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Introduction

Compressor is used to increase the pressure ofair from low pressure to high pressure by usingsome external energy.

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Objectives

Explain the function of the compressor Discuss the concept of compression ratio List common compressors found in refrigeration

systems Describe the component parts of reciprocating

compressors

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Applications of compressed air

For filling the air in tube of vehiclesIn automobile service station to clean vehicles.For spray painting in paint industries.In vehicle to operate air brakes.

For cleaning workshop machines.For supercharging of an IC engines.For operation of pneumatic tools i.e. rock drills,vibrators etc.

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SLIDING VANELIQUID PISTONHELICAL LOBE

SCREWLOBED BLOWER

COMPRESSOR

INTERMITTENT FLOW CONTINOUS FLOW

POSTIVE DISPLACEMNT DYNAMIC EJECTOR

RECIPROCATINGROTARY

MECHANICALPISTON

RADIAL FLOW

MIXED FLOW

AXIALFLOW

CENTRIFUGAL MIXEDFLOW

AXIAL

Classification

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Definitions related to compressor

Compression ratio:-It is defined as the ratio of volume of air before

compression to the volume of air after compression.

Compressor capacity:-

It is the quantity of air actually delivered by acompressor in m 3 per minute.

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Definitions related to compressor

Free air Delivered(FAD):-

It is the volume of air delivered by compressor underthe conditions of temperature and pressure existing at thecompressor intake.

Swept Volume:-The volume displaced or swept by piston whenit moves between top dead center and bottom deadcenter.

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Types of Compressors

Positive Displacement (PD) : Operate by trapping aspecific volume of air and forcing it into a smallervolume

2 Basic Designs for PD CompressorsRotaryReciprocating

Centrifugal : Operate by accelerating the air andconverting the energy to pressure

2 Basic Designs for Centrifugal CompressorsCentrifugalAxial

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Reciprocating Compressor

In a reciprocating compressor, a volume of air is

drawn into a cylinder, it is trapped, and compressedby piston and then discharged into the dischargeline. The cylinder valves control the flow of airthrough the cylinder; these valves act as checkvalves.There are two types of reciprocating compressor.

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Single Acting compressorIt is a compressor that has one discharge per

revolution of crankshaft. Double Acting Compressor

It is a compressor that completes two discharge strokesper revolutions of crankshaft. Most heavy-dutycompressors are double acting..

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Suction line Discharge line

Valve plate

Head Discharge valve

Suction valve Piston

Rings

CrankshaftConnecting Rod

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Different parts of double acting compressor arelisted below.

Suction valve.Suction air jacket.Piston.Cylinder.

Discharge valve.Discharge air jacket

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Suction valveopen

Discharge valveclosed

Piston moving downward in the cylinder

Pressure of the Airin the cylinder is

equal to thesuction pressure

Suction airpulled into thecompression

cylinder

Most of the energy that entering the compressor in the suction cylinder is latent heat.

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Suction valveclosed

Discharge valveopen

Piston moving up in the cylinder

The air leaving the compressor is

very warm.

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Two stage compressor

FIRST STAGE SECOND STAGE

Low pressure Medium Pressure

Suction

Discharge Discharge

Suction

High Pressure

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Advantages of multi-staging

Reduction in power required to drive thecompressor.

Better mechanical balance of the whole unit anduniform torque.Increase in volumetric efficiency.Reduced leakage loss.

Less difficulty in lubrication due to low workingtemperature.Lighter cylinders can be used.Cheaper materials can be used for construction as theoperating temperature is lower.

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Intercooling

Perfect intercooling.

Imperfect intercooling.

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Rotary air compressor

Rotary compressors (get their name from therotating motion of the transfer element) compressgases with lobes, screws, and vanes into smallervolumes.4 Primary Types of Rotary Compressors:

Rotary Screw

Sliding VaneLobeCentrifugal

Axial flow

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Screw Compressors

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Screw Compressors

Commonly used in industry.It operates with 2 helical rotors that rotate toward eachother, causing the teeth to mesh.As the left rotor turns clockwise, the right rotor rotatescounterclockwise. This forces the gases to becometrapped in the central cavity.The 2 rotors are attached to a drive shaft and drive thatprovide energy to operate the compressor.Have an inlet suction line and outlet discharge port.

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Vane Compressor

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Vane Compressor

Uses a slightly off-center rotor with sliding vanes tocompress air.Inlet air flows into the vanes when they are fullyextended and form the largest pocket. As the vanesturn toward the discharge port, the gases arecompressed.

As the volume decreases, the pressure increases untilmaximum compression is achieved. Then the air isdischarged out the compressor.

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Lobe Compressor

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Lobe Compressor

Characterized by 2 kidney-bean shaped impellersused to trap and transfer air.The 2 impellers move in opposite directions onparallel mounted shafts as the lobes sweep acrossthe suction port.Compressed gases are released into the discharge

line.The lobes do not touch each other. A few clearingexists between the casing and lobes.

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Centrifugal Compressor

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Centrifugal compressors accelerates the velocity

of the gases (increases kinetic energy) which is

then converted into pressure as the air flow

leaves the volute and enters the discharge pipe.

Usually operate at speeds > 3,000 rpm.

Deliver much higher flow rates than positive

displacement compressors

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Basic ComponentsImpellers, Vanes, Volutes, Suction Eyes,

Discharge lines, Diffuser Plates, Seals, Shaft,CasingSuction Vane Tips = Part of the impeller

vane that comes into contact with air first.Discharge Vane Tips = Part of the impellervane that comes into contact with air last

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Axial flow compressor

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Composed of a rotor that has rows of fanlike

blades.In industry, axial compressors are used a lot highflows and pressures are needed.Air flow is moves along the shaft.

Rotating blades attached to a shaft push air overstationary blades called stators.Stator blades are attached to the casing.

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As the air velocity is increased by the rotating blades, thestator blades slow it down. As the air slows, kinetic

energy is converted into pressure.air velocity increases as it moves from stage to stage untilit reaches the discharge.Multi-Stage axial compressors can generate very high flow

rates and discharge pressures.Axial compressors are usually limited to 16 stages (due totemperature/material limitations)Pound for pound, axial compressors are lighter, more

ffi i t d ll th t if l

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