From the outside, a screw compressor is a cabinet with a screen, a power supply and an air outlet. Inside, its overall structure remains simple: it is made up of three circuits, the air circuit, the oil circuit and the electrical circuit. It is only when you look at each one in detail that you see everything behind it.
Understanding these three circuits helps you maintain the machine properly, read its alarms and choose the right air treatment.
1. The air circuit
Ambient air enters through the intake filter, which traps dust before it reaches the screw element. In the screw element, two helical rotors turn against each other and progressively reduce the volume of the air: its pressure rises.
When it leaves the screw element, the compressed air is laden with oil (see the oil circuit below). It passes into the separator tank, then through the air/oil separator cartridge, which retains the oil. The air, now free of its oil, then flows through the minimum pressure valve, located above the separator, before being sent to the air receiver (the air tank) and on to your network.
Treating the air at the outlet
Even after the separator, the air still contains water and very fine oil particles. Depending on the application, a dryer and fine filters (precision filters) are added: this is filtration. A basic set, suited to 8 bar installations, has three filters of different grades:
- the first provides initial filtration, in particular of the moisture present in the compressed air;
- the second refines the filtration and acts as additional protection;
- the third retains the very fine oil particles and oil aerosols still present.
For a 16 bar compressor, such as those in the L series for laser cutting, a system of five precision filters is generally used, and these must be models designed for high pressure. Our guide to compressed air quality according to ISO 8573-1 explains how to choose the level of filtration, and our article Fine filters - how do you filter the air at the compressor outlet? goes into the details.
2. The oil circuit
In an oil-injected screw compressor, the oil is injected into the screw element. It lubricates the rotors and bearings, seals the gap between the rotors and carries away the heat produced by compression.
It leaves the screw element mixed with the air. The air/oil separator recovers it: the oil stays inside the compressor and returns to the lubrication circuit to be reused, while the air continues on its way. On its return path, the oil is cooled and passes through the oil filter.
That is why screw compressor maintenance revolves around four parts: the air filter, the oil filter, the oil and the separator cartridge. A saturated separator lets oil into the network and increases oil consumption; a clogged separator forces the motor to work harder. The service intervals are detailed in our article When should you service a screw compressor?.
3. The electrical circuit
The electrical circuit drives the machine and controls it. On our variable speed models, the screw element is direct-driven by a rare-earth permanent magnet motor, powered by a frequency inverter.
Pressure regulation
The control panel manages everything automatically. You set the desired pressure: the compressor builds up to that value, then stops by itself. When the network pressure drops below the set threshold, it restarts on its own to restore it. This is also why a stopped compressor can start up at any moment: before any work on it, it must be locked out and depressurised.
Far more diagrams than you might think
A complete unit does not fit on a single wiring diagram. There is one for the compressor's control panel, one for the electrical cabinet, one for the frequency inverter and one for the drive motor. On top of these come the circuits for the temperature sensors, the pressure sensors and the solenoid valves. That is why electrical troubleshooting should be entrusted to a technician trained on the machine: our after-sales service takes care of it.
Why an 8 bar compressor cannot be turned into a 16 bar one
It is a question we are often asked, and the answer is no. For a compressor designed for standard pressure to reach a higher pressure, many components would have to be replaced, including internal parts that cannot be seen from the outside. The separator, the minimum pressure valve, the filters, the air receiver and its safety devices are all sized for a given pressure.
If your application requires 16 bar, you need a machine designed for 16 bar from the outset, with an air receiver and equipment rated for that pressure. That is the case with the L series. For a conventional workshop, the S series covers 8 to 13 bar depending on the model.
In summary
| Circuit | Role | Parts to monitor |
|---|---|---|
| Air | draw in and compress the air, remove its oil and send it to the air receiver | intake filter, air/oil separator, precision filters |
| Oil | lubricate, seal and cool the screw element, then be reused | oil, oil filter, air/oil separator |
| Electrical | drive the screw element and regulate the pressure | control panel, inverter, sensors, solenoid valves |
Not sure which model or which air treatment you need? Send us your air flow and pressure requirements: we will propose the right configuration.