Image: Atlas Copco
Overview
Rotary screw air compressors are the most widely used air compressor models in the industrial marketplace. Rotary screw compressors provide continuous compressed air for precision tasks and they are extremely efficient and remarkably quiet. Screw compressors utilize some of the most advanced technology available in the compressor industry and have a very low energy consumption. These machines are built to run 24 hours a day, 7 days a week, for many years!
Types
There are many different types of rotary screw compressors. The classifications vary based on stages, cooling method, drive type, among others.
The most widely used type is the single stage, oil-injected rotary screw compressor. The main difference is that oil-lubricated compressors inject oil into the compression chamber to serve a few different purposes.
Oil-free compressors can be used in specific applications where the tiny amounts of excess oil in the air would contaminate the product or process.
Oil-injected models are by far the most common and for the purpose of this article we will focus on this type.
Compression
The power (and name) behind these remarkable machines comes from two counter-rotating screws which are housed in a chamber, formally known as an airend. Outside air first travels through a filter to catch any harmful particles and debris that could cause damage. Once filtered, the air goes through an inlet valve and into the space between the interlocking screws. As the rotors turn, the air moves along and travels to the other end of the compression chamber. The area in which the air is contained gets increasingly smaller as the air moves along, and the smaller space increases the pressure. This process is one smooth continuous motion, and therefore produces minimal pulsing or surging which can occur with piston compressors. The result is a high-volume, steady stream of compressed air.
Did you know?
Rotary screw technology is the same technology utilized by superchargers in high-performance engines!
Oil-Injected
In an oil-injected rotary screw air compressor, the oil serves five key purposes: it cleans, cools, lubricates, seals, and protects. Therefore, what is produced by the airend is a compressed air/oil mixture. This mixture flows into a separator tank where the two are, well… separated! Primarily, a mechanical separation takes place using a filter. Further, oil can be removed using centrifugal force or a rapid change in direction. In the case of centrifugal force, rapid spinning allows the heavier oil particles to drop to the bottom while the lighter air spins around on top. Simultaneously, the separated elements leave the tank.
The air leaves the tank and travels through a minimum-pressure valve, and (in some cases) a cooler. The minimum-pressure check valve is dual-purpose and ensures there is sufficient air pressure for proper operation while also working to keep the air pressure from coming back into the system so, when necessary, the motor can start without a load.
The compressed air can still be very hot and might need to go through a cooler to bring it down to 12-20 degrees Fahrenheit above ambient temperature. This cooling process causes water vapor to condense inside the cooler, and therefore, in most compressors there is a water separation stage before the air leaves the package. After these last stages, compressed air finally leaves the compressor ready for treatment, regulation, storage, and to the customer’s processes.
Meanwhile, as the oil was collected from the tank earlier, a line transports the oil back to the compression chamber to be used again. But first, it must be filtered and possibly cooled. A thermostatic valve determines if the oil is too hot and, if necessary, sends the oil through an oil cooler before being filtered. Once filtered, the oil is ready to begin the journey over again.
Check out this cool animation from Atlas Copco detailing the process in an oil-injected rotary screw compressor:
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In a start/stop control scheme, compressor controls actuate relays to apply and remove power to the motor according to compressed air needs.
Thank you for the feedback, Peter. Most of the time, a Start/ Stop control scheme is utilized with piston compressors. For rotary screw compressors, we usually see load/ no-load, inlet valve modulation, dual/ auto-dual, effective rotor length adjustment, or variable speed drive. That all being said, many start/ stop compressors are now using a simple pressure switch to control the motor.
I’m looking for an air compressor. What are the best brands of Rotary Screw Air Compressor that you could recommend. WOuld help a lot. Thanks
Sadly, there is no best brand of rotary screw air compressor. I wish it were that simple! There are many key factors to consider. Depending on what is most important to you, as well as the requirements you have for the unit, most brands have products that will serve your needs well. I apologize for being so vague in my answer, but if you need further assistance, we will be glad to help narrow things down for you!
Hmmm, thanks for your answer. My need is around home-use, such as filling the air and some nailing. Still looking around for a good air compressor.
A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism. They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers.
The best one will be Atlas Copco. We have been using in our factory and most of the air compressors used are Atlas Copco. Less service needed and currently we are changing the air compressors to GA 90-VSD which is Variable Speed Drive. This in turn saves a lot of energy for the whole factory
Thanks for your sharing. Your clip is very insteresting. Can you write more about how to maintaince a air compressor?
Absolutely! We plan on going into much more depth in upcoming articles. Thank you for your feedback. Please check back frequently!
my compressor is not producing any air,the rotary screw type
If it is not loading, we need to figure out if the inlet valve is opening or not. Please call us if you would like us to help you troubleshoot your unit.
my industrial screw type compressor is giving me a problem. even when the storage tank is full ( i use 8 bar max and 6.5 bar min pressure) the compressor keeps on loading and unloading. Though the pressure in the tank is at 8 bar, the compressor led shows it drop to minimum and than restarts loading. every 5 seconds it loads and unloads thereby pushing up the pressure in the storage tank. Any suggestions on what i can do to fix the problem. \thanks
The first thing I would do is verify that the air tank is not full of water! When we use a compressor that loads and unloads, it is reacting to changes in the pressure at the discharge of the compressor. When we see a rapid cycling, it means that the compressor is nearly immediately achieving the required pressure. This can be due to a receiver tank full of water, high differential pressure across a filter (not just inline filters, but the separator and oil filter in the compressor too), or an isolation valve that is closed too close to the unit.
Thanks for your response. The air tank has a timed discharge unit and also the air passes through an air dryer installed before the air tank. One problem i have is that the oil cooling unit has a couple of leaks, could this be the reason? I will check if there is a problem with the separator unit and also the isolation valve. Thanks once again.
Usually, oil and cooling issues are separate from loading issues. I would start with the separator filter.
Very informative article about the functioning process of air compressor. The air compressor does not work properly in the high temperature environment. I don’t know whats the reason behind it. I want to understand the reason behind it.
Most air compressors utilize ambient air to remove the heat of compression. In higher ambient conditions, this becomes more and more difficult.
Thanks for your sharing. Your clip is very insteresting. Can you write more about how to maintaince a air compressor?
We are working on a maintenance series in the near future. Please check back with us!
My compressor is not producing any air,the rotary screw type
Is it running, but not producing air?