A Roots blower is a valve-less displacement compressor without internal compression.
Blowers are usually air-cooled and oil-free. Their low efficiency limits these blowers to very low-pressure applications and compression in a single stage, even if two- and three-stage versions are available. Roots blowers are frequently used as vacuum pumps and for pneumatic conveyance.
Root blower Compressor Diagram :
Construction and Working of Root Blowers :
- The two-lobe type roots blower is shown in Fig. One of the rotors is connected to an external drive and the second rotor is driven from the first.
- The lobes of the rotors are of cycloid or involute form giving correct mating to seal the delivery side from the inlet side.
- The high-pressure delivery side is sealed from the low-pressure suction side at all angular positions and this sealing continues until delivery commences.
- To reduce wear, a small clearance (i.e 0.01 cm to 0.02 cm) is provided between the lobes and casing.
- The clearance however forms a leakage path and has an increasingly adverse effect on efficiency as the pressure ratio increases.
- During rotation, the volume of air ‘V’ at atmospheric pressure ‘P’ is trapped between the left-hand rotor and the casing, and this air is positively displaced at constant pressure without a change in volume until the space opens to high-pressure region.
- Theoretically, there should be no pressure rise; because the flow area does not decrease from inlet to exit.
- But when the outlet port opens, the high-pressure air in the receiver offers resistance to the delivery of blower discharge causing in instantaneous irreversible pressure rise.
- The air in space ‘A’ and ‘C’ is at atmospheric pressure P1 and the air in the space B increases in pressure to the value of receiver pressure P2.
- The irreversible pressure rise from P1 to P2 at constant volume is shown in the P-V diagram.
Efficiency of Roots blower
Roots efficiency is defined as the ratio of isentropic work done to actual work done and it is given by,
Roots efficiency = ( Isentropic Work done / Actual Work done )
This equation shows that the efficiency of the root decreases with the increase in pressure ratio.
Applications of Root Blowers :
Application of Root blowers are as follows :
- Scavenging and supercharging of I.C. Engines.
- Delivery capacity: 0.14 m3/min to 1400 m3/min.
Pressure ratio: 2:1 for a single-stage and 3:1 for a two-stage.
- Electrolytic Tank – Deliver air into the electrolytic tank so as to circulate the electrolyte for achieving better plating quality.
- Paper Feeding for Printer – For paper segregation, paper handling, and paper feeding.
- Cleaning the Piping – To blow out dust or metal from the piping, so as to keep the piping clean.
- Drying the Conveyer Belt – For drying small conveyor belts.
- Wastewater Treatment – Stirring the sediment, or purification of a water treatment plant.
Advantages of Root Blowers :
Root blowers has some advantages over another type of compressor, they are as follows,
1) It has only two moving parts (i.e two rotors) which are identical in shape and size.
2) Its operation is entirely rotary.
3) As the rotors are symmetric about their center of rotation, the operation is dynamical balanced.
4) Discharge of the compressed gas is complete and there is no clearance volume.
- In the range of partial load, the conveying speed is higher, i.E. Wear on the conveyor pipe and breakage of the conveying material will be greater.
- A conveying speed once chosen can only be altered with considerable expenditure.
- Due to the low-frequency noises (pulsating conveying airflow), expensive noise dampening equipment will be necessary.
- In order not to exceed the operating pressures, a control device must be installed; on account of the narrow piston clearance.
- The roots-blower is sensitive to foreign matter, i.E., Filter cleaning of the conveying air is required.
- After a longer period of use, the piston clearance becomes larger and leads to capacity losses.
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