The benefits of dry screw, liquid ring and rotary vane vacuum pumps.
by Uli Merkle
August 1, 2018
  • dry compression, no contamination or reaction possible between process gas and operating fluid
  • high vacuum level
  • energy efficient
  • can be designed for nearly all process gases thanks to material selection and temperature regulation

Disadvantages

  • sensitive to particles entering the system
  • requires special considerations when used with process gases that tend to be reactive at high temperatures

Liquid Ring Vacuum Pumps

Liquid ring vacuum pumps are rotating positive displacement pumps with an impeller that is eccentrically placed in a cylindrical housing. A liquid sealant flows through the vacuum pump, and the rotation of the impeller creates a liquid ring on the inside of the housing that seals the spaces between the individual impeller blades. The gas is conveyed in the spaces between the shaft, the individual blades and the liquid ring. Due to the eccentric placement of the impeller, the volume of these spaces initially increases, drawing vapor in through the inlet. As the impeller continues to rotate, the volume of these spaces is reduced, the vapor is compressed and then discharged through the exhaust port. The liquid ring vacuum pump can be operated as a simple continuous sealant flow system, or a partial or total recirculation sealant system.

Image 2. Two-stage liquid ring vacuum pump operating principle Image 2. Two-stage liquid ring vacuum pump operating principle

These pumps have proven to be robust and reliable in chemical processes. The operating fluid in the compression chamber continuously dissipates the compression heat, so the vacuum pump operates nearly isothermally. This means the process gas does not heat up to a notable degree and the vacuum pump operates at relatively low temperatures, significantly reduceing the risk of unwanted reactions.

Low operating temperatures also facilitate condensation of vapor, which increases the nominal pumping speed of the vacuum pump but adds process liquid to the seal fluid. This condensed process fluid may affect the vacuum capability and/or capacity of the pump as well as generating a sealant that must be treated prior to disposal.

Water is usually used to create the liquid ring. Ethylene glycol, mineral oils or organic solvents are also often used in practice. The ultimate pressure of the vacuum pump depends on the vapor pressure of the seal liquid, and the density and viscosity of the sealant will impact the power consumption of the vacuum pump. Liquid ring vacuum pumps are available in different configurations and shaft seal arrangements, and in many materials of construction, from simple to exotic.

Advantages

  • not sensitive at all to vapor or liquid entering the system
  • materials of construction can be tailored to the process gas

Disadvantages

  • possible contamination of the operating fluid with condensate from the process gas, affecting performance and making it necessary to subsequently treat the operating fluid before its disposal
  • high energy consumption
  • ultimate pressure depends on the vapor pressure of the operating fluid

Once-Through Oil-Lubricated Rotary Vane Vacuum Pumps

These are among the mechanical vacuum pumps used in the chemical and pharmaceutical industry. A two-stage once-through oil-lubricated rotary vane vacuum pump was developed in the 1960s and was designed for chemical and pharmaceutical processing. The rotary vane vacuum pumps have three distinguishing features when compared to other vacuum pumps that operate with the rotary vane principle:

Two compression stages are stacked and connected to each other, facilitating initial compression of the process gas in the first stage and secondary compression in the subsequent stage, achieving a lower ultimate pressure.

Image 3. Once-through oil-lubricated rotary vane vacuum pump operating principle Image 3. Once-through oil-lubricated rotary vane vacuum pump operating principle

A defined amount of operating fluid, oil or other media-compatible fluid is injected into the compression chamber. Other such pumps use oil circulating lubrication.

These pumps are water cooled, allowing the operating temperature to be regulated within a certain range.

These rotary vane vacuum pumps are rotating positive displacement pumps. The vanes are placed in slots in a rotor, which rotates eccentrically in a cylindrical housing. The rotating creates centrifugal force that causes the vanes to slide out to the cylinder wall, creating spaces with different volumes and generating the suction and compression effect.

As stated, a small amount of lubricant is continuously injected into the pumping chamber and vane slots to provide lubrication for the vanes and improve the seal. The vanes do not directly contact the cylinder wall or vane slot, but slide on a lubricant film that is continually regenerated by the injected lubricant. This process takes place in both compression stages before the process gas is discharged with the operating fluid via the outlet where the lubricant is collected in the silencer for draining. The injected lubricant continually flushes the vacuum pump during operation, protecting it from corrosion and deposits.

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