Dry vacuum pump technology — especially variable pitch chemical dry pumps — offers clear, measurable advantages in various critical applications. Dry screw vacuum pumps that are correctly designed can ensure processing needs are met through safe, reliable, and cost-effective solutions.
Dry screw vacuum pumps do not require water or oil for sealing or lubrication at vacuum levels. Therefore, these dry vacuum systems avoid the generation of wastewater, contamination, and high disposal costs.
Dry screw vacuum pumps consist of two parallel, non-contact helical screws that rotate in synchronization at high speeds through precision gears. They rotate in opposite directions, capturing a certain volume of gas at the intake port and transporting it to the exhaust port and into the discharge channel. The special shape of the cylinder walls and the intermeshing screws form a compression chamber or cavity for conveying gas. The small clearances between the screws and the cylinder walls, as well as between the intermeshing screws, ensure minimal reverse leakage to the intake port compared to the forward flow generated by the screw cavities.
The length of the sealing boundary (i.e., the number of helices and tight clearances) prevents the backflow of the pumped gas. In pumps equipped with a backing plate, when the outlet valve or port is initially exposed, the gas slightly back-expands into the screw. Under the action of the screw, the captured gas volume gradually reduces to zero, allowing for rapid discharge.
The reverse flow of gas is mainly controlled by the width of the "sealing zone" at the end of the screw profile. These wide areas lie close to the stator, minimizing reverse gas leakage. The ultimate pressure of screw pumps can be less than 0.01 torr (0.01 mBar). In variable pitch models, as the pitch changes, the gas is compressed, providing additional compression before the pump discharge. This distributes thermal load more uniformly along the rotor's length. In single-pitch models, greater compression is achieved in the last half turn against the backing plate or valve, resulting in heat generation concentrated near the exhaust port. In dry pumps, the temperature must be high enough to prevent overall condensation while low enough to avoid spontaneous ignition and polymerization. The gradually increasing gas temperature towards the exhaust port in variable pitch pumps significantly helps prevent condensation of pumped vapors. Variable pitch screw pumps also utilize power more effectively than single-pitch screw pumps.
Cooling is carried out through a surrounding jacket. Pumps can be configured for direct or indirect closed-loop cooling. The latter has many advantages, as it means the cooling water for the apparatus never comes into direct contact with the pump material, preventing jacket clogging or corrosion from poor quality cooling water.
There are gas ballast ports. If needed, gas ballasts can help quickly heat up cold pumps or dry wet pumps, keep flammable vapors out of their flammable range, and help remove solids from the pump, especially during solvent flushing.
Thermal management is crucial for the reliable operation of chemical dry screw vacuum pumps.
In pumps too cold for a given process, corrosive vapors may condense, leading to corrosion, lubricant dilution, and seal swelling. This damage is severe but only occurs if vapors are allowed to condense into the liquid phase. Conversely, if the pump's operating temperature is too high for a given process, unnecessary reactions like polymerization or spontaneous ignition may occur, and bearing temperatures may rise, potentially leading to thermal seizure.
The various impacts mentioned above can be slightly mitigated by internal coatings, but this method should never be solely relied upon. Coatings protect the pump well during initial storage and system commissioning, but they can only survive long-term at the temperatures and vacuum levels where the pump is predominantly used.
The key is to ensure process vapors remain in the gas phase. Some strategies to ensure this include controlling the temperature/flow rate of pump coolants, using nitrogen purging to alter the process dew point, and using inlet condensers to remove vapors upstream of the pump.
To further enhance reliability when the system faces challenges, additional features can be added to the pump system to help guarantee reliability. One example is a solvent flushing system to keep the pumping mechanism clean and clear. Another is a demister tank and filters to capture unavoidable liquid or powder agglomerates.
Dry screw vacuum pumps are very simple, yet technically advanced, reliable, and efficient. Dry and non-contact operation eliminates the need for lubrication inside the pump chamber. This translates into a major advantage: avoiding any contamination of the process and environment during pump operation. Thanks to the oil-free and non-contact screw design, dry vacuum pumps can safely and reliably handle corrosives, organics, inorganics, and solvents. Major applications include:
Distillation (Conventional, Short Path, and Molecular Distillation)
Drying (Filter, Freeze, and Transformer Drying)
Evaporation Vacuum in Filtration Chambers (Central or General/Laboratory Vacuum Service, Pilot Plants)
Reactor Servicing Solvent Recovery (Fuel Vapors)
Sterilization (Ethylene Oxide)
Problematic Gases (Flammable, Low-Spontaneous Combustion Temperature, Corrosive Gases, and Hydrogen)
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