2. Contents
Theory of compression
Purpose of compression
Understanding of key terms
Kinetic theory of gases
Compressibility
Methods of compression
Types of compressors
Centrifugal compressors
Advantages and disadvantages
Basic construction
Understanding compressor curves
Basic operation
Compressor arrangements
Maintenance
3. Theory of compression
Compression
Process of increasing the pressure of a fluid in order to make it travel to another place
Compressor
Mechanical equipment that is used to increase the pressure of a compressible fluid.
Why compressors are required
Increase temperature and pressure of gas for certain reactions to take place.
Liquefaction of gases for storage purposes
Refrigeration effect
Pressurized air is used for measuring and control
Applications in industry
In every field of life.
Purpose of compression
5. Theory of compression
Capacity of a compressor
Understanding of key terms
Volume of gas handled in a unit time.
Its units are cubic feet per minute
Capacity is always measured at compressor suction flange.
SCFM and ACFM
9. Theory of compression
The average kinetic energy of the
molecules does not change with time.
The average kinetic energy of the
molecules is proportional to absolute
temperature.
Kinetic theory of gases
11. Theory of compression
Trap quantities of gas, reduce the volume, push the compressed gas out
of the enclosure.
Trap gas, carry it without volume change to the discharge opening.
Compress the gas by backflow from discharge system, then push the
compressed gas out of the enclosure
Compress by the mechanical action rotating impellers or bladed rotors
that impart velocity and pressure
Entrain the gas in a high velocity jet of the same or another gas and
convert the high velocity of the mixture into pressure in a diffuser.
Methods of compression
15. Types of compressors
Continuous and intermittent
Compressors can be subdivided into two broad groups based on
compression mode.
Intermittent
This compression is cyclic in nature. A specific quantity
of gas is ingested by the compressor, acted upon and
discharged, before the cycle is repeated.
Continuous
The compression mode in which gas is moved into the
compressor, acted upon, moved through compressor,
and discharged without interruption of the flow at any
point in the process.
16. Types of compressors
Compressors
Positive displacement Dynamic
Reciprocating Rotary Centrifugal Axial
Single acting Double acting
Diaphragm
Lobe Screw Vane
Liquid ring Scroll
Ejector
19. Centrifugal Compressors
Working Principle
The working principle is based upon
imparting kinetic energy to a fluid
stream and conversion of this dynamic
energy into static energy, i.e., from
velocity into pressure.
The specific compression energy
transmitted to the gas by the impeller
is called "head."
The discharge pressure of a turbo
compressor, which is affected by gas
inlet conditions, is a function of head.
20. Centrifugal Compressors
Less efficient for small volumes.
Discharge pressure limitation.
Effect of gas density and temperature.
Problem of surge phenomenon.
Discharge flow is relatively free of pulsation.
Mechanical design permits high through puts, capacity limitation is
rarely a problem.
Centrifugal Compressors are capable efficient performance over a wide
range of pressure and capacities even at constant speed operation.
These are relatively small, occupy less space, operate with minimum
attention and quieter.
Less contamination due to lubricants.
Disadvantages
Advantages
21. Centrifugal Compressors
Major components of a centrifugal compressor include:
Impeller
The part of centrifugal compressor that moves the gas is the impeller. As the impeller
rotates, it moves the gas toward the outer rim of the impeller and its velocity
increases.
Diffuser
As the gas leaves impeller, it flows into a passage-way called the diffuser. The diffuser
being larger in volume, the velocity of gas decreases and its pressure increases
Volute
Gas passes from diffuser into the volute. In the volute, the conversion of velocity
energy to pressure energy continues.
Casing
It is the outer cover of a centrifugal compressor which contains inlet and discharge
nozzles.
Construction
22. Centrifugal Compressors
A centrifugal compressor has two main type of parts;
Rotor assembly
Main shaft
Impellers
Thrust runner
Disc spacer
Balance drum
Stationary parts
Casing
Nozzles
Volutes
Bearings
Shaft seals
Construction
51. Centrifugal Compressors
Incoming flow to compressor decreases below surge limit
Understanding Compressor Curves
Flow
Head Surge control
point
Surge
Stone wall
59. Centrifugal Compressors
Start up Sequence
All jobs completed and blinds removed
All instrument calibrated and in service
System Purging
Lube oil Circulation
Cooling water circulation
System pressurization
Sealing system
Start up
Basic Operation
69. Centrifugal Compressors
Start up Procedure
Open suction valve
Open discharge valve (discharge check valve closed).
Open bypass or vent valve
Start and bring up to speed
Close bypass or vent
Place bypass or vent on automatic control
Basic Operation
71. Centrifugal Compressors
Thermodynamic condition
A compressor's efficiency is reflected by its thermodynamic
performance Evaluation of performance requires acquisition of the
following data.
Suction and discharge pressures of each stage
Suction and discharge. temperatures of each stage
Volume flow
Density
Operating speed
Power consumption
Cooling water Flow rate,
Fouling
Basic Operation
72. Centrifugal Compressors
Mechanical condition
The most critical components of a compressor are the rotor, its bearings
and seals. Monitoring of mechanical compressor functions, therefore,
will center around the behavior of these components. The following
data allow assessment of the mechanical condition:
Radial vibration
Shaft position
Bearing temperature
Lube-oil supply pressure and temperature
Seal-oil supply temperature, and differential pressure between oil supply
and reference gas
Seal buffer-gas pressures and differential pressures
Compressor speed (with variable speed driver)
Basic Operation
73. Centrifugal Compressors
Common Problems
Dirty Intercoolers - Increased gas temperature reduces the density into the
compression stage.
Hot Coolant - Increased gas temperature reduces the gas density into the compression
stage.
Dirty Inlet Filter - Reduces the pressure into the first stage.
Driver Input Speed - The ability for a centrifugal compressor stage to make pressure
falls as the square of the stage speed. A reduction in the driver input speed can
dramatically affect the total machine ratio.
Discharge valve closure - Discharge valve closure decreased the flow through the
compressor hence increase the pressure and causes surging .
Suction valve closure- Suction valve closure or suction vent opening causes pressure
drop in suction reducing flow and surging the machine.
Inter stage vent malfunction- In a multistage compressor if inter stage vent valve
malfunction and opens the down stream stage will surge.
Change in gas composition. If the gas composition change it effects the pressure thus
the compressor can go into surge at the same speed
Recycle valve malfunction. Recycle valves are normally open during reduced flow
operation. if closes by accident or malfunction it will surge the compressor.
Basic Operation
75. Centrifugal Compressors
Surging
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 or “Clanking” noise
Trips may occur
Conventional instruments and human operators may fail to recognize
surge
Basic Operation
76. • Rapid flow oscillations
• Thrust reversals
• Potential damage
• Rapid pressure oscillations with
process instability
• Rising temperatures inside
compressor
FLOW
PRESSURE
TIME (sec.)
1 2 3
TEMPERATURE
TIME (sec.)
1 2 3
TIME (sec.)
1 2 3
Centrifugal Compressors
Trend of major process parameters during surge
77. Qs, vol
Pd
Machine shutdown
no flow, no pressure
• Electro motor is started
• Machine accelerates to nominal
speed
• Compressor reaches performance
curve
• Note: Flow goes up faster because
pressure is the integral of flow
• Pressure builds
• Resistance goes up
• Compressor “rides” the curve
• Pd = Pv + Rlosses
A
• Compressor reaches surge point A
• Compressor looses its ability to make
pressure
• Suddenly Pd drops and thus Pv > Pd
• Plane goes to stall - Compressor surges
B
• Because Pv > Pd the flow reverses
• Compressor operating point goes to point B
C
• Result of flow reversal is that pressure goes
down
• Pressure goes down => less negative flow
• Operating point goes to point C
D
• System pressure is going down
• Compressor is again able to overcome Pv
• Compressor “jumps” back to
performance curve and goes to point D
• Forward flow is re-established
Pd
Pv
Rlosses
Pd = Compressor discharge pressure
Pv = Vessel pressure
Rlosses = Resistance losses over pipe
• Compressor starts to build pressure
• Compressor “rides” curve towards surge
• Point A is reached
• The surge cycle is complete
• From A to B 20 - 50 ms Drop into surge
• From C to D 20 - 120 ms Jump out of surge
• A-B-C-D-A 0.3 - 3 seconds Surge cycle
Centrifugal Compressors
Surge representation on compressor curves
78. Centrifugal Compressors
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
Some surge consequences
79. Centrifugal Compressors
Surge
Factors leading to onset of surge
Startup
Shutdown
Operation at reduced throughput
Operation at heavy throughput with:
- Trips - Power loss
- Operator errors - Process upsets
- Load changes - Gas composition changes
- Cooler problems - Filter or strainer problems
- Driver problems
Surge is not limited to times of reduced throughput. Surge can occur at
full operation
80. Qs, vol
Rc
stable zone
of operation
adding
control
margins
Actual available
operating zone
Centrifugal Compressors
Anti surge control
90. Centrifugal Compressors
Commonly used drivers for a centrifugal compressor are:
Steam Turbine
Electric motor
Induction
Synchronous
Gas Turbine
Expander
Compressor drivers
