Intelligent Compressed Air: What is an Air Compressor?

Example of the supply side of a compressed air system

One thing that is found in virtually every industrial environment is an air compressor. Some uses for the compressed air generated are: powering pneumatic tools, packaging, automation equipment, conveyors, controls systems, and various others. Pneumatic tools are favored because they tend to be smaller and more lightweight than electric tools, offer infinitely variable speed and torque, and can be safer than the hazards associated with electrical devices. In order to power these devices, compressed air must be generated.

types of compressors

There are two main categories of air compressors: positive-displacement and dynamic. In a positive-displacement type, a given quantity of air is trapped in a compression chamber. The volume of which it occupies is mechanically reduced (squished), causing a corresponding rise in pressure. In a dynamic compressor, velocity energy is imparted to continuously flowing air by a means of impellers rotating at a very high speed. The velocity energy is then converted into pressure energy.

Of the positive-displacement variety they are broken down further into two more categories: reciprocating and rotary. A reciprocating compressor works like a bicycle pump. A piston reduces the volume occupied by the air or gas, compressing it into a higher pressure. There are two types of reciprocating compressors, single or double-acting. Single-acting compressors are the most common and are available up to 30HP at 200 psig. Their small size and weight allow them to be installed near the point of use and avoid lengthy piping runs. These are the types of compressors that would be commonly found in your garage. The double-acting reciprocating compressor is much like its single-acting brethren, only it uses both sides of the piston and cylinder for air compression. This doubles the capacity of the compressor for a given cylinder size. They are much more efficient than single-acting compressors, but are more expensive and do require a more specialized installation and maintenance.

Rotary compressors are available in lubricant-injected or lubicrant-free varieties. These types of compressors use two inter-meshing rotors that have an inlet port at one end and a discharge port at the other. Air flows through the inlet port and is trapped between the lobes and the stator. As the rotation continues, the point intermeshing begins to move along the length of the rotors. This reduces the space that is occupied by the air, resulting in an increase in pressure. In the lubricant-injected compressors, the compression chamber is lubricated between the intermeshing rotors and bearings. This takes away the heat of compression and also acts as a seal. In the lubricant-free varieties, the intermeshing rotors have very tight tolerances and are not allowed to touch. Since there is no fluid to remove the heat of compression, they typically have two stages of compression with an intercooler between and an after cooler after the second stage. Lubricant-free compressors are beneficial as they supply clean, oil-free compressed air. They are, however, more expensive and less efficient to operate than the lubricant-injected variety.

On the other side of the coin, we have the dynamic compressors. These are comprised of two main categories: axial and centrifugal. These types of compressors raise the pressure of air or gas by imparting velocity energy and converting it to pressure energy. In a centrifugal air compressor, air continuously flows and is accelerated by an impeller. This impeller can rotate at speeds that exceed 50,000 rpm. Centrifugal air compressors are generally much larger and can accommodate flow ranges of 500-100,000 CFM. They also provide lubricant-free air.

Axial compressors are used for situations that require lower pressure but high flow rates. They do not change the direction of the gas, it enters and exits the compressor in an axial direction. It is accelerated and then diffused which creates the increase in pressure. A common application that would be served by this type of compressor is to compress the air intake of gas turbines. They have a relatively high peak efficiency, however their large overall size and weight as well as the high starting power requirements pose some disadvantages.

Just as you can find a wide variety of makes and models of automobiles, the same can be said for air compressors. The size, type, and features will be dictated by the types of applications that you’ll be needing the compressed air for in your facility. A quick chat with your local air compressor supplier will help you to determine which type is most suitable for you.

Of course, any of these types of compressors can be used to supply air to your engineered Intelligent Compressed Air Products. If you have an application in your facility that could benefit from an engineered solution, give us a call. An Application Engineer would be happy to discuss your options with you and see to it that you’re getting the most out of your compressed air!

Tyler Daniel
Application Engineer
Twitter: @EXAIR_TD


Images Courtesy of  the Compressed Air Challenge and thomasjackson1345 Creative Commons.

About Double Acting Reciprocating Air Compressors

My colleague, Lee Evans, wrote a blog “About Single Acting Reciprocating Compressors”, and I wanted to extend that conversation to a more efficient relative, the double acting reciprocating compressor.   As you see in the chart below, this type of compressor falls within the same family under the category of positive displacement compressors.

Compressor Types

Positive displacement compressors increase air pressure by reducing air volume within a confined space.  The reciprocating type of air compressor uses a motor that turns a crank which pushes a piston inside a cylinder; like the engine in your car.  In a basic cycle, an intake valve opens to allow the ambient air into the cylinder, the gas gets trapped, and once it is compressed by the piston, the exhaust valve opens to discharge the compressed volume into a tank.  This method of compression happens for both the single and double acting reciprocating compressors.  With a single acting compressor, the air is compressed only on the up-stroke of the piston inside the cylinder.  The double acting compressor compresses the air on both the up-stroke and the down-stroke of the piston, doubling the capacity of a given cylinder size.  This “double” compression cycle is what makes this type of air compressor very efficient.  A single acting compressor will have an operating efficiency between 22 – 24 kW/100 cfm of air while the double acting compressor has an operating efficiency between 15 – 16 kW/100 cfm.  Therefore, electricity cost is less with a double-acting reciprocating air compressor to make the same amount of compressed air.

To explore the internals a bit closer, the mechanical linkage used to move the piston is slightly different as well as the additional intake and exhaust valves.   Instead of the connecting rod being attached directly to the piston as seen inside a single acting compressor, a crosshead is added between the compression piston and the connecting rod (view picture below).  The rod that connects the crosshead to the compression piston can be sealed to keep the cylinder completely encapsulated.  For every rotation of the electric motor, the air is being compressed twice.  With the added heat of compression, the double acting compressors are generally water-cooled.  Also, with the added mechanism between the crank and the piston, the rotational speeds are typically less.  Because of the larger size, water jackets, and added parts, the initial cost is more expensive than the single acting compressor, but the efficiency is much higher.

Double Acting Reciprocating Air Compressor

Double acting compressors are generally designed for rugged 100% continuous operations.  Dubbed the work horse of the compressor family, they are also known for their long service life.  They are commonly used in high pressure services in multistage styles and can come in lubricated and non-lubricated configurations.   With the dual compression, slow speed and inter-cooling, it makes this type of air compressor very proficient in making compressed air.

John Ball
Application Engineer
Twitter: @EXAIR_jb


Photos:  used from Compressed Air Challenge Handbook

About Single Acting Reciprocating Compressors

Whether you’re new to the field of compressed air, an experienced technician, or just in the market for a new compressor, you may find yourself coming into contact with various compressor types.  Within the world of compressed air supply there are two types of compressors: positive displacement and dynamic.  These two compressor types branch off into several different variations, as shown in the chart below.

Compressor types

Positive displacement compressors increase air pressure by reducing air volume within a confined space.  In a positive displacement compressor mechanical linkage is used to reduce the volume of air (the fluid), which results in a change to the air pressure.  To think of it another way, the energy which is used to displace the air volume is converted into an increase in air pressure.

Dynamic compressors, on the other hand, utilize an increase in air velocity to cause a change in air pressure.  For a dynamic compressor, the fluid (air) is accelerated to a high velocity through a rotor or impeller.  The kinetic energy of the air is then converted to an increased potential energy/static pressure by slowing the flow through a diffuser.  The air at the outlet of the diffuser is the compressed air which is used to perform work.

The internals of a single acting reciprocating compressor.

Within this vast field of compressed air generation, one of the most common types of compressors is the single acting reciprocating compressor.  The term “single acting” refers to manner in which the cylinder inside of the compressor motor interacts with the working fluid (the air).  When the fluid (air) acts only on one side of the piston, the motor is referred to as “single acting”.  This type of motor relies on the load of the motor, a flywheel, springs, other cylinders, or some other device/momentum to return the piston back to its original location.

Single acting compressors can be air-cooled or water cooled, lubricated or non-lubricated, and packaged to provide a wide range of pressure and flow capacities.  Because of this adaptability, single acting compressors are quite common and serve a variety of industrial needs.

No matter the type of compressor on the system’s supply side, having engineered products on the demand side improves overall system performance and efficiency.  If you’d like to discuss engineered solutions for your compressed air system, EXAIR Application Engineers are ready and waiting.

Lee Evans
Application Engineer


Compressor internals image courtesy of h080, Creative Commons License.

What Type of Compressor Is Right for EXAIR Products?


A frequent question (and rightfully so) for compressed air products is “How much compressed air does it use?”  Fortunately for EXAIR, we can provide these values with confidence, knowing the research and development, testing, and quality control that goes into the products we make.

For many applications, this question is cut and dry.  For others, particularly those that do not currently have compressed air on site, the question leads to further conversation.  One of the questions that is often asked, is “What type of compressor should we use with these products?”

While the end-use products (EXAIR products) will operate regardless of the compressor type, there are benefits and advantages to various compressor types in different applications.

For short-term or intermittent use, a reciprocating compressor can be an excellent choice.  The size and weight, maintenance requirement (relatively low), and ease of procurement make them very suitable for small demand applications.  They are also suitable for high pressures. Keep in mind that reciprocating compressors typically have higher noise levels and higher cost of compression when compared to screw compressors.

When the compressed air need is high volume, and the demand requires a continuous supply of compressed air, a rotary screw compressor can be a better choice.  Rotary screw compressors are designed for more regular use in industrial applications, are (generally) more maintenance intensive, feature partial load capability allowing to align supply and demand, and can be found in a variety of sizes. You can expect to pay more for these models than the reciprocating compressors.

From an engineering standpoint, reciprocating compressors are dynamic devices, and screw compressors are positive displacement devices.  Click here for a more in depth look at screw compressor operation.

EXAIR manufactures many, many compressed air driven devices with a concentration on solving problems, conserving compressed air and making it safe by meeting OSHA standards.  And, although we do not supply or support any specific compressor manufacturer, our Application Engineers are well versed in compressed air generation and suited to discuss those needs with our customers.

If you have a compressed air related question, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer


Image courtesy of Vic. Creative Commons License