. Vacuum pump discharges the pressurized gas water mixture to gas separator.
Knock-out Vessel, also known as a Collection Pot or Catch Pot, is a vessel that is often used before a liquid ring vacuum pump in industrial applications. The purpose of the catch pot is to collect any liquid or solid particles that may be present in the gas stream before it enters the vacuum pump.
When gas containing liquid or solid particles is introduced into the vacuum pump, it can cause damage to the pump by creating a "slugging" effect or by causing erosion to the impeller. The knock-out vessel helps to prevent this by allowing the liquid or solid particles to settle out of the gas stream and collect in the vessel.
The knock-out vessel typically has a drain valve at the bottom that allows the collected liquid to be drained or removed from the system. By removing the liquid and solid particles from the gas stream before it enters the vacuum pump, the catch pot helps to ensure the longevity and optimal performance of the pump.
Gas Separator is commonly used after a liquid ring vacuum pump in order to separate the gas and liquid mixture that exits the pump. Liquid ring vacuum pumps use a ring of liquid to create a vacuum by forming a seal between the pump inlet and outlet. As the pump operates, it generates a mixture of gas and liquid, which exits the pump at the outlet.
The gas separator is used to separate the gas from the liquid in this mixture. This is important because the gas may be vented or exhausted, while the liquid may be recycled or disposed of separately. Additionally, the gas separator can help to prevent damage to downstream equipment, such as heat exchangers or process vessels, which may be sensitive to liquid carryover.
Gas separators work by using a centrifugal force to separate the liquid from the gas. The mixture is fed into the separator at a high velocity, and the centrifugal force causes the heavier liquid to be flung outward and collected in a separate chamber, while the lighter gas is directed toward the outlet.
In sugar refineries, water trapped at gas separator is sent to cooling tower for circulation. Water gets heated due to compression and centrifuging at vacuum pump. This water is then cooled and recirculated to be used again as liquid ring in vacuum pump.
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