Compressors are mechanical devices that compress gases. There are two main types: dynamic compressors which use rotating impellers like centrifugal and axial compressors, and positive displacement compressors which trap and displace a fixed volume of gas like reciprocating and rotary screw compressors. Centrifugal compressors are widely used and can achieve high pressures but are sensitive to changes, while reciprocating compressors can achieve high pressures but require more maintenance. Proper compressor selection depends on the application pressure and flow requirements.

1. COMPRESSORS
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2. What are compressors?
Compressors are mechanical devices that
compresses gases. It is widely used in
industries and has various applications
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3. How they are different from 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
Centrifugal pump
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4. What are its applications?
Compressors have many everyday uses, such as in :
• Air conditioners, (car, home)
• Home and industrial refrigeration
• Hydraulic compressors for industrial machines
• Air compressors for industrial manufacturing
Refrigeration compressor
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5. What are its various types?
Compressor classification can be described by following flow chart:
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6. What are dynamic compressors?
The dynamic compressor is continuous flow compressor is characterized by rotating
impeller to add velocity and thus pressure to fluid.
It is widely used in chemical and petroleum refinery industry for specific services.
There are two types of dynamic compressors
 Centrifugal Compressor
 Axial Flow Compressor
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7. Centrifugal Compressor
Achieves compression by applying inertial forces to the gas by means of
rotating impellers.
It is multiple stage ; each stage consists of an impeller as the rotating
element and the stationary element, i.e. diffuser
 Fluid flow enters the impeller axially and discharged radially
 The gas next flows through a circular chamber (diffuser), where it loses
velocity and increases pressure.
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8. Axial flow compressor
Working fluid principally flows parallel to the axis of rotation.
 The energy level of air or gas flowing through it is increased by the
action of the rotor blades which exert a torque on the fluid
Have the benefits of high efficiency and
large mass flow rate
Require several rows of airfoils to achieve
large pressure rises making them complex and
expensive
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9. What are positive displacement compressors?
Positive displacement compressors causes movement by trapping a fixed
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.
 Rotary Compressor
 Reciprocating compressor
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10. Rotary compressors
The gas is compressed by the rotating action of
a roller inside a cylinder.
The roller rotates off-centre around a shaft so
that part of the roller is always in contact with the
cylinder.
 Volume of the gas occupies is reduced and the
refrigerant is compressed.
High efficient as sucking and compressing
refrigerant occur simultaneously.
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11. Rotary compressors
These compressors use rotors in place of pistons, giving
a pulsating free discharge air. These rotors are power
driven. They have the following advantages over
reciprocating compressors:
o They require a lower starting torque
o They give a continuous, pulsation free discharge air
o They generally provide higher output
o They require smaller foundations, vibrate less, and have
lesser parts, which means less failure rate
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12. Subtypes:
Depending on the action and geometry of the rotors,
these are classified as follows:
Rotary
compressors
Lobe
compressors
Screw
compressors
Vane
compressors
Scroll
compressors
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13. 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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14. Rotary screw compressors:
Rotary screw compressors use two meshed rotating
positive-displacement helical screws to force the air into
a smaller space (see figure).
These are usually used for continuous operation in
commercial and industrial applications and may be either
stationary or portable.
Because of simple design and
few wearing parts, rotary screw
air compressors are easy to
install, operate, and maintain.
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15. Rotary vane compressors:
One of the oldest compressor technologies, rotary vane
compressors consist of a rotor with a number of blades
inserted in radial slots in the rotor.
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16. • The rotor is mounted offset in a larger housing that is either
circular or a more complex shape. As the rotor turns, blades slide
in and out of the slots keeping contact with the outer wall of the
housing. Thus, a series of decreasing volumes is created by the
rotating blades
• They can be either stationary or portable, can be single or multi-
staged, and can be driven by electric motors or internal
combustion engines. They are well suited to electric motor drive
and is significantly quieter in operation than the equivalent
piston compressor. They can have mechanical efficiencies of
about 90%
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17. Scroll compressors:
A scroll compressor, also known as scroll pump
and scroll vacuum pump, uses two interleaved
spiral-like vanes to pump or compress air.
Often, one of the scrolls is fixed, while the other
orbits eccentrically without rotating, thereby
trapping and pumping or compressing pockets of
fluid or gas between the scrolls (see figure).
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18. • Due to minimum clearance volume between the fixed
scroll and the orbiting scroll, these compressors have a
very high volumetric efficiency. They operate more
smoothly, quietly, and reliably than other types of
compressors in the lower volume range.
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19. Reciprocating compressor
It is a positive-displacement compressor that
 Uses pistons driven by a crankshaft to deliver
gases at high pressure.
The intake gas enters the suction manifold, then
flows into the compression cylinder
It gets compressed by a piston driven in a
reciprocating motion via a crankshaft,
Discharged at higher pressure
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20. RECIPROCATING COMPRESSORS
Detailed Analysis
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21. Block diagram of reciprocating compressor
It is a piston and cylinder
device with (automatic)
spring controlled inlet and
exhaust valves
There is a clearance
between the piston crown
and the top of the
cylinder.
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22. • Widespread use, many applications
• Gas is accelerated outwards by rotating impeller
• Can be built for operation as low pressure
requirement 0f 4 to 6 bar
• Sizes range from 300 hp to 50,000 hp
Centrifugal compressors
Single Case Compressor Centrifugal Impeller
IMPELLERS
Picture of horizontal split
Cross section of barrel
type compressor
Picture of barrel
type compressor
Cross section of
bull gear compressor
Picture of (bull)
gear and impellers
Picture of bull
gear compressor
Cross section of horizontal split
DIFFUSERS
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23. Compressor inlet
nozzle
Thrust bearing
Journal bearing
Shaft and labyrinth
seal
Impeller inlet
labyrinth seals
Discharge volutes
Impellers
Drive coupling
Casing
(horizontally split
flange) Compressor discharge
nozzle
Cross section of horizontal split
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28. Picture of horizontal split
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29. Axial compressors
• Gas flows in direction of rotating shaft
• Can be built for higher pressures ratio of 10 to 12.
• High flow rate
• Efficient
• Not as common as centrifugals
Rotor
Blades
Casing
Stator
Blades
Stator Blades
Rotor Blades
Casing
Shaft
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31. Compressor outlet
nozzle
Rotor blades
Labyrinth seals
Guide-vane actuator
linkage
Compressor rotor
Compressor inlet
nozzle Thrust bearing
Adjustable guide vanes
Cross section of axial compressor
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32. Picture of axial compressor
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36. Advantages and Disadvantages of dynamic compressors
Advantages Disadvantages
Dynamic
Compressors
Centrifugal •Wide operating range
•High reliability
•Low Maintenance
•Instability at reduced flow
•Sensitive to gas composition
change
Axial •High Capacity for given size
•High efficiency
•Heavy duty
•Low maintenance
•Low Compression ratios
•Limited turndown
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37. Advantages and disadvantages of positive displacement
type compressor
Advantages Disadvantages
Positive displacement compressor
Reciprocating •Wide pressure ratios
•High efficiency
•Heavy foundation required
•Flow pulsation
•High maintenance
Diaphragm •Very high pressure
•Low flow
•No moving seal
•Limited capacity range
•Periodic replacement of diaphragm
Screw •Wide application
•High efficiency
•High pressure ratio
•Expensive
•Unsuitable for corrosive or dirty
gases
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38. Stable zone
of operation
Developing the compressor curve
Minimum speed
Power limit
Maximum speed
Process limit
Qs, vol
Adding control
margins
Stonewall or
choke limit
Surge limit
Rc
Actual available
operating zone
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39. Developing the compressor curve
Pd Discharge Pressure (P2)
DPc Differential Pressure (Pd - Ps) or (P2 - P1)
Rc Pressure Ratio (Pd/Ps) or (P2/P1)
Hp Polytropic Head
Rc
Qs, normal
Qs, mass
Qs, vol
Compressor curve for
a specific speed N1
Rprocess,1
Q1
Rc1
Rprocess,2
Q2
Rc2
O.P.
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40. Surges in centrifugal compressors
 What are surges?
o Surge is defined as the operating point at which centrifugal compressor peak
head capability and minimum flow limits are reached.
o When the plenum pressure behind the compressor is higher than the
compressor outlet pressure, the fluid tends to reverse or even flow back in the
compressor.
o As a consequence, the plenum pressure will decrease, inlet pressure will
increase and the flow reverses again.
o This phenomenon, called surge, repeats and occurs in cycles with frequencies
varying from 1 to 2 Hz.
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41. Effects on performance
o Surging can cause the compressor to overheat to the point at which the
maximum allowable temperature of the unit is exceeded.
o Also, surging can cause damage to the thrust bearing due to the rotor shifting
back and forth from the active to the inactive side. This is defined as the surge
cycle of the compressor.
Surge points for centrifugal compressors running at varying speeds
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42. Surge description
• Flow reverses in 20 to 50 milliseconds
• Surge cycles at a rate of 0.3 s to 3 s per cycle
• Compressor vibrates
• Temperature rises
• “Whooshing” noise
• Trips may occur
• Conventional instruments and human operators may fail to
recognize surge
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43. Some surge consequences
• Unstable flow and pressure
• Damage in sequence with increasing severity
to seals, bearings, impellers, shaft
• Increased seal clearances and leakage
• Lower energy efficiency
• Reduced compressor life
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44. • Rapid flow oscillations
• Reversal flow leads to
reversal thrust
• Potential damage
FLOW
PRESSURE
TEMPERATURE
TIME (sec.)
1 2 3
TIME (sec.)
1 2 3
TIME (sec.)
1 2 3
Major Process Parameters during Surge
• Rapid pressure oscillations
with process instability
• Rising temperatures
inside compressor which
can be seen at the
Discgarge
Operators may fail to
recognize surge
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45. Graph showing operating regions of various compressors
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46. Table showing operating conditions of various compressors
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