AIR COMPRESSOR: TYPES AND WORKING PRINCIPLES
Air compressors have been used in industry for well over 100 years because air as a resource is safe,
flexible, clean and convenient. These machines have evolved into highly reliable* pieces of equipment
that are almost indispensable in many of the applications they serve. Compressors can come in a wide
variety of different types and sizes.
It is a device which compresses the gases to higher pressure and than this high pressure gas is used for
various application like Tyre inflation, powering a turbine or used to do some mechanical work. The air
compressor is generally powered by electric motor, diesel or gas engine.
Working Principle
Since we know that air is compressible and we only need some mechanical device to do that and for
that we have air compressor. It takes air form one end and than squeeze this air to high pressure and
deliver it to the other end for various application.
Types of Air Compressor
According to the pressure delivered
1. low-Pressure Air Compressors (LPACs): It can discharge pressure at 151 psi or less.
2. Medium-Pressure Air Compressors : Can discharge pressure in between 151 psi to 1000 psi.
3. High-Pressure Air Compressors (HPACs): having discharge pressure above 1000 psi.
According to the type of compression, it can be classified as
1. Positive Displacement : Piston type, rotary screw, rotary vane air compressor
2. Dynamic Displacement: Centrifugal and axial flow air compressor
1. Positive Displacement Air Compressor
In positive displacement compressor, the air is sucked into a chamber whose volume gets decreased to
compress the air. When the maximum pressure is reached in the chamber, the discharge valve opens
and air is discharged to the storage tank. Once the pressure of the storage tank reaches to its desired
upper limit the compressor stops. The compressed air in the tank is used to perform various works. As
the pressure in the tank reaches to its minimum limit, the compressor again starts and begins the
process of compressing air.
The positive displacement compressor can be of following type
1. Piston Type or Reciprocating Air Compressor
reciprocating air compressor
Reciprocating or Piston compressors are the most common machines available on the market. They are
positive displacement compressors and can be found in ranges from fractional to very high horse
powers. Positive displacement air compressors work by filling an air chamber with air and then reducing
the chamber’s volume (Reciprocating, Rotary Screw and Rotary Sliding Vane are all positive
displacement compressors). Reciprocating compressors work in a very similar manner as does as
internal combustion engine but basically in a reverse process. They have cylinders, pistons, crankshafts,
valves and housing blocks.
advantages
* simple design
* low initial cost
* easy to install
* two stage models offer the highest efficiency
* no oil carryover
* large range of horse power
* special machines can reach extremely high pressures
disadvantages
* higher maintenance cost
* many moving parts
* potential for vibration problems
* foundation may be required
* many are not designed to run at full capacity 100% of the time
2. Rotary Screw Compressor
Rotary Screw Compressors work on the principle of air filling the void between two helical mated screws
and their housing. As the two helical screws are turned, the volume is reduced resulting in an increase of
air pressure. Most rotary screw compressors inject oil into the bearing and compression area. The
reasons are for cooling, lubrication and creating a seal between screws and the housing wall to reduce
internal leakage. After the compression cycle, the oil and air must be separated before the air can be
used by the air system.
Positive displacement compression of air takes place in this compressor. It uses two helical types of
screws matching each other, when it rotates it takes the air from the atmosphere and guides it to a
chamber whose volume keeps on decreasing as the screw turns.
advantage
* simple design
* low to medium initial cost
* low to medium maintenance cost
* two stage designs provide good efficiency
* easy to install
* few moving parts
* most popular compressor design in plants
disadvantages
* limited airend life
* airends are not field serviceable
* high rotational speeds
* shorter life expectancy than other designs
* oil injected designs have oil carryover
* single stage designs have lower efficiency
* two stage, oil-free designs have higher initial cost.
* difficulty with dirty environments
3. Rotary Vane Compressor
Rotary Sliding Vane Compressors like Reciprocating and Rotary Screw compressors are positive
displacement compressors. The compressor pump consists primarily of a rotor, stator, and 8 blades. The
slotted rotor is eccentrically arranged within the stator providing a crescent shaped swept area between
the intake and exhaust ports. As the rotor turns a single revolution, compression is achieved as the
volume goes from a maximum at the intake ports to a minimum at the exhaust port. The vanes are
forced outward from within the rotor slots and held against the stator wall by rotational acceleration. Oil
is injected into the air intake and along the stator walls to cool the air, lubricate the bearings and vanes,
and provide a seal between the vanes and the stator wall. After the compression cycle, the oil and air
must be separated before the air can be transferred to the air system.
It is also a positive displacement compressor in which the rotor has blades. As the rotor rotates it traps
the air in between the vanes (blades) and air gets compressed. A question hits now in our mind that
How this air gets compressed by the vane compressor. Vanes of the compressor is adjustable according
to the casing and the rotor in the casing is eccentric (i.e. it is not located in the center but its center is
shifted away from the casing center). when rotor rotates, the air gets traped between two adjacent
vanes. The casing of compressor gets on decreasing as the rotor rotates, and due to the decrease of
casing the air gets compressed.
advantages
* simple design
* easy to install
* low to medium cost
* low maintenance cost
* field serviceable air-end
* long life air-end
* low rotational speeds
* very few moving parts
* forgiving to dirty environments
disadvantages
* oil injected designs have oil carryover
* single stage designs have lower efficiency
* difficulty with high pressures over 200 psi
* oil free designs are unavailable
2. Dynamic Displacement Air Compressor
In dynamic displacement compressor, it has a rotating part which imparts its kinetic energy to the air
and converts it into pressure energy. The impart of the kinetic energy is done with the help of the
centrifugal force.
This type of air compressor includes centrifugal compressor and axial flow compressor.
1. Centrifugal Compressor
Centrifugal Compressors are not positive displacement compressors like the Reciprocating, Screw or
Vane Compressors. They use very high speed spinning impellers (up to 60,000 rpm) to accelerate the air
then diffuser to decelerate the air. This process, called dynamic compression, uses velocity to cause an
increase in pressure. In most Centrifugal compressors, there are several of these impeller/diffuser
combinations. Typically, these machines have intercoolers between each stage to cool the air as well as
remove 100% of the condensate to avoid impeller damage due to erosion.
In this compressor centrifugal force is used to compress the air. It contains mainly three main parts i.e.
impeller, diffuser and involute casing. It is the most commonly used type of air compressor in different
areas.
advantages
* high efficiencies approaching two stage reciprocating compressors
* can reach pressure up to 1200 psi
* completely packed for plant or instrument air up through 500 hp
* relative first cost improves as size increases
* designed to give lubricant free air
* does not require special foundation
disadvantages
* high initial cost
* complicated monitoring and control system
* limited capacity control modulation, requiring unloading for reduced capacities
* hign rotational speeds require special bearing and aophisticated vibration and clearance monitoring
* specialized maintenance considerations
2. Axial Flow Compressor
It is a compressor in which the air which is to be compressed moves axially during its compression. Axial
flow compressor can continuously provide compressed gas.
