Compressor GATE mechanical engineer

8/17/2019 Compressor GATE mechanical engineer

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What arecompressors?Compressors are mechanical devices that

compresses gases. It is widely used inindustries and has various applications


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How they are diferent rom pumps?

Major difference is that compressors handles the gases and pumps handles

the liquids.

As gases are compressible, the compressor also reduces the volume of gas.

Liquids are relatively incompressible while some can be compressed

Centriugal pump


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What are its applications?

Compressors have many everyday uses, such as in !

•Air conditioners, "car, home#

•$ome and industrial refrigeration

•$ydraulic compressors for industrial machines

•Air compressors for industrial manufacturing

Rerigeration compressor


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What are its various types?

Compressor classification can be described by following flow chart!


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What are dynamic

compressors?%he dynamic compressor is continuous flow compressor is characteri&ed by rotating

impeller to add velocity and thus pressure to fluid.

It is widely used in chemical and petroleum refinery industry for specific services.

%here are two types of dynamic compressors

Centrifugal Compressor

A'ial (low Compressor


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Centrifugal Compressor

Achieves compression by applying inertial forces to the gas by means ofrotating impellers.

It is multiple stage each stage consists of an impeller as the rotating

element and the stationary element, i.e. diffuser

(luid flow enters the impeller a'ially and discharged radially

%he gas ne't flows through a circular chamber "diffuser#, where it loses

velocity and increases pressure.


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Axial ow compressor

)or*ing fluid principally flows parallel to the a'is of rotation.

%he energy level of air or gas flowing through it is increased by the

action of the rotor blades which e'ert a torque on the fluid

$ave the benefits of high efficiency andlarge mass flow rate

+equire several rows of airfoils to achieve

large pressure rises ma*ing them comple' and

e'pensive


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What are positive displacementcompressors?

ositive displacement compressors causes movement by trapping a fi'ed amount of

air then forcing "displacing# that trapped volume into the discharge pipe.

It can be further classified according to the mechanism used to move air.

+otary Compressor

+eciprocating compressor


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Rotary compressors%he gas is compressed by the rotating action of

a roller inside a cylinder.

%he roller rotates off-centre around a shaft so

that part of the roller is always in contact with the

cylinder.

olume of the gas occupies is reduced and the

refrigerant is compressed.

$igh efficient as suc*ing and compressingrefrigerant occur simultaneously.


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Rotary compressors

These compressors use rotors in place o pistons,

giving a pulsating ree discharge air. These rotorsare power driven. They have the ollowingadvantages over reciprocating compressors:

o They reuire a lower starting torue

o They give a continuous, pulsation ree dischargeair

o They generally provide higher output

o They reuire smaller oundations, vi!rate less, andhave lesser parts, which means less ailure rate


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Subtypes:

"epending on the action and geometry othe rotors, these are classi#ed as ollows:

Rotarycompressors

Lobecompressors

$crew

compressors

%ane

compressors

$croll

compressors


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Lobe compressor Also called the roots blower, it’s essentially a low pressure

blower and is limited to a discharge pressure of 1 bar in a

single-stage and 2.2 bar in two stage-design.

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Rotary screw compressors:

Rotary screw compressors use two meshedrotating positive&displacement helical screwsto orce the air into a smaller space 'see#gure(.

These are usually used or continuous

operation in commercial and industrialapplications and may !e either stationary orporta!le.

)ecause o simple design and

ew wearing parts, rotary screw

air compressors are easy to

install, operate, and maintain.13


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Rotary vane compressors:*ne o the oldest compressor technologies,

rotary vane compressors consist o a rotorwith a num!er o !lades inserted in radialslots in the rotor.

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• The rotor is mounted ofset in a larger housing thatis either circular or a more comple+ shape. s therotor turns, !lades slide in and out o the slots-eeping contact with the outer wall o the housing.

Thus, a series o decreasing volumes is created !ythe rotating !lades

• They can !e either stationary or porta!le, can !esingle or multi&staged, and can !e driven !y electricmotors or internal com!ustion engines. They arewell suited to electric motor drive and is

signi#cantly uieter in operation than the euivalenpiston compressor. They can have mechanicaleciencies o a!out /01

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Scroll compressors: A scroll compressor, also nown as scroll pump

and scroll !acuum pump, uses two interlea!edspiral-lie !anes to pump or compress air.

"ften, one of the scrolls is fi#ed, while the other

orbits eccentrically without rotating, thereby

trapping and pumping or compressing pocets offluid or gas between the scrolls $see figure%.

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• "ue to minimum clearance volume !etweenthe #+ed scroll and the or!iting scroll, these

compressors have a very high volumetriceciency. They operate more smoothly,uietly, and relia!ly than other types ocompressors in the lower volume range.

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Reciprocating

compressorIt is a positive-displacement compressor that

/ses pistons driven by a cran*shaft to deliver gases at

high pressure.

%he inta*e gas enters the suction manifold, then flows into

the compression cylinder

It gets compressed by a piston driven in a reciprocating

motion via a cran*shaft,

0ischarged at higher pressure


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REC!R"CA#$%

C"&!RE''"R'"etailed nalysis

http://var/www/apps/conversion/tmp/scratch_7/Nomenclature%20of%20reciprocating%20compressor..pdfhttp://var/www/apps/conversion/tmp/scratch_7/Nomenclature%20of%20reciprocating%20compressor..pdf


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(loc) diagram of reciprocatingcompressor

It is a piston and cylinder

device with "automatic#

spring controlled inlet and

e'haust valves

%here is a clearance

between the piston crown

and the top of the

cylinder.


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• Widespread use, many applications

• 2as is accelerated outwards !y

rotating impeller• Can !e !uilt or operation as low

pressure reuirement 0 3 to 4 !ar

• $i5es range rom 600 hp to 70,000 hp

Centrifugal compressors

'ingle Case CompressorCentrifugal mpeller

&!ELLER'

Picture of horizontal split

Cross section of barreltype compressor

Picture of barreltype compressor

Cross section ofbull gear compressor

Picture of (bull)

gear and impellers

Picture of bullgear compressor

Cross section of horizontal split

*++,'ER'


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Compressorinlet no--le

#hrust bearing

.ournal bearing

'haft andlabyrinth seal

mpeller inletlabyrinth seals

*ischarge volutes

mpellers*rive coupling

Casing/hori-ontallysplit ange0 Compressor

discharge no--le

Cross section of hori-ontal split


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!icture of hori-ontal split


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Axial compressors

• 2as 8ows in direction o rotating shat

• Can !e !uilt or higher pressures ratio o 90 to9.

• High 8ow rate

• ;cient

•


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Compressoroutlet no--le

Rotor blades

Labyrinth seals

%uide1vane

actuator lin)age

Compressor rotor

Compressorinlet no--le #hrust bearing

Ad2ustable guide vanes

Cross section of axial compressor


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!icture of axial compressor


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Advantages and disadvantages ofpositive displacement type

compressorAdvantages *isadvantagesositive displacement compressor

+eciprocating •)ide pressure ratios•$igh efficiency

•$eavy foundation required•(low pulsation

•$igh maintenance

0iaphragm •ery high pressure•Low flow• 2o moving seal

•Limited capacity range•eriodic replacement of diaphragm

1crew •)ide application•$igh efficiency•$igh pressure ratio

•3'pensive•/nsuitable for corrosive or dirtygases


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'table -oneof operation

*eveloping the compressor curve

&inimumspeed

!ower limit

&aximum speed

!rocess limit

3s4 vol

Addingcontrol

margins

'tonewall orcho)e limit

'urge limit

Rc

Actual availableoperating -one


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*eveloping the compressor curve

!d *ischarge !ressure /!50!c *i6erential !ressure /!d 1 !s0 or /!5 1

!70

Rc !ressure Ratio /!d8!s0 or /!58!709p !olytropic 9eadRc

3s4 normal3s4 mass3s4 vol

Compressorcurve for aspeci:c speed$7

Rprocess47

37

Rc7

Rprocess45

35

Rc5

";!;


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'urges in centrifugal

compressors

What are surges?o $urge is de#ned as the operating point at which centriugal

compressor pea- head capa!ility and minimum 8ow limits arereached.

o When the plenum pressure !ehind the compressor is higher thanthe compressor outlet pressure, the 8uid tends to reverse or

even 8ow !ac- in the compressor.

o s a conseuence, the plenum pressure will decrease, inletpressure will increase and the 8ow reverses again.

o This phenomenon, called surge, repeats and occurs in cycles

with reuencies varying rom 9 to H5.

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E6ects on performanceo $urging can cause the compressor to overheat to the point at

which the ma+imum allowa!le temperature o the unit is

e+ceeded.o lso, surging can cause damage to the thrust !earing due to

the rotor shiting !ac- and orth rom the active to the inactiveside. This is de#ned as the surge cycle o the compressor.'urge points for centrifugal compressors running at varying spe

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'urge description


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'urge description

• >low reverses in 0 to 70 milliseconds

• $urge cycles at a rate o 0.6 s to 6 s per cycle

• Compressor vi!rates

• Temperature rises

• Whooshing@ noise

• Trips may occur

• Conventional instruments and human operatorsmay ail to recogni5e surge

'


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'ome surge conse


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%raph showing operating regionsof various compressors


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#able showing operating conditionsof various compressors

Compressor GATE mechanical engineer

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