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.

Centrifugal Pic 1
Sample Centrifugal Compressor

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
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Image Courtesy of  the Compressed Air Challenge

Intelligent Compressed Air: Avoid Pressure Drop

A critical component to optimal performance of any compressed air operated product is ensuring sufficient compressed air flow. Simply put, inadequate air flow won’t allow you to get the job done.

As compressed air moves through the distribution system, it encounters friction inside of the walls of the pipe, tube, hose, etc. The diameter of the pipe, length, number of direction changes, and finish surface of the inner wall all play a part in this. A drop in air pressure will occur as a result of this friction. In addition to pressure drops experienced due to the distribution system, they can also occur at the point of use.

4597315810_fb8e3e4d26_o
Common analog pressure gauge

When designing and maintaining your compressed air system, pressure measurements should be taken across varying points to identify (and fix) any issues before they create a greater problem down the road. According to the Compressed Air Challenge, these are the places you should take regular pressure measurements to determine your system operating pressure:

  • Inlet to compressor (to monitor inlet air filter) vs. atmospheric pressure
  • Differential across air/lubricant separator
  • Interstage on multistage compressors
  • Aftercooler
  • At treatment equipment (dryers, filters, etc.)
  • Various points across the distribution system
  • Check pressure differentials against manufacturers’ specifications, if high pressure drops are noticed this indicates a need for service

*More recent compressors will measure pressure at the package discharge, which would include the separator and aftercooler.

Once you’ve taken these measurements, simply add the pressure drops measured and subtract that value from the operating range of your compressor. That figure is your true operating pressure at the point of use.

If your distribution system is properly sized and the pressure drops measured across your various equipment are within specifications, any pressure drop noticed at the point of use is indicative of an inadequate volume of air. This could be due to restrictive fittings, undersized air lines, hose, or tube, or an undersized air compressor. Check that the point of use product is properly plumbed to compressed air per the manufacturer’s specifications.

EXAIR Products are designed to minimize this pressure drop by restricting the flow of compressed air at the point of use. The more energy (pressure) that we’re able to bring to the point of use, the more efficient and effective that energy will be. The photo below shows two common examples of inefficient compressed air usage. With an open-ended blow off, a pressure drop occurs upstream inside of the supply line. If you were to measure the pressure directly at the point of use, while in operation, you’d find that the pressure is significantly lower than it is at the compressor or further up the line. In the other photo with modular style hose, some pressure is able to be built up but if it gets too high the hose will blow apart. These types of modular style hose are not designed to be used with compressed gases.

open end blow offs
They may be inefficient, but they sure are loud…

EXAIR’s Super Air Nozzles, on the other hand, keep the compressed air pressure right up to the point of discharge and minimize the pressure drop. This, in addition to the air entrained, allows for a high force while maximizing efficiency. If you’d like to talk about how an EXAIR Intelligent Compressed Air Product could help to minimize pressure drop in your processes give us a call.

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@EXAIR.com
Twitter: @EXAIR_TD

 

Pressure gauge photo courtesy of Cliff Johnson via Flickr Creative Commons License

Robert Boyle And The Scientific Method

How do we know something is true? In grade school, you may remember being taught a process by which an observation elicits a question, from which a hypothesis can be derived, which leads to a prediction that can be tested, and proven…or not) These steps are commonly known as the Scientific Method, and they’ve been successfully used for thousands of years, by such legendary people of science as Aristotle (384 – 322 BC,) Roger Bacon (1219 – 1292,) Johannes Kepler (1571-1630,) Galileo Galilei (1564-1642) and right up to today’s scientists who run the CERN Large Hadron Collider.  The collider is the largest machine in the world, and its very purpose is the testing and proving (or not) of hypotheses based on questions that come from observations (often made in the LHC itself) in ongoing efforts to answer amazingly complex questions regarding space, time, quantum mechanics, and general relativity.

The Scientific Method is actually the reason (more on this in a minute) for the name of a fundamental law of physics: Boyle’s Law.  It states:

“For a fixed amount of an ideal gas kept at fixed temperature, pressure and volume are inversely proportional.”

And can be mathematically represented:

PV=k, where:

  • P = is the pressure of a gas
  • V = is the volume of that gas, and
  • k = is a constant

So, if “k” is held constant, no matter how pressure changes, volume will change in inverse proportion.  Or, if volume changes, pressure will change in inverse proportion.  In other words, when one goes up, the other goes down.  It’s also quite useful in another formulaic representation, which allows us to calculate the resultant volume (or pressure,) assuming the initial volume & pressure and resultant pressure (or volume) is known:

P1V1=P2V2, where:

  • P1  and P2 are the initial, and resultant, pressures (respectively) and
  • V1  and V2 are the initial, and resultant, volumes (respectively)

This is in fact, what happens when compressed air is generated, so this formula is instrumental in many aspects of air system design, such as determining compressor output, reservoir storage, pneumatic cylinder performance, etc.

Back to the reason it’s called “Boyle’s Law” – it’s not because he discovered this particular phenomenon.  See, in April of 1661, two of Robert Boyle’s contemporaries, Richard Towneley and Henry Power, actually discovered the relationship between the pressure and volume of a gas when they took a barometer up & down a large hill with them.  Richard Towneley discussed his finding with Robert Boyle, who was sufficiently intrigued to perform the formal experiments based on what he called “Mr Towneley’s hypothesis.”  So, for completing the steps of Scientific Method on this phenomenon – going from hypothesis to law –  students, scientists, and engineers remember Robert Boyle.

Russ Bowman
Application Engineer
EXAIR Corporation
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IMGP6394 image courtesy of Matt Buck, Creative Commons License

Video Blog: How To Calculate Air Consumption At A Pressure Other Than Published Values

The below video shows how to calculate the air consumption when operating at any pressure.

If you want to discuss efficient compressed air use or any of EXAIR’s engineered compressed air products, give us a call or email.  We would enjoy hearing from you!

Steve Harrison
Application Engineer
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The Value Of A Pressure Regulator At Every Point Of Use

regulator
EXAIR Pressure Regulator

To understand the value of a having a Pressure Regulator at every point of use we should start with identifying the two types of Pressure Regulators, Direct Acting & Pilot Operated.  Direct Acting are the least expensive and most common (as shown above), however they may provide less control over the outlet pressure, especially if they are not sized properly.  However when sized properly they do an outstanding job.  Pilot Operated Regulators incorporate a smaller auxiliary regulator to supply the required system pressure to a large diaphragm located on the main valve that in turn regulates the pressure.  The Pilot Operated Regulators are more accurate and more expensive making them less attractive to purchase.  The focus of this Blog will be on the Direct Acting Pressure Regulator.

The Direct Acting Pressure Regulator is designed to maintain a constant and steady air pressure downstream to ensure whatever device is attached to it is operated at the minimum pressure required to achieve efficient operation.  If the end use is operated without a regulator or at a higher pressure than required, it result’s in increased air demand and energy use. To clarify this point, if you operate your compressed air system at 102 PSI it will cost you 1% more in electric costs than if the system was set to run at 100 PSI! Also noteworthy is that unregulated air demands consume about 1% more flow for every PSI of additional pressure.  Higher pressure levels can also increase equipment wear which results in higher maintenance costs and shorter equipment life.

Sizing of the Air Regulator is crucial, if it is too small to deliver the air volume required by the point of use it can cause a pressure drop in that line which is called “droop”.  Droop is defined as “the drop in pressure at the outlet of a pressure regulator, when a demand for compressed air occurs”.  One commonly used practice is to slightly oversize the pressure regulator to minimize droop.  Fortunately we at EXAIR specify the correct sized Air Regulator required to operate our devices so you will not experience the dreaded “droop”!

Standard Air Knife Kit
EXAIR Standard Air Knife Kit Which Incudes Shims, Properly Sized Pressure Regulator & Filter Separator

Another advantage to having a Pressure Regulator at every point of use is the flexibilty of making pressure adjustments to quickly change to varying production requirements.  Not every application will require a strong blast sometimes a gentle breeze will accomplish the task.  As an example one user of the EXAIR Super Air Knife employs it as an air curtain to prevent product contamination (strong blast) and another to dry different size parts (gentle breeze) coming down their conveyor.

EXAIR products are highly engineered and are so efficient that they can be operated at lower pressures and still provide exceptional performance!  This save’s you money considering compressed air on the average cost’s .25 cents per 1000 SCFM.

Super Air Knife Performance
EXAIR Super Air Knife Performance Specifications At 5 Different Pressures.

If you would like to discuss Air Regulators or quiet and efficient compressed air devices, I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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Pressure – The Inner Working of the Basic Pressure Gauge

Everyday here at EXAIR we talk about pressure, specifically compressed air pressure. The other day I was looking up our model 9011, 1/4″ NPT Pressure Gauge , and it got me to wondering just how does this small piece of industrial equipment work. The best way to find out is to tear it apart.

9011_exair

Most mechanical gauges utilize a Bourdon-tube. The Bourdon-tube was invented in 1849 by a French watchmaker, Eugéne Bourdon.  The movable end of the Bourdon-tube is connected via a pivot pin/link to the lever.  The lever is an extension of the sector gear, and movement of the lever results in rotation of the sector gear. The sector gear meshes with a spur gear (not visible) on the indicator needle axle which passes through the gauge face and holds the indicator needle.  Lastly, there is a small hair spring in place to put tension on the gear system to eliminate gear lash and hysteresis.

When the pressure inside the Bourdon-tube increases, the Bourdon-tube will straighten. The amount of straightening that occurs is proportional to the pressure inside the tube. As the tube straightens, the movement engages the link, lever and gear system that results in the indicator needle sweeping across the gauge.

Pressure Gauge Top

The video below shows the application of air pressure to the Bourdon-tube and how it straightens, resulting in movement of the link/lever system, and rotation of the sector gear –  resulting in the needle movement.

If you need a pressure gauge 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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Compressed Air Regulators: The Design and Function

Regulator

Compressed air regulators are a pressure reducing valve that are used to maintain a proper downstream pressure for pneumatic systems.  There are a variety of styles but the concept is very similar; “maintain a downstream pressure regardless of the variations in flow”.  Regulators are very important in protecting downstream pneumatic systems as well as a useful tool in saving compressed air in blow-off applications.

The basic design of a regulator includes a diaphragm, a stem, a poppet valve, an orifice, compression springs and an adjusting screw.  I will break down the function of each item as follows:

  1. Diaphragm – it separates the internal air pressure from the ambient pressure. They are typically made of a rubber material so that it can stretch and deflect.  They come in two different styles, relieving and non-relieving.  Relieving style has a small hole in the diaphragm to allow the downstream pressure to escape to atmosphere when you need to decrease the output pressure.  The non-relieving style does not allow this, and they are mainly used for gases that are expensive or dangerous.
  2. Stem – It connects the poppet valve to the diaphragm. This is the “linkage” to move the poppet valve to allow compressed air to pass.  As the diaphragm flexes up and down, the stem will close and open the poppet valve.
  3. Poppet valve – it is used to block the orifice inside the regulator. It has a sealing surface to stop the flowing of compressed air during zero-flow conditions.  The poppet valve is assisted by a spring to help “squeeze” the seal against the orifice face.
  4. Orifice – it is an opening that determines the maximum amount of air flow that can be supplied by the regulator. The bigger the orifice, the more air that can pass and be supplied to downstream equipment.
  5. Compression springs – they create the forces to balance between zero pressure to maximum downstream pressure. One spring is below the poppet valve to keep it closed and sealed. The other spring sits on top of the diaphragm and is called the adjusting spring.  This spring is much larger than the poppet valve spring, and it is the main component to determine the downstream pressure ranges.  The higher the spring force, the higher the downstream pressure.
  6. Adjusting screw – it is the mechanism that “squeezes” the adjusting spring. To increase downstream pressure, the adjusting screw decreases the overall length of the adjusting spring.  The compression force increases, allowing for the poppet valve to stay open for a higher pressure.  It works in the opposite direction to decrease the downstream pressure.

With the above items working together, the regulator is designed to keep the downstream pressure at a constant rate.  This constant rate is maintained during zero flow to max flow demands.  But, it does have some inefficiencies.  One of those issues is called “droop”.  Droop is the amount of loss in downstream pressure when air starts flowing through a regulator.  At steady state (the downstream system is not requiring any air flow), the regulator will produce the adjusted pressure (If you have a gage on the regulator, it will show you the downstream pressure).  Once the regulator starts flowing, the downstream pressure will fall.  The amount that it falls is dependent on the size of the orifice inside the regulator and the stem diameter.  Charts are created to show the amount of droop at different set pressures and flow ranges (reference chart below).  This is very important in sizing the correct regulator.  If the regulator is too small, it will affect the performance of the pneumatic system.

The basic ideology on how a regulator works can be explained by the forces created by the springs and the downstream air pressures.  The downstream air pressure is acting against the surface area of the diaphragm creating a force.  (Force is pressure times area).  The adjusting spring force is working against the diaphragm and the spring force under the poppet valve.  A simple balanced force equation can be written as:

Fa  ≡ Fp + (P2 * SA)

Fa – Adjusting Spring Force

Fp – Poppet Valve Spring Force

P2 – Downstream pressure

SA – Surface Area of diaphragm

If we look at the forces as a vector, the left side of the Equation 1 will indicate a positive force vector.  This indicates that the poppet valve is open and compressed air is allowed to pass through the regulator.  The right side of Equation 1 will show a negative vector.  With a negative force vector, the poppet valve is closed, and the compressed air is unable to pass through the regulator (zero flow).

Let’s start at an initial condition where the force of the adjusting spring is at zero (the adjusting screw is not compressing the spring), the downstream pressure will be zero.  Then the equation above will show a value of only Fp.  This is a negative force vector and the poppet valve is closed. To increase the downstream pressure, the adjusting screw is turned to compress the adjusting spring.  The additional spring force pushes down on the diaphragm.  The diaphragm will deflect to push the stem and open the poppet valve.  This will allow the compressed air to flow through the regulator.  The equation will show a positive force vector: Fa > Fp + (P2 * SA).  As the pressure downstream builds, the force under the diaphragm will build, counteracting the force of the adjusting spring.  The diaphragm will start to close the poppet valve.  When a pneumatic system calls for compressed air, the downstream pressure will begin to drop.  The adjusting spring force will become dominant, and it will push the diaphragm again into a positive force vector.  The poppet valve will open, allowing the air to flow to the pneumatic device.  If we want to decrease the downstream air pressure, the adjusting screw is turned to reduce the adjusting spring force.  This now becomes a negative force vector; Fa < Fp + (P2 * SA).  The diaphragm will deflect in the opposite direction.  This is important for relieving style diaphragms.  This deflection will open a small hole in the diaphragm to allow the downstream air pressure to escape until it reaches an equal force vector, Fa = Fp + (P2 * SA).  As the pneumatic system operates, the components of the regulator work together to open and close the poppet valve to supply pressurized air downstream.

Compressed air is expensive to make; and for a system that is unregulated, the inefficiencies are much greater, wasting money in your company.  For blow-off applications, you can over-use the amount of compressed air required to “do the job”.  EXAIR offers a line of regulators to control the amount of compressed air to our products.  EXAIR is a leader in manufacturing very efficient products for compressed air use, but in conjunction with a regulator, you will be able to save even more money.  Also, to make it easy for you to purchase, EXAIR offer kits with our products which will include a regulator.  The regulators are already properly sized to provide the correct amount of compressed air with very little droop.   If you need help in finding the correct kit for your blow-off application, an Application Engineer at EXAIR will be able to help you.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb