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What a Rotary Compressor Is and How to Buy the Right Type for Your Needs

Rotary air compressors are integral to many industrial processes. Learn what it is, how it works and where to buy the best one for your project.

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What is a rotary compressor

The industrial air compressor market is expected to be worth USD 16.36 billion by 2024. Air compression machines are integral to the proper execution of many industrial processes. There are two ways to compress air - positive displacement and dynamic displacement.


Positive displacement makes use of Boyle’s Law and dynamic displacement makes use of Bernoulli’s principle. The result is that you get a range of air compression machines with different operating characteristics. One type of air compressor could be preferred over another depending on the use case, technical requirements and budget.


Rotary compressors are a type of positive displacement compressor. We will go over how it works further on.

Why you would select a rotary compressor

In the end both methods - positive displacement and dynamic displacement - achieve the same objective.


They compress and raise the pressure of gas.


So when selecting the right air compressor for the job engineers pick based on technical criteria and limitations in their project.


Technical constraints to consider are:

  • Type of gas to be compressed (molecular weight)
  • Type of application of the machine
  • Inlet capacity
  • Maximum discharge pressure
  • Adiabatic efficiency
  • Operating speed
  • Maximum power

While technical requirements are critical during selection, engineers often end up selecting a machine that is best for the project and not just the application.


Any project will have technical constraints, but it also has commercial and schedule considerations, “off design” conditions and practical future expectations.


These are non-technical constraints and they are often the deciding factor when selecting the right type of air compressor.


Some non-technical requirements to consider:

  • Operating conditions (present and future)
  • Quality of the equipment brand 
  • Available skills for maintenance
  • Staff’s familiarity with the machine
  • Automation and digital controls
  • Availability of parts
  • Delivery time
  • Price and the Life-Cycle Cost

The most common reason we select a positive displacement compressor over a dynamic compressor type is because they are more tolerant of changes in gas composition (molecular weight etc.) than dynamic compressors.


Once we conclude that gas composition is a make or break factor then the choice is between reciprocating compressors or rotary compressors; the two subcategories of positive displacement compressors.


Reciprocating compressors are optimal for applications where air is required in really short bursts of time, with a 30 to 45 minute break in between uses on average.


But reciprocating compressors require a higher degree of maintenance, are susceptible to flow pulsation and vibration problems, and they are also sensitive to liquids in the gas stream.


Their main drawback is the frequency of replacing diaphragms, and cylinder and piston maintenance work that is required.


The working principle of reciprocating compressors itself is harsh on the mechanical parts and the compressor requires heavy foundations due to unbalanced forces. And if it is oil lubricated there is an additional cost for installing oil separation systems and the maintenance of those systems.


Rotary compressors are best if the project application of the air compressor requires air all day or requires air on a shift basis multiple times a day.


This is because the rotating elements within these types of compressors are meant to run continuously and at a constant rate.


Rotary compressor designs generate less vibration than reciprocating compressors.


They have high mechanical energy transfers, are adaptable to different working conditions, they are more mobile and tend to have flexible control options with automation.


As you can see there’s a lot that goes into selecting the right compressor for your project- technical and non-technical things.


Let’s quickly go over how a rotary compressor works.

The working principle of a rotary compressor

The image is of a type of rotary compressor that uses rotating screws designed by Atlas Copco Group.

Rotary compressors use different rotating elements and chamber designs to manipulate Boyle’s Law in order to create pressurized air.

Boyle’s law states that the absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.

The idea is to draw gas through an inlet, and then move the gas through a cavity or chamber that decreases in size, compresses it and in the process increases its pressure.

Then once maximum pressure is achieved inside the cavity or chamber, open a valve and release the high pressure gas into an outlet system.

But with rotary compressors they typically don’t have inlet or outlet valves.

Instead they use suction and discharge ports that are alternately exposed and covered by the rotating elements or sliding elements.

Have a look at how it works in this video. This is an example of how a rotary screw compressor would work.

In the video you’ll notice oil being injected into the compressor.


The rotary compressor process creates adiabatic heating, which is a natural rise in the temperature of the gas as it becomes pressurized.


In order to dispose of this excess heat some form of air or water or oil cooling is used in the rotary compressor system.


In the case of oil injected compressors, the oil helps by lubricating the rotors, sealing clearances, removing heat of compression and lubricating bearings.


After passing through the compressor the oil needs to be separated from the gas and returned to the lubricating points as well as to the compressor inlet through a separate system.

Types of rotary compressors available


Screw Compressor

This type of rotary compressor compresses the air between two helical screw elements.

The male and a female rotary screw elements are driven either by the male rotor or by a timing gear.


Scroll Compressor

This type is an orbiting displacement compressor that typically operates oil-free.

The compressor consists of two elements - a stator spiral fixed in a housing, and a motor-driven eccentric orbiting spiral.

The spirals are fitted with a 180° phase displacement that forms air pockets with a gradually varying volume.


Rotary Lobe Compressor

This type of compressor is similar to the rotary screw design except that they are twin lobe elements.

The lobe elements are cylindrical in length but shaped like fidget spinners along the width.

They produce less pressure than screw pumps can.

They’re often referred to as blowers because they produce low pressure at a high volume (see Bernoulli’s principle).

Not all rotary screw compressors are created equal

Depending on the manufacturer’s design there may be additional parts and systems that give their products an edge over the competition.


For example, many Atlas Copco Screw Compressors come with Variable Speed Drive technology built in.


Most industrial applications of screw compressors have fluctuating air demand and that is where variable speed drive technology helps.


A compressor with VSD technology simply adjusts its motor and elements speed to match your demand. 

Some Atlas Copco Screw Compressors come with remote monitoring software as well.


Remote monitoring software gives the customer real-time data insights.


With this data they can observe their equipment closely, correct inefficiencies, schedule maintenance in advance, and improve their uptime overall.

Interested in saving 30 to 50% on your electricity costs?

The brand of rotary compressor you chose can either help or hurt your bottom line.


10 to 30% of all energy used in the average industrial facility goes towards generating compressed air. In the worst cases we’ve found that upto 90% of that energy can be lost through:

  • Heat
  • Air leakage
  • Friction between screw compressor parts
  • Bad compressor design

The good news is that Atlas Copco specializes in energy savings. Our award-winning compressor technology is designed to improve your energy efficiency, increase uptimes and reduce your costs.


Book a free consultation with us today and let’s talk about how we can help you.

Atlas Copco Compressors deliver on their promise

Silverstream is the first company to demonstrate fuel savings using a patented air lubrication system that reduces hull drag on cruise ships and merchant vessels. Their system incorporates Atlas Copco’s air compressor and blower.

The system uses a constant and dependable supply of compressed air. Atlas Copco’s ZS55 oil free screw compressors provide this – giving us the right combination of cost, performance and reliability.

Stephen Potts , Head of Supply Chain at Silverstream

Hyundai Motor’s manufacturing plant in Piracicaba, Brazil, contacted Atlas Copco to understand how to improve the energy efficiency of their compressed air equipment. 

We have experienced 1.2 million savings per year in energy alone. In addition, we have reduced our maintenance costs by 40% to 50%. In terms of reliability and breakdowns, our rates have dropped to zero.

Cesar Henrique dos Santos , Facilities and Utilities Senior Manager at Hyundai


With 6400 active patents globally, representing about 2100 inventions, we continuously set new industry standards. And since part of our mission is to minimize the impact of our products on the environment we design them to be as efficient as possible.

Innovation is in our DNA. 

We’ve designed our Atlas Copco rotary screw compressors to give you energy savings of about 30 to 50%. And we’ve made them simple to dismount and maintain so that you save on labor costs as well. Expect to have a plug-and-play style of experience!

You will also receive a complete solution that includes training, performance analysis and support from our engineering department.

Our customers maintain a long term working relationship with us because we give them peace of mind and do everything in our power to make sure their production processes run smoothly at all times.

We promise to continue to design products that provide tangible benefits in terms to you of productivity, energy efficiency and lower life cycle cost.


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