The volume ratio of a machine can be changed using a couple of different methods. Some compressor manufacturers offer “Variable Vi”, which means the slide valve/slide stop assembly can be moved along the axis of the rotors. This feature provides an infinite number of Vi settings, within the available range, to obtain the closest internal compression ratio to the system ratio. If we refer back to Figure 10, we see an example of a machine with variable Vi by moving the slide valve/slide stop. Example (A) shows the low volume ratio machine with the slide valve/slide stop moved into the minimum Vi position. Example (B) shows the same machine with the slide valve/slide stop assembly moved to the maximum Vi position for high compression ratio
requirements.
Other manufacturers provide “Fixed Vi”, where different slide valves are available to meet the various requirements. On fixed Vi machines, the slide valve will need to be changed out to achieve a more optimal Vi for the system. Most manufacturers offering fixed Vi machines will provide three or four different slide valves to meet the varying requirements. The fixed Vi method limits the number of actual settings that can be achieved. On these machines we need to pick the Vi that provides the closest ratio to our system requirements.
Figure 11 shows a comparison of the same machine with two different slide valves for different Vi settings. View (A) is the low Vi machine where View (B) represents the high Vi compressor. Notice how the cut in the slide valve in View (A) is deeper, resulting in a shorter stroke length and a lower compression ratio. These illustrations are not drawn to scale and are intended to assist in understanding the concepts being discussed.
Capacity Control
Capacity control on a screw compressor package can be handled using several different methods. The first, and most significant is the reduction of flow within the machine using the slide valve. This method is exclusive to the screw compressor and is not available on reciprocating machines.
Up to this point, we have seen how the slide valve controls the volume ratio of a screw compressor through the position of the radial discharge port. The slide valve is also responsible for the throughput of gas in the machine. It can reduce the total amount of gas being compressed from 100% down to roughly 10%, depending on manufacturer and machine sizes. The major significance to unloading with the slide valve is the power savings that are associated with the reduced flow rates.
Other methods of capacity control include reducing driver speed, discharge to suction bypass, and inlet pressure control. These three methods are commonly used on reciprocating machines as means of automatic capacity control. Piston machines can also utilize variable volume pockets to manually adjust throughput. Each of these methods has some various drawbacks.
On engine drive units, speed reduction is limited to roughly 75 – 80% of full speed in order to provide constant torque. Speed reduction on electric drive units is done using a variable frequency drive, which is very costly. Discharge to suction bypass can provide 100% turndown but there are no power savings associated with the reduced capacity. All of the gas is still being compressed within the machine. Cooler sizing must also be considered when using bypass as capacity control. Typically the machines are designed for suction temperatures in the range of 60-80°F, but during bypass operation the gas will return to the compressor suction at roughly 120°F, or whatever outlet temperature the gas cooler was designed for. This will cause high temperature problems within the machine. The last method of automatic capacity control is a suction pressure control valve. This method is also inefficient from a power standpoint. The suction pressure of the gas must be reduced, and then re-compressed again. The variable volume pockets on reciprocating machines can be opened to provide capacity reduction as well. This method must be done manually and is limited to roughly 40% turn down of the machine.
In Figure 11 above, we saw how compression will occur along the bottom of the rotors beginning on the left side, or suction end of the machine. In all of the examples we have reviewed, the compression process was able to continue all the way from the suction end of the machine through to discharge without interruption. We have also seen how the maximum suction volume of a flute occurs when the rotors completely unmesh from each other and close off from the suction port.
Now we will see what happens if we open up an internal pocket along the bottom of the machine just as the compression process is about to begin. If we move the slide valve towards the discharge end of the machine, we open an internal recirculation slot for the gas to vent back to the suction port. This results in a lower suction volume of gas within the machine when the compression cycle begins.
Figure 12 shows a compressor at full capacity and at minimum capacity. The large flute volume represents the maximum input volume as seen before. As we move the slide valve away from the stop we open an internal recirculation slot back to the suction port. Here the rotors have nothing to seal the gas against so no compression work is done. The compression process can not begin until the flute has something to seal against. For the purpose of illustration, we have brought the slide valve in front of the rotor so we can illustrate the movement and recirculation slot. In reality, it would be located between the rotors and we would not be able to see it as well.
The position of the slide valve can be controlled automatically or manually to provide an infinite number of positions. This gives the machine the ability to flow the exact amount of gas required by the process. A hydraulic piston, located at the suction end of the machine, is used to move the slide valve back and forth. A pressure sensor is used to monitor either suction or discharge pressure. This sensor will indicate whether or not the machine is operating above or below the set point.
