A cross-sectional view of two pairs of rotational elements, called rotors, of the screw compressor with two different profiles is shown in Figure 5.1. The male rotor here has four lobes and the female rotor.six gullies, and this combination of numbers of lobe/gullies is most common. Other combinations, such 3/5 and 5/7 are sometimes available. Another view of the rotors presenting the third dimension is shown in Figure 5.2.
Some of the popular nominal diameters1 of the rotors are 125, 160, 200, 250, and 320 mm. Manufacturers often offer two or three rotor lengths for each rotor diameter and the length-to-diameter ratios usually fall in a range of 1.12 to 1.70. The rotors slip into a housing as indicated by the exploded view, as shown in Figure 5.3, that also shows some of the main elements of the compressor.
The separate processes experienced by the vapor in passing through the compressor are (1) filling of a cavity with suction gas, (2) sealing of gas between the rotors and housing, (3) reducing the volume of the cavity to perform the compression, and (4) uncovering the discharge opening to expel the compressed gas to the discharge line. One way to picture these processes is by observing a side view of the screws in Figure 5.4 whose threads move to the right as the rotors turn. The suction vapor enters the top of the rotors, and as the rotors turn a cavity appears at 1. Cavity 2 is continuing to fill, and cavity 3 is completely filled. Cavity 4 has now trapped gas between its threads and the housing. Cavity 5 is in the compression process with the volume shrinking as the cavity bears against the end of the housing. When the thread of the rotor reaches the discharge port, the compressed gas flows into the discharge line. A translation process is indicated in Figure 5.4, which is an interval occurring between the time the cavity is sealed until compression begins. This translation process takes up about 30° of the rotation of the rotor.