CoC Oral Exam Preparation (Part – 8): Air Compressor

Air Compressor

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Effects of leaking valves in Air Compressor:

First Stage Suction:

  • Reduce air delivery
  • Reduce 2nd stage suction pressure
  • Unload the compressor
  • Increase running time.

First Stage Delivery:

  1. Reduce air delivery
  2. Increase discharge temperature
  3. Less air drawn in, due to high-pressure air leaking back into cylinder.

Second Stage Suction:

  1. Reduce air delivery
  2. High temperature & pressure in 2nd stage suction line
  3. Increase running time

Second Stage Delivery:

  1. Reduce air delivery
  2. Increase suction pressure in 2nd stage
  3. Increase delivery pressure from 1st stage.
  4. Back pressure from air bottle.

How to check Air Compressor Efficiency?

  1. Regular overhauling of valves done or not.
  2. Check Air Bottle filling time.
  3. Compare test results and records.

How to check Air Compressor capacity is sufficient?

  • Total no. of Air Compressors must be sufficient to fill the empty Air Bottle to maximum pressure within 1 hour.
  • Must be sufficient to start at least 12 times for Reversible Engine, and at least 6 times for Non-Reversible Engine.

Bursting Disc:

  1. Fitted on the shell of Intercooler at waterside.
  2. Relieves pressure if the tubes burst.
  3. Rolled Copper Alloy and relief pressure is set while the disc is at softest condition.
  4. Material tends to harden due to time and surrounding temperature, and set pressure also increased.
  5. Bursting Disc needed to be annealed, to regain correct relief pressure.

Compound Valves, why used in Air Compressor?

  1. Give large area of opening and small amount of valve lift.
  2. Improve Volumetric Efficiency, as valve open and close in minimum time.
  3. Reduce bumping clearance.
  4. Reduce wear and tear.

LP Relief Valve opening:


  1. HP suction valve
  2. Intercooler choked.
  3. Relief valve, jammed by foreign particles, in open position.

HP Relief Valve opening:


  1. HP discharge valve, in closed position.
  2. After cooler choked.
  3. Relief valve, jammed by foreign particles, or spring sticking in open position.

(Relief Valves opening pressure are set at  ≯ 10% above stage pressure.)

Why Intercooler is fitted?

  1. Reduce air temperature and volume, and increase air density for next stage.
  2. Increase Compressor Capacity and Volumetric Efficiency.
  3. Better lubrication for cylinder and rings.
  4. Water and excess oil can be drained out, preventing fouling of Intercooler and pipes, Air Bottle corrosion, and starting airline explosion.
  5. Work done is saved.
  6. Metal stresses reduced, due to control of temperature.

Volumetric Efficiency:

VE  = (Volume of Air drawn into Cylinder) / (Stroke Volume of LP Piston)


VE = (Volume of Air discharged as ‘free air’) / (Stroke Volume of LP Piston)

Why Multistage Air Compressor is built?

  1. To obtain near to ideal isothermal compression, compressor is to be constructed of multistage with inter-stage cooling.
  2. Inter-stage cooling reduces air temperature and volume after 1st stage compression, thus increase mass of air for 2nd
  3. Workdone is saved and air compressor efficiency increased.

Other advantages are:

  1. Easy to get high final air pressure.
  2. Easy to control air temperature.
  3. Easy to maintain correct lubrication.
  4. Better compressor balancing.
  5. Reduction in size.
  6. reduction in clearance volume loss.

Capacity of air compressor:

  1. Capacity is checked upon how much filling time is lowered.
  2. Compressor should have enough filling capacity so that sufficient stopping time should be provided between fillings.
  • 12 consecutive starts in reversible engine  and
  • 6 consecutive starts in non-reversible engine.

What is Free Air Delivery, FAD?

  1. Capacity of Air Compressor is stated in terms of [ m³/ hr ].
  2. Volume of air actually discharged in 1 hour, that would occupy if expanded down to atmospheric pressure and cooled to atmospheric temperature.

Safety devices on Main Air Compressor:

  1. Bursting Disc on Intercooler: (At water side)
  2. Bursting Disc and Fusible Plug (121°C) on Aftercooler
  3. Relief valves on LP and HP stages. (Set to lift at 10% rise above normal stage pressure.)
  4. Automatic Moisture Drain Valve.
  5. Cooling water supply failure alarm.
  6. Low LO pressure alarm.
  7. Relief valve on crankcase LO pump.
  8. Delivery air HT cut out and alarm on Aftercooler outlet. (Max. 93°C)
  •   LP discharge pressure 4 bars and HP discharge pressure 30 bar
  • Intercooler inlet air 130°C and intercooler outlet air 35°C
  • Aftercooler inlet air 130°C and Aftercooler outlet air 35°C:

Intercooler is single pass type:  Aftercooler, double pass U-tube type:}

Uses of Compressed Air: 

  • Engine Starting 20 to 25 bar
  • Boiler Soot Blowing 20 to 25 bar
  • General Service (Whistle, Pneumatic Tools, Lifeboat , service air for cleaning and Pilot ladder) 7 to 10 bar
  • Instrumentation and Control 5 bar

Air Filter: 

  1. Material: Felt, Metal gauze or Nylon strands
  2. Removes contaminants from air. Dirt and dust act as abrasives and increase wear.
  3. Contaminant deposits on valves can become hot and source of ignition.

Hazard of Dirty Filter: 

  1. Very dirty filter or muffling a filter results in large pressure drop.
  2. Air has to be compressed over higher range.
  3. In extreme case, discharge air temperature may exceed flash point, or auto-ignition temperature resulting in an
  4. As a safety against explosion, air temperature is limited to 93° Fusible Plug (121°C) or a High Temperature cut out (93°C) is provided on Compressor.

Pressure Test on Air Compressor:

  • Cylinders, cylinder cover, Inter & After coolers are hydraulically tested to:

Air Side:  1.5 x max. Working Pressure.
Water Side: 4 bar or 1.5 x max. WP (whichever is greater)

Air receiver:

  • Total capacity of air receivers must be sufficient to give at least 12 starts for reversible engine, and at least 6 starts for non-reversible engine, without refilling the receivers.
  • There must be two identical main air receivers and one emergency bottle for every vessel.

 Mountings of Air receiver:  

  1. Fusible plug: 
  • composition – Bismuth 50%, Tin 30%, Lead 20%,
  • Melting point: 220΄F (104.4΄C).    Fitted at the reservoir’s bottom or on reservoir at ship side, when relief valve (safety valve) is not directly fitted on the reservoir.
  1. Atmospheric relief valve: provided for back-up of fusible plug. In case of ER fire when CO₂ flooding is required, this valve is opened before evacuating ER.
  2. Spring loaded safety valve: setting pressure: 32 bar (for 30 bar working pressure), with ≯ 10% rise in accumulation of pressure. May be fitted directly or with extension.
  3. Compensation ring: when a hole is cut or machined in pressure vessel, higher stresses will subject to the material around the hole, and to reduce this, compensation rings are fitted. It is a flange on which a valve or fitting is usually mounted.
  4. Manual Drain valve or Automatic Drain valve.
  5. Pressure gauges.
  6. Access doors.
  7. Main starting air valve, auxiliary starting air valve, filling valve, service air or whistle air valve.

Internal surface coating:

Graphite suspension in water, Linseed oil, Copal vanish or Epoxy coating having basic properties of anti-corrosive, anti-toxic or anti-oxidation.

Safety devices on Main Air Bottle:

  1. Fusible plug.
  2. Pressure Relief Valve
  3. Atmospheric Relief Valve.
  4. Low Air Pressure alarm.
  5. Automatic or remote control Moisture Drain Valve.
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