A Roots blower

A rotary lobe blower, Roots Blower refers to a specific design of positive displacement compressor with two lobed impellers with a roughly figure "8" shape which are mechanically linked via gears such that they rotate in opposite directions. The figure "8" shape allows the impellers to be close to but never in contact with both each other and the pump walls at every position of their rotation. The close tolerances of the blower allow the impellers to move without a lubricant producing an oil free process flow. The machine has been modified and improved to include European and Asian designs with three lobe blowers that produce lower pulsations and lower noise.

Isaiah Davies invented the design principle of the rotary lobe blower in 1848, but it was some twenty years later before Francis and Philader Roots applied it in practice in the US, first as a water wheel in their woolen mill. The term Roots Blower derives from the Roots brothers. Still in business today, Rootsâ„¢ vacuum pumps are a diverse line of pumps manufactured by Roots division of industrial giant Dresser Industries.

Applications

Roots blowers are typically used in applications where a large volume of gas must be moved across a relatively small pressure differential. This includes low vacuum applications, with the roots blower acting alone, or use as part of a high vacuum system, in combination with other pumps. Roots blowers are also used as superchargers.

Roots blowers are also used in reverse to measure the flow of gases or liquids, for example, in gas meters.

Technical considerations

Because rotary lobe pumps need to maintain a clearance between the lobes, a single stage roots blower can only pump gas across a limited pressure differential. If the pump is used outside its specification, the compression of the gas generates so much heat that the lobes expand to the point that they jam, damaging the pump.

Roots pumps are capable of pumping large volumes but as they only achieve moderate compression, it is not uncommon to see multiple roots blower stages, frequently with heat exchangers (intercoolers) in between to cool down the gas. The lack of oil on the pumping surfaces allows the pumps to work in environments where contamination control is important. The high pumping rate for hydrocarbons also allows the roots pump to provide an effective isolation between oiled pumps, such as rotary compression pumps, and the vacuum chamber.

Comparative advantages

The roots pump is never used as a stand alone pump in high vacuum applications but its high pumping speed can be used to reduce the end pressure and increase the pumping speed of other vacuum pump systems in the medium to low vacuum range.