Vacuum plays an essential role in chemical and pharmaceutical processes. Whether in vacuum conveying, inertization, distilling or drying processes, vacuum is used to make processes safer, faster and more economical or to make them possible.
There are versatile technologies for generating vacuum. Liquid ring vacuum pumps and steam ejectors have been robust workhorses for many decades when it comes to generating vacuum. However, like rotary vane vacuum pumps with recirculating oil lubrication, they have one disadvantage: they require an operating fluid that comes into contact with the process gas.
Screw vacuum pumps hit the market in the mid-1990s. The major difference from the vacuum pumps known at the time was that screw vacuum pumps did not require any operating fluid to compress the process gas, and this is why they are called dry screw vacuum pumps (Image 1).
Dry screw vacuum technology is now also widely used in the chemical and pharmaceutical industries.
In a screw vacuum pump, two interlocking screw-shaped rotors rotate in opposite directions (Image 2). The process gases are drawn in, trapped between the cylinder and screw chambers, compressed and transported to the gas discharge. During the compression process, the screw rotors do not come into contact with each other or the cylinder. Precise manufacturing and minimal clearances between the moving parts enable this operating principle and, in addition, ensure a low ultimate pressure of up to 0.01 millibar (absolute).
Screw vacuum pumps operate with a cooling system that ensures even heat distribution, greater thermal efficiency and stability throughout the pump body. This allows the temperature to be selected so that it is high enough not to condense the process gas, but low enough to avoid potential temperature-related problems such as gas deposition or spontaneous ignition. The absence of operating fluid allows a compression of the process chamber without contamination
Protecting the Vacuum System
Depending on the process gas, the vacuum pump can be exposed to certain risks. Therefore, it is important that the process gases are sufficiently known in order to minimize these risks. Different components are often required, which can be installed on the inlet or pressure side, in order to convey the process gas without damaging the vacuum pump. This is called a vacuum system, which can also consist of several vacuum pumps (Image 3).
For safe operation of the vacuum system, it is important to protect it from corrosion and deposits caused by crystallization or polymerization, and to increase the material resistance.
Protection Against Corrosion
Various measures can be effective in protecting the vacuum system or the individual vacuum pumps against corrosion. The first possible measure is to prevent corrosive substances from entering the interior of the vacuum pump. This can be implemented by upstream condensers or gas scrubbers.
The second possible measure to avoid corrosion is to keep the process stream in the gas phase. In a screw vacuum pump, this can be implemented by setting a certain operating temperature. In addition, the process gas can be diluted by a supplied ballast gas to reduce the partial pressure of the condensable gases.
So, the following logic applies: suction in gaseous form and ejection in gaseous form. The minimum temperature must be selected so that it is high enough to prevent gases from condensing out. The maximum temperature must be selected so that the vacuum pump is not damaged or so the maximum permissible temperature according to ATEX classification (for equipment capable of causing an explosion) is not exceeded.
A third possible measure is to use compatible materials for the vacuum pump. An example is a vacuum pump where all parts in contact with the process are made of ductile cast iron by default and have a special coating that is resistant to almost all chemicals.
Protection From Particles Entering the System
Screw vacuum pumps should always be operated with an inlet screen or an inlet filter. This is to prevent particles from entering the inside of the vacuum pump. Due to the precise manufacturing of screw vacuum pumps with the associated small clearances and tolerances, there is a certain sensitivity to entrained particles. Dry screw vacuum pumps are frequently used with particulate dryers, especially in the pharmaceutical industry. A certain number of such particles can easily pass through the vacuum pump together with the process gas or be flushed out at the end of the process. Nevertheless, it is advisable to take appropriate precautions in order to prevent particles from being sucked in on a regular basis.
Leak-Tightness of the Vacuum Pump/Vacuum System
Vacuum pumps and vacuum systems in a chemical environment must be so tight that no air or a minimum of ambient air can enter and create a potentially explosive atmosphere, or toxic or explosive gases can escape. Polymer O-rings are generally used to prevent leaks between two stationary parts. The resistance depends on the selected polymer.
The seal material also needs to be adapted to possibly process gases.
For most applications, it is recommended that the vacuum pump is warmed up for a certain lead time before process operation. This allows the specified temperature to be set. After the end of the process, it is recommended to purge the vacuum pump with noncondensable inert gas to completely remove the process gas from the vacuum pump before switching it off.
Nitrogen is normally used for this flushing process. Flushing the vacuum pump with a cleaning liquid at the end of the process is also possible and recommended if there is a risk of deposits forming inside the vacuum pump
With different sealing systems, various coatings and appropriate accessories, some screw vacuum pumps can be configured to be compatible with virtually any chemical. Also, any other national regulation can be adapted for these vacuum pumps like the National Electric Code’s explosion-proof regulations in U.S. or Korean Occupational and Safety Health Association (KOSHA) in South Korea.
If adapted, these vacuum pumps can also be used worldwide in potentially explosive areas and for conveying explosive gases and vapors. Flame arresters may also be integrated if necessary.