DRY VACUUM PUMPS & SYSTEMS FOR TOUGH INDUSTRIAL ENVIRONMENTS

• 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. Therefore, trapping a quantity of gas at the inlet (5). Then, transporting 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 orr (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. 

Do you want to learn more about the technology behind our dry screw vacuum pumps & systems Visit our technology page or watch the video below!