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

Webinar Replay: SCFM, ACFM, ICFM, CFM – Why So Many Terms For Air Flows?

EXAIR’s latest addition to the Fall Webinar series was a discussion on the topic of volumetric air flow terms: SCFM, ACFM, ICFM, and CFM. In the compressed air world, these terms are used often to quantify the performance of a compressor or the point-of-use equipment on the supply side of your system. Since conditions will vary from one site location to another, it’s important that we understand how certain variables can change the performance of your system. The webinar is available to view on demand on the EXAIR.com.

The term SCFM (Standard Cubic Feet Per Minute) is used to allow us to make an apples to apples comparison across different equipment. The performance is rate at a set of “standard” conditions to remove any potential variables from the equation. CAGI, or the Compressed Air and Gas Institute, uses the standard conditions of: 14.5 psia, 0% relative humidity (RH), and 68°F. This allows us to compare different devices without needing to make any sort of adjustments.

Variables such as elevation (barometric pressure), relative humidity, and temperature all change the performance and must be considered.

With elevation, we’re looking at the atmospheric or barometric pressure at the location of operation. One way to illustrate this to consider a balloon. If you inflated a balloon at sea-level, or 14.5 psia, then carry that same balloon up to the top of Mt. Everest what would happen? Using Boyle’s Law (P1 x V1 = P2 x V2), we’re able to calculate the exact volume of the balloon. At the peak of Mt. Everest, pressure is significantly lower at roughly 4.5 psi. The balloon when taken to the peak at 4.5 psi would become 3.2x it’s original size as the pressure acting on the outside of the balloon decreases.

Relative humidity tells us how much moisture content is contained within a specific volume of air. Water molecules cannot be compressed, so when the air is compressed this water takes up the same volume. The water condenses in the inter-coolers and after-coolers or is removed via drains and dryers downstream. So, 1 cubic foot of air coming into the compressor weigh more than 1 cubic foot of air out due to this water vapor loss.

As temperature increases, so does air pressure as the molecules in the air speed up and come into contact with one another and the walls of its container at a more rapid pace. Air can also hold a greater volume of moisture at higher temperatures. So, the balance between RH and temperature is an important consideration when determining actual performance, or ACFM.

In the webinar, we walked through two different examples to highlight the changes in these variables and how it impacts the performance of a compressed air system. If you were unable to attend live, the webinar is available to view on demand on the EXAIR website. We have this latest webinar posted there on the website along with all prior webinars as well! There, we talk about topics ranging from compressed air system optimization, static electricity, OSHA Compliance, and more! Check out the available webinars on the Resources tab of the EXAIR.com page today for all the knowledge you’ll need about your compressed air system and processes.

Tyler Daniel, CCASS

Application Engineer

E-mail: TylerDaniel@EXAIR.com