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How Dry Screw Vacuum Pumps Work 

Dry vacuum pump technology – especially variable pitch screw chemical dry pumps – offer clear, measurable advantages in a wide variety of essential applications. A properly designed dry screw vacuum pump ensures the processing needs are accomplished with a safe, reliable, and cost-effective solution.

Dry screw vacuum pumps require no water or oil for sealing or lubrication in the vacuum stages. Consequently, these dry vacuum systems eliminate effluent generation, pollution, and high treatment costs.

Dry Screw Vacuum Pump Operation

Dry Screw Vacuum Pump Operating Principles
  • A dry screw vacuum pump consists of two parallel, non-contacting helical screw-shaped rotors (1) and (2), Fig. 1, rotating synchronously at high speeds via precision gears (3). They rotate in opposite directions, and in so doing, trap a quantity of gas at the inlet (5) and transport it towards the exhaust port (6) and into the exhaust channel (7). The walls of the stator (9) and the special shape of the intermeshing screws form the compression chambers or pockets (4) that transport the gas.
  • Small clearances between the screws and the stator, as well as small clearances between the intermeshing screws, ensure that the amount of reverse leakage towards the inlet is small in comparison to the forward flow of gas generated by the screw pockets.
  • Reverse flow of the pumped gases is prevented by the length of the sealing boundary, (i.e., the number of spirals and tight clearances). On pumps fitted with a compression plate a slight reverse expansion of gas into the screws occurs when the outlet valve or port is first exposed. This is quickly expelled as the trapped volume is progressively reduced to zero by the action of the screws.
  • The reverse flow of gas is primarily controlled by the width of the “sealing lands” on the tips of the screw profile. These wide lands run in close proximity with the stator and minimize the reverse leakage of gas. Ultimate pressures in screw pumps can be less than 0.01 torr (0.01 mBar). 
  • In variable pitch models, the gas is compressed as the pitch changes to give additional compression before the pump exhaust. This spreads the heat load more evenly across the length of the rotors. In single pitch models, more compression is achieved in the last half-turn against a compression plate or valve, biasing the heat generation towards the exhaust. In dry pumps, temperatures have to be high enough to avoid condensation throughout and low enough to avoid auto-ignition and polymerization. Progressively higher gas temperature towards the exhaust in variable pitch pumps assists greatly in preventing condensation of pumped vapors. Variable pitch screw pumps also use power more efficiently than single pitch ones. 
  • Cooling is achieved via the surrounding jacket (8).  Pumps can be configured for direct or indirect closed-loop cooling. There are many advantages with the latter, as it means the plant’s cooling water is never in direct contact with the pump material and the jacket cannot silt up or corrode due to poor cooling water quality.
  • A gas ballast port (10) is available. If required, a gas ballast can help to warm-up a cold pump or dry a wet pump faster, take a flammable vapor out of its flammable range and help to clean solids out of a pump, particularly during solvent flushing.

Watch This Animation To See A Dry Screw Vacuum Pump In Action

Thermal Management In Dry Screw Vacuum Pumps

Thermal management is essential for reliable operation of chemical dry screw vacuum pumps.

In a pump that is too cool for a given process, aggressive vapors may condense, leading to corrosion, the dilution of lubricants, and the swelling of seals.  This damage is serious but can only happen if the vapor is allowed to condense into the liquid phase.

Conversely, if the operating temperatures in the pump are too high for a given process, unwanted reactions such as polymerization or auto-ignition are possible, with the addition of high bearing temperatures or thermal seizure. 

The effects mentioned above can be mitigated slightly by internal coatings but this is something that should never be relied on. Coatings work great to protect the pump during initial storage and system commissioning but they can only survive so long at the temperatures and vacuum levels where the pumps spend most of their time.

The key is to ensure that the process vapors stay in the vapor phase [Green area] in the figure below.  A few strategies to ensure this occurs include temperature/flow control of the pump coolant, use of nitrogen purge to change process dew point, and the use of inlet condensers to remove the vapor upstream of the pump.  

To further improve reliability where system challenges are always present, additional features can be added to the pump system to help guarantee reliability. One example is a solvent flush system to keep the pumping mechanism free and clean. Another being knock-out pots (KOPs) and filters to capture liquid or powder slugs when unable to prevent them. 

Thermal Management in Dry Screw Vacuum Pumps

Oil-free Vacuum For Industrial Applications 

NASH dry screw vacuum pumps are remarkably simple, yet sophisticated, reliable, and highly efficient. The dry and contact-free operation requires no lubrication in the pumping chamber. This translates into major advantages: no process contamination and no pollution caused by the pump operation. NASH dry vacuum pumps can safely and reliably handle corrosives, organics, inorganics, and solvents because of its oil-free, non-contacting screw design. Key applications include:

  • Distillation (normal, short path & molecular)
  • Drying (filter, freeze, and transformer drying)
  • Evaporation
  • Filtration
  • House vacuum (central or general/laboratory vacuum service, pilot plants)
  • Reactor service
  • Solvent recovery (fuel vapor)
  • Sterilization (ethylene oxide)
  • Problem gasses (flammable, low auto-ignition temperatures, corrosive gasses, and hydrogen)
  • Conveying

Additional Applications Include

  • Crystallization 
  • Deodorization 
  • Degassing 
  • Desorption 
  • Fluid handling 
  • Impregnation
  • Pervaporation 
  • Polymerization 

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