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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 compressor’s 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.

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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 pressure
goes 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 completely
free 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 the
design 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 power
around 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 are
more 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 compressors–one 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 limit–about 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 the
lubricating 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 a
uniform 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 compressor

can 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 pressure–3.0
scfm 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 compressor
pump 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
Pressure
against
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 better

efficiency 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 drip
cylinder 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 the
cylinder 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 of

low 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 any
residual 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;

(g -1)/g
T2 = T1. (P2/P1)
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 clean
o -very effective due to large surface area of large
number small diameter tubes
o -plugging of failed tube allos cooler to continue in
service with little loss in efficiency
o -must allow for thermal expansion by having one
tube plate floating
• 'U' tube-
o -suitable for higher pressures than plain tube
o -self compensating for thermal expansion
o -efficent due to large nomber small diameter tubes
o -failed tubes may be plugged
o -more expensive than plain tube and diificult to clean
• Coil tube
o -self compensating for expansion
o -suitable for high pressures
o -difficult to clean
o -inefficicent due to large tube diameter
o - not easy to plug
o -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

Reciprocating Compressor
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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 Compressor

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 reciprocating
compressors (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 = [ ( Ve / Va ) * 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 practically
impossible, 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 inter
cooling 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
Compression
During 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
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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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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