With respect to the control air supply system for main engine control:

The control air (instrument air) supply for a pneumatic control system must satisfy the following essential conditions:

1. Cleanliness: the air must be free of solid particles (dirt, rust, scale) which could block the small orifices and damage the valves and instruments.
2. Dryness: the air must be free of moisture (water vapour), which could condense in the lines and instruments, causing corrosion, freezing, and malfunction. The air must be dried to a suitable dew point.
3. Oil-free: the air must be free of oil vapour/contamination, which could cause deposits, sticking of valves, and malfunction.
4. Correct pressure: the air must be supplied at a stable, correct pressure (e.g. 7 bar) for the pneumatic devices to operate accurately; the pressure must be regulated and stable.
5. Correct temperature: the air should be at a suitable temperature to prevent condensation and freezing.
6. Adequate capacity: the air supply must have sufficient capacity (flow) to meet the demand of the control system.
7. Reliability: the air supply must be reliable (with a standby compressor/receiver) so that the control system is not lost.

These conditions are achieved by proper compression (oil-free or with good oil separation), filtration, drying (air dryers), and pressure regulation.

[Sketch notes: The control air supply arrangement consists of: (1) the air compressor; (2) an aftercooler; (3) a moisture separator; (4) a filter; (5) an air dryer (refrigerant or desiccant); (6) an air receiver; (7) a pressure-reducing valve; (8) a final filter; (9) the control air distribution to the pneumatic devices.]

The control air is compressed by the compressor, cooled in the aftercooler, and the moisture is separated. The air then passes through a filter to remove particles, and through an air dryer to remove the moisture. The dry, clean air is stored in the air receiver and supplied through a pressure-reducing valve (to the control pressure) and a final filter to the control system.

Positioning of dryers and filters: The dryer is positioned after the compressor and the aftercooler (and the moisture separator) so that the bulk of the moisture is removed by cooling and separation before the dryer, making the dryer more efficient. The filter is positioned before the dryer (to protect it from particles) and a final filter is positioned after the pressure-reducing valve (just before the control system) to remove any particles picked up in the receiver and the piping. The dryer is placed before the receiver so that the stored air is dry, and the final filter is placed close to the point of use to ensure the air reaching the instruments is clean. This arrangement ensures the control air is clean, dry, and at the correct pressure.