Vacuum Sources
Many factors help determine the best type for your application
By Henry H. Hesser
In the recent past, a process engineer had few things to consider when choosing a vacuum source for a process requiring less than atmospheric pressure. Cost, reliability, vacuum level and experience in similar processes were about all that mattered. These considerations have not disappeared, but like everything else in life, many more complications have arisen. A list of concerns today might include:
- effluent considerations (Clean Air Act and Clean Water Act)
- potential for recycling or recovery of solvents and materials removed with the vacuum stream
- energy requirements and methods of reduction
- expandability and flexibility of adding or subtracting capacity
- accuracy of vacuum control
The engineer selecting a vacuum source to meet these needs has a large variety of equipment to choose from:
- steam or air ejectors
- roots-type rotary lobe boosters
- rotary vane vacuum pumps
- screw-type vacuum pumps
- claw-type vacuum pumps
- liquid-ring vacuum pumps
- rotary piston pumps
Many other types of vacuum pumps are available, but the remaining types (diffusion pumps, cryogenic pumps, turbo-molecular pumps, ion-pumps, etc.) are generally not considered practical or economical for rough vacuum uses, such as those found in chemical and pharmaceutical applications. These pumps are normally applied in high vacuum industries like semiconductor manufacturing.
Henry H. Hesser was a staff technical specialist at Busch, Inc. when this article was first published in Pumps & Systems in August 1993. Hesser earned a Bachelor of Science in chemical engineering from the University of Texas and a master’s degree from the University of Delaware. At the time, he had 20 years of experience in cost estimating, financial analysis, government compliance standards and major engineering projects for the chemical and pharmaceutical industries.
Selecting the right vacuum technology for chemical and pharmaceutical processing applications is often difficult. First, a vacuum system must deliver the required pumping speed at operating pressure and ensure the required pump downtime. Second, it cannot be sensitive to process gases and must 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.
Dry Screw Vacuum Pumps
Dry screw vacuum technology is widely used in the chemical and pharmaceutical industries. The first dry screw vacuum pump for use in the chemical and pharmaceutical markets was launched in the 1990s. These pumps have an advantage since they do not require operating fluid to compress the process gas. In a screw vacuum pump, two screw-shaped rotors rotate in opposite directions. 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 contact 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 less than 0.1 Torr.
Modern screw vacuum pumps have a variable pitch screw, which results in even compression of the process gas across the entire length of the screw.
This has the advantage of ensuring the same temperature throughout the entire compression chamber, which can easily be monitored and controlled. Screw vacuum pumps use a cooling jacket, ensuring even temperature distribution and greater thermal efficiency and stability throughout the pump body.
Image 1. Modern screw vacuum pump operating principle (Images courtesy of Busch Vacuum Pumps and Systems)Generally, dry screw vacuum pumps operate at temperatures sufficient to prevent condensation of the process gas. This enables the process gas to avoid contamination by, or reaction with, an operating fluid, as well as preventing corrosion due to process liquids attacking the pump materials.
Ductile iron is the standard material used for process wetted parts that contact the pumped medium. The metal has special coating to make it resistant to nearly all chemicals.
In most applications, it is recommended to warm the pump prior to process operation and purge the pump with inert noncondensable gas (generally nitrogen) to remove the process vapor prior to shutdown. In some applications, it is also recommended to flush the vacuum pump with a suitable cleaning fluid to remove process material that are in the pump prior to shutdown to avoid deposits forming as the pump cools.
Flushing capability is another characteristic of the dry screw pump that arises, because the pump does not use an operating fluid. With different compression systems and various coatings, screw vacuum pumps can be configured to be compatible with any chemical.
Advantages
- 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 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 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.
Both stages use a water jacket for cooling. Versions with once-through water cooling and water circulation are available. The cooling jacket controls the pump operating temperature needed for the specific application. The once-through water version controls the temperature by controlling the water flow with a temperature-controlled valve. The recirculating coolant version uses a thermostat to control the pump temperature.
Advantages
- high vacuum level
- extremely robust and reliable
- easy servicing
- suited for conveying acid vapors and monomers or products that lead to polymerization when other vacuum technologies are used
Disadvantages
- operating fluids must be treated or correctly disposed of
Conclusion
It is important to consult with a vacuum expert and consider process conditions, process gases and integration into process control, as well as economic efficiency, safety and reliability. Often, these factors lead to a customized vacuum system that is tailored to the requirements.