The vessel air system comprises, normally, of Starting air system, Control air system and General service/working air system.
The starting air system is supplied by two or three main air compressors which provide air, usually, to two main air reservoirs and is used to start the main engine and the generator engines. The generators may also be supplied with starting air from an auxiliary air reservoir which is replenished from the main air reservoirs and on some of the vessels directly from emergency air compressor (obviously in case of an emergency).

Additionally, it is channeled through a pressure decreasing valve to the air typhoon, engine room alarm horns, the working/control air system, and the generator engine diesel oil emergency pump. Air is provided to the oil-fired boiler via the starting air receivers for soot blowing operations.

On the modern vessels and engines compressed air from the main air system is also supplied to the generators jet system and main engine top bracing cylinders. On some vessels equipped with twin 4 stroke engines the main air system is used for the engine pneumatic break.

The two starting air receivers are supplied by an oil/water separator installed in the compressor’s common receiver filling line. Normally one main air reservoir is designated as the duty air reservoir, while the other one is fully pressurized with its outlet valves closed when at sea. When the ship is maneuvering both reservoirs are used.

Each compressor is equipped with two safety valves, one following the first stage of compression and the other following the second stage. In case of water cooled type compressors, the cooling water jacket of the cylinder block is equipped with a safety plate that blows out if the cooling water system is subjected to excessive pressure.
The low temperature fresh water cooling system provides cooling water. When the lubricating oil (LO) pressure falls below a predetermined value, a pressure switch attached to the automatic control system shuts down the compressor. The bearings are lubricated using a gear pump located at the crankshaft’s end and at the compressor outlet, a high temperature sensor trips the compressor if the temperature exceeds a predetermined limit.
Each compressor is normally equipped with an automatic unloader that acts in conjunction with the compressor’s start and stop cycles. This enables the compressor to start and stop off load, hence lowering the load on the electric drive motor and compressor running gear. Where automatic drain traps are fitted to sections of starting air line or services, the isolating valve for this drain trap must remain open. If a drain does not have an automatic drain trap the drain valve should normally be closed. This type of manual drain must be opened periodically, at least once each week, in order to drain any water from the line. Manual drain valves on supply lines should be opened before services on the supply line are operated.

Automatic operation of the compressors is generally selected at the local control panel, with the selection switch set to AUTO for automatic operation or MANU for manual operation. The emergency compressor is powered from the emergency switchboard.
When selected for AUTO operation the compressors are started and stopped by pressure switches, which are fitted to the inlet line to the starting air receivers.
A three-position switch permits the duty compressor to be selected from any of the others, while the others serve as the first and second follow-on compressors. When manual control is set, each compressor must be started manually; however, stopping is automatic via the pressure switch; the compressor may also be stopped manually using a pushbutton on the compressor control panel. At all times during operation, the space heater at each compressor control panel must be turned on.

For filling the starting air receivers and supplying the main and generator engines with starting air, generally the following procedure applies:

• The air receiver filling valve must be open to replenish a starting air receiver.
• As the compressors operate automatically and are controlled by pressure switches the receiver valve and the line valve for the pressure switches must be open for the air receiver which is being filled.
• Ensure that all pressure gauge and instrumentation valves are open and that gauges and instruments are reading correctly.
• Check the oil level in the compressors and replenish if necessary.
• Check the compressor sumps for water.
• Ensure that the low temperature central fresh water cooling system is operating and supplying cooling water to the starting air compressors.
• The automatic drain valves on the starting air compressors must be operational and the manual drain valves to the automatic drain traps must be open. Because the compressors are to operate automatically the drains must also operate automatically.
• Check that the compressor unloader gear is functioning.
• Set one air compressor to auto and the others to standby.

The first follow-up compressor will operate if the air pressure falls below a predetermined value and will function if the duty air compressor fails or is unable to maintain the receiver pressure due to excessive air demand. The second follow-up compressor will operate if the duty and first follow-up compressors fail to maintain pressure. Compressor selection for duty/follow on should be changed over weekly in order to even out the running hours on the compressors.

The control air system is supplied from the control and general service air reservoir. This reservoir is replenished by the service air compressors or the control air system may be supplied from the main starting air system via the pressure reducing unit. The pressure in the control air system is 7 to 8 bar. The air supply from the main air system is a back-up and the valves are normally closed. Control air is processed through a refrigerated control air dryer and associated filters before supplying the control air services.
Control air dryer operating principle is as follow: The humid air flows into the air inlet connection and is pre-cooled in the heat exchanger before it enters the evaporator. As the air passes through the evaporator, which is cooled by the liquid refrigerant, the air temperature drops to 10°C, which is the dew point at which the moisture in the air is condensed. The condensed water is now separated from the air and is purged out of the system through the automatic drain trap. The high pressure liquid refrigerant now passes through the expansion valve and is evaporated in the evaporator, before returning to the compressor to continue the refrigeration cycle. The control systems are supplied via grouped outlet valves for particular areas or systems.
It is important that the air is dried before entering the control system, as any moisture in the control air can cause problems at actuators or other parts leading to failure at these devices. Only in the event of failure of the control air dryer should the dryer bypass valve be opened.

Air is supplied to the working air system via the control and working air reservoirs. The reservoirs for control and working air can be refilled using either the main starting air system or one or two control and working air compressors.

Compressors for the control and working air supply air to the control and working air reservoirs. Compressors are controlled by reservoir pressure, loading and unloading as necessary. At the control panel, the compressors can be configured for operation; features such as starting and stopping pressures, as well as timed start and stop, are all possible. The compressors can be operated LOCALLY or REMOTELY through a selector switch on the control panel. The compressor is started and stopped in local control mode via the local panel. The compressor is started and stopped automatically in remote control mode based on the pressure in the working air reservoir. In case of two compressors, a selector switch enables master/standby air compressors to be selected as a pair, No.1/No.2 or No.2/No.1.

Normally, the duty working air compressor starts at a pressure of 7.0kg/cm2 and stops at a pressure of 8.0kg/cm2.

As mentioned at the beginning, the working air system can be supplied from the starting air system, through a reducing valve. On the modern systems, the working air supply line from the control/working air reservoir to the working air system is equipped with an automatic shut-off valve (usually operates at a pressure of 6.5kg/cm2), which will be closed automatically by low pressure in the control/working air reservoir. This saves air which is required for operation of the control systems. If the automatic shut-off valve operates it means that the control/working air compressors are unable to maintain system pressure. This may be due to excessive use of working air in which case closure of the shut-off valve will allow the compressors to restore system pressure. If the low pressure is due to failure of the control/working air compressors air must then be supplied from the starting air system via the reducing valve.

The general service air system supplies the following services: deck services; CO2 alarm horns; quick-closing air receiver and fire fan flap damper control; engine room services; accommodation service; generator engines emergency air-driven MDO pump; purifiers; emergency generator room; turbocharger cleaning unit; boiler; diesel generators; workshop; engine room air horns and ship’s main air horn; stern tube seal air control unit; ballast pumps and bilge, fire and GS pumps priming unit; local firefighting unit, etc.

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