The Compression Process in a Screw Gas Compressor
Most people are familiar with the basic operation of a reciprocating compressor. To help understand the operation of a screw compressor, we will compare the compression process to a piston type machine.
Lets think of the male rotor lobe as a piston and the female flute as a cylinder. On a reciprocating compressor, as the piston begins to pull away from top dead center, the suction pressure overcomes the pressure inside the cylinder, forcing the suction valve open and allowing gas to enter. We need to remember that a screw compressor does not have any suction or discharge valves. On a screw compressor, as the rotors begin to unmesh the male rotor lobe will roll out of the female rotor flute. The volume vacated by the male rotor will fill with suction gas. As the rotors continue to unmesh, the volume in each flute will increase. Figure 3 shows a comparison of the beginning of the suction process on a reciprocating compressor and a screw compressor. View (A) represents a single acting reciprocating cylinder. View (B) illustrates a top view from the suction end showing the rotors unmeshing and allowing the natural gas to enter. View (C) shows a side view of the screw compressor at the same point. We can only show the rotor closest to you as the other one is hidden. In order to keep the illustrations simple, we will only show one rotor flute in operation.
On a reciprocating machine, gas will continue to enter the cylinder until the piston reaches the end of the stroke, or bottom dead center. At this point the suction valve will close and the input volume of the cylinder is established. If we multiply the input volume of each cylinder by the number of cylinders and then by the compressor rpm, we will establish the compressor displacement.
On a screw compressor, gas will continue to enter each flute until the rotor lobes roll out of mesh with each other. As the rotors finish unmeshing with each other, the flute passes by the edge of the suction port, closing it off from the system. The point where the flute travels past the edge of the suction port is where the maximum volume of that flute occurs. This represents the suction volume of the flute. The suction volume is the volume of trapped gas within the flute at the end of the suction process. Multiplying the volume of input gas in the male and female flute by the number of lobes on the male rotor and then by the rotor rpm establishes the displacement of the screw compressor. Figure 4 shows a comparison of the reciprocating and screw machines at the end of the suction process. Once again, view (A) represents the reciprocating cylinder, view (B) and (C) are the screw compressor. The volume of trapped gas in the cylinder is still at suction pressure because the compression process has not begun yet.
Once the suction process is completed and the input volume is established, the compression process can begin. Figure 5 shows the same comparison of the reciprocating and screw machines during the compression process. On the reciprocating machine, the piston begins to move upward away from bottom dead center, reducing the volume in the cylinder and causing an increase in pressure of the trapped gas. Here the screw compressor is not unlike the recip machine. As the rotors continue to rotate, they begin to mesh together along the bottom. Once again, lets think of the male rotor lobe as a piston and the female flute as a cylinder. As the rotors mesh together the male rotor lobe moves into the female flute and reduces the volume in the flute. View (B) now shows the bottom of the rotors to illustrate where the compression occurs. Keep in mind, we are still looking at the rotors from the suction end. The compression will continue as the gas moves toward the discharge port. View (C) shows the volume in the flute beginning to reduce through compression.
The compression process will continue in the reciprocating machine until the internal pressure within the cylinder exceeds the discharge pressure of the system. At this point the discharge valve will open and allow the trapped gas within the machine to escape. The discharge process of a screw compressor is very different than a reciprocating machine. There are no valves in the screw to allow discharge gas to escape from the flute. The location of the discharge port along the axis of the rotors is critical as it determines when the compression process is complete, and the discharge process begins. If we look at the individual flute containing the trapped gas we see two rotor lobe tips, one on the discharge side of the flute and one on the backside of the flute. The tip on the discharge side is called the leading tip, as it will be the first one to reach the discharge port. The second tip is referred to as the trailing tip. Figure 6 shows the leading and trailing tip of the rotor lobes. As the leading tip of the rotor passes by the edge of the discharge port, the compression process is complete and the gas will be forced into the discharge line.
We saw how the suction volume of the compressor was established in Figure 4. The discharge volume of the machine is the volume of gas trapped in the flute just before the leading tip of the rotor enters the discharge port. Figure 6 shows a comparison of the reciprocating machine and the screw compressor at the beginning of the discharge process.
The discharge process will be completed on a reciprocating compressor when the piston reaches top dead center and the discharge valve closes again. There must be a small clearance between the top of the piston and the head to avoid piston damage. There will always be some inefficiency in the reciprocating machine due to this trapped pocket of gas left behind by the piston. The volume of gas that remains in the cylinder will re-expand during the next suction process. This will reduce the amount of gas entering the cylinder, causing a reduction in volumetric efficiency of the machine. On high compression ratio applications, this will cause a significant decrease in efficiency, resulting in reduced flow rates.
