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.
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.