Compressors

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Compressors

• CM4120
• Julie King

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Presentation Outline
Introduction Types Compressor System References
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Introduction

• 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

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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

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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
  volumes.
• 4 Primary Types of Rotary Compressors
• Rotary Screw
• Sliding Vane
• Lobe
• Liquid Ring

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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
  compressor.
• Have an inlet suction line and outlet discharge
  port.

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Rotary Screw Compressor
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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.

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Sliding Vane Compressor
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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
  line.
• The lobes do not touch each other. A few
  thousands of an inch clearing exists between the
  casing and lobes.

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Lobe Compressors

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

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Lobe Compressor
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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.

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Liquid Ring Compressors

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

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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.

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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

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Positive DisplacementReciprocating Compressors

• Can have 1 to 4 cylinders. One shown only has one
  cylinder.

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Mulitstage Compressors

• Discharge from Stage 1 is suction for Stage 2.

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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.

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Centrifugal Compressors

• 2 Types of Centrifugal Compressors
• Single- Stage Compress the gas once
• Use for high gas flow rates, low discharge
  pressures
• 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
  pressures

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Centrifugal Compressors

• Basic Components
• Impellers, Vanes, Volutes, Suction Eyes,
  Discharge lines, Diffuser Plates, Seals, Shaft,
  Casing
• 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

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Centrifugal Compressor
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Centrifugal Compressor Axial Design

• Composed of a rotor that has rows of fanlike
  blades.
• 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.

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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
  pressure.
• 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
  compressors.

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Axial Compressor
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Typical Compressor System
Other equipment needed in a process system.
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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
  technicians.

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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.

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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
  correctly.
• Turn on the compressor.
• Monitor equipment and process until conditions
  steady out.

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Reference

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