8th November 2018
Selecting the right vacuum technology for chemical and pharmaceutical processing applications is often difficult. Firstly, a vacuum system has to deliver the required pumping speed at operating pressure and thus ensure the required pump-down time.
Secondly, it cannot be sensitive to process gases and has to meet all requirements when it comes to CIP (clean-in-place) cleaning and gas recovery. Reliability and economic efficiency also play a significant role when deciding which vacuum technology to use.
Here, we will outline the three vacuum technologies most often used in chemical and pharmaceutical processing technologies.
- Liquid ring vacuum pumps
- Dry screw vacuum pumps
- Oil-lubricated rotary vane vacuum pumps
Liquid ring vacuum pumps
Liquid ring vacuum pumps (fig. 1) are used in many applications. They are rotating positive displacement pumps with an impeller that is eccentrically placed in a cylindrical housing (fig. 2). Water is usually used as the operating fluid. The rotation of the impeller creates a liquid ring on the inside of the housing that seals the spaces between the individual blades. The gas is conveyed in the spaces between the centre, the individual blades and the liquid ring. Thanks to the eccentric placement of the impeller, the volume of these spaces increases, thus sucking the medium in through the inlet. As the impeller continues to rotate, the volume of the spaces is reduced, the medium is compressed and then discharged again through the discharge. The liquid ring vacuum pump can be operated as a simple continuous flow system, or as a partial or total recirculation system.
Dolphin liquid ring vacuum pump from Busch
Two-stage liquid ring vacuum pump operating principle
Over many years, these vacuum pumps have proven themselves to be robust and reliable vacuum generators in chemical processes. The operating fluid in the compression chamber continuously dissipates the compression heat, so the vacuum pump operates nearly isothermally. This means that the process gas does not heat up to a notable degree and the vacuum pump operates at relatively low temperatures. This significantly reduces the risk of unwanted reactions or an explosion. Low operating temperatures also facilitate condensation of vapours and gases, which increases the nominal pumping speed of the vacuum pump.
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 vapour pressure and viscosity of the liquid. The viscosity of the operating fluid will impact the power consumption of the vacuum pump.
Liquid ring vacuum pumps are available on the market in different versions, materials and shaft seals.
Advantages of liquid ring vacuum pumps:
- Not sensitive at all to vapours or liquids entering the system
- The different material versions enable them to be tailored to the process gas
- Possible contamination of the operating fluid with condensate from the process gas, which makes it necessary to subsequently treat the operating fluid before its disposal
- High energy consumption
- Ultimate pressure depends on the vapour pressure of the operating fluid
Dry screw vacuum pumps
Dry screw vacuum technology is also very widely used in the chemical and pharmaceutical industries. However, this is relatively new compared to liquid ring technology.
In the 1990s, Busch launched the first dry screw vacuum pump on the market, the COBRA AC. The major difference compared to the liquid ring vacuum pump described above is that screw vacuum pumps (fig. 3) do not require operating fluid to compress the process gas. This is why it is called a “dry” screw vacuum pump.
COBRA NC screw vacuum pump
In a screw vacuum pump, two screw-shaped rotors rotate in opposite directions (fig. 4). The pumped medium is trapped between the cylinder and screw chambers, compressed, and transported to the gas outlet. During the compression process the screw rotors do not come into contact with each other or the cylinder. Precise manufacturing and minimal clearance between the moving parts enable this operating principle and, in addition, guarantees a low ultimate pressure of <0.1 mbar.
Modern screw vacuum pump operating principle
Screw vacuum pumps operate using water cooling, which ensures even temperature distribution throughout the pump body and thus thermal stability in the entire process.
Modern screw vacuum pumps feature variable pitch screws, which results in pre-compression of the process gases. The advantage is that both gas temperatures and power consumption of the vacuum pump can be reduced significantly. In older generations of screw vacuum pumps, the pitch of the screws is the same across the entire length. This leads to compression of the process gas in the last half rotation of the screw, which generates excessive thermal load there. Thus, adjusting to the ideal operating temperature with water cooling is more difficult. Generally, dry screw vacuum pumps operate at higher temperatures than liquid ring vacuum pumps. Condensation of process gas elements is thus largely eliminated. This enables the process gas to be conveyed through the vacuum pump without contamination or causing a reaction with an operating fluid. Cast iron is the standard material used for work pieces for all parts that come into contact with the pumped medium. It is either untreated or treated with a special coating to make it resistant to nearly all chemicals. After the end of the process, we recommend flushing the vacuum pump with a suitable cleaning fluid and to purge it with nitrogen to avoid corrosion and deposit forming during stand-still.
With different compression systems and various coatings, screw vacuum pumps from Busch can be configured to be compatible with any chemical.
Advantages of dry screw vacuum pumps:
- 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 of dry screw vacuum pumps:
- Sensitive to particles entering the system
- Cannot be used with process gases that tend to be reactive at high temperatures
Once-through oil-lubricated rotary vane vacuum pumps
Oil-lubricated rotary vane vacuum pumps have been successfully used in many fields for decades. Today, they are amongst the most widely used mechanical vacuum pumps in the industry. Busch already developed the Huckepack, a two-stage once-through oil-lubricated rotary vane vacuum pump in the 1960s, which was specially designed for chemical and pharmaceutical processing technology. Busch has constantly further developed this vacuum pump, which continues to enjoy great acceptance in processing technology today thanks to its robustness.
Huckepack rotary vane vacuum pumps (fig. 5) have three significant distinguishing features when compared to other vacuum pumps that operate according to the rotary vane principle:
- Two compression stages are stacked and connected to each other which facilitates initial compression of the process gas in the first stage and secondary compression in the subsequent second stage. This makes it possible to achieve a lower ultimate pressure.
- These vacuum pumps feature an oil-lubrication which means that a defined amount of operating fluid, oil or other media-compatible fluid is injected into the compression chamber. In contrast, other rotary vane vacuum pumps use oil circulating lubrication.
- Huckepack rotary vane vacuum pumps are water cooled, thus allowing the operating temperature to be regulated within a certain range.
Huckepack once-through oil-lubricated rotary vane vacuum pump
Huckepack once-through oil-lubricated rotary vane vacuum pump operating principle
Huckepack 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. Because of the centrifugal force created by the rotating motion of the rotor the vanes slide out of the slots and they come into contact with the cylinder wall. This creates spaces with different volumes, which in turn generate the suction and compression effect. To reduce friction and improve the seal, oil is continuously injected into the compression chamber. This process takes place in both compression stages before the process gas is discharged together with the operating fluid via the outlet and can subsequently be removed. Both stages are water cooled. Versions with once-through water cooling and water circulation are available.
Because the lubricant only flows through the vacuum pump once, nearly all liquids with a viscosity in the range of 150 centistokes (cSt) can be used. These constantly flush the vacuum pump during operation, protecting it from corrosion and deposits. Busch offers vanes made of three different materials to ensure resistance to most solvents.
Advantages of once-through oil-lubricated rotary vane vacuum pumps:
- High vacuum level
- Extremely robust and reliable
- Easy servicing
- Perfectly suited for conveying acid vapours and monomers or products that lead to polymerization when other vacuum technologies are used
- Operating fluids must be treated or correctly disposed of
All of the vacuum generation technologies discussed here have advantages and disadvantages. There is no single ideal solution for all applications. It is therefore important to seek consultation from a vacuum expert and take into account all important parameters in the process, starting with process conditions, process gases and integration into process control, through to economic efficiency, safety and reliability of future vacuum generation. In most cases, consideration of these factors leads to a customized vacuum system that is directly tailored to the requirements.