Most commercial and industrial companies rely on compressed air, considering it a fourth utility (joining water, electricity and gas) that powers air tools, pneumatics and material handling equipment.
It’s safer, cleaner, more consistent and more reliable than most other power sources. (In fact, you can read more about the advantages of compressed air and the wide range of industrial applications in our recent post, “Advantages of Industrial Air Compressors.”
Plus, it has some unique advantages that make it very desirable in industries like semiconductor, electronics and healthcare.
Because they are exceptionally reliable and cost-efficient, most air compressors used in commercial and industrial settings are rotary screw models. For that reason, learning how rotary screw air compressors work is crucial for the people responsible for maintaining their company’s systems. And even workers who regularly use air-powered tools and equipment will benefit from a better understanding of how they operate.
This knowledge is a game changer, providing insight into the design, operation and ongoing maintenance of a compressed air system. As an organization, we are committed to providing this information.
There are two main elements of an air compressor system:
- The conversion of atmospheric air into compressed air
- The flow of oil that keeps compressors cool and running efficiently
We will handle each of those separately in the main sections—Compressing the Air and Oil—The Lifeblood of the System—below.
Compressing the Air
Air Enters Through the Inlet Air Filter
Atmospheric air enters a rotary screw air compressor through an inlet air filter and an inlet valve, which controls how much air is drawn into the unit (note: atmospheric air pressure varies greatly, depending on elevation). The inlet valve is wide open when your compressor has demand or is loaded and is closed when it has no demand and is in the no-load state.
When the compressor reaches a designated pressure (unload pressure), the compressor package will relieve the internal pressure to reduce power consumption while the compressor is running in a no-load/no-demand state. As often happens, the compressor may switch back into the loaded state before the transition to the no-load state is complete. If that happens too frequently, it can lead to rapid cycling, potentially damaging the unit. If you are seeing rapid cycling in your air compressor, we strongly recommend you consult an air compressor professional.
The air goes through the air filter to remove particles. Then it goes through the inlet valve and into the airend for compression.
Air is Compressed in the Airend
As the intake air enters the compression chamber, two interlocking helical rotors or screws mesh, trapping the air between the rotors and casing. The rotors move in opposite directions, reducing the space and compressing the air. An electric motor typically drives the male rotor, which then drives the female rotor.
Oil-lubricated compressors inject oil into the compression chamber to add a coating to the rotors, helping to seal in the air, reduce wear and tear and prolong compressor life. It also helps reduce the temperature of the air and the compressor. (Oil-free compressors are available for special applications, but most companies choose oil-lubricated models because they are less expensive and more efficient.)
The lubricant that is injected into the compression chamber is discharged along with the compressed air. That oil must be removed.
Oil is Removed in the Air-Oil Separator
As the air-oil mixture enters the sump or air-oil separator tank, it immediately slams into a series of baffles, which are devices used to redirect the air in a centrifugal motion. The resulting vortex effect pushes most of the oil out of the air onto the sides of the tank. Any remaining oil is removed by a filter element at the top of the separator.
The air leaving the separator contains less than 3 parts of oil per million—more than clean enough to meet the needs of most customers. However, we can cut that down to .01 ppm by adding a few additional filters for companies with more demanding requirements, such as those in the semiconductor, food and beverage and medical equipment industries.
Unfortunately, compressing air generates heat, so the compressed air leaving the separator is still too hot to be released into the air compressor system. Plus, it is saturated with moisture that needs to be removed to prevent damage to the equipment downstream.
The Air is Cooled in the Aftercooler
The compressed air then goes to an air-cooled aftercooler, a heat exchanger with a fan blowing ambient air over finned tubes. As the aftercooler lowers the pressure dew point of the air, water vapor condenses and creates condensate. A typical aftercooler has a 15-18°F approach temperature.
Water is Removed in the Moisture Separator
In the final step before the compressed air is released into the system, the water is extracted with a moisture separator and is removed from the separator through a drain valve.
Depending on the pressure dewpoint and air quality a customer needs, a rotary screw air compressor may include a refrigerated air dryer or a desiccant air dryer to remove additional moisture. That’s especially important in industries like automotive, semiconductor and electronics.
In addition to the air compression cycle, a rotary screw air compressor has a critically important oil circuit.
Oil—The Lifeblood of the System
Oil is Added in the Compression Chamber
Oil is the lifeblood of your compressor and plays an essential role in the life of your machine. Obviously, it reduces the friction among all the moving parts, also reducing the heat buildup.
As mentioned above, oil is added to the compression chamber, where it coats and seals the rotors, reducing wear and tear on the compressor and keeping it and the air from getting too hot.
Tiny droplets of oil mix with the air as it compresses. To deliver the clean air most applications require, that oil must be removed.
Oil is Removed in the Air-Oil Separator
When the air-oil mixture enters the sump or air-oil separator tank, the centrifugal motion of the air flow forces most of the oil out of the air. The oil runs down the side walls of the separator tank and collects in the bottom, where it is drawn into an oil return line. Any remaining oil that is trapped in the airstream is removed by a coalescing filter element and a scavenge line returns the removed oil back to the compression cycle.
Oil Cooled and Recycled
In most applications, a thermostatic valve directs oil that is sufficiently cool back into the airend, while the oil that is too hot is sent through an oil cooler. Some newer VSD fan-cooled compressors flow all lubricant through the oil cooler and adjust the fan speed to maintain proper lubricant temperature.
The oil is then filtered to remove contaminants that could harm the compressor and is returned to the airend, where the cycle starts again.
The Importance of Maintenance
Understanding the anatomy of an air compressor makes it easy to see the importance of maintenance. You’ll want to change filters regularly, both oil and air. And inspect drains and sumps to prevent blockages.
Following the oil analysis routine prescribed by the manufacturer allows you to routinely monitor the condition of the oil in your system. Plus, it can alert you to potentially costly repairs and downtime before they happen and can suggest proactive steps to take.
Finally, you should pay special attention to your compressor’s oil levels as well. The oil in your compressor declines steadily over time as droplets are absorbed into the air and either escape or are removed by the coalescing air filters and drainage system. So it’s wise to check the oil level before starting your compressor each day, using the sight glass or other means that are typically found on the separator tank. And changing the compressor oil is an absolute must to maintain peak operating efficiency and maximize production uptime.
Consult your manufacturer’s instructions to determine the proper intervals for changing filters and oil.
If you suddenly find your compressor is using more oil than usual, you may need expert help. Excessive oil use could indicate a problem: oil might be leaking somewhere. Or the operating temperature may be too high.
Consulting Help
Developing a relationship with a local air compressor professional can help you select the right compressor and configure it for optimum operation in your application.
Kaishan works with a nationwide network of independent distributors, who can provide on-site help and consultation. These factory-trained air compression experts have an investment in their local communities and can service your air compressor system without a problem.
Key Takeaways
- Knowing how rotary screw air compressors work is crucial for the people responsible for maintaining them.
- Small amounts of oil are injected into the air as it is compressed. But you need to remove it to meet the requirements of end-use applications.
- Oil plays an essential role in the life of your compressor. That’s why we call it the lifeblood of your machine.
- Routine oil maintenance is the most effective way to ensure compressor longevity.
- Understanding the anatomy of an air compressor makes it easy to see the importance of maintenance.
Let Us Help
We can help you design your air compressor system and maintain your rotary screw air compressor to optimize its operation in your plant. Get in touch with the experts at Kaishan. Contact us today.