Efficiency of air compressor on ships

What is volumetric efficiency of an air compressor?

  • It is the ratio of the actual volume of air drawn in each suction stroke to the stroke volume.
  • Volumetric efficiency =  (Actual volume of air drawn in suction stroke) / (Stroke volume)

What are the factors affecting the volumetric efficiency of an air compressor?

  1. The bumping clearance (the larger the bumping clearance the less air is discharged per stroke).
  2. Sluggish opening and closing of suction and delivery valves.
  3. Leakage past compressor piston rings.
  4. L.P inlet air temperature too high.
  5. L.P inlet filters dirty and choked.
  6. Inlet cooling water temperature too high.
  7. Insufficient cooling water owing to fouling of coolers.

What is the difference between the intercooler and aftercooler?


  1. Fitted intermediate stages.
  2. Saving in power. 
  3. Increase volumetric efficiency.
  4. Reduce temperature for next stage and it can avoid the danger of explosion in the compressor cylinder.
  5. Can provide good lubrication of the compressor piston.
  6. No carbonized material forming of discharge valve.
  7. Moisture separation is easier through intercooler drain.


  1. Fitted at the final stage-discharge side.
  2. Reduce final discharge air temperature to room temperature.
  3. Increase volumetric efficiency.
  4. Air bottle size is smaller.
  5. Moisture separation is easier through after cooler drain.

Why need bumping clearance in an air compressor?

  • This is a clearance must be provided between the piston top and the cylinder head when the piston is top dead centre.
  • It must be kept as small as possible for achieving the best compressor efficiency.
  • To prevent mechanical damage to the compressor.
  • To provide for thermal expansion
  • To provide the necessary space for valve operation.

How to adjust the bumping clearance?

It can be adjusted in two ways.

  1. By altering the head gaskets (cover joint ) thickness.
  2. By adding and subtracting shims between the connecting rod foot and bottom and bearing.

Main air compressor bumping clearance taking procedure?

  1. Stop the compressor and (lock off) take out fuse.
  2. Drain the cooling water.
  3. Remove the cylinder cover.
  4. Clean the cylinder head face and piston crown.
  5. Place the lead wire ball on the top face of the piston. That is a larger size than expected by clearance.
  6. Cylinder cover is placed on the cylinder with correct joint thickness and tightens the head bolts.
  7. Turn the compressor slowly by hand over top centre so that lead wire ball is pressed.
  8. Then remove cylinder cover again.
  9. Remove the compressed lead wire ball.
  10. Take the measurement of lead wire thickness by using a micrometre.

At what condition fusible plug is fitted main air bottle?

  • Fusible plug is fitted at the main air bottle when the relief valve is fitted indirectly to the air bottle.
  • It can cause any obstruction by human error.

What is the purpose of the bursting disc? Where is it fitted?

  • It is fitted on each cooler shell of air compressor, to give ample relief of pressure when cooler tubes burst.

Why bursting disc need to be annealed?

  • Due to heating and time expanded bursting disc is harden so to get back the normal relief pressure. It must be annealed.

What is first start arrangement on ships?

  1. Firstly start the emergency air compressor to fill up the air to the emergency air bottle up to desired pressure obtained.
  2. Drain out oil and water.
  3. Open emergency air bottle outlet valve and pressed up desired pressure.
  4. Then start the generator with the correct procedure.
  5. Close the breaker and switch on the lighting.
  6. Start the required machinery ( especially G/E FO booster pump, SW cooling pump, E/ R blower).
  7. Start the main air compressor and press up to main air bottle.
  8. After that, M/E warming by using generator jacket water and start the L.O pump.

Why provide air compressor on board ships?

  • Starting Diesel engines
  • Instrumentation and control systems, 
  • Various pneumatic tools and cleaning equipment, 
  • Boiler soot blowing and 
  • Ship’s whistle.

 Why need minimum lubrication for air compressors?

  • To prevent carbonizing of the valves
  • To avoid loss of compressor efficiency due to sluggish action of the valve
  • To avoid loss of cooler efficiency due to deposition of oil on the cooling surface
  • To prevent airline explosion, to prevent air bottle corrosion

Leaky Valves in Air Compressors

Valves can leak and finally fail. Valves may fail to owe to wear and fatigue, insufficient of over-lubrication, presence of foreign particles, oil decomposition and excess valve lift.

What are the effects of air compressor valve leaking on ships?

First stage suction valve leaking

  1. Reduce air delivery
  2. Reduce second stage suction pressure
  3. Increase running time
  4. Unload the compressor (Motor take less load)

First stage delivery valve leaking

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

Second stage suction valve leaking

  1. High pressure in the suction line of the 2nd stage
  2. The high temperature in the suction line of the 2nd stage
  3. Reduce delivery
  4. Increase running time

Second stage delivery valve leaking

  1. Increase suction pressure in 2nd stage
  2. Reduce delivery in 2nd stage
  3. Increase delivery pressure from the 1st stage

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