1. air compressor

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Learning Objective:Understand the air compressor and reservoirs types, construction and operation on board.

Specific Learning Objectives: Explain the uses of compressed air onboard. State the compressor types that may be found

onboard. Explain the operation of single and multi-stage air

compressor Sketch and describe the inter and after cooling. Describe the air compressor components assembly

and materials. With the aid of diagram, explain the air reservoir

construction. Describe the safety features an air compressor. Describe the design features of air reservoir. Describe the safety features of air reservoir.


AIR COMPRESSOR


Introduction

Air compressor is a thermal machine used for repeated compression of gasses known as atmospheric air to produce high-pressure air.

Air is compressed of mainly 23% oxygen and 76% nitrogen by mass along with small quantities of their gasses such as carbon dioxide, argon, helium, neon and water vapour

It is a main constituent of air near perfect gases; a mixture of them will behave as a perfect gas, following Boyle and Charles law. When air compressed its temperature and pressure increase as it volume is reduced.


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 the starting and reversing of diesel engines by 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 or two stage-reciprocating units.

Medium/low pressure air 4 and 5 bar pneu-press and grinell sprinkler by single or two stage-reciprocating units or by rotary vane units.

Low-pressure air 2 bar pneumatic control systems by single stage or rotary vane units


Uses of Compressed Air

Onboard merchant vessels, the compressed air is now actually the lifeblood of the ship for various operations ranging from diesel engine starting to cleaning purposes.

On land itself, Air compressors of various designs are widely used throughout facilities in numerous applications including the operation of equipment and portable tools.

However compressed air may divided into three different uses as follow;

Starting operation

Controller

General usage


Starting operation(average 25 to 40 Bar)

Starting the compression ignition diesel engine required total capacity normally 30 Bar or related to it requirement with total capacity of the air bottle must generally be sufficient to start 12 times consecutively of a reversible engine and 6 time for non reversible engine.

On generator starting air bottle system normally connected to emergency air compressor driven by small engine or manual operated

Prior to starting and finished with engine, blow down or blow true is taken to remove any condensate or residue in cylinder through opened indicator cock in order to avoid damage due to hydraulic lock


Controller - Control Air(region1.4 to 7 Bar)

Pneumatically controlled controller or instruments such as pressure, temperature,

level, speed, flow etc controller and valve

positional with working pressure usually fall into

the region1.4 to 7 Bar.

This variety of pressure may be obtained by using a small reducing valve or solenoid valve to

supply air at the correct value


General usage(4 to 7 Bar)

Generally compressed air are used for any kind of tools which is operated by compressed air to perform their duty such as drilling machine, impact wrenches, hand grinder, lifting gear etc considered as pneumatic tools.

Some additional auxiliary or deck machinery operated by compressed air such as Wilden pump, lifeboat davit etc

Hydraulic jack for even tightening any of the studs now are commonly used also use compressed air even some engine makers design fuel injector testing using compressed air system to ensure the injectors condition. Water Pressure Test too.

A part from those usages, compressed air widely used for cleaning purpose such as dust and cleaning at the narrow space in such situation where brush and finger tips cannot be reach the place where the cleaning should be made.

Cleaning any dust from the electrical machinery e.g. alternator, motor etc


Requirements

Two starting air compressors must be fitted, of sufficient total capacity to meet the engine requirements. Each compressor must be able to press up air receiver from 15 to

25 bar in thirty minutes

Two air receivers must be provided

Total air receiver capacity is to be sufficient for twelve starts of reversible engines and six stars for non-reversible engine

Additional one diesel driven or hand operated (or if possible both) emergency air compressor must be fitted to start auxiliary engines of a dead ship

Safety valves or preferably bursting discs must be fitted on the cooling water casing to give ample relief of pressure, should an air cooling tube burst

Cont.

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Each compressor must have a safety valve designed so that the accumulated pressure, with the outlet valve closed, will not exceed 10% of the maximum working pressure .

The air compressor cylinders, covers, intercoolers and after coolers are tested by hydraulic pressure to twice their working pressure

The casing of the inter cooler and after cooler is hydraulic tested 1.5 times cooling water pressure. Thus for 4 Bar cooling water pressure, the casing will be tested at 6 Bar pressure

A water separator or drain should be fitted on the air side of each intercooler. The drain cock should be used at regular intervals to prevent water from one stage being carried into the following stages

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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

Compressor can be classified according to the principle of compression as follow; Positive displacement compressor

Dynamic compressor

In addition some of special designed compressor are used for specific purpose 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.

Lubrication difficulties will be minimised by keeping air temperatures as low as possible.

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Temperatures in excess of 200C tend to carbonise oil-leaving gum and carbon deposits on piston rings and delivery valves. This in turn leads to further problems and an impairment of the efficiency of the machine.

Rotary vane units are limited by gas slip past the seals and a differential of about 7 bars per stage is about the limit.

The capacity of an air compressor is measured by the number of cubic metres of free air discharged per minute.

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The volumetric efficiency of an air compressor is measured by the number of cubic metres of free air discharged per minute compared with the displacement of the LP piston in cubic metres per minute. Modern air compressors have a volumetric efficiency of between 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.

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Thank You

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1. air compressor

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