In this section, we will delve deeper into the mechanics of a single screw compressor.
As with all compressors, single screw compressors have a fixed swept volume. Swept volume refers to the theoretical volume of gas that is compressed per revolution.
The theoretical volume is obtained by integrating the gate rotor surface area that meshes with a flute by the angle of screw rotor rotation from suction to discharge.
Figure 4 - Single Screw Rotor Compression Steps
The swept volume of a screw rotor is determined by several physical parameters, the screw and gate rotor outer diameter, and the groove flute/chamber width. Someone would ask, “But how can this complex shape be manufactured?” The old-school manufacturing method for the screw rotor tooth profile involves machining with a cut tool similar to the one used for the gate rotor. As the screw rotor revolves, the cutting tool revolves at the equivalent gate rotor motion, removing material in linear motion. The ratio VG / VS determines their motion relationship. Where VG and VS denote the speed of the gate rotor and screw rotor, respectively.
Figure 5 - Screw Rotor Geometry - Linear Cutting Process
The disadvantage of this method is that it requires special machines and equipment to manufacture the geometry of a screw rotor. Furthermore, existing cutting tools cannot be used for different screw sizes; new cutting tools are required, which increases costs. It is evident that only a handful of manufacturers can produce the screw geometry using this method.
Finally, the new screw rotor method of manufacturing employs a 5-axis CNC machine, which solves the linear cutting process issues.
Figure 6 - Screw Rotor Geometry - 5-axis CNC Machine cutting Process
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For someone unfamiliar with this concept, it may be easier to imagine a compression flute as a piston moving within a cylinder, traveling between two extremes. If we take the trajectory shown in Figure 7 and straighten it, using our imagination, the same concept becomes apparent. |
 Figure 7 - Flute Trajectory |
Figure 8 - Screw Rotor
The star gate rotor is made of a glass-fiber–reinforced nylon composite that is machined to the required gate-rotor geometry. The gate-rotor materials are selected to withstand sliding contact and high loads while providing excellent wear resistance. Each star gate rotor is supported by a rotor shaft. The primary function of the gate rotor is to seal the flutes as the working fluid transitions from suction to discharge pressure.
As mentioned above, depending on the compressor arrangement, single-screw compressors may use either a single gate rotor or two gate rotors. In the double-gate-rotor design, each “star” has 11 teeth, and these teeth engage with three rotor flutes on each side, resulting in a total of six tooth-to-flute meshing points. It is important to note that, in order to use two gate rotors, the angular groove length (wrap angle) must be approximately 300°, and the gate rotors must be positioned 180° apart. Consequently, the discharge ports are offset at an angle. With this configuration, the radial forces generated during compression are effectively canceled out, as illustrated by the blue arrows in Figure 10.
It is evident that when the radial forces are eliminated, only the axial forces remain, allowing the compressor to operate more smoothly. Since the internal pressure at the discharge end of the flute is higher than in the rest of the screw, this pressure acts toward the suction side. As a result, the remaining axial load must be accounted for in the bearing design.
Figure 9 illustrates the gate rotor tooth shape in the front view, sealing the root of the screw rotor.
During the compression process, as the gas becomes trapped and carried from the suction side to the discharge end, it exerts compression forces on both the top face and the root face of the flute, attempting to escape toward any lower-pressure region. As is evident, the manufacturing tolerances of these components, along with their wear resistance, play a crucial role in the overall efficiency of the compressor. In addition, the compression process imposes shear forces on the gate-rotor shaft, which must be properly accounted for, as well as high stresses on the bearings. These effects are illustrated by the red arrows in Figure 10.
Figure 9 - Gater rotor teeth mesh with the main rotor and seal the gas, preventing leakage to lower-pressure flutes
Figure 10 - Single Screw Rotor Assembly Members
One important term in compressors is the compression ratio. Each compressor has a built-in PR pressure ratio determined by the swept volume. The pressure ratio is the ratio of the discharge pressure P2 to the suction pressure P1. Likewise, the volume V1 is the suction volume compressed to discharge volume V2. Additionally, the Greek letter γ refers to the isentropic index of the concerned gas.
The pressure ratio of a compressor is directly linked to the compressor’s efficiency; therefore, the compressor will exhibit different efficiency levels at a given pressure ratio.
The gas progression is displayed in the figure11(graph) below. As the gas is drawn from the suction to the discharge, it is compressed, and the volume of the gas is reduced while the pressure increases until it reaches the discharge pressure and escapes from the discharge port.
The most popular industry that uses single screw compressors is the refrigeration industry. The compressors can operate under various refrigerants, a few to name R123, R1234ZE, R454C, N31(Ammonia), etc.