How a ship’s main air compressor works
CHIEF. LEARN · Guide 1 of 5
The air path of a two-stage marine air compressor, its drains and unloading, the safety valves and the 10-minute blow-down, and what the starter does at each step.
The example throughout is No.1 main air compressor of a ship: a 30 kW motor on a 440 V, 60 Hz supply with a relay starter panel (SP-MAC1) and 220 V control voltage, charging two 3 m³ receivers to 30 bar.
- ETO
- electro-technical officer
- AMS
- alarm and monitoring system
- LP / HP
- low pressure / high pressure (first / second stage)
- NRV
- non-return valve
The air path
Suction and the first stage
- Mechanics
- Air enters through the suction filter-silencer FL1 and is compressed in the low-pressure cylinder C1 to about 5 bar. SV1 on the first stage lifts at 9 bar if the second stage cannot take the air, for example with a leaking HP suction valve.
- Electrics
- Nothing electrical in the air itself. What the panel controls are the drains and the motor.
- What you see and hear
- On the compressor: the filter on the suction, the LP cylinder, the LP gauge.
Intercooler and the LP separator
- Mechanics
- Compression heats the air. The water-cooled intercooler IC cools it before the second stage; the water that condenses out collects in separator SEP1 with drain solenoid YV1.
- Electrics
- YV1 is normally open: with no voltage it drains SEP1 and unloads the first stage. 220 V on X1:16–X1:17 closes it.
- What you see and hear
- A plume from the SEP1 drain whenever the compressor turns with YV1 open.
Second stage, aftercooler, HP separator
- Mechanics
- The high-pressure cylinder C2 takes the air to receiver pressure plus about half a bar. The aftercooler AC cools it again; SEP2 with YV2 collects the condensate. SV2 lifts at 31.5 bar.
- Electrics
- YV2 is normally open as well, fed in parallel with YV1 from wire 29 (X1:18–X1:19).
- What you see and hear
- HP gauge, the second drain, the safety valve SV2 on top.
Fusible plug, non-return valve, receivers
- Mechanics
- The fusible plug FP1 melts at 121 °C if the delivery air gets dangerously hot. The non-return valve NRV keeps the receivers from blowing back. The discharge valve V3 is open in service. Receivers AR1/AR2, 3 m³ each, 30 bar.
- Electrics
- PS1 and PS2 on the receiver line start the compressor in AUTO; the AMS alarms STARTING AIR PRESSURE LOW at 16 bar.
- What you see and hear
- Receiver gauge with the green band 27–30 bar and the red mark at 16 bar.
Running and blow-down
Running loaded
- Mechanics
- The watchkeeper checks stage pressures, oil pressure, water and air temperatures, leaks, knocks and the water sight glass.
- Electrics
- The ETO checks the current on the ammeter, heating of the cable and the contactor, and that the drains work.
- What you see and hear
- LP about 4.5 bar, HP just above the receiver, oil 2.2 bar.
The 10-minute blow-down
- Mechanics
- Every 10 minutes both separators are drained for 3 s: a short blast, LP and HP dip.
- Electrics
- KT2 runs while wire 28 is live. During its 3 s pulse its 15-16 opens: wire 29 goes dead, YV1 and YV2 open.
- What you see and hear
- A short “pssht” from both drains, the gauges dip.
Back to pumping
- Mechanics
- The drains close and the stages load again.
- Electrics
- KT2 15-16 closes again: 220 V back on X1:16–X1:17 and X1:18–X1:19.
- What you see and hear
- Gauges back up.
For training only. Models are simplified; class rules and the maker’s manual are the authority.