An air compressor compresses atmospheric air to produce high-pressure air for uses onboard ships such as starting diesel engines, pneumatic tools, and instrumentation. It can be classified as positive displacement or dynamic based on its compression method. Positive displacement compressors include reciprocating and rotary types that compress air intermittently, while dynamic compressors like centrifugal compressors compress air continuously. Multi-stage compressors cool air between stages for higher efficiency and lower temperatures.

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AIR COMPRESSOR

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• Learning Objective: Know the basic design features and
functions of various marine auxiliary machinery
• Machinery: Air Compressor
• Specific Objectives:
• Recognize the various names and locations of
• auxiliary machinery found on board
• Describe the basic operation of the machinery
• Identify the main parts of the machinery
• Sketch and label the main parts

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Introduction
• Air compressor is a thermal machine used for repeated
compression of gas known as atmospheric air to produce
high-pressure air.
• Air Composition: mainly 23% oxygen and 76% nitrogen by
mass (small quantities of other gases such as carbon
dioxide, argon, helium, neon and water vapour)
• A mixture of them will behave as a perfect gas, following
Boyle and Charles law.
• When air is compressed, its temperature and pressure
increase as its volume is reduced.

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Uses of Compressed Air
• Air compressors of various designs are widely used
in numerous applications including the operation of
equipment and portable tools.
• Ship board compressed air usage may divided as
follows:
– Starting operation
– Control & Instrumentation
– General Service/ Utilities

5. DME/MECC/Marine Engineering Knowledge / Jan 2007 /RBAir Compressor 5
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• Compressed air is used onboard ship for a
number of purposes and at varying pressures
depending on that purpose.
- High-pressure air – 25 to 40 bar – for starting
and reversing of diesel engines (two or three
stage-reciprocating units)
- Medium pressure air – 7 bar – for general service
air, deck air pneumatic systems, power
positioners, servo mechanisms and air puff soot
blowers (single/two stage reciprocating units &
rotary units)
- Medium/low pressure air – 4 and 5 bar – Utilities-
‘pneu-press’ and ‘grinell sprinkler’ by single/two
stage-reciprocating units or by rotary units.
- Low-pressure air – 2 bar – Control &
Instrumentation pneumatic control systems by
single stage or rotary units

6. Starting operation
(average 25 to 40 Bar)
• Starting the compression ignition diesel engine
require normally 30 Bar or related with total
capacity of the air bottle
• On generators, starting air system is normally
connected to emergency air compressor driven by
small engine or manually operated
• Prior to starting and FWE, engine is blown through
with air to remove any condensate or residue in
cylinder

7. Control & Instrumentation
(1.4 to 7 Bar)
• Pneumatically controlled instruments for
pressure, temperature, level, speed, flow
etc., with working pressure in the range of
1.4 to 7 Bar.
• The range of pressure is obtained by using
a small reducing valve or solenoid valve to
supply air at the correct value

8. General Service
(4 to 7 Bar)
• Service air is used for kind of tools (considered as pneumatic tools)
which are operated by compressed air (e.g., drilling machine, impact
wrenches, hand grinder, lifting gear etc.,)
• Used on auxiliary or deck machinery operated by compressed air
(e.g., Wilden pump, lifeboat davit, de-mucking winches etc.,)
• Pneumatic wrenches for even tightening of studs
• Fuel injector testing
• Water Pressure Test
• Used for cleaning purposes. Dust cleaning/clearing at narrow spaces
where brush and finger tips cannot reach
• Cleaning any dust from the electrical machinery e.g. alternator, motor
etc.,

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Mechanical piston
Reciprocating
Sliding vane
Liquid ring
helical screw
straight lobe
Rotary
Positive Displacement
INTERMITTENT
FLOW
centrifugal
Radial flow
Mixed flow
Mixed flow
Axial
Axial flow
Dynamic Ejector
CONTINUOUS
FLOW
AIR COMPRESSOR
Chart of air compressor

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Classification
• Compressors can be classified based on
operational principle of compression as follows:
– Positive displacement compressors
– Dynamic compressors
• Specially designed compressors used for specific
purpose are further defined by:
– The number of compression stages
– Method of cooling (air, water, oil)
– Drive method (motor, engine, steam, other)
– How they are lubricated (oil, oil-free)
– Packaged or custom-built

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Multi stage compressors
• They are used for three reasons:
– By cooling between stages the air is
kept in a moderate temperature range,
– By cooling between stages less work is
required to compress a given quantity
of air to a required pressure (closer to
isothermal)
– Lubrication difficulties minimized (air
temperatures low)

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• The capacity of an air compressor is
measured by the number of cubic
metres of free air discharged per
minute (FAD).
• Air Receivers (Air Bottles) are provided
as per Regulations. Safety features
include Fusible plugs and Relief valves

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• The Volumetric Efficiency of an air
compressor is measured by the
number of cubic metres of free air
discharged (FAD) per minute
compared with the displacement of
the LP piston in cubic metres per
minute.
• Modern air compressors have a
Volumetric Efficiency of 80 to 90%.

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Summary
• Uses of Compressed Air onboard.
• Compressor Types
• Operation of Single and Multi-stage
Air Compressor
• Components of Air Compressor
• Safety features an Air Compressor
and Air Reservoir.