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An air pocket is formed in the center of the casing. ... Centrifugal Compressor: Axial Design As the gas velocity is increased by the rotating blades, ... – PowerPoint PPT presentation

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Title: Compressors

  • CM4120
  • Julie King

Presentation Outline
Introduction Types Compressor System References
  • Compressors used to increase the pressure of a
    gas (compressible fluid)
  • Examples
  • Increase the pressure for instrument air systems
    (to get control valves to operate), transport
    gases such as hydrogen, nitrogen, fuel gas, etc.
    in a chemical plant

2 Types of Compressors
  • Positive Displacement (PD) Operate by trapping
    a specific volume of gas and forcing it into a
    smaller volume
  • 2 Basic Designs for PD Compressors
  • Rotary
  • Reciprocating
  • Centrifugal Operate by accelerating the gas and
    converting the energy to pressure
  • 2 Basic Designs for Centrifugal Compressors
  • Centrifugal
  • Axial

Positive Displacement Compressors Rotary Design
  • Rotary compressors (get their name from the
    rotating motion of the transfer element) compress
    gases with lobes, screws, and vanes into smaller
  • 4 Primary Types of Rotary Compressors
  • Rotary Screw
  • Sliding Vane
  • Lobe
  • Liquid Ring

Rotary Screw Compressors
  • Commonly used in industry.
  • It operates with 2 helical rotors that rotate
    toward each other, causing the teeth to mesh.
  • As the left rotor turns clockwise, the right
    rotor rotates counterclockwise. This forces the
    gases to become trapped in the central cavity.
  • The 2 rotors are attached to a drive shaft and
    drive that provide energy to operate the
  • Have an inlet suction line and outlet discharge

Rotary Screw Compressor
Sliding Vane Compressors
  • Uses a slightly off-center rotor with sliding
    vanes to compress gas.
  • Inlet gas flows into the vanes when they are
    fully extended and form the largest pocket. As
    the vanes turn toward the discharge port, the
    gases are compressed.
  • As the volume decreases, the pressure increases
    until maximum compression is achieved. Then the
    gas is discharged out the compressor.

Sliding Vane Compressor
Lobe Compressors
  • Characterized by 2 kidney-bean shaped impellers
    used to trap and transfer gases.
  • The 2 impellers move in opposite directions on
    parallel mounted shafts as the lobes sweep across
    the suction port.
  • Compressed gases are released into the discharge
  • The lobes do not touch each other. A few
    thousands of an inch clearing exists between the
    casing and lobes.

Lobe Compressors
  • Designed to have constant volume discharge
    pressures and constant speed drivers.
  • Lobe Compressors can be used as compressors or
    vacuum pumps.

Lobe Compressor
Liquid Ring Compressors
  • It has one moving transfer element and a casing
    that is filled with water or seal liquid.
  • As the rotor turns, the fluid is centrifugally
    forced to the outer wall of the elliptical
    casing. An air pocket is formed in the center of
    the casing.
  • As the liquid ring compressor rotates, a small
    of the liquid escapes out the discharge port.
    Make up water or seal liquid is added to the
    compressor during operation. The liquid helps
    cool the compressed gases.

Liquid Ring Compressors
  • Used to compress hazardous and toxic gases as
    well as hot gases.

Positive Displacement Reciprocating Compressors
  • Most common type of compressors.
  • Work by trapping and compressing specific volumes
    of gas between a piston and a cylinder wall.
  • The back and forth motion incorporated by a
    reciprocating compressor pulls gas in on the
    suction (or intake) stroke and discharges it on
    the other.
  • Spring-loaded suction and discharge valves
    open/close automatically as the piston moves up
    and down in the cylinder chamber.

Positive Displacement Reciprocating Compressors
  • Basic Parts of are
  • Piston
  • Connecting Rod
  • Crankshaft
  • Diver
  • Piston Rings
  • Suction Line
  • Discharge Line
  • Spring -Loaded Suction and Discharge Valves

Positive DisplacementReciprocating Compressors
  • Can have 1 to 4 cylinders. One shown only has one

Mulitstage Compressors
  • Discharge from Stage 1 is suction for Stage 2.

Centrifugal Compressors
  • Centrifugal compressors accelerates the velocity
    of the gases (increases kinetic energy) which is
    then converted into pressure as the gas flow
    leaves the volute and enters the discharge pipe.
  • Usually operate at speeds gt 3,000 rpm.
  • Deliver much higher flow rates than positive
    displacement compressors.

Centrifugal Compressors
  • 2 Types of Centrifugal Compressors
  • Single- Stage Compress the gas once
  • Use for high gas flow rates, low discharge
  • Multi- Stage Take the discharge of one stage
    and pass it to the suction of another stage
  • Use for high gas flow rates, high discharge

Centrifugal Compressors
  • Basic Components
  • Impellers, Vanes, Volutes, Suction Eyes,
    Discharge lines, Diffuser Plates, Seals, Shaft,
  • Suction Vane Tips Part of the impeller vane
    that comes into contact with gas first.
  • Discharge Vane Tips Part of the impeller vane
    that comes into contact with gas last

Centrifugal Compressor
Centrifugal Compressor Axial Design
  • Composed of a rotor that has rows of fanlike
  • In industry, axial compressors are used alot high
    flows and pressures are needed.
  • Gas flow is moves along the shaft.
  • Rotating blades attached to a shaft push gases
    over stationary blades called stators.
  • Stator blades are attached to the casing.

Centrifugal Compressor Axial Design
  • As the gas velocity is increased by the rotating
    blades, the stator blades slow it down. As the
    gas slows, kinetic energy is converted into
  • Gas velocity increases as it moves from stage to
    stage until it reaches the discharge.
  • Multi-Stage axial compressors can generate very
    high flow rates and discharge pressures.
  • Axial compressors are usually limited to 16
    stages (due to temperature/material limitations)
  • Pound for pound, axial compressors are lighter,
    more efficient, and smaller than centrifugal

Axial Compressor
Typical Compressor System
Other equipment needed in a process system.
Typical Compressor System
  • Safety valves and pressure relief valves used to
    remove excess pressure that could damage
    equipment and people.
  • Silencers are mounted on the inlet and outlet of
    a compressor to reduce the noise. Compressors
    are very noisy. Exxon had one for a refinery
    light ends stream nicknamed Old Snort by the

Typical Compressor System
  • Demister removes moisture (liquid) from the gas
    stream. The liquid falls to the bottom of the
    demister and is removed. The clean gases goes
    out the top of the demister.
  • Dryer sometimes used on the compressor discharge
    line to remove any liquids (moisture). Silica gel
    and molecular sieves (3A mole sieve) often used.

Typical Compressor Start Up Procedures
  • Check valve line up on the compressor and
    associated equipment.
  • Check compressor oil levels and bearing cooling
    water systems.
  • Be sure all the compressor controls are set
  • Turn on the compressor.
  • Monitor equipment and process until conditions
    steady out.

  • The Process Technology Handbook, by Charles E.
    Thomas, UHAI Publishing, Berne, NY, 1997.