## Air Compressors: Centrifugal Type.

There are two main ways to compress air for supplying pneumatic systems; Positive Displacement and Dynamic.  Positive Replacement reduces the volume of air within a confined space to generate pressure.  The dynamic type raises the air pressure by using kinetic energy and velocity with rotating impellers that continuously brings in airflow.  In this blog, I will cover the centrifugal type of the dynamic branch.

As mentioned, the centrifugal compressor works by transforming kinetic energy and velocity into pressure.  Ambient air passes through guide vanes into the center of a rotating Impeller with radial blades and is then pushed outward by a centrifugal force. This radial velocity of air results in an increase in pressure due to kinetic energy.  Let’s look at the equation for kinetic energy in Equation 1:

Equation 1:

K = ½ * m * V2

K – Kinetic Energy (J)

m – mass (Kg)

V – velocity (m/s)

As you can see, the energy increases with the square of the velocity.  How do we increase the velocity?  Let’s look at Equation 2:

Equation 2:

V = w * r

V – linear velocity (m/s)

As you can see, as the air travels along the impeller towards the outside, the radius increases.  Since the rotations per second are constant, the velocity will increase.  In combination with Equation 1, you can see how the energy will increase, thus increasing the pressure.

With the increase in pressure, you will get an increase in heat.  It is a natural occurrence with air compressors.  Heat from the centrifugal compressor is dissipated with heat exchangers before moving onto the next stage.  Multiple stages are required to raise the pressure to a sufficient level for typical industrial plant requirements.  The most common centrifugal air compressors have two to four stages to generate pressures up to 100 to 150 PSIG.  Centrifugal compressors are near the middle of the road regarding efficiency.  Their typical operating cost is 16 to 20 kW/100 CFM.

• Up to 1500 HP systems are available
• Price per Horsepower drops as system size increases
• Supplies lubricant-free air
• Special installation pads are not required for installation

• Costs more Initially
• Requires specialized maintenance

No matter the type of air compressor that you use, they are very costly to operate.  To help you use them efficiently and safely, EXAIR offers a range of products that can clean, cool, blow, conserve, and convey.  This would include our Super Air Knives, Super Air Nozzles, Safety Air Guns, Cabinet Coolers, and much more.  If you want to save energy, increase safety, and cut costs no matter what size air compressor you have; you can contact an Application Engineer at EXAIR.  We will be happy to help you.

John Ball
Application Engineer
Email: johnball@exair.com

Images Courtesy of the Compressed Air Challenge

## Centrifugal Air Compressors: What are they?

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.

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. For the purposes of this blog, I’m going to highlight the most common style of dynamic compressor: the centrifugal air compressor.

Dynamic compressors are composed 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, although they’re more commonly used in the range of 1,000 CFM to 5,000 CFM.

In a centrifugal compressor, kinetic energy is transformed into pressure energy inside of the diffuser. The air passes through the inlet guide vanes and is drawn into the center of a rotating impeller. The impeller has radial blades that push outward from the center due to centrifugal force. This radial movement of air causes an increase in pressure and the generation of kinetic energy. This kinetic energy is then also converted into pressure as it passes through the diffuser.

According to the Compressed Air Challenge, some advantages of the centrifugal air compressor include:

• Completely packaged for plant or instrument air up through 1,000 HP
• Relative first cost improves as the size increases
• Designed to deliver lubricant-free air
• Do not require special foundations
• Ability to deliver large volumes of air (up to 100,000 CFM)

• Limited capacity control
• High rotational speeds require special bearings, sophisticated monitoring of vibrations and clearances resulting in specialized maintenance considerations
• High initial purchase cost

A centrifugal air compressor is just one of the many different styles utilized in industry to supply a variety of point of use 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

Image courtesy of the Compressed Air Challenge

## How a Centrifugal Compressor Works

Continuing our series on different types of air compressors, today’s blog will feature the centrifugal compressor.  The centrifugal compressor is classified as a dynamic compressor.  Dynamic compressors are designed to work with  a continuous flow of air that has its velocity increased by an impeller rotating at a very high speed.

The centrifugal compressor works by transforming the kinetic energy and velocity into pressure energy in the diffuser.  The air passes through the inlet guide vanes being drawn into the center of a rotating Impeller with radial blades and is then pushed outward from the center by centrifugal force. This radial movement of air results in a pressure rise and the generation of kinetic energy.  The kinetic energy is also converted into pressure by passing through the diffuser.

Multiple stages are required to raise the pressure to a sufficient level for typical industrial plant requirements.  Each stage takes up a part of the overall pressure rise of the compressor unit.  Depending on the pressure required for the application, a number of stages can be arranged in a series to achieve a higher pressure.

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

Centrifugal compressors are the near middle of the road regarding efficiency, their typical operating cost is 16 to 20 kW/100 CFM.  The most efficient compressor type is the double-acting reciprocating and costs 15 to 16 kW/100 SCFM and the least is the Sliding Vane which costs 21 to 23 kW/100 SCFM.

Advantages of the centrifugal air compressor:

• Up to 1500 HP systems are available
• Price per HP drops as system size increases
• Supplies lubricant-free air
• Special installation pads are not required for installation

Disadvantages of the centrifugal air compressor

• Costs more Initially
• Requires specialized maintenance
• Due to high rotational speeds (can exceed 50,000 RPM) precision high speed bearings and vibration monitoring are required

EXAIR recommends contacting a reputable air compressor dealer in your area to discuss your volume and pressure requirements to determine the best size & type air compressor for your needs.

Regardless of the type of air compressor you have, EXAIR’s Intelligent Compressed Air Products® can minimize your compressed air consumption, potentially reducing the size of compressor needed, reduce noise and still deliver powerful results!   If you would like to discuss highly efficient and quiet point of use compressed air products or any EXAIR product, we would enjoy hearing from you.

Steve Harrison
Application Engineer
Send me an email
Find us on the Web

Image Courtesy of  the Compressed Air Challenge

## 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.

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.

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

• Higher initial investment costs
• Has specialized maintenance requirements

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

Send me an email
Find us on the Web