Compression cycle Multistage compressors Inter-cooling Compressor cylinder head

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• An air compressor is a mechanical device that increases the pressure of air by reducing volume.
• Air is compressible, the compressor reduces the volume of air and induces pressure in the air
• An air compressor converts electrical energy into kinetic energy in the form of the air
Compressed air Usage onboard
1. M/E Starting
2. A/E Starting
3. Control air
4. Service Air
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What is a Compressor ?

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According to number of stages
Single stage, double stage, three stage of multiple stage
According to action
Single acting or double acting
According to position of cylinder w.r.t. crankshaft
Cylinders inline, vertical, radial position, V-type cylinder arrangement
According to prime mover
Electric motor drive or IC engine drive, Gas turbine drives
According to cooling medium
Air cooled, water cooled air compressors
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Classification of air compressor

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Configurations
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Compression cycle

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Single stage single acting
Single stage
Single Stage Double Acting
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Multistaging
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P-V Diagram
851

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Intercooling effect
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𝜂𝜂𝑣𝑣= _______________________
Volumetric Efficiency
Theoretical suction volume Vs
Actual suction volume Vx
853

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Compressor Valves

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Compound Valves, why used in Air Compressor?
• Give large area of opening and small amount of valve lift.
• Improve Volumetric Efficiency, as valve open and close in
minimum time.
• Reduce bumping clearance.
• Reduce wear and tear.
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Compressor Valves

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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:
• Reduce air delivery
• Increase discharge temperature
• Less air drawn in, due to high-pressure air leaking back into cylinder.
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Compressor Valves

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Effects of leaking valves in Air Compressor:
Second Stage Suction:
• Reduce air delivery
• High temperature & pressure in 2nd stage suction line
• Increase running time
Second Stage Delivery:
• Reduce air delivery
• Increase suction pressure in 2nd stage
• Increase delivery pressure from 1st stage.
• Back pressure from air bottle
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Compressor Valves

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Compressor Valves
How to check Air Compressor Efficiency?
• Regular overhauling of valves done or not.
• Check Air Bottle filling time.
• 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.
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Safety devices on Main Air Compressor
• Bursting Disc on Intercooler: (At water side)
• Bursting Disc and Fusible Plug (121°C) on Aftercooler
• Relief valves on LP and HP stages. (Set to lift at 10% rise
above normal stage pressure.)
• Automatic Moisture Drain Valve.
• Cooling water supply failure alarm.
• Low LO pressure alarm.
• Relief valve on crankcase LO pump.
• 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:
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Bursting Disc
• Fitted on the shell of Intercooler at waterside.
• Relieves pressure if the tubes burst.
• Rolled Copper Alloy and relief pressure is set while
the disc is at softest condition.
• Material tends to harden due to time and
surrounding temperature, and set pressure also
increased.
• Bursting Disc needed to be annealed, to regain
correct relief pressure
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Hazard of Dirty Filter
• Very dirty filter or muffling a filter results in large
pressure drop.
• Air has to be compressed over higher range.
• In extreme case, discharge air temperature may
exceed flash point, or auto-ignition temperature
resulting in an
• 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.
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Air receiver
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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Mountings
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.
2. 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.
3. 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.
4. 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.
5. Manual Drain valve or Automatic Drain valve.
6. Pressure gauges.
7. Access doors.
8. Main starting air valve, auxiliary starting air valve, filling valve, service air or whistle air valve.
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