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

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

Two types of Positive Displacement Compressors:◦ Reciprocating ◦ Rotary

Two Types of Dynamic Compressors◦ Centrifugal◦ Axial

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

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

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

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

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

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

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

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

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

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

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

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 shaftVideo

Axial Compressors◦ High Volume◦ High Efficiency◦ High Cost◦ Common Uses

Gas Turbines Jet engines Power stations

◦ Nickname – “Superchargers”

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?