Centrifugal Air Compressors: How Do They Work?

Centrifugal air compressors are one example of dynamic style air compressors. The dynamic type of compressors have a continuous flow of air that has its velocity increased in an impeller that is rotating at a higher speed. The kinetic energy of the air is increased due to the increase in velocity and then becomes transformed into pressure energy through the use of a volute chamber, or a diffuser. The volute chamber is a curved funnel that increases in surface are as it approaches the discharge port. This converts the kinetic energy into pressure by allowing the velocity to reduce while the pressure increases. Approximately 1/2 of the energy is developed in the impeller and the other half is developed in the volute chamber or diffuser.

1 – Basic Centrifugal Air Compressor

The most common centrifugal air comppressor has between two and four stages in order to generate pressures up to 150 psig. A water cooled inter-cooler and separator is placed between each stage in order to remove condensation and cool the air down prior to being passed on to the next stage. These compressors still have advantages and some disadvantages. The list below showcases just a few.

Advantages:

  • Lubricant-free air is generated
  • Complete packages up to 1,500 hp
  • Initial costs decrease with increase in compressor size
  • No special foundations or reinforcements needed

Disadvantages:

  • Specialized maintenance requirements
  • Higher initial investment
  • Unloading/waste of air required to drop system pressures

To determine which type of compressor may be best suited for your facility, we suggest to locate and contact a compressor sales company in your geographic area. When it comes to determining the volume of air required to operate the EXAIR products and even some other point of use compressed air applications, EXAIR’s Application Engineers can help you determine the volume you will need to ensure the compressor is sized appropriately. If you would like to discuss any other point of use application, please contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

1 – Dugan, Tim PE – Basic Centrifugal Air Compressor, Compressed Air Best Practices; retrieved from https://www.airbestpractices.com/technology/air-compressors/centrifugal-air-compressor-controls-and-sizing-basics

Five Things To Know About Single Acting Reciprocating Compressors

With the development of highly efficient air compressors, there’s still a place for the most basic design: the single acting reciprocating compressor.  When the piston moves out of the cylinder, it draws air in, at atmospheric pressure.  When it moves in to the cylinder, it reduces the volume that air occupies, increasing its pressure.  These machines are durable, effective, relatively inexpensive, and pretty easy to maintain.  Here are a few interesting things to know about them:

1. Popularity. Because of the simplicity of their design, they’re the most common air compressor in the 10HP and under sizes.  You can get them from a number of sources, and they’re not going to set you back as much as some other types.
2. Oil free air (part 1) While the most basic design uses oil to lubricate the piston rings in the compression cylinder, oil-less reciprocating compressors have cylinders with very smooth (and hard) bore surfaces, like nickel or chrome plating. A series of guide rings around the whole circumference of the piston prevent metal-to-metal contact, eliminating the need for liquid lubrication in the compression cylinder.
3. Oil free air (part 2) If oil in your compressed air is a problem, an oil-free (as opposed to oil-less) compressor is another option. While an oil-less compressor doesn’t use lubricant for the piston movement, an oil-free compressor’s moving parts are oil lubricated, but that oil is kept away from the compression cylinder(s) with connecting rod(s) so that the oil is confined to the lower moving parts…the crankshaft and bottom ends of the connecting rods, and away from the pistons & compression cylinders.
4. Foundation. Reciprocating machinery, as the name implies, has parts that move back and forth. The sudden reversal of direction of heavy metal pistons & rods, dozens of times a minute, means that their operation is inherently unbalanced. This out-of-balance condition, though, can be absorbed by properly securing the compressor to a properly prepared foundation.
5. Higher pressures. If your facility’s compressed air usage primarily entails pneumatic tools, cylinders, and blow off devices like air guns, the system header pressure is likely maintained at around 100psig. While a one-stage reciprocating compressor is usually rated for discharge pressures up to 125psig, a second stage can increase that to 175psig. Multi-stage compressors are used for applications that require up to 3,000psig compressed air. Examples of these are scuba breathing air, pneumatic excavators, and my personal favorite: ballast tank blowing air, used to surface a submarine.

4-stage reciprocating compressors charge 3,000psig air tanks that are used to rapidly push water from a submarine’s ballast tanks to create positive buoyancy.  Because keeping your ‘diving-to-surfacing’ ratio at 1:1 is important.

At EXAIR Corporation, helping you get the most out of your compressed air system is important to us.  If you’ve got questions about how to do just that, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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Compressed Air Receiver Tanks On The “Demand” Side

Most any air compressor is going to have a receiver tank…from the “pancake” types that might hold a gallon or so, to the large, multi-tank arrangements that facilitate both cooling and drying of compressed air in major industrial installations.  The primary purpose of these receiver tanks is to maintain proper operation of the compressor itself…they store a pressurized volume of air so that the compressor doesn’t have to run all the time.  Receiver Tanks, however, can also be used to eliminate fluctuations at points of use, especially in facilities where there might be a lot of real estate between the compressor and the compressed air consuming products.

I recently had the pleasure of discussing an Line Vac Air Operated Conveyor application with a caller.  The need was to move wood chips, from inside to outside the plant, into trailers.  The facility has plenty of compressed air to operate the Line Vacs (the application calls for several) but because the point of operation is so far from the header, they’ll need a “stash” (the caller’s words…we call it “intermediate storage” but he’s not wrong) of compressed air to keep the Line Vacs supplied for operation without any dips in performance.

Enter the Model 9500-60 60 Gallon Receiver Tank.  When an application requires an intermittent demand for a high volume of compressed air, the Receiver Tank provides intermediate storage (or a “stash” – that word’s growing on me) to prevent pressure fluctuations and the associated dips in performance.

Model 9500-60 60 Gallon Receiver Tank

The Model 9500-60 has a small footprint for where floor space is at a premium, and meets ASME pressure vessel code specifications. It comes with a drain valve so you can discharge condensate and contaminants.  A check valve (not included) can be installed upstream to maintain the tank at max pressure so it doesn’t ‘back feed’ other upstream uses.

Use of intermediate storage near the point of use is one of our Six Steps To Optimizing Your Compressed Air System.  If you’d like to find out more about getting the most out of your compressed air, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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Pressure – Absolute, Gauge, and Units of Both

Compressed air is a common utility used throughout industrial facilities and it has to be measured like any other utility in order to know just how much a facility is using. When dealing with compressed air a common unit of measurement that readily comes up is psi, pound-force per square inch. This unit of measure is one of the most basic units used to measure pressure in the compressed air industry. There are other means to measure this though, so let’s discover the difference.

Again, the pressure is a force distributed over an area, the Earth’s atmosphere has pressure, if it didn’t we would all balloon up like the Violet from Willy Wonka, just without eating some prototype gum causing internal pressure. PSIA is a unit of measure that is relative to a full vacuum. It is pounds per square inch absolute (PSIA). The absolute pressure is calculated as the sum of the gauge pressure plus the atmospheric pressure. If you were to travel into space, the atmospheric pressure would be absolute zero which is actually a vacuum. There is nothing pushing from the outside in so the inside pushes out, hence the ballooning.

The atmospheric pressure on earth is based on sea level. This is 14.7 pounds per square inch absolute pressure. This pressure will change along with the weather and the altitude at which the measurement is taken.

So how do we get to the pressure that is displayed on a pressure gauge?  When shown open to room air, my pressure gauge reads zero psi. Well, that is zero psi gauge, this already has the atmosphere showing. It is not showing the Absolute pressure, it is showing the pressure relative to atmospheric conditions. This is going back to the fact that gauge pressure is the summation of absolute pressure and atmospheric conditions, for sea level on earth that is 14.7 psia. So how do we increase this and get the gauge to read higher levels?

We compress the air the gauge is measuring, whether it is using a screw compressor, dual-stage piston compressor, single-cylinder, or any other type of compressor, it is compressing the ambient, atmospheric air. Some materials do not like being compressed. Air, however, reacts well to being compressed and turns into a form of stored energy that gets used throughout industrial facilities.  By compressing the air, we effectively take the air from atmospheric conditions and squeeze it down into a storage tank or piping where it is stored until it is used. Because the air is being compressed you can fit larger volumes (cubic feet or cubic meters) into a smaller area. This is the stored energy, that air that is compressed always wants to expand back out to ambient conditions. Perhaps this video below will help, it shows the GREAT Julius Sumner Miller explaining atmospheric pressure, lack of it, and when you add to it.

Lastly, no matter where you are, there is a scientific unit that can express atmospheric pressure, compressed air pressure, or even lack of pressure which are vacuum levels. To convert between these scientific units, some math calculations are needed. While the video below is no Julius Sumner Miller, it does a great job walking through many of the units we deal with daily here at EXAIR.

 

If you want to discuss pressures, atmospheric pressure, how fast the air expands from your engineered nozzle to atmospheric, why all the moisture in the air compresses with it, and how to keep it out of your process, contact an application engineer and we will be glad to walk through the applications and explanations with you.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

1 – Willy Wonka & the Chocolate Factory – Violet Blows Up Like a Blueberry Scene (7/10) | Movieclips, Movieclips, retrieved from https://youtu.be/8Yqw_f26SvM

2 – Lesson 10 – Atmospheric Pressure – Properties of Gases – Demonstrations in Physics,  Julius Sumner Miller, Retrieved from https://www.youtube.com/watch?v=P3qcAZrNC18

3 – Pressure Units and Pressure Unit Conversion Explained, Chem Academy, retrieve from https://www.youtube.com/watch?v=2rNs0VMiHNw