Compressed Air Basics Part 2: Different types of air compressors

This week we will be going over the different types of air compressors.  As we discussed last week, an air compressor takes ambient air and decreases its volume.  This causes the pressure and temperature to rise.  Decreasing the volume is the "compressing" that air compressors do.  I should qualify that this is how positive displacement compressors increase the pressure.  There are dynamic compressors that use a different method. 

The two main types of compressors are dynamic and positive displacement.

The positive displacement compressor is probably the one you're familiar with.  It traps gas in a volume and then decreases that volume.  The decrease in volume causes a rise in pressure.    In layman's terms we're squeezing the air.

A dynamic compressor uses a rotating element (usually called an impeller) which increases the velocity of a gas stream and that is converted to pressure by then slowing it through a diffuser. I guess in layman's terms you'd look at this as a traffic jam on the interstate and the cars are the air particles.  Everybody is moving fast and has good spacing (well...not everybody), when a wreck blocks traffic in one lane.  The cars then slow down and get closer together.  When the same amount of air molecules that were spaced out are now jammed closer together - that's pressure. 

From there it breaks down into different types of technology.  Here's chart that breaks it down:

The positive displacement compressors break down into two main categories:  Rotary and Reciprocating.

Rotary compressors use one or more rotating elements in a housing (called an air end) to decrease the volume and increase the pressure.

Reciprocating compressors use a piston in chamber called a cylinder to decrease the volume and increase the pressure.  They are commonly referred to as "piston compressors."


Each technology has it's advantages and disadvantages.  What technology you choose should be chosen based on:
1.  What pressure do you need?
2.  What flow do you need?
3.  What are the ambient conditions? (temperature, humidity, altitude, hazardous location, ect.)
4.  What is the level of purity you need and what dew point is required?
5.  What is the media you are compressing? (air, nitrogen, natural gas, helium, refrigerant, ect.)
6.  Are there sound or vibration limits that you have to meet?

Every situation is different so there may be other factors, as well.

This blog will go over some technologies briefly and some in depth.  Next week we'll look in depth at reciprocating compressors with a focus on the single-acting ones.

Compressed Air Basics Part 1: The Fourth Utility

This will be the beginning of a series on the basics of compressed air.  We will begin on why compressed air use is so common and its benefits.

Compressed Air is often called "The Fourth Utility" because it an essential element to millions of businesses around the world.  However, unlike your electricity, water and gas utilities, your compressed air system is something you have complete control of.

Compressed air is the energy of choice to power a great variety of applications. Sanders, grinders and paint guns in automotive service shops, presses and accessory equipment in dry cleaning stores and commercial laundries all depend on a reliable supply of compressed air. Theme parks depend on compressed air to run roller coasters, fountains, and animated characters. Construction and road crews use compressed air to power jackhammers and repair our roads.  Manufacturers use air compressors to run their equipment.  Every hospital has an air compressor for medical air and sometimes for climate control.  Power companies use them for engine starting.  Universities use compressors for lab air.  Bottling companies use compressed air to make the plastic bottles that you drink out of.  Compressed air in the tires of your car.

Compressed air is everywhere.

The reasons why compressed is so widely used include:

• Compressed air can be easily stored for its intended purpose, using storage tanks located in places where no other power is available or practical.
• Compressed air can be used where other energy sources cannot be used due to explosion hazard or fire risk.
• Equipment operated by compressed air can function at extreme temperature.
• Compressed air can have a high degree of cleanliness, where quality, hygiene and safety are essential.
• Compressed air can be stored in bottles and used where no pipe system exists.
• As a power source, compressed air does not interfere with electrical monitoring equipment.
• Air tools are often much lighter than the equivalent electrical models, making them easier for an operator to handle.
• Air tools are ideal for challenging applications such as in steel mills and foundries where the conditions are almost destructible; however, with periodic maintenance the tools can continue to run at their peak efficiency.

Compressed air has been used by man since at least the 3rd Century BC.  The Greek inventor Ctesibius used compressed air in an alarm clock and in a cannon that shot arrows.  His protege, Hero, used it to open the gates at the Temple of Alexandria.

So what is compressed air?

Compressed air is simply atmospheric air under pressure.  That pressure is energy stored in the air, and now that energy is available to do work.

There is a relationship between volume, pressure and temperature:

You can see that as volume decreases, both pressure and temperature increase.  So what air compressor does is take in ambient air and decrease its volume, which creates a rise in pressure and a rise in temperature.

This pressure is either used immediately to do work or stored in tanks for future work. There are many different types of compressors that use various methods to compress the air.  We'll go over the different types next week.