At the end of my last blog, I mentioned the slide valve operation on a screw compressor. A slide valve is the basis of throughput control for a screw compressor. Throughput control is a term used to describe the process of controlling the energy input to the compressor in order to reach the control objective (output pressure and/or flow). No matter the type of compressor, throughput control is achieved by using speed control, suction throttling, discharge throttling, or recycle control. There are a few other methods of controlling throughput, but these four are the most common, and throughput control is a common practice used to dial in the needs of a compressed air system/application.
The first, speed control, is the most common and most efficient method. Essentially, the output flow and pressure are regulated by adjusting the speed of the motor driving the compressor unit. Increasing the speed of the motor driving the compressor will result in an increased output flow at a constant pressure, or an increased output pressure at a constant flow. Speed control can also be coupled with other control methods to fine tune the throughput of the compressor.
Suction valve throttling is exactly what it sounds like. The incoming air flow and pressure are restricted by installing a control valve immediately upstream of the compressor inlet, and the valve’s position is controlled as a function of the exhaust discharge pressure and/or flow. When the valve is activated and the suction is “throttled” or restricted, the output flow will decrease (because there is less air taken in by the compressor), and the output pressure will subsequently increase.
Discharge valve throttling restricts the pressure from the compressor to match the process requirements at a constant flow. As a result of this setup, the compressor must work harder than the process requires and this control scheme is extremely inefficient.
Recycle control uses a valve to return compressor discharge flow back to the suction port of the compressor. As many people know, compressing a gas can generate a good amount of heat, and this heat is often transferred into the compressed air. Because of this, a cooler is usually (and should be) installed in the line between the recycle control and the suction valves. The recycle valve can modulate from fully open to fully closed, which gives a full range of control over the discharge flow and can help with loading/unloading of the compressor.
These control methods are all fairly straightforward and on their surface aren’t too intimidating. They remind me of rudimentary PID controllers, which can be dialed in to a tee. Think of the way an elevator car reaches the intended floor without slamming to a stop or jolting when it starts moving. That’s achieved though PID control, and similar methodology is applied to compressor load and unload as well as operation. But if I get under the surface of compressor control and see PID diagrams, I’m getting the professor!