5. Compressor
Introduction
• In the seventeenth century it was discovered that air had weight and
was compressible. Its practical use in a compressed form started at
that time.
• Early documented used of compressed air was for the reed type of
musical wind instrument, which later became the pipe organ.
• The first compressor constructed in the United States was in 1865.
Theory of Gas Compression
• Air has weight and it is the weight of the column of air over a particular
location that determines the atmospheric pressure at that particular
location.
•
At sea level and under average temperature and moisture conditions, a
one square inch column of air extending up to the uppermost limit of the
atmosphere weighs about 14.7 pounds.
•
Atmospheric pressure at sea level is, therefore, about 14.7 pounds per
square inch at 60oF and 36% relative humidity.
Theory of Gas Compression
• Consider a confined volume of gas. The gas molecules are distributed
throughout the volume and are widely separated as compared to their
size.
• They move at high velocity and collide frequently with each other and
with the walls of the vessel.
• The continuous bombardment of the enclosing walls produces pressure
and the intensity of pressure depends on the number, mass, and velocity
of the molecules.
• Temperature is a measure of the kinetic energy of the molecules which,
in turn, depends on their mass and velocity.
• If the confined gas is heated, its stored energy will be increased and the
molecules will move with increased speed.
• Therefore both pressure and temperature will increase.
• The rise in temperature is evidence of an increase in the amount of
internal energy stored in the gas.
Theory of Gas Compression
• If the enclosing vessel is fitted with a piston so that the air can be
compressed into smaller volume, the moving piston delivers energy to
the molecules, causing them to move with increased velocity.
• As with heating, this results in a temperature increase. Thus, the work of
compression is stored as internal energy in the air.
• Further more, all of the molecules have been forced into smaller space
which results in an increased number of collisions on a unit area of the
wall. This, together with increased molecule velocity, results in increased
pressure.
• Compression may be thought of as forcing a confined volume (or weight)
of gas into a smaller space to increase pressure,
• It is accompanied by a rise in temperature (and an increase of stored
internal energy).
Purpose of Gas Compressor
Purpose of gas compressor is to compress a gas from an initial or suction
pressure to a final or higher discharge pressure.
Compression of gases to adequate higher pressure
is necessary to perform operational functions.
• Transmission – move gas from place to place or from one part of a
process operation to another.
• Recovery – mixture of gases remaining after separating condensable
component are compressed for further liquification.
• Air compression - conveying, for power tools, process operations, etc.
.
How Compressors Works
• Individual gas always travel at high speed, at normal temperature, they
strike against the walls of an enclosing vessel and produce what is
known as pressure.
• When heat is added, the molecules even travel faster, so they hit the
containing walls of the vessel harder and more often. This shows up as
greater pressure.
• If the enclosing vessel is fitted with a piston so that the gas can be
squeezed into a smaller space, molecule travel is restricted. The
molecules hit the walls with greater frequency increasing the pressure.
The moving piston also delivers energy to the molecules, causing them
to move with increased velocity.
• As with heating, this results in temperature increase. Furthermore, all
the molecules have been forced into a smaller space, which results in
an increased number of collisions on a unit area of wall. This , together
with increased molecules velocity results in increased pressure.
How Compressors Work
• The compression of gases to higher pressure can result in very high
temperature creating problems in compressor design.
• All basic compressor elements, regardless of type, have certain limiting
operating conditions.
• Basic elements are single stage.
• When a temperature limitation is involved it becomes necessary to
multiple stage the compression process.
Major Design Classification
There are 2 major design classification of
compressor:
1.
Positive displacement
2.
Dynamic
1. Positive Displacement Compressors
In positive displacement compressor, successive volumes of air are
confined within a closed space and pressure is increased by reducing
volume of space
Two types of positive displacement compressors:
1.
Reciprocating
2.
Rotary
Positive Displacement Compressors
1. Reciprocating
Reciprocating Compressor-principal elements
The principal elements of a reciprocating gas compressor are:
1.
Cylinder, heads, pistons, inlet and discharge valves
2.
Power-transmitting parts such as crankshaft, crossheads,
connecting rods, flywheel
3.
Lubricating system.
Reciprocating Compressor
Reciprocating compressor uses automatic spring loaded valves which
open only when the proper differential pressure exist across the valve.
Operations
1. Inlet valves open drawing in gas when the pressure in the cylinder is
slightly below the intake pressure
2. During the compression stroke the inlet and discharge valves are
closed until the pressure in the cylinder is slightly above the
discharge pressure
3. The discharge valves open and the gases flow out until the
discharge stroke is completed
4. As the piston moves back in the expansion stroke both the inlet
and discharge valves remain closed
5. The gases trapped in the clearance space increase in volume
causing a reduction in pressure
6. When the pressure in the cylinder once again is slightly below the
inlet pressure the inlet valves open drawing in gas and the
compression cycle is repeated.
Reciprocating Compressor
Reciprocating Compressor – single acting
Reciprocating Compressor – double acting
Reciprocating Compressor-multi-stage
Positive Displacement Compressor
2. Rotary Compressor
•
•
•
•
•
•
•
In rotary compressors, force is given to the gas or air by a rotating
impeller.
Design of rotary compressors are numerous and there are many
modifications of the rotary principle
Generally one-stage machines, compressing to moderate
pressures.
Most of these units have no provision for cooling water.
In some machines, it is necessary to have the rotary elements –
cams, drums, blades or gears – form an airtight contact with the
casing so that the air can not leak neither around the ends of the
rotary member nor past the peripheral surface.
Volume of air increases with speed
Lubrication consist of supplying a film of oil to the surface of all
sliding internal parts and to the bearing supporting the rotary parts.
Purpose is to prevent wear and abrasion of part in contact and as
sealing film to prevent air leakage.
Rotary Compressor
Sliding vane Rotary Compressor
Rotary Compressor
Two Impeller Rotary Compressor
Liquid Piston type Rotary Compressor
Rotary Compressor
Screw Type Rotary Compressor
2. Dynamic Compressors
Dynamics compressors use rotating elements to accelerate the gas by
diffusing action, velocity is converted to static pressure. Total energy in
A flowing gas stream is constant. Entering an enlarged section, flow
speed is reduced and some of the velocity energy turns into pressure
energy. Thus, static pressure is higher in the enlarged section.
Two types of dynamic compressors:
1. Centrifugal
2. axial
Dynamic Compressors - Centrifugal
Centrifugal compressor is a dynamic displacement machine
wherein inertial forces is applied to a gas transmitted by an imeller
which, by dynamic centrifugal forces is applied to the gas
transmitted by an impeller which, by dynamic centrifugal motion,
adds velocity energy through acceleration of the gas.
This velocity energy (called kinetic energy) is retarded in a vaned
or vaneless diffuser, transforming most of that velocity energy into
additional static pressure.
In the diffuser, the same as in the additional compressor
components, like the inlet collector, exit collector, stationary vanes
to guide the flow, etc, there are pressure losses. Therefore, the
impeller has to produce enough energy to satisfy the pressure
requirements plus those losses.
Dynamic Compressors - Centrifugal
• Centrifugal compressors or blowers consist of a casing in which
revolve one or more impellers (bladed wheels) mounted on a shaft
supported by one or more bearings.
• Gas enters the impeller near the shaft and is discharged at the outer
end of the impeller blades.
• When the shaft is rotated, the effect of centrifugal force upon the gas
within the impeller causes its compression and, at the same time,
induces it to flow through the impeller.
• The gas passing through the impeller is accelerated, and the increase
in velocity is a form of energy convertible into additional pressure.
• The conversion is produced by the gradual and orderly Deceleration of
the gas either in a bladed or open diffuser, or in a volute or scroll
surrounding the impeller.
Centrifugal Compressor-Longitudinal Section
Centrifugal Compressor-Gas Flow
Dynamic Compressors-centrifugal
Centrifugal Compressor Elements
• Impeller – Gas is given an outward thrust or radial velocity
kinetic energy added to the gas by increasing speed
• Diffusers – Reduces velocity of gas gradually converting velocity
energy to pressure
• Volute – Collects compressed gas and directs to outlets.
Dynamic Compressors - Centrifugal
Three main types of impeller used in centrifugal compressors
are:
• Open Impeller
• Semi-open Impeller
• Enclosed Impeller
Dynamic Compressors - Centrifugal
Open impeller is used in single stage
compressors to produce high head with
but small flow (capacity)
Dynamic Compressors - Centrifugal
Semi-enclosed impeller is used in
single staged compressors or in the
first stage of multi-stage
compressors to produce a large flow
Dynamic Compressors - Centrifugal
The enclosed impeller is used in
multi-stage compressors where
pressure is increased in to a
high discharge pressure
Operation of Centrifugal Compressor
• Compression of gas is accomplished by drawing the gas into the
center of the impeller and discharging it at the periphery with
considerable velocity
• This velocity is converted into pressure in the diffuser passage
which in turn guide the gas into the inlet of the next impeller
• Shaft sealing of centrifugal compressors is usually accomplished by
labyrinth rings.
• If the vapor being compressed is volatile or flammable, a pressure
draw-off before the last ring in the compressors suction line of a
liquid or gas seal on the gland to the atmosphere is frequently used.
Dynamic Compressors-Centrifugal
Surge Point
• There is for every speed and pressure of a centrifugal compressor a
certain minimum volume below which the machine does not operate
properly. This volume is called the surge point
• Below it delivery of gas becomes irregular, reversing itself at frequent
intervals with a characteristic noise known as surging.
Surge condition
head
N
Choke condition
(stone wall)
Volume Flow
Compressor Performance Curve
Dynamic Compressor - Centrifugal
SURGE
Pressure
Operating zone
Surge zone
B
A OP
C
D
D
E
Negative flow
Surge Cycle
Positive flow
Dynamic Compressor - Centrifugal
Surge (con’t)
Refer to - Surge cycle figure
Consider a compressor operating in steady state at point A. If the load
is reduced, the OP (operating point) must move toward B. the SURGE
POINT. At B the compressor is producing more flow than the load can
absorb. This fluid is temporarily stored in the discharge volume, but the
discharge pressure cannot rise above B. The only relief for these
conditions is for the OPERATING POINT to jump to point C. This is the
flow reversal often observed during surge.
With negative flow the discharge pressure drops (traject C-D). At point
D we find that the flow is insufficient to build up the pressure necessary
to reach B, so the OPERATING POINT jumps to E. Now the flow is in
excess of the load and the OPERATING POINT will move up the curve
to reach B again. This completes one SURGE CYCLE. The typical
duration of one SURGE CYCLE is 0.5 to 2.0 seconds.
Dynamic Compressors - Centrifugal
Surge (con’t)
The consequences of surge are severe. Besides process
disturbance and the eventual process trips and disruption, surge can
damage the compressor:
- Damage to seals and bearings is common.
- Internal clearance are altered, leading to internal recycle
- Lowering of compressor efficiency
- Destruction of compressor rotor
Dynamic Compressors - Centrifugal
Surge (con’t)
Protection method
As shown earlier, a combination of high discharge pressure and low
flow can result in surge. Avoiding one or both of these situations
prevents a compressor from going into surge. A working solution can
be found in a RECYCLE or BLOW-OFF line. Operating a valve,
positioned in this line, reduces the discharge pressure and
increases the load thus preventing surge.
Various surge control systems are not included in this general
course.
Dynamic Compressors-Axial
• In axial flow compressors, gas moves generally parallel to the shaft
axis.
• The axial compressor or blower is a dynamic type of machine, identified
by the use of moving and stationary blading to accomplish the velocitypressure conversion
• for pressure increase. In general, axial compressor design is based on
the theory of 50% reaction
• This means that half of the pressure rise is accomplished in the rotor
blade and half in the stator blade
• As gas flows through the rotating blades, pressure and velocity both
increase
• Each row of stationary blades converts the energy of the increased
velocity to additional pressure, acting as a diffuser for the gas flowing
out of the preceding
• row of rotating blades. Also, the stationary blades act as nozzle to guide
the gas into the next row of rotating blades.
• Each stage consists, therefore, of one rotating and one stationary row
of blading.
Safety Devices
Typical Compressor Safety Devices
Name
Relief valves
Overspeed
Shutdown
Oil failure
shutdown
Jacket-water
Valve
Over-pressure
Shutdown
Excessive temp
Shutdown
Main bearing
Protection
Function
- On the discharge side to relieve excessive
pressure
- Trips driver when compressor exceed
predetermine safe speed
- For system fitted with pressure
lubrication. Protects bearing by stopping
unit when oil pressure fails
- Shutdown compressor if water pressure fails.
Operated either by pressure or temperature.
- Stops compressor when discharge pressure
goes above pre-set safe valve
- Automatically stops unit on a pre-set high discharge
temperature
- Thermal shutdown device stops compressor if
bearing temperature goes too high
Simple Over-speed Safety Stop
Over-speed safety Stop
• In a typical over-speed safety stop, the revolving part is connected
to the magneto drive shaft. If the engine reach a pre-set point (rpm),
the weight W is thrown out, overcoming the spring S. The weight hits
the plunger P which snaps down and presses the copper disk C
against the contact A. This grounds the magneto and stops the
engine.
Guides – How to Prevent Compressor Failure
Guide: Starting of electric motor driven compressors:
1.
2.
3.
4.
5.
6.
If work has been done on a compressor, before starting, turn over
by hand at least one revolution to make sure everything is clear
Always be sure cooling water is circulating through compressor
before attempting to start. If the operator neglects to turn on the
cooling water and starts the compressor first, do not turn cooling
water into compressor, but shutdown and allow compressor to
cool before re-starting machine
Always be sure ventilating system(s) are in operation before
attempting to start
Refer to manufacturer’s instruction book for means of
determining lubrication requirement
Watch any bearing that is taken up for a reasonable time to be
certain it is not too tight
Never open gas cylinder without first purging it.
Gas Turbine
Fuel
Compressed air
Exhaust Gases
Hot gases
Combustion chamber
Compressor
HPT
LPT
Load
Air in
Flow Diagram of a Simple Cycle 2 Shaft Gas Turbine
Gas Turbine
Gas turbine
Principle of Gas Turbine operation
The gas turbine portion of the mechanical drive gas turbine unit is that part
in which fuel and air are used to produce shaft horsepower.
The compressor/high pressure turbine rotor is initially brought to 20% speed
by a starting device. Atmospheric air drawn into the compressor is piped to
the combustion chambers where fuel is delivered under pressure.
A high voltage spark ignites the fuel-air mixture. [Once ignited, combustion
will remain continuous in the air stream for as long as fuel is delivered to the
combustion chamber]. The hot gases increase the speed of the
compressor/high pressure turbine (HPT) rotor. This in turn increases the
compressor discharge pressure.
Gas Turbine
Gas turbine
Principle of Gas Turbine operation (con’t)
As the pressure begins to increase, the low pressure turbine rotor
will begin to rotate and both turbine rotors will accelerate to
operating speed. The products of combustion (high pressure/high
temperature gases) expand first thru the high pressure turbine and
then thru the low pressure turbine and exhausted to atmosphere.
As the expanding gases pass thru the high pressure turbine amd
impinge on the turbine buckets, they causes the turbine to spin; thus
rotating the compressor and applying a torgue output to the driven
accessories. The gases also spin the low pressure turbine before
exhausting; thus rotating the load. The rotor spins in a
counterclockwise direction when viewed from the inlet end.
Gas Turbine
Gas turbine
Principle of Gas Turbine operation (con’t)
1. Compressor section
In the compressor, air is confined to the space between the rotor
and stator blading where it is compressed in stages by a series of
alternate rotor and stator airfoil shaped blades. The rotor blades
supply the force needed to compress the air in each stage and the
stators blades guide the air so that it enters the following rotor stage
at the proper angle. The compressed air exists through the
compressor discharge casing to the combustion chambers.
Air is extracted from the compressor for turbine cooling, for bearing
lube oil sealing, and for pulsation control during start up/shutdown.
Gas Turbine
Gas turbine
Principle of Gas Turbine operation (con’t)
2. Combustion section
During operation, air from the compressor flows into the combustion
wrapper and into the annular space between the liner and the flowshied. This air flows into the liner, is mixed with fuel, and ignited.
The combustion chamber has the difficult task of burning large
quantities of fuel, supplied by the compressor, and releasing the
heat in such a manner that the air is expanded and accelerated to
give a smooth stream of uniformly heated gas at all conditions
required by the turbine
Gas Turbine
Gas turbine
Principle of Gas Turbine operation (con’t)
2. Combustion section
Combustion process
Air from the engine compressor enters the combustion chamber at
a high velocity, and because of this high velocity the air speed is far
too high for combustion. The first thing that the chamber must do is
to diffuse it, i.e. decelerate it and raise its static pressure. Because
the speed of burning fuel at normal mixture ratios is not high, any
fuel lit even in the diffused air stream, which now has a velocity of
about 80% less, would be blown away.
A region of low axial velocity has therefore to be created in the
chamber, so that the flame will remain alight throughout the range
of engine operating conditions.
Gas Turbine
Gas turbine
Principle of Gas Turbine operation (con’t)
2. Combustion section
Combustion process
In normal operation, the overall air/fuel ratio of a combustion
chamber can vary between 45:1 and 130:1, so the fuel must be
burned with only part of the air entering the chamber, what is called
a primary combustion zone.
This is achieved by means of a flame tube (combustion liner) that
has various devices for metering the airflow distribution along the
chamber.
About 28% of the compressed air is utilized to burn the fuel in the
primary zone. The surplus of the compressed air not being used for
complete combustion is sent down the annular space and
eventually mixed up with the hot gases.
Gas Turbine
Gas turbine
Principle of Gas Turbine operation (con’t)
2. Combustion section
Typical Combustion Chamber
Flame tube
Air casing
8%
10% 10%
Comp. air
18%
28%
Perforated flare
Fuel
Burning
total
72%
Dilution total
Primary zone
Swirl vanes
Nozzle
Dilution hole
Gas Turbine
Principle of gas turbine (con’t)
About 28% of the compressed air is utilized to burn the fuel in the
primary zone. About 18% is taken in by the entry section.
Immediately downstream of the entry section are swirl vanes and a
perforated flare through which air passes into the primary
combustion zone. The swirling air induces a flow upstream of the
center of the flame tube for the desired recirculation.
Through the wall of the flame tube, adjacent to the combustion zone,
are a selected number of holes through which a further 10% of the
main flow of air passes into the primary zone.
The air from the swirl vanes and that from the primary air holes
interacts and creates a region of low velocity recirculation.
Gas Turbine
Principle of gas turbine (con’t)
This takes the form of a toroidal vortex similar to a smoke ring, and
has the effect of stabilizing and anchoring the flame.
It is arranged that the conical fuel spray from the burner intersects
the recirculating vortex at its center. This action, together with the
general turbulence in the primary zone, greatly assists in breaking
up the fuel and mixing it with the incoming air.
The temperature of the combustion gases released by the
combustion zone is about 1,800 to 2,000oC, which is far too hot for
entry to the nozzle guide vane of the turbine. The air not used for
combustion is introduced progressively into the flame tube.
Approximately half is used to lower the gas temperature and the
other half is used for cooling the walls of the flame tube.
Gas Turbine
Principle of gas turbine (con’t)
3. Turbines
The turbine has the task of providing the power to drive the
compressor and accessories and providing shaft power for gas
compressors, generator rotors, etc.
It does this by extracting energy from the hot gases released from
the combustion system and expanding them to a lower pressure and
temperature.
To produce the driving torque, the turbine may consist of several
stages, each employing one row of stationary nozzle guide vanes
and one row of moving blades. The number of stages depends on
whether the engine has one shaft or two and on the relation
between the power required from the gas flow, the rotational spped
at which it must be produced and the diameter of turbine permitted.
Gas Turbine
Principle of gas turbine (con’t)
3. Turbines
The number of shafts varies with the type of engine; high
compression ratio engines usually have two shafts, driving high and
low pressure compressors. The design of the nozzle guide vane and
turbine blade passages is based broadly on aerodynamic
consideration and to obtain optimum efficiency compatible with
compressor and combustion design, the nozzle guide vanes and
turbine blades are of a basic aerofoil shape (like an airplane wing).
The relationship and juxtaposition of these shapes are such that the
turbine functions partly under impulse and partly under reaction
conditions; that is to say the turbine blades experience an impulse
force caused by the initial impact of the gas on the blades and a
reaction force resulting from the expansion and acceleration of the
gas through the blade passages. Normally, gas turbine engines do
not use either pure impulse or pure reaction turbine blades.
Gas Turbine
Principle of gas turbine (con’t)
3. Turbines
With an impulse turbine, the total pressure drop across each stage
occurs in the fixed nozzle guide vanes and the effect on the turbine
blades is one of momemtum only; whereas with a reaction turbine,
the total pressure drop occurs through the turbine blade passages.
The proportion of each principle incorporated in the design of a
turbine is therefore largely dependent on the type of engine in which
the turbine is to operate, but in general it is about 50% impulse and
50% reaction.
Gas Turbine
Principle of gas turbine (con’t)
3. Turbines
nozzle
turbine
nozzle
turbine
Turbine driven by the impulse of the
gas flow and its subsequent reaction
as it accelerates through the
converging blade passage