What is a Compressor?

1
Compressors

• What is a Compressor?
• A mechanical device that increases the pressure
  of a gas by reducing its volume.
• Similar to a pump Increases the pressure on a
  fluid and transport it through a pipe.
• What is key difference between a Fluid and a Gas?
• Compressibility a gas is compressible
• What happens to gas volume as it is compressed?
• Decreases
• What happens to the Temperature of the Gas as it
  is compressed?
• Increases

2
Compressors

• Compressors are classified by how they work
• Two Categories of Compressors
• Positive Displacement
• Dynamic
• What is a Positive Displacement Compressor?
• A compressor that confines successive volumes of
  gas within a closed space in which the pressure
  of the gas is increased as the volume of the
  closed space is decreased.
• Intermittent Flow
• What is a Dynamic Compressor?
• A compressor using a rotating mechanism to add
  velocity and pressure to gas.
• Continuous Flow

3
Compressors

• Two types of Positive Displacement Compressors
• Reciprocating
• Rotary
• Two Types of Dynamic Compressors
• Centrifugal
• Axial

4
Compressors

• Reciprocating Compressors
• How does it work?
• Piston movement in a cylinder connected to a rod
  and crankshaft
• Downward piston motion, low pressure gas enters
  the chamber
• Upward piston motion, gas is compressed and exits
  the chamber
• Video

5
Compressors

• Reciprocating Compressors
• High Horsepower Applications
• Common in natural gas transmission lines.
• Processes for high pressure delivery of gasses
• Air Conditioning Compressors
• Some manufactures (Frigidaire) use rotary
  compressors
• AC Compressors (and other small appliance
  applications) are Hermetic or Semi-Hermetic

6
Compressors

• Hermetically Sealed and Semi Hermetically Sealed
  Compressors
• The compressor and motor driving the compressor
  are integrated, and operate within the
  pressurized gas envelope of the system.
• The motor is designed to operate and be cooled by
  the gas or vapor being compressed.
• The hermetic uses a one-piece welded steel casing
  that cannot be opened for repair.
• If the hermetic fails it is simply replaced with
  an entire new unit.
• A semi-hermetic uses a large cast metal shell
  with gasketed covers that can be opened to
  replace motor and pump components.
• The primary advantage of a hermetic and
  semi-hermetic is that there is no route for the
  gas to leak out of the system.
• Open compressors rely on either natural leather
  or synthetic rubber seals to retain the internal
  pressure
• Require a lubricant such as oil to retain their
• sealing properties

7
Compressors

• Rotary Compressors
• How do they it work?
• When a rotating mechanism spins past the inlet
  valve, it creates a vacuum.
• The fluid flows out of the valve behind it,
  filling the vacuum.
• As it approaches the outlet valve, the chamber
  shrinks, creating more pressure on the fluid.
• The fluid has nowhere to go but out of the outlet
  valve, so it shoots out of it.
• Then the rotating mechanism continues on to draw
  more fluid at the inlet valve.
• How it works Video

8
Compressors

• Common Types of Rotary Compressors
• Screw
• Vane
• Scroll
• Screw
• Two meshing helix screws
• Rotors
• Compact and smooth running
• 2 types - Oil Free and Oil Flooded
• Oil Free No assistance from oil to cool and
  assist in sealing .
• Oil Flooded Oil injected to aid in sealing and
  provide cooling.
• Separator downstream to capture the oil

9
Compressors

• Vane Compressors
• Vane housing on a off centered shaft
• Vanes slide in an out always making contact with
  the compressor walls
• Gas enters in the largest opening
• Exits the smallest
• Good for low pressure applications
• Efficient
• Heat controlled by
• oil injection

10
Compressors

• Scroll Compressor
• How it works
• 2 Spirals
• 1 stationary, 1 orbits without rotating
• 1st orbit entraps inlet gas
• Subsequent orbits compresses gas and exited out
  the center
• Generally 2-3 orbits for a full cycle
• Video
• Advantages
• Compact
• Steady flow
• Low energy use
• Quiet
• Smooth operation

11
Compressors

• What is a Dynamic Compressor?
• A compressor using a rotating mechanism to add
  velocity and pressure to gas.
• Continuous Flow
• What are the Two Types of Dynamic Compressors?
• Centrifugal
• Axial

12
Compressors

• Centrifugal Compressors
• Rotating disk (impeller) forces gas to the rim
  of the impeller, increasing velocity
• The diffuser converts the velocity energy to
  pressure energy.
• Primarily used for continuous, stationary service
  in industries such as refineries, chemical plants
  and snow making operations
• Single Stage and Multi-Stage Compressors
• How a Centrifugal Compressor Works

13
Compressors

• Impeller
• Most critical part of a centrifugal compressor
• Compressor performance determined by impeller
• Size
• Shape
• Speed
• 3 types of Impellers
• Closed
• Most common
• Shroud covering both sides of the blade
• Center eye hole for gas to enter
• Used in Multi-stage compressors
• Semi open
• Open

14
Compressors

• Multi-Stage Compressors
• Diaphragm
• Specially designed casing wall separating the
  stages
• Gas passes through the difuser
• Passes through the return channel in the
  diaphragm
• Controlling Axial Load on the Shaft
• Bearing Review
• Thrust Bearing

15
Compressors

• Axial Compressors
• Gas flows parallel to the axis of rotation
• Unlike centrifugal that has radial components
• Has rotating and stationary components
• Rotating airfoil rotor
• Stationary airfoil stator
• Similar number of these on
• a shaft
• Video

16
Compressors

• Axial Compressors
• High Volume
• High Efficiency
• High Cost
• Common Uses
• Gas Turbines
• Jet engines
• Power stations
• Nickname Superchargers

17
Homework

1. What is a Compressor?
2. What happens to the temperature and the volume of
   a gas as it is compressed?
3. What are the 2 categories of Compressors?
4. Explain the difference between a positive
   displacement and radial compressor.
5. Explain how a reciprocating compressor works.
6. Describe a Hermetically sealed compressor.
7. Explain how a rotary compressor works.
8. Describe how a scroll compressor works.
9. Explain how a multi stage compressor works.
10. What type of compressor is found on jet aircraft?