Sliding Vane Air Compressors

Over the last few months, my EXAIR colleagues have blogged about several different types of air compressor types including single and double acting reciprocating and rotary screw. (You can select the links above to check those out.) Today I will review the basics of the sliding vane type, specifically the oil/lubricant injected sliding vane compressor.

The lubricant injected sliding vane compressor falls under the positive displacement-type, the same as the other types previously discussed.  A positive displacement type operates under the premise that a given quantity of air is taken in, trapped in a compression chamber and the physical space of the chamber is mechanically reduced.  When a given amount of air occupies a smaller volume, the pressure of the air increases.

Each of the previous positive displacement type compressors use a different mechanism for the reduction in size of the compression chamber.  The single and double acting reciprocating use a piston that cycles up and down to reduce the compression chamber size. The rotary screw uses two inter-meshing rotors, where the compression chamber volume reduces as the air approaches the discharge end.  For the lubricant sliding vane type, the basic design is shown below.

Sliding Vane2
Air enters from the right, and as the compression chamber volume reduces due to counterclockwise rotation, the pressure increases until the air discharges to the left

The compressor consist of an external housing or stator, and the internal circular rotor, which is eccentrically offset.  The rotor has radially positioned (and occasionally offset) slots in which vanes reside.  As the rotor rotates, the centrifugal forces on the vanes cause them to move outwards and contact the inner surface of the stator bore.  This creates the compression areas, formed by the vanes, rotor surface and the stator bore.  Because the rotor is eccentrically offset, the volume of the compression area reduces as the distance between the rotor surface and the stator reduces.  As the rotor turns counterclockwise, the vanes are pushed back into the rotor slots, all the while in contact with the stator surface.  The shrinking of the compression area leads to the increase in air pressure.

Oil is injected into compression chamber to act as a lubricant, to assist is sealing, and to help to remove some of the heat of compression.

The advantages of the lubricant sliding vane compressor type is very similar to the lubricant injected rotary screw.  A few key advantages include:

  • Compact size
  • Relatively low initial cost
  • Vibration free operation- no special foundation needed
  • Routine maintenance includes basic lubricant and filter changes

A few of the disadvantages include:

  • Lubricant gets into the compressed air stream, requires an air/lubricant separation system
  • Requires periodic lubricant change and disposal
  • Less efficient than rotary screw type
  • Not as flexible as rotary screw in terms of capacity control in meeting changing demands

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 compressed air 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.

Jordan Shouse
Application Engineer

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Photo Credit to Compressed Air Challenge Handbook

Round and Round They Go…. Rotary Compressors: How They Work

Positive-displacement and dynamic displacement compressors are the two high level principles for the generation of compressed air or gas. Positive displacement types are the most common found in industrial facilities. These units draw ambient air into a chamber which it seals off and then works to compress and squeeze it down into a smaller volume. The air is then discharged into the outlet system of the compressor. Out of the many types of positive displacement compressors, today we are going to discuss the rotary style positive displacement air compressors.

1 – Simplified Rotary Vane Compressor



These are available in both lubricant-free as well as lubricant-injected versions. The main function of the compressor is the same, both have two inter-meshing rotors what pull air into the inlet port and then after the rotational processing of the compressor the air is discharged through a discharge port. The in between of the ports is where the important information lies. The air after being sucked into the inlet gets pulled in between two lobes and the stator. As the air is being trapped the space between the lobes becomes increasingly smaller, thus increasing the pressure of the air transferring it to the discharge port.

The lubricated versions will often help to dissipate the heat that is created as the air is being compressed. This lubricant is then possible to transfer into the compressed air stream and must be removed before a point of use product if lubricant-free air is needed for the process. These compressors rank amongst the lower efficiencies in the positive displacement air compressors.

If you are visual learner, feel free to take a few minutes for the video below.

2 – Rotary Compressor Operation


If you would like to discuss the way to get the most out of your compressor, no matter the type, contact an Application Engineer and let us help you determine the most efficient way to use the air effectively.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

1 – Rotary vane.png – R. Castelnuovo, 10/20/2005 – retrieve from https://commons.wikimedia.org/wiki/File:Rotary_vane.png

2 – Rotary compressor operation – HVACRinfo.com – 8/1/2016 – retrieved from https://www.youtube.com/watch?v=fxDEK3Ymx30

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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