Rotary Air Compressors: How Do They Work?

Ingersoll Rand Rotary Screw Compressor

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, control 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. We’ve discussed the different styles of air compressors here on the EXAIR Blog in the past. Today I’d like to highlight the rotary compressors, one of the positive-displacement types of compressors.

Positive-displacement compressors are broken into two categories: reciprocating and rotary. The rotary compressors are available in lubricant-injected or lubricant-free varieties. Both styles utilize two inter-meshing rotors that have an inlet port at one end and a discharge port at the other. Air flows through the inlet port and is trapped between the lobes and the stator. As the rotation continues, the point inter-meshing begins to move along the length of the rotors. This reduces the space that is occupied by the air, resulting in an increase in pressure.

In the lubricant-injected varieties, the compression chamber is lubricated between the inter-meshing rotors and bearings. This lubricant protects the inter-meshing rotors and associated bearings. It eliminates most of the heat caused by compression and acts as a seal between the meshing rotors and between the rotor and stator. Some advantages of the lubricant-injected rotary compressor include a compact size, relatively low initial cost, vibration free operation, and simple routine maintenance (replacing lubricant and filter changes). Some drawbacks to this style of compressor include lower efficiency when compared with water-cooled reciprocating compressors, lubricant carry over must be removed from the air supply with a coalescing filter, and varying efficiency depending on the control mode used.

In the lubricant-free varieties, the inter-meshing rotors have very tight tolerances and are not allowed to touch. Since there is no fluid to remove the heat of compression, they typically have two stages of compression with an inter-cooler between and an after cooler after the second stage. Lubricant-free compressors are beneficial as they supply clean, oil-free compressed air. They are, however, more expensive and less efficient to operate than the lubricant-injected variety.

Each of these compressors can deliver air to your Intelligent Compressed Air Products. If you’re looking to reduce your compressed air consumption and increase the safety of your processes contact an EXAIR Application Engineer today. We’ll be happy to discuss the options with you and make sure you’re getting the most out of your compressed air usage.

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




Photo courtesy of Ingersoll Rand CC BY 3.0, https://en.wikipedia.org/w/index.php?curid=32093890

Compressed Air Quality Classes – ISO 8573-1

ISO 8573-1:2010 is the international standard for Air Quality Classes. It lays the ground rules for acceptable levels of pollutants, particulate, moisture, and oil in a compressed air source. 

Image Courtesy of  the Compressed Air Challenge

Though the standard has detailed standards for maximum particle size, maximum pressure dew point and maximum oil content for different industries and/or environments (see Slide show above) we can generalize a bit and express the levels of air quality like this:

Plant Air – general plant compressed air used for air tools, nozzles etc.
Instrument Air – found in laboratories, paint and powder coat booths, used for climate control.
Process Air – used in food and pharmaceutical applications, electronics applications.
Breathing Air – used for breathing respirators, breathing tanks and hospital air systems.

Achieving the different levels of air quality can be done with 3 basic types of filtration.
     1. Particulate – a filter element removes particles larger than the opening in the filter material. Typically done with particles greater than 1 micron.
     2. Coalescing – use different methods to capture the particles; 1) direct interception – works like a sieve, 2) Inertial impaction – collision with filter media fibers, 3) Diffusion – particles travel in a spiral motion and are captured in the filter media.
     3. Adsorption – the filter element holds the contaminants by molecular adhesion.

Filters
EXAIR FILTER SEPARATORS

The higher the class your air needs to be the more of these filtration methods you will use. Adsorption will remove more and finer particles than a simple particulate filter. And many applications will use a combination of these methods.

EXAIR products, all of which need a source of “clean, dry air” will operate very well utilizing a source of plant air and only a particulate filter. Your process, dictate if you need to supply additional filtration methods for better air quality. For example, an automotive plant using compressed air to blow parts off will not need the kind of filtration a food handling facility will need while blowing a food product off. If you are using a lubricated compressor or have lubricant in your compressed air lines from another source, you will want to use a coalescing oil removal filter.

EXAIR stocks 5 micron particulate filters which are properly sized for each individual product as an option for our customers if they choose. We also stock coalescing oil removal filters for customers who may need to remove oil from the air. Replacement filter elements are also available and should be replaced at least twice a year, depending on the quality of your air.

Oil Removal Filter
EXAIR Oil Removal Filter

Remember to ask about filtration if you have any concerns about your air quality. We can assist in sizing up the proper filters to get the air quality we recommend for proper operation and longevity of our products. 

If you would like to see how we might be able to improve your process or provide a solution for valuable savings, please contact one of our Application Engineers.

Jordan Shouse
Application Engineer
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Image Courtesy of  the Compressed Air Challenge

Compressor Room Updates Improve Performance

I’d like to start out by saying a common theme I have observed over the past six months is a huge spike in DIY projects around the home. While everyone has been sent home to work and kids have been sent home to learn remotely, the home has become more than just a resting place. It is an office, school, recreation center, even movie theater. This led to an amazing year for home improvement big box stores and lots of people are tackling projects that they may have thought were beyond their level. At this point in the year we are also seeing a lot of manufacturing that either hasn’t stopped or is starting back up safely, there are lots of projects around an industrial facility that can be tackled during downturns as well.

Compressor Room – 1

The main focus today will be on a critical room that generally gets shoved back into a deep dark corner, the compressor room. The air compressor is a piece of capital equipment that generates a companies 4th utility, compressed air. This is then sent throughout most of the facility and utilized at critical points within production. Air compressors have changed their look over the years and are still often crammed into a small dimly lit room that no one wants to venture into. Having an outdated compressor room can also be causing undesirable performance and lack luster performance as well. Here’s a few items that can more often than not be addressed pretty simply to improve the overall appearance and most importantly the performance of the compressors.

Clean air intake on a screw compressor – 2

First, clean air intake. Rather than letting the compressor suck air in from the room that may be stagnant or even worse, just sucking in the hot air coming off the heat exchangers on the compressor and causing elevated compressed air temps. This fix can include ducting clean air from outside of the facility to ensure micro-debris from within the facility isn’t being pulled in. While pulling in ambient air from outside the facility will still require a filter that will need to be maintained. If a large single source is used, that is perfectly acceptable. To step this project up multiple smaller inlets that are each controlled by a damper would permit variability to match ambient conditions on temperature.

Industrial exhaust fan – 3

Second, install an exhaust fan that feeds the air not just out of the room, yet out of the facility if at all possible. This helps to promote a through-flow of air with the clean air intake and keep from recirculating dirty already cycled air. This will also help any form of system based air treatment that relies on an exchange of heat, such as a refrigerant dryer. Again, a fan that stays on constantly would be the base level fix, step this up by adding a thermostatically controlled system so the fan doesn’t run continuously.

Third, if you heat your facility throughout the winter, use that hot exhaust air from the compressors to reclaim the heat of the compression cycle and optimize your return on using electricity. This can be done by strategic routing of the exhaust ductwork mentioned above, and can be stepped up to have thermostatically controlled dampers on the ducts to open and flow the air through an adjacent room for cooler months rather than exhaust straight out during the warm Summer months.

If you would like to discuss any of these topics or any of your compressed air point of use applications, feel free to contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

 1 – Air Compressor in Engine Room – retrieved from, Work With Sounds / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) – https://commons.wikimedia.org/wiki/File:Air_compressor_in_engine_room.JPG

2 – Screw Compressor 1 – retrieved from, Endora6398 / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) – https://commons.wikimedia.org/wiki/File:Screw_compressor_1.jpg

3 – Industrial Exhaust Fan – retrieved from , Saud / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) – https://commons.wikimedia.org/wiki/File:Industrial_Exhaust_Fan.jpg

Sliding Vane Compressors: How they Work

When it comes to air compressors there are many different types to choose from. Each compressor performs differently and have their own benefits and downfalls. In this case we will be taking a look a Sliding Vane Compressor which is a positive-displacement compressor. Positive-displacement type compressors have a given amount of air or gas that gets trapped in a compression chamber. From there the volume of that air is mechanically reduced causing an increase in the pressure. Sliding vane compressors use a circular stator that is housed in a cylindrical rotor; the rotor contains radially positioned slots where the vanes reside. These vanes are what create the compression in the “cells”.

Diagram of a sliding vane compressor

The inlet port on the system is positioned in a way that allows the air flow into each cell, optimizing the amount of air that each cell can hold. Once the air enters the system the cell size is reduced down farther and farther as rotation continues and each vane is pushed back into its original slot in the rotor. Compression will continue until each cell reaches the discharge port. One of the more common forms of a sliding vane compressor is the lubricant injected variety. These compressors inject a lubricant into the chamber to lubricate the walls and the vanes; this removes the heat of compression, as well as provide a seal on the cell. These air compressors are generally sold in a 10 – 200 HP range with capacities running between 40 – 800 acfm.

Advantages of a lubricant injected sliding-vane compressor include:

  • Compact size
  • Relatively low purchase cost
  • Vibration-free operation does not require special foundations
  • Routine maintenance includes lubricant and filter changes

Some of the disadvantages that come with this type of compressor:

  • Less efficient than the rotary screw type
  • Lubricant carryover into the delivered air will require proper maintenance of an oil-removal filtration system
  • Will require periodic lubricant changes

If you have any questions about compressed air systems or want more information on any of EXAIR’s products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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Photo Credit to Compressed Air Challenge Handbook