5. Compressor by ywp5Yn

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									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
                                           N


   head                                Choke condition
                                       (stone wall)




                             Volume Flow
                 Compressor Performance Curve
                  Dynamic Compressor - Centrifugal
     SURGE


           Pressure

                          Surge zone        Operating zone
                                       B
 C                                          A OP




                      D
                          D
                                                                   E



Negative flow                                      Positive flow
                              Surge Cycle
               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 velocity-
  pressure 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                Function
Relief valves       - On the discharge side to relieve excessive
                       pressure
Overspeed          - Trips driver when compressor exceed
Shutdown              predetermine safe speed
Oil failure        - For system fitted with pressure
shutdown              lubrication. Protects bearing by stopping
                     unit when oil pressure fails
Jacket-water        - Shutdown compressor if water pressure fails.
Valve                Operated either by pressure or temperature.
Over-pressure      - Stops compressor when discharge pressure
Shutdown              goes above pre-set safe valve
Excessive temp      - Automatically stops unit on a pre-set high discharge
Shutdown              temperature
Main bearing       - Thermal shutdown device stops compressor if
Protection            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.    If work has been done on a compressor, before starting, turn over
      by hand at least one revolution to make sure everything is clear
2.    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
3.    Always be sure ventilating system(s) are in operation before
      attempting to start
4.    Refer to manufacturer’s instruction book for means of
      determining lubrication requirement
5.    Watch any bearing that is taken up for a reasonable time to be
      certain it is not too tight
6.    Never open gas cylinder without first purging it.
                             Gas Turbine




                           Fuel                   Exhaust Gases


Compressed air
                                           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 flow-
   shied. 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                          Dilution hole
                                  Flame tube
     Air casing




                     8%
                           10% 10%
                   18%
 Comp. air
                                      28%                         72%

Perforated flare                     Burning           Dilution total
                    Fuel             total


                                               Primary zone
                    Swirl vanes
   Nozzle
                            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
                                               turbine
            nozzle   turbine        nozzle




                                    Turbine driven by the impulse of the
                                    gas flow and its subsequent reaction
                                    as it accelerates through the
                                    converging blade passage

								
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