Project of Air Compressor

8/8/2019 Project of Air Compressor

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-: INTRODUCTION:-

Air compressors have relatively simple design and can be sized

all the way from the extremely powerful to ones that can just

drive small tools. They have few parts and hence last for a long

time with little maintenance. The factors make them drivers of

choice from heavy equipment on manufacturing shop floors, to

cleaning areas in gas stations and now, inside homes and small

garages as parts of handy gadgets.

An air compressor works by transforming electrical energy in to

kinetic energy in the form of air that is held in a highly

compressed space before sudden release. The energy released

when the compressed air is let out can be used for inflation,

cleaning under pressure, turning, generating torque, driving or

other similar movement by force.

The principle of an air compressors functioning is like that of

an internal combustion unit. A crankshaft moves a piston

through a connecting rod, in a linear fashion along the length of

a cylinder in which it fits snugly. Air is drawn in through an

open valve as the piston moves upwards, providing ample

space for a high volume of air. The valve closes and the piston

moves down, compressing the air. The latter has

kinetic energy as it gains tremendous pressure because of the

restricted space in to which it has been forced. The valve opens

again to release the energy.

ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES

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Cylinders may be placed in series, so that the compressed air is

fed to an intake where it is compressed further. The pressuregoes up further and hence the power is increased in proportion.

All air compressors must have back up systems to release

extra pressure in the event of a malfunction in the valve

mechanism. This is required to prevent an accident such as in

an explosion.

The efficiency of air compression depends on air temperature,

atmospheric pressure and relative humidity. The rating of an

air compressor therefore depends on ambient conditions. This

aspect should be taken in to account if an air compressor is

used in very cold weather, very wet weather or at an high

altitude.

Oil is used to lubricate the piston as it moves inside the

cylinder. Rings act as seals to reduce the amount of oil that can

mix with the compressed air. Some oil contamination,

especially in atomized form, will inevitably escape ring seals

and hence a more secure oil containment design is required in

case the compressed air energy source has to be completelyfree of oil. Painting and other functions related to finishing a

surface have this kind of requirement as even a faint oil trace

can affect appearance.

Air compressors have relatively few parts and hence require

little maintenance. They last for years and are versatile and

compact as well. This is why new uses for air compressors keep


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surfacing so many years after the first unit was built. Some

models have a second cylinder and sizes can be adjusted at thedesign stage to increase the range of applications and uses.

Years ago, it was common for shops to have a central power

source that drove all the tools through a system of belts,

wheels and driveshafts. The power was routed around the work

space by mechanical means. While the belts and shafts may be

gone, many shops still use a mechanical system to move poweraround the shop. It's based on the energy stored in air that's

under pressure, and the heart of the system is the air

compressor.

You'll find air compressors used in a wide range of situations

from corner gas stations to major manufacturing plants. And,

more and more, air compressors are finding their way into

home workshops, basements and garages. Models sized to

handle every job, from inflating pool toys to powering tools

such as nail guns, sanders, drills, impact wrenches, staplers

and spray guns are now available through local home centers,

tool dealers and mail-order catalogs.

The big advantage of air power is that each tool doesn't need

its own bulky motor. Instead, a single motor on the compressor

converts the electrical energy into kinetic energy. This makes

for light, compact, easy-to-handle tools that run quietly and

have fewer parts that wear out.


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Air compressor types


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While there are compressors that use rotating impellers to

generate air pressure, positive-displacement compressors aremore common and include the models used by homeowners,

woodworkers, mechanics and contractors. Here, air pressure is

increased by reducing the size of the space that contains the

air. Most of the compressors you'll run across do this job with a

reciprocating piston.

Like a small internal combustion engine, a conventional piston

compressor has a crankshaft, a connecting rod and piston, a

cylinder and a valve head. The crankshaft is driven by either an

electric motor or a gas engine. While there are small models

that are comprised of just the pump and motor, most

compressors have an air tank to hold a quantity of air within a

preset pressure range. The compressed air in the tank drives

the air tools, and the motor cycles on and off to automatically

maintain pressure in the tank.

At the top of the cylinder, you'll find a valve head that holds the

inlet and discharge valves. Both are simply thin metal flaps

one mounted underneath and one mounted on top of the valve

plate. As the piston moves down, a vacuum is created above it.

This allows outside air at atmospheric pressure to push open

the inlet valve and fill the area above the piston. As the piston

moves up, the air above it compresses, holds the inlet valve

shut and pushes the discharge valve open. The air moves from


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the discharge port to the tank. With each stroke, more air

enters the tank and the pressure rises.

Typical compressors come in 1- or 2-cylinder versions to suit

the requirements of the tools they power. On the

homeowner/contractor level, most of the 2-cylinder models

operate just like single-cylinder versions, except that there are

two strokes per revolution instead of one. Some commercial 2-cylinder compressors are 2-stage compressorsone piston

pumps air into a second cylinder that further increases

pressure.

Compressors use a pressure switch to stop the motor when

tank pressure reaches a preset limitabout 125 psi for many

single-stage models. Most of the time, though, you don't need

that much pressure. Therefore, the air line will include a

regulator that you set to match the pressure requirements of

the tool you're using. A gauge before the regulator monitors

tank pressure and a gauge after the regulator monitors air-line

pressure. In addition, the tank has a safety valve that opens if

the pressure switch malfunctions. The pressure switch may

also incorporate an unloader valve that reduces tank pressure

when the compressor is turned off.

Many articulated-piston compressors are oil lubricated. That is,

they have an oil bath that splash-lubricates the bearings and


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cylinder walls as the crank rotates. The pistons have rings that

help keep the compressed air on top of the piston and keep thelubricating oil away from the air. Rings, though, are not

completely effective, so some oil will enter the compressed air

in aerosol form.

Having oil in the air isn't necessarily a problem. Many air tools

require oiling, and inline oilers are often added to increase auniform supply to the tool. On the down side, these models

require regular oil checks, periodic oil changes and they must

be operated on a level surface. Most of all, there are some tools

and situations that require oilfree air. Spray painting with oil in

the airstream will cause finish problems. And many new

woodworking air tools such as nailers and sanders are designed

to be oilfree so there's no chance of fouling wood surfaces with

oil. While solutions to the airborne oil problem include using an

oil separator or filter in the air line, a better idea is to use an

oilfree compressor that uses permanently lubricated bearings

in place of the oil bath.

A variation on the automotive-type piston compressor is a

model that uses a one-piece piston/connecting rod. Because

there is no wrist pin, the piston leans from side to side as the

eccentric journal on the shaft moves it up and down. A seal

around the piston maintains contact with the cylinder walls and

prevents air leakage.


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Where air requirements are modest, a diaphragm compressorcan be effective. In this design, a membrane between the

piston and the compression chamber seals off the air and

prevents leakage.

~:Compressor power:~

One of the factors used to designate compressor power is

motor horsepower. However, this isn't the best indicator. You

really need to know the amount of air the compressor can

deliver at a specific pressure.

The rate at which a compressor can deliver a volume of air is

noted in cubic feet per minute (cfm). Because atmospheric

pressure plays a role in how fast air moves into the cylinder,

cfm will vary with atmospheric pressure. It also varies with the

temperature and humidity of the air. To set an even playing

field, makers calculate standard cubic feet per minute (scfm)


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as cfm at sea level with 68 degrees F air at 36% relative

humidity. Scfm ratings are given at a specific pressure3.0scfm at 90 psi, for example. If you reduce pressure, scfm goes

up, and vice versa.

You also may run across a rating called displacement cfm. This

figure is the product of cylinder displacement and motor rpm.

In comparison with scfm, it provides an index of compressorpump efficiency.

The cfm and psi ratings are important because they indicate

the tools that a particular compressor can drive. When

choosing a compressor, make sure it can supply the amount of

air and the pressure that your tools need.

Air Compressors


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Graph of

Pressureagainst

volume in a

reciprocating

compressor

Volumetric efficiency Vh = Actual suction volume Vx/

Theoretical suction volume Vs

For greater efficiency air compression should be isothermal as

this requires the minimum work input. In practice Isothermal

compression is not possible, an ideal Isothermal cycle requires

sufficient time to allow all the required heat to be transferred

out of the cylinder, practicality dictates that the piston must

have a relatively high speed to give a reasonable output,

Cylinder cooling on a single stage compressor gives betterefficiency but there is a limitation in the surface area to

cylinder volume that can be used for cooling effect, but

multistage compressors with an efficient extended surface

interstage cooler gives cycle improved compression efficiency

better approaching that of the isothermal. In theory the greater

the number of stages the closer the curve will approach the

ideal isothermal compression curve, however there is an


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increase in cost, complexity, and the law of diminishing returns

limit the number.

Compression in stages has the following advantages;

1. The compression ratio at each stage is lower and so the

final temperature is lower. This reduces problems with

lubrication

2. . The machine is smaller and better balanced

3. water can be drained off at each stage

4. Compression better approaches the ideal isothermal

It is important that the compressor clearance volume is kept

small as possible in order to improve overall volumetric

efficiency as the air trapped in this space must expand to

below suction pressure before new air can enter, this is an

effective loss of stroke.

A clearance is required in order to prevent the piston striking

the cylinder cover when starting or stopping off load. The

clearance volume is sometimes referred to as the 'Bump

Clearance'.

Crankcase lubrication

Lubrication of the crankcase in a compressor does not pose any

specific problems and normally consist of splash lubrication

with pressurised oil being fed to shell bearings. Where dripcylinder lubrication is used, this should be kept to a minimum

conducive with liner wear. A standard mineral oil similar to that

used in the main engine may be used, although due to carbon

deposits, higher quality oils are generally used with the most

effective being specifically designed synthetics which have

allow a considerable reduction in maintenance but are costly.


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Mineral oils contain a blend of lighter elements such as

paraffin's, and heavier elements such as asphaltenes. During

compression the lighter elements are vaporised leaving the

heavy ends, these coat the piston rings and discharge valves in

combination with oxidised oil deposits. These deposits also coat

passage ways and coolers resulting in higher interstage air

temperatures. Deposits on discharge valves cause them to

become sticky and leak resulting in hot air being drawn back

into the cylinder for recompression. This increases the

temperature and hence causes greater oxidation and deposits,

and so the condition deteriorates with increasing rapidity.

Temperature can become very high, this may result in oily

deposits at the discharge valves carbonising. Eventually this

carbon could glow red and cause detonation. It is more likely,

however, that oily deposits will be carried over to the air

receiver and air start manifold to be ignited by blowpast at thecylinder air start valve.

Deposits at piston rings cause leakage allowing oil to enter the

cylinder from the crankcase thus increasing the danger it is

essential that crankcase lubrication be kept to a minimum

compatible with an acceptable wear rate. Regular maintenance

will minimize oily deposits build up and hence the risk of

explosion

Materials and design of a reciprocating compressor

The compressor casing, cylinder covers and piston rings are

generally of cast iron. Pistons may be of cast iron, steel of

aluminium. Aluminium being the preferred material for use on

the LP piston due to its larger diameter. Valves are usually

made so that parts can be interchanged between the suction

and discharge valves. Seats are of mild steel with small


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diameter air passages to prevent the fragments of broken

valve plate from entering the cylinder. Valve plates are of

vanadium steel heat treated and ground to provided the

required hardness and surface finish. Springs should be

arranged such that they lift and seat squarely. Uneven spring

force or deposits on the seat cause valves to bend resulting in

fatigue cracking.

For compressors designed for starting air requirements a water

jacket relief valve is fitted.


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Rotary Compressor


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The rotary compressor may be of the impeller type similar to

that used in the turbocharger , scroll, twin rotating lobes or of

the sliding vane type similar to the one shown above. In

practice there would be several more vanes than shown.

Rotary compressors are capable of handling large quantities oflow pressure air much more efficiently than a reciprocating

compressor. In order to produce increased pressures it is

possible to stage rotary compressors but leakage problems

increase at higher pressures as well as stress on the vanes.

The sliding vane compressor consists of a slotted rotor with its

axis offset from that of the cylindrical casing. Vanes fit in the

slots and have contact with the casing

On the suction side the space contained between the casing,

the rotor and an adjacent pair of vanes is gradually increasing

allowing air to be drawn in.

On the compression side this same space is gradually reduced

causing the pressure increase. When the leading vane

uncovers the discharge port air will flow to outlet. Larger


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compressors of this type are water-cooled, smaller

compressors tend to be air cooled.

The main problems related with sliding vane compressors

concern wear at the vane tips and sealing of the ends.

Rotary/reciprocating Compressor

Rotary compressors in general do not require internal

lubrication but they are not suitable alone for providing air at a

pressure for starting duties. They can, however, be linked to

reciprocating stages to produce a hybrid compressor.

The compressor is lighter, more compact and better balanced

than an equivalent all reciprocating unit. In basic terms the

rotary first stage supplies air to the reciprocating second and

subsequent stages. All stages being driven by the same shaft


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Safety Valve

Materials

Cast iron-Casing, Liners, Pistons( the LP piston is sometimes

made from an aluminium alloy, Cylinder covers Steel-

Crankshaft, Conrods, Pistons, Valve seats Vanadium Steel-

Valve plates

Starting air compressor circuit

Starting and stopping sequence is adjustable, the magnetic

valves are open when the compressor is stopped so anyresidual pressure is blown off. On starting the magnetic valve

are sometimes delayed to close so as to allow the compressor

motor to reach full speed before the compressor is loaded up.

The non-return valves prevent HP air leaking back from the

receiver on which the filling is also of the non return type.


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Calculation of required cylinder compression for a

multistage reciprocating compressor

r = stage pressure ratio

R = compression pressure ratio

for a two stage

r = R1/2

for a three stage

r = R1/3

for example, a 3 stage compressor requiring a final pressure of

64bar would have the following interstage pressures 1st stage

1bar compressed to 4bar

2nd stage 4bar compressed to 16bar

3rd stage 16bar compressed to 64bar

It would appear that most of the work is being carried out in

the final stage, this is untrue as with the increase in pressure is

a complimentary reduction in volume, if the temperature


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conditions remain the same then work will be equally divided

between the stages.

By reducing the suction pressure, the cylinder is required to do

more work on the air before the discharge valve opens. This

means that the air will be delivered at a higher pressure. The

higher temperature can lead to problems with the cylinder

lubrication as well as a drop in efficiency as well as carbonising

of the oil and increased deposits on the valves and piston rings

and interstage passages. In the extreme it can lead to seizure

and possible diesel detonation of the oil laden air.

The reduction in pressure at the suction can be due to a

partially blocked suction filter or partially choked suction valve.

The lower pressure conditions in the cylinder at the start of

compression can cause oil laden air to be drawn from the

crankcase up the liner. This oil can lead to increased deposits

in the compressor as well as further downstream in the

distribution system

(P1.V1)/ T1 = (P2. V2)/T2

and

(P1/P2).(T2/T1) = (V2/V1)

P1.V1g = P2.V2g

and

P1/P2 = V2g/V1g From these we get;

T2 = T1. (P2/P1) (g -1)/g

g = 1.4

and if we take for and example

P1 = 0.4 bar

P2 = 1 bar


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Pf = 5 bar

Tinitial = 300 K we end with final temperatures for the

two compression's of

T1= 617 K and T2 = 475 K

from the graph it can be clearly seen that losses due to

the bump clearance has increased and the period of

constant pressure delivery has been reduced.

Coolers

Plain Tube-o -easy to cleano -very effective due to large surface area of large

number small diameter tubeso -plugging of failed tube allos cooler to continue in

service with little loss in efficiencyo -must allow for thermal expansion by having one

tube plate floating 'U' tube-

o -suitable for higher pressures than plain tubeo -self compensating for thermal expansiono -efficent due to large nomber small diameter tubeso -failed tubes may be pluggedo -more expensive than plain tube and diificult to clean

Coil tubeo -self compensating for expansion

o -suitable for high pressureso -difficult to cleano -inefficicent due to large tube diametero - not easy to plugo -expensive


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Reciprocating Compressor

In a reciprocating compressor, gas is compressed by

mechanical variation of the volume of space inside the

cylinder, by reciprocating motion of the piston.

For a cycle of operation, there are two strokes such as,

1.) Suction stroke, and

2.) Compression stroke


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As the piston moves down, air is sucked from atmosphere to

the cylinder through suction valve (a non-return valve). As

piston moves up, air is compressed and at the end of

compression stroke, air is delivered through delivery valve

(which is also a non-return valve).

Topmost portion the piston can travel inside the cylinder is is

called Top Dead Center (TDC), and bottom most portion the

piston can

reach inside the cylinder is called as Bottom Dead Center(BDC).

RECIPROCATING AIR COMPRESSOR

Screw Compressor

Screw compressors (also called as helical lobe compressors)

are positive displacement machines in which gas is being

compressed is forced through the casing by two screws. Unlike

the reciprocating compressors which are also positive

displacement machines, screw compressors does not typically

require internal suction or discharge valves. In addition the flow

from screw compressor is generally more uniform and has

fewer pulsations than the flow from a reciprocating

compressor.


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A twin screw compressor consist of two meshing helical rotors

mounted on counter rotating parallel shafts that are enclosed

within close-clearance casing. One screw is called driving screw

which is coupled with a drive, say an electric motor, while the

other screw is called as the driven screw, since it is driven by

the driving screw. Gears used for driving the screws are called

timing gears, since they are properly timed to maintain the

close-clearance between the screws.

SCREWS OF A TWIN SCREW COMPRESSOR

For screw compressors oil is injected into the screws while

operation. There are mainly 3 functions for the oil, they are,

1) Sealing of the screws to prevent leakage of the gas

2) Lubrication of the parts, especially the screws, and

3) Cooling of the gas compressed, which results in increased

efficiency of the system

Materials used for construction



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Crank-case and body - Cast Iron

Crankshaft - Spheroidal graphite cast iron or stainless steel

Connecting rod - Forged Steel

Piston - Aluminium Alloy or Cast / Ductile Iron

Piston Rings - Cast Iron

Screw Compressor

Casing - Cast or Ductile Iron

Screw - Steel or Stainless Steel or Nickel Alloy


Reciprocating compressors are characterized with higher

pressures and reduced mass flow rate. They are mainly used in

high pressure applications since it can deliver air at about 30 -

40 bar.

1) For diesel engine starting, where electric motor starting

becomes costly and impractical.

2) Refrigeration compressors are normally reciprocating type

(Single Stage) with a discharge pressure around 10 bar.

3) Air conditioning systems also uses reciprocatingcompressors (Nowadays trend is changed to screw

compressors).

Screw Compressors

Screw type compressors provide air at increased mass flow

rate but with reduced discharge pressure around 8 bar. Hence

applications are also in low pressure systems, such as,


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1) Service air compressors used in industry (For cleaning air,

etc.)

2) Air conditioning systems nowadays employ screw

compressors. (which have low power consumption and

increased mass flow rate as advantages)

3) For low pressure air required for running pneumatic tools,

pneumatic-hydraulic equipments, etc.

Cycle of operation

Consider one cycle of operation in a reciprocating compressor.

ONE COMPLETE CYCLE OF OPERATION

The process that occurs in a cycle 1-2-3-4-1 are explained

below

(3) - (4) - As piston travels from BDC to TDC air trapped

inside the cylinder is compressed.

(4) - (1) - As piston approaches TDC discharge valve opens


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and compressed air is delivered.(1) - (2) Undelivered air trapped in the clearance space is

expanded as piston moves down.(2) - (3) - When trapped air in the clearance space is

expanded to atmospheric pressure, further downward

movement of the piston creates a vacuum inside the cylinder

and thereby atmospheric air enters through suction valve.

Again cycle repeats.In fig. 'Va' indicates the volume corresponding to actual stroke

of the piston from TDC to BDC (also called stroke volume).

Similarly 'Ve' indicates the volume corresponding to the

effective stroke of piston, when atmospheric air enters the

cylinder.

The ratio of effective stroke volume to the swept volume is

known as the volumetric efficiency of the compressor,

Voleff= [ ( V

e/ V

a) * 100 ] %

Then why clearance space?

It was already seen from the above equation that

volumetric efficiency is 100% when

Effective stroke = Actual stroke

In other words, no clearance volume exists. This is practicallyimpossible, because some clearance space is required

otherwise piston hits on the cylinder head as it travels. Also

expansion of piston occurs as it travels and very little clearance

may cause the same problem. Also increased clearance space

reduces compressor efficiency and increase its running hours.

So clearance volume must be kept around a reasonable value

as instructed by the manufacturer.


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How to measure clearance volume

The clearance between piston and cylinder head while piston at TDC is called bumping clearance. This can be measured in

different ways. One common method is, remove the valves

from the top of the piston. Put a lead ball of sufficient diameter

into the cylinder. Slowly turn the flywheel one revolution by

hand. Take out the lead piece and measure its thickness, which

gives the bumping clearance.

Why cooling is required?

Reciprocating compressors are generally cooled with air or

water. The cylinders in air cooled compressors often include

large external fins that increase the surface area available for

heat transfer.

In water cooled compressors, freshwater is circulated through

jackets that are built into the walls of the cylinders and cylinder

heads.

The compression cycle is illustrated graphically down below.


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A SINGLE STAGE COMPRESSION

Above cycle compresses gas from atmospheric pressure to 8

bar in a single stage. The area enclosed by the points 12341

represents the work of compression in a single stage

compressor. Also see the cycle or Pressure-Volume diagram (P-

V) below which compresses gas from atmospheric pressure to

8 bar in two stages.


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TWO STAGE COMPRESSION

Here first stage compresses gas from atmospheric pressure to

3 bar and then gas is cooled isobarically (at constant pressure,

refer diagram above). Now gas is again compressed to 8 bar.

Now we can see that the work of compression corresponding to

the shaded area in the diagram is saved by incorporating

an inter cooling between two stages. Hence when comparing

with a single stage compressor work can be reduced by intercooling in a multistage compressor.

Work can be reduced further by increasing number of stages

and inter cooling, but as the number of stages increases

design becomes complex, constructional cost

increases, maintenance cost also increase, which may nullify

the effect of work saved during operation. This is the limiting

factor for more number of stages.


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So how it happens?

ISOTHERMAL, PLOYTROPIC AND ADIABATIC

COMPRESSION

The indicator card (PV diagram) above shows 3 types (or

processes) of compression possible.

Isothermal

CompressionDuring the process of compression, whatever heat produced is

taken away by a cooling medium. In other words, it is the

compression , keeping temperature of the gas constant. For a

process to be isothermal, the process must be very slow, which


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is impractical. From the indicator card, it is clear that, work of

compression is minimum in isothermal compression.

Also,

EQUATION FOR ISOTHERMAL COMPRESSION

Adiabatic Compression

Whatever heat produced during compression is kept inside the

gas only, or heat transfer is zero in an adiabatic compression.For a perfect adiabatic process, process must be very fast. All

the thermodynamic process resembles adiabatic process. It can

be seen from the indicator card that, work of compression is

maximum in adiabatic compression.

Also,


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EQUATION FOR ADIABATIC COMPRESSION

Specific heat is defined as the heat energy required to raise the

temperature of unit mass of substance by unit degree.

Polytropic compression is neither isothermal nor adiabatic. It

comes in between.Also,

EQUATION FOR POLYTROPIC COMPRESSION

Work of compression can be minimized by isothermal

compression. But compression is practically a fast process. So

it better resembles an adiabatic process. Jacket cooling of

compressor makes the compression polytropic.


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Polytropic Compression


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Now the only way to make the compression more isothermal is,

by dividing the process to a number of stages. Between every

stage inter cooling of gas is done. Hence the work can be

saved substantially. Refer the diagrams below.

SINGLE STAGE & MULTISTAGE COMPRESSION

Reference :

http://www.popularmechanics.com

http://www.essortment.com

http:// wikipedia.com


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