Rotary Tooth Compressors
One type is illustrated in Figures 23 and 24. The rotors are parallel in section and are externally meshed through timing gears; there is no possibility of one tooth driving the other as in rotary screws. They are suitable for producing oil-flee air. They are available as low pressure single-stage blowers up to 2.5 bar and as two stage compressors up to 7 bar. They are claimed to be suitable for pressures up to 10 bar. The rotors are of comparatively simple construction in that they do not require helical machining so they can be made from materials that would otherwise be difficult to manufacture, such as stainless steel.
The inlet and delivery ports are situated in the end plates of the compressor housing with no inlet or delivery valves. There is very little end thrust on the rotating elements, which ensures low bearing loads. With the tooth form shown, there can be no drive between the rotors so there has to be external gearing.
Because of the straight tooth design, and the axial inlet and delivery ports, axial loads on the rings are eliminated. The compressor illustrated has rotor speeds between 3000 and 6000 r/min. Note that in this design, portions of the teeth intersect.
An alternative design has no intersecting part (Figure 25), which makes it closer to a Roots blower. This compressor element can be direct coupled to an electric motor with a rotary speed to 1500 r/min and below.