Turbomolecular high vacuum pump; Source: Busch Vacuum Pumps and SystemsHigh vacuum pumps provide evacuation of chambers or systems into the high vacuum (10-3 to 10-8 torr) or ultra-high vacuum (<10-8 torr) range of pressure. Mechanical pumps and venturi generators operate in the medium to rough vacuum ranges.

There are a number of pump designs made which can generate high or ultra-high vacuums. Each of these types operates using either the gas-transfer or gas-capture principle.

Gas-Transfer

Vacuum pumps that operate on the gas-transfer principle create vacuums by moving gases through the pump. These pumps can be used continuously without the need for regeneration. All gas-transfer pumps are either positive displacement or kinetic (momentum transfer). Positive displacement (gas-displacement) pumps displace gas from sealed areas to the atmosphere to a downstream pump stage. Kinetic pumps displace gas by accelerating it in the pumping direction, either mechanically (mechanical vacuum pumps) or via an adjacent vapor stream (venturi jet vacuum pumps).

Turbomolecular Pumps

Turbomolecular pumps use a rotating disk with a series of rotors and flow stabilizing, stationary stators to impart a preferential motion to gas molecules and create molecular flow through the pump. The drive shaft of the disk may operate at speeds from 25,000 to 90,000 rpm. They are often described as "molecular bats." They are also known as axial flow turbines. Turbomolecular pumps can pump into the 10-10 Torr range on a suitable system and are known as the workhorses of high vacuum technology.

Molecular Drag Pumps

Molecular drag pumps (also called molecular pumps or drag pumps) are similar to turbomolecular pumps, except a rotor drum with a ridged surface and cylindrical stator are used in place of stator and rotor blades to impart a preferential motion to gas molecules and create molecular flow through the pump. Through this mechanism, gas molecules are physically dragged along the surface, hence the name molecular "drag" pump. These pumps can only reach ultimate pressures as low as 10-6 Torr. These pumps are used in applications with limited conductance (pressure drop or resistance through the system) and for frequent cycling operations.

Selection Tip: Drag pumps will start pumping at inlet pressures several torr higher than turbopumps, and they do not require as low backing pressures for full operation. They also have much lower pumping speeds than turbomolecular pumps with the same inlet size.

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