Vacuum Pumps Department Editor: Kate Torzewski - Chemical Engineering

Vacuum is any system of reduced pressure, relative to local (typically atmospheric) pressure.

Achieved with a pump, vacuum systems are commonly used to:

♦ Remove excess air and its constituents
♦ Remove excess reactants or unwanted byproducts
♦ Reduce the boiling point
♦ Dry solute material
♦ Create a pressure differential for initiating transport of material

Liquid-ring and dry pumps offer the most advantages for the chemical process industries (CPI). Both of these pump types have bearings sealed off from the pumping chamber and do not require any internal lubrication because the rotors do not contact the housing. Both, when
employing a coolant system, prevent the coolant from contacting the process fluid and causing contamination, and both use mechanical shaft seals for containment.

Liquid-Ring Pumps
In the cylindrical body of the pump, a sealant fluid under centrifugal force forms a ring against the inside of the casing (Figure 1).

The source of that force is a multi-bladed impeller whose shaft is mounted so as to be eccentric to the ring of liquid. Because of this eccentricity, the pockets bounded
by adjacent impeller blades (also called buckets) and the ring increase in size on the inlet side of the pump, and the resulting suction continually draws gas out of the vessel being
evacuated. As the blades rotate toward the discharge side of the pump, the pockets decrease in size, and the evacuated gas is compressed, enabling its discharge.

The ring of liquid not only acts as a seal; it also absorbs the heat of compression, friction and condensation. Popular liquid choices include water, ethylene glycol, mineral oil and organic solvents.

Dry Pumps
Rotary-claw, rotary-lobe and rotary-screw pumps dominate as dry pumps in the CPI, particularly in larger-size pump applications.

Rotary Claw. The geometric shape of this pump allows for a greater compression ratio to be taken across the rotors at higher pressures (Figure 2). Two claw rotors rotate in opposite directions of rotation without touching, using timing gears to synchronize the rotation.The gas enters through an inlet port after it has been uncovered and fills the void space between the rotors and pump housing. On the next rotation, that same trapped sample of gas is
compressed and discharged as the discharge port opens.

A minimum of three stages in series is required to achieve pressures comparable to those of an oil-sealed mechanical pump. Some dry designs use two technologies in combination; for example, a rotarly lobe as a booster for a claw pump.

Rotary Lobe. The rotary-lobe pump (Figure 3) is typically used as a mechanical booster operating in series with an oil-sealed piston or vane pump to boost pumping capacity at
low pressures.

This pump consists of two symmetrical two-lobe rotors mounted on separate shafts in parallel, which rotate in opposite directions to each other at high speeds.Timing gears are used to synchronize the rotation of the lobes to provide constant clearance between the two.

Rotary Screw. Two long helical rotors in parallel rotate in opposite directions without touching, synchronized by helical timing gears (Figure 4). Gas flow moves axially along the screw without any internal compression from suction to discharge. Pockets of gas are trapped
within the convolutions of the rotors and the casing, and transported to the discharge. Compression occurs at the discharge port, where the trapped gas must be discharged against atmospheric pressure. Each convolution of the rotor acts similarly to a stage in series with the one behind it; at least three convoluted gas pockets in the rotor are required to achieve acceptable vacuum levels.

References
1. Vilbert, P., Mechanical Pumps for Vacuum Processing, Chem. Eng. October 2004, pp. 44–51.
2. Aliasso, J., Choose the Right Vacuum Pump, Chem. Eng. March 1999, pp. 96–100.