Types of compressors

Classification of Compressors
Generally, compressors are classified according to the method of air
Compression.
(1)Positive displacement devices
(2)Dynamic displacement devices
Displacement refers to the quantity of air that passes through a
compressor in a single revolution.

Positive displacement devices
Air is confined within an enclosed space where it is compressed by
decreasing its volume.
A cavity inside the machine stores the air brought from outside, and
then slowly compresses the cavity to increase the air pressure and
potential energy.
Displacement refers to the quantity of air that passes through a
compressor in a single revolution.

Dynamic displacement devices
Air is accelerated by rapidly rotating elements, such as rotor blades,
thus causing an increase in pressure.
Some of these compressors are completely oil-free for highly
sensitive environments.

Classification of Compressors
According to the type of motion of the compressing element
1.Rotary type.
2. Reciprocating type.
According to the number of times air is compressed in each
revolution
1.Single acting
2.Double acting.

Terms and Definitions
Delivery Volume
Delivery volume is the quantity of air that a compressor delivers to a
system.
It is specified in terms of air at normal atmospheric conditions.
There are two types of delivery volumes
I. theoretical delivery volume
II. effective delivery volume.

Theoretical delivery volume
Ex:
For a reciprocating compressor, the theoretical delivery volume is
the product of volume of air swept in one revolution of its rotating
element and the number of revolutions per unit time.
Effective delivery volume
Depends on the type of compressor.
To pneumatic personnel, only the effective delivery volume is of
interest as this air volume is available for driving and controlling
pneumatic equipment.

Pressure
In the compressor context, pressure is of two types
• working pressure.
• operating pressure.
Working pressure is the compressor outlet pressure or the pressure
in the receiver tank and in the pipelines.
Operating pressure is the pressure that is required at the operating
position. In most pneumatic systems, the preferred operating
pressure is 6 bar.

Drive
Compressors are driven by either electrical motors or internal
combustion engines.
Cooling
When air is compressed, work is done, and a part of this work
appears as heat in the compressed air.
The heat produced must be removed by cooling the air.
Good cooling extends the life of the compressor.

Cooling
Cooling methods
Selection of a suitable cooling system depends on the amount of
heat produced.
Cooling fins on smaller air-cooled compressors permit the heat to be
removed by radiation.
A large compressor is usually equipped with an additional fan to take
away the heat. In the case of a compressors with a drive power in
excess of 30 kw are equipped with a water circulation cooling
system.

Regulation
To match the delivery volume of the compressor with the fluctuating
air consumption and as a safety measure, it is necessary to regulate
the pressure developed by the compressor.
Various methods of regulation are employed for this purpose.

Regulation methods
Start/Stop (On-off Regulation)
Load/Unload
Modulating Control
Variable Displacement
Variable Speed Drive (VSD)

Regulation methods
Pressure (switching) limits Pmax and Pmin that can be set on a
regulator. The drive motor is switched off on reaching a preset
maximum pressure, Pmax The motor is switched on again when
the pressure has dropped to a preset minimum pressure, Pmin.

Regulation methods
Load/Unload (online/offline control)
It keeps the motor running continuously, but unloads the
compressor when the discharge pressure is adequate.
Unloaded rotary screw compressors typically consume 15-35% of
their full load power demand, while producing no useful
compressed air output.
Optional unload timers are available that will save energy by
automatically turning off the compressor and placing it in standby if
the unit runs unloaded for a period of time (usually 15 minutes).

Regulation methods
Modulating Control
This mode of control varies the compressor output to meet flow
requirements by adjusting the inlet valve, resulting in air restrictions
to the compressor.
Even fully modulated at zero flow rotary screw compressors
typically consume about 70% of their full load power demand.
Modulating control is unique to lubricated screw compressors and
is the least efficient way to operate these units.

Regulation methods
Variable Displacement
Some lubricated rotary screw compressors vary their output
capacity using special capacity control valves, also called spiral,
turn or poppet valves.
With a variable displacement control scheme, the output pressure
and compressor power consumption can be closely controlled
without having to start/stop or load/unload the compressor.

Regulation methods
Variable Speed Drive (VSD)
This control method varies the speed of the compressor to respond
to changes in air demand.
Both lubricated and oil free screw compressors can be purchased
equipped with variable speed drive controls that continuously adjust
the drive motor speed to match the variable demand requirements
and maintain constant pressure.

Types of compressors
Reciprocating Piston Compressor

A single-acting
reciprocating
compressor has a
set of valves at the
top and there is
one compression
cycle for every
revolution of the
crankshaft.

The double-acting reciprocating
compressor has two sets of valves, one
each at the top and the bottom of the
cylinder, and there are two
compression cycles for every revolution
of the crankshaft. When the piston is
compressing the gas on one side, the
gas is sucked into the cylinder on the
other side of the piston.

Reciprocating piston compressors are very
common and provide a wide range of
pressures and delivery volumes.
They are employed where high pressures
and low delivery volumes ( <10,000 m3/h)
are needed.
Single stage up to aprrox. 600kPa ( 6 bar )
Two stage up to aprrox. 1500kPa ( 15 bar )

For pressures exceeding 30 bar, multi-
stage compressors with inter-cooling
between stages of compression are used.
Cooling between stages reduces the input
work requirement and the volume of air to
be handled by the next stage, and this can
dramatically increase compressor
efficiency.

Single stage double cylinder Two stage

A single-stage reciprocating compressor has one or multiple
cylinders connected parallelly. Here the air is compressed only once
before it is moved to the final storage tank.
In the case of a two-stage or dual-stage reciprocating compressor,
there will be two cycles of compression occurring in two cylinders
that are connected in series. The gas is compressed in one cylinder
(first stage), and the discharged gas is further compressed in the
second cylinder (second stage), to a higher pressure.

Single-Stage vs Dual-Stage
Both single and dual-stage compressors can have multiple cylinders.
The key difference between a single-stage compressor and a dual or
multi-stage compressor is the number of times the air is compressed.
That means, in a multi-stage device the air compressed from the first
stage is sent to the next cylinder for further compression.
The other difference is the presence of inter-cooling between the two
stages of compression. A multistage reciprocating compressor, with a
good lubrication system, can achieve 250 PSI or more.

Multi-Stage Compressors
Multistage (more than two-stage) reciprocating compressors have
more than one cylinder and compress the gas in multiple stages.
However, all multi-cylinder reciprocating compressors may not be
multistage compressors, since there are single-stage reciprocating
compressors with more than one cylinder.

Diaphragm Compressor
In piston compressors, there is a likelihood of small amounts of
lubricating oil from the piston walls contaminating the compressed
air.
This very small contamination may prove harmful in food,
pharmaceutical, and chemical industries.
For applications in such industries, diaphragm compressors may be
used as a power source.
Here a flexible diaphragm, partitions the compressor chamber and
the actuating piston.
This feature facilitates the isolation of lubricating oil from the
compressed air supply.

Diaphragm Compressor(membrane compressor)

Diaphragm Compressor
The diaphragm type compressor (also called membrane compressor) is
a variant of the reciprocating compressor, and the compression of the
gas happens due to the up and down movement of a flexible diaphragm
or membrane.
The up and down movement of the diaphragm is achieved by
connecting it with a piston driven by the connection rod and crankshaft.

Rotary Compressor
1.Screw Compressor

Screw Compressor
Rotary screw air compressors generate energy via two internal rotors
that turn in opposite directions. The air gets trapped between the two
opposing rotors, and builds up pressure within the housing.
A screw compressor consists of two helically grooved screws meshing
with each other, leaving a negligible clearance of about 0.05 mm. The
design of the screws makes it possible to move air from the inlet to the
outlet of the compressor.

Screw Compressor
Compression is achieved by pushing the trapped air into a progressively
smaller volume as the screws move ahead. Because there are no
surfaces that actually make contact with one another, this type of
compressor does not require cooling and is characterized by low noise
level and small loss of efficiency.
They have the benefit of simplicity with fewer moving parts rotating at a
constant speed and a steady delivery of compressed air without
pressure fluctuations.

Screw Compressor

Rotary Compressor
2.Sliding vane Compressor
The sliding vane compressor consists of an external
casing called a cylinder, the rotating element inside
called the rotor, and sliding vanes. The rotor is
eccentrically located in the cylinder.
The rotor typically has longitudinal slots that hold the
vanes. As the rotor turns the vanes slide outward, due to
centrifugal force, to reach the inner walls of the cylinder.
The vanes will trap a pocket of gas as they pass by the
compressor inlet. As the rotor continues to rotate, the
volume of that pocket of gas is decreased due to the
eccentricity of the rotor.

Rotary Compressor
2.Sliding vane Compressor

Dynamic Compressors

Axial Compressors
Axial air compressors are not typically used in construction projects but
are instead found in high-speed engines on ships or planes.
They have a high-efficiency rate but are much more expensive than
other types of air compressors, and can get up to many thousands of
horsepower, which is why they are mainly reserved for aerospace
research.

Axial Compressors

Centrifugal Compressors
Centrifugal air compressors slow and cool the incoming air through a
diffuser in order to build up potential energy. Because of the multi-
phase compression process, centrifugal compressors are able to
produce a high amount of energy in a relatively small machine.
They require less maintenance than the rotary screw or reciprocating
compressors and some types can produce oil-free air. They are
typically used for more demanding construction sites such as
chemical plants or steel manufacturing centers

Centrifugal Compressors

Types of compressors

Recommended

Recommended

More Related Content

Similar to Types of compressors

Similar to Types of compressors (20)

Recently uploaded

Recently uploaded (20)

Types of compressors