Many technologies have been used to provide vacuum for chemical and pharmaceutical processes. Some examples include steam jet ejector systems using the Venturi effect to draw a vacuum, liquid-ring vacuum pumps using water as a sealant and rotary vane vacuum pumps using a once-through sealing system. Each of these technologies continues to have a place in chemical and pharmaceutical applications, but dry screw vacuum technology is preferred in most chemical and pharmaceutical applications since no operating fluid is required to compress the process gas. In a screw vacuum pump, two interlocking screw-shaped rotors rotate in opposite directions. The process vapors are drawn in, 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 and guarantee an ultimate pressure of <0.1 Torr. Dry screw vacuum pumps in the early 1990s used constant pitch screws. With constant pitch screws, gas is transported through the pump without compression and then is compressed to atmospheric pressure at the exhaust port. With almost all compression occurring at this location, the heat of compression is concentrated, causing high gas temperatures, which can lead to vapor decomposition and can exceed the autoignition temperature of the process gases. Modern screw vacuum pumps include a variable pitch screw, which results in compression of process gas across the entire length of the screw. This has the advantage of ensuring a more consistent temperature throughout the compression chamber, which can easily be monitored and controlled. Screw vacuum pumps use a cooling jacket, which ensures an even temperature distribution, and provides greater thermal efficiency and stability throughout the pump body. Generally, dry screw vacuum pumps operate at temperatures warm enough to prevent condensation of the process gas, preventing corrosion due to process liquids attacking the pump, but cool enough to prevent potential issues such as vapor decomposition or autoignition. Since there is no seal fluid in the pumping chamber, the process vapors exit the pump without contamination, allowing the condensation of the process vapors to be recycle or used downstream, should the financial returns prove worthwhile. Ductile iron is the standard material used for process-wetted parts in contact with the pumped medium. The metal is given with a Teflon-impregnated special coating to make it resistant to nearly all chemicals. For extreme cases, other coatings are available to give additional protection. In most applications, it is important to warm up the vacuum pump prior to process operation and to purge the pump with an inert, noncondensable gas (generally nitrogen) to remove the process vapor prior to shutdown. In some applications, it is helpful to flush the vacuum pump with a cleaning fluid to avoid deposits of process material forming as the pump cools. Dry screw vacuum pumps are also amenable to the use of variable speed drives (VSDs) to control the operating pressure and to match the pumping speed to the required process flow. VSDs provide the added benefit of optimizing the energy usage of the vacuum pump. With different compression systems, various coatings and appropriate accessories, screw vacuum pumps can be configured to be compatible with virtually any chemical.
Advantages of Dry Screw Vacuum Pumps:
- dry compression, no contamination or reaction possible between process gas and operating fluid
- ultimate vacuum
- 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