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The Roots Mechanism:
The Roots mechanism is a valve less positive displacement device in which a pair of lobed rotors, interlocked and synchronized, rotate in opposite directions moving past each other and the stator wall with small clearance. Gas is trapped in front of each lobe and swept around the stator periphery from inlet to outlet. Very large displacement (above 20000 m3/h) and the mechanism has proven very effective in the pressure region from 10 to 10³ mbar, but only when delivering against low pressure differentials.
The graph shows how the attainable pressure ratio for this mechanism varies as a function of outlet pressure. In this example a maximum pressure ratio of 25:1 is obtained at an outlet pressure of about 1 mbar. The fall off in pressure ratio either side of the peak can be attributed to conductance effects and to back-leakage of a proportion of the pumped gases. This latter effect increases with increasing outlet pressure.
As a displacement machine and not a true compressor, the Roots mechanism is not suited to discharging to near atmospheric pressure ( i.e with significant differential pressure). The low pressure ratio possible results in a large amount of work being done on the pumped gas, which leads to problems of overheating.
It is possible to obtain operation at higher pressure by providing a pressure relief valve/hydrokinetic drive/Variable frequency drive such that a maximum pressure differential is not exceeded.