In this article we will discuss about:- 1. Rotary Vane Compressor 2. Screw Compressor 3. Roots Blower (Lose Type).
Rotary Vane Compressor:
Rotary Vane compressor is also called as ‘sliding vane compressor’. It consists of a rotor eccentrically housed in the casing. Rotor has several radial slots in it, each housing a spring loaded vane. These vanes are made of steel or synthetic fibrous material. Larger the number of vanes, internal leakage of air decreases due to small pressure difference prevailing between the adjacent spaces around the rotor.
High pressure ratio requires large number of vanes (20 – 30). The casing has intake and delivery openings. These compressors are often used for capacities upto 150 m3/min and for pressure ratios upto 8.5. For a given pressure ratio and FAD, vane compressor requires less work input than that for roots blower.
When the rotor rotates, vanes are driven out of the rotor towards the casing due to centrifugal force. The space between the two adjacent vanes, rotor and the casing increases creating vacuum. Thus, the gas is drawn in, from the suction opening. When the rotor crosses the point just opposite to its eccentricity, suction starts. As the rotor continues to rotate, the entrapped gas is compressed due to reduction in volume.
The high pressure gas is then discharged through delivery opening. Usually, half of the total pressure rise is developed during the internal reversible compression and the remaining pressure rise occurs irreversibly when the entrapped gas is released to the delivery side, due to back flow of high pressure air from the receiver.
Screw (Lysholm) compressor consists of two rotors meshing with each other closely. Each rotor is helical with large pitch. The male rotor has three or four lobes while the female rotor has four, five or six recesses. The entrapped air is continuously compressed as it flows axially, due to narrowing of passages formed by the rotors. The inlet and discharge openings are somewhat oblique as shown in the 16.7.
The cycle of working of screw compressor consists of four processes namely:
When the meshing rotors begin to disengage near the suction opening, gas is drawn in due to low pressure created. This is continuous so long as rotors unmesh.
As the rotation of the rotor continues, the space filled with gas isolates from suction opening. This space is then gradually transported around the outer periphery’ at constant pressure until the male lobe starts meshing with the space being transported.
When rotors remesh from suction end gradually, the entrapped gas is compressed.
Soon after the space filled with entrapped gas is exposed to discharge opening, the gas is driven out into the delivery line due to continuous penetration of male rotor into the space.
The compressor is run at very high speeds, 3000 r.p.m. for large compressors and upto 32000 r.p.m. for small units. Rotors have peripheral velocity of 50 m/s to 100 m/s. The rotors made of Carbon steel need precision machining. High pressure ratio increases internal leakage of gas back to suction opening and reduces volumetric efficiency.
These compressors have capacity ranging between 3 m3/min to 1000 m3/min and pressure ratios of 4 (in single stage) and 8 – 11 (in double stage). The compressor does not need any lubrication, except for bearings. Thus, there is no contamination of gas with lubricating oil. These compressors are widely used in food and chemical industries.
The rotors are dynamically balanced hence, it runs vibration free and does not need heavy foundation. Its main drawback is operating noise of high frequency.
AA = Cross-sectional area between two lobes of male rotor, m2
AB = Cross-sectional area between two lobes of female rotor, m2
nA= Number of lobes in the male rotor
L = Length of rotor, m
NA = Speed of male rotor, r.p.m.
ηvol = Volumetric efficiency
∴ Volume of gas sucked/rev. = (AA + AB) nA L NA x ηvol, m3/min. …(5)
Roots Blower (Lose Type) Compressor:
Figure 16.4 (a) shows a twin lobe roots blower. It has two lobes each mounted on separate shaft. One of these shafts is connected to the external prime mover (electric motor) while the other is gear driven from the first. The lobes of the rotor are of cycloidal or envolute profile. Throughout all angular positions, the high pressure delivery side remains sealed from the low pressure suction side by the closely mating lobes. The wear between the lobes is avoided by a small clearance of 0.1 – 0.2 mm. The clearance results in leakage of air thereby reducing volumetric efficiency of the blower.
Suction takes place through the intake port. The entrapped air between the lobes and casing is carried forward during the rotation and is finally discharged to the delivery port. There is no change in the flow area and no reduction of volume of air. However, when the delivery port is open, blower discharges air into the high pressure reservoir causing irreversible pressurise as shown in the p-V diagram [Fig. 16.4 (b)]. The dotted line in p-V diagram shows compression process of a reciprocating compressor. The area represents excess work required due to irreversible pressurise.
From Eq. (3), it is seen that with increase of pressure ratio rp, roots efficiency ηroots decreases. Roots blower have capacity ranging between 0.14 m3/min to 1400 m3/min and can develop pressure ratio of 2 in a single stage and of 3 in two stage.
The roots (lobes) are made of steel and are finished with close precision. Extremely fine clearances are required to be maintained during their manufacture and assembly.