Working principle of a Roots-type lobe blower |
That essential innovation was the introduction of blowers using internal compression instead of external compression. By designing compressor screws dedicated for low pressure (0.5bar(e)), dramatic energy savings are achieved resulting from improved energy efficiency and lower air outlet temperatures.
Lobe blowers
Roots-type blowers are positive displacement machines consisting of a pair of two lobed or three lobed rotors, rotating inside an oval shaped casing. One rotor is driven by external power while the other rotor is driven by synchronisation gears.
As the rotors rotate, air is drawn into inlet side and forced out the outlet side against the system pressure. There is no change in the volume of the air within the machine but it only displaces the air from the suction end to the discharge end against the discharge system resistance.
At the lobe-type blower delivery side, air at a higher pressure is present. When the rotor lobes uncover the exit port, air from the delivery side flows back into the flute space between rotor and casing. This back flow of air equalises pressure and compresses the entrapped air externally at constant volume. Furthermore, the air is forced to the discharge line against the full system pressure.
Due to dynamic losses at the inlet and discharge side and from leakages and friction, the real compression work is increased and, subsequently, the adiabatic efficiency of the blower will be reduced.
Delivering a stable flow is a key consideration for aeration systems. The theoretical maximum efficiency in the case of Roots-type blowers is 76.5% at a pressure ratio of 2, while a tuned screw blower can achieve 100%.
Due to dynamic losses at the inlet and discharge side, as well as leakages and friction, the real compression work is increased and, subsequently, the adiabatic efficiency will be reduced.
All of these effects are taken into account when defining levels of energy efficiency. The oil-free screw blower is a positive displacement machine consisting of male and female rotor elements which move towards each other while the volume between them and the housing decreases.
The rotors do not make contact and are synchronised by timing gears. Each screw blower has a fixed, integrated internal pressure ratio. To attain optimum efficiency, the internal pressure ratio must be adapted to the required working pressure.
At the beginning of the compression cycle, air at suction pressure fills the flute spaces as the rotors unmesh under the suction flange. Air continues to fill the flute spaces, until the trailing lobe crosses the inlet port and the air is trapped inside the flute space. As the lobe meshes, the flute volume is reduced, causing the pressure to increase. Air is discharged from the flute space when the leading lobe crosses the discharge port. Further rotation and meshing of the rotors forces this air to the discharge line.