A Review of Centrifugal Air Compressors

Over the last few months, my EXAIR colleagues and I have blogged about several different types of air compressor types including single and double acting reciprocating, rotary screw, sliding vane and rotary-scroll air compressors. You can click on the links above to check those out. Today, we will examine centrifugal air compressors.

The types of compressors that we have looked at to date have been of the Positive Displacement type.  For this type, an amount of air is drawn in and trapped in the compression area, and the volume in which it is held is mechanically reduced, resulting is rise in pressure as it approaches the discharge point.

types of compressors

The centrifugal air compressors fall under the Dynamic type. A dynamic compressor operates through the principle that a continuous flow of air has its velocity raised in an impeller rotating at a relatively high speed (can exceed 50,000 rpm.) The air has an increase in its kinetic energy (due to the rise in velocity) and then the kinetic energy is transformed to pressure energy in a diffuser and/or a volute chamber. The volute is a curved funnel that increases in area as it approaches the discharge port. The volute converts the kinetic energy into pressure by reducing speed while increasing pressure. About one half of the energy is developed in the impeller and the other half in the diffuser and volute.

Centrifugal Compressor
Centrifugal Compressor Components

The most common centrifugal air compressor has two to four stages to generate pressures of 100 to 150 PSIG.  A water cooled inter-cooler and separator between each stage removes condensation and cools the air prior to entering the next stage.

Some advantages of the Centrifugal Air Compressor-

  • Comes completely packaged fort plant air up to 1500 hp
  • As size increases, relative initial costs decrease
  • Provides lubricant-free air
  • No special foundation required

A few disadvantages-

  • Higher initial investment costs
  • Has specialized maintenance requirements
  • Requires unloading for operation at reduced operational capacities

EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.

If you would like to talk about air compressors or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Intelligent Compressed Air: What is an Air Compressor?

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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
E-mail: TylerDaniel@exair.com
Twitter: @EXAIR_TD

 

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