Control air (instrument air) is used in modern shipboard machinery for pneumatic control systems, pneumatic actuators, valves, and instruments (e.g. for the main engine control, boiler controls, and various pneumatic devices). The quality of the air is essential for reliable operation because:

1. Moisture: if the air contains water vapour, it can condense in the lines and instruments, causing corrosion, freezing (in cold conditions), blockage of small orifices, and malfunction of the pneumatic devices. Water in the air can also wash away the lubricant and cause sticking of valves and actuators.
2. Oil: oil vapour/contamination in the air can cause deposits, sticking of valves, and malfunction of the instruments; it can also be a fire hazard and contaminate the system.
3. Dirt/particles: solid particles in the air can block small orifices, wear the valves and actuators, and cause malfunction.
4. Pressure stability: the air must be supplied at a stable, correct pressure for the pneumatic devices to operate accurately.
5. Temperature: the air should be at a suitable temperature to prevent condensation and freezing.

Therefore, the control air must be clean, dry, oil-free, and at the correct pressure and temperature. This is achieved by proper air compression (oil-free or with good oil separation), filtration, drying (air dryers), and pressure regulation. Poor air quality causes unreliable operation, malfunction of the controls, and increased maintenance, which is unacceptable in modern automated machinery.

The main types of air dryers are:

1. Refrigerant (refrigeration) dryers: the air is cooled in a refrigeration circuit to condense and remove the moisture, then reheated. They are the most common for general instrument air. They are relatively energy efficient because they use a refrigeration compressor, but they cannot achieve very low dew points (typically down to about 2-3 deg C pressure dew point).
2. Desiccant (adsorption) dryers: the air is passed through a bed of desiccant (e.g. silica gel, activated alumina, molecular sieve) which adsorbs the moisture. They can achieve very low dew points (down to -40 deg C or lower). They are regenerated (by heat or by a purge of dry air) in two towers (twin-tower, heatless or heated). They are less energy efficient than refrigerant dryers because the regeneration consumes energy (heat or purge air).
3. Membrane dryers: the air passes through a membrane that selectively removes the water vapour. They are simple and compact but have a limited capacity and a purge air loss.
4. Deliquescent dryers: the air passes through a chemical that absorbs the moisture. They are simple but the chemical is consumed and the dew point is limited.

The refrigerant dryer is generally more energy efficient than the desiccant dryer for most marine applications, because:

1. It uses a refrigeration compressor which is efficient for removing the bulk of the moisture.
2. It does not require the energy-intensive regeneration (heating or purge air) that the desiccant dryer needs.
3. For the typical dew point requirement of instrument air (about 2-3 deg C), the refrigerant dryer is sufficient and uses less energy.

The desiccant dryer is more energy intensive because of the regeneration, but it is used where a very low dew point is required (e.g. for critical instruments or in very cold conditions). So, for most marine control air, the refrigerant dryer is the more energy-efficient choice.