A Beginner’s Guide to Reading Rotary Screw Compressor Performance Curves
March 19, 2025The Eight Key Components of a Rotary Screw Compressor
Kaishan USA | March 26, 2025| Uncategorized
There are eight key components of a rotary screw air compressor.
Identifying the functions of its different parts is a helpful step in operating a rotary screw air compressor efficiently.
Knowing what happens at each step and how the different components work together will enable you to get the quality and quantity of compressed air you need to meet the needs of your end-use applications.
The goal of this post is to present the most essential parts of your compressor and explain the role each plays in the compression process.
We’ll start at the beginning with the air inlet.
1. Air Inlet
The inlet valve is the gatekeeper for your compressor. It allows air into the compression chamber, opening fully during the load portion of the cycle and closing during the unload part.
The air inlet valve is one of the most important parts of a rotary screw air compressor.
As such, it is one of the most important and complicated parts of a rotary screw air compressor. No part of the compressor so uniquely differentiates the various manufacturers.
It regulates two significant functions:
- The air entering the compression cycle. If the volume of air entering the chamber is not sufficient, your compressor will struggle to provide enough air pressure. If the inlet valve brings too much air into the chamber, it could cause over-pressurization, damaging the compressor and nearby equipment. Underscoring the importance of this function, Kaishan’s KROF two-stage oil-free compressor even has hydraulic inlet valves to increase reliability in contaminated environments, with closed-system hydraulics ensuring positive sealing and controlling valve actuation through all operating pressures.
- The oil lubricating the airend. When your unit compresses air, we use air pressure as a fluid-motion device, creating oil pressure. Even when your compressor is in the unload state, you need to keep the oil flowing to the airend.
Kaishan is one of the few manufacturers who offer an adjustable inlet valve, available on our premium KRSP and KRSP2 rotary screw air compressors. The ability to adjust the inlet valve allows you to keep the oil pressure where you want it. Adjust it too high, and you’ll waste power. But if the pressure’s too low, you run the risk of locking up and causing significant damage to the airend. For more information on the air inlet valve, read our blog post, “What Is the Air Compressor Inlet Valve, and How Does It Work?”
Next is the control panel.
2. Control Panel
Today’s rotary screw air compressors feature a control panel, which displays all the information you need to know regarding the status of your compressor. It’s the user interface for the control system that regulates the operation of the compressor.
The control panel of a typical compressor can contain as many as five different voltages, so only a qualified electrician or compressor repair person should open the cabinet.
Every Kaishan compressor displays all relevant operational data on the screen of your compressor controls: pressure, temperature, voltage and any amps that are present. As a result, you don’t need to open the control panel to have the critical status information you need. More proof that we do everything we can to help you operate our equipment safely. For more on safety, see our blog post, “Four (and a Half) Critical Compressed Air Safety Tips.”
If you want to keep an eye on key parameters and receive notifications via text, you can even monitor your compressor remotely through the Internet of Things. We offer options like AirWatch, which digitally creates a twin online using Modbus protocol to display pressure, temperature, faults, status and other key indicators.
To make the process easier and safer, Kaishan units display all relevant data on the screen of your compressor controls: pressure, temperature, voltage and any amps that are present.
The controls adjust the operation of the motor.
3. Motors
Rotary screw air compressors have a main motor driving the rotors in the airend. Most have electric motors, but the driving function can also be fulfilled by a belt drive, as in our KRSB belt-drive rotary screw air compressors.
The motor's horsepower determines the compressor's capacity—the more horsepower, the greater its capacity. You need enough horsepower (HP) to generate all the energy needed to deliver compressed air at the required flow and pressure.
The main motor is not the only motor in your compressor—secondary motors may also drive cooling fans or oil pumps. Water-cooled machines also have very small motors for package flow, keeping the hot air circulating out of the package. Their coolers, however, do not need fans to move the package air out.
All our compressors are equipped with totally enclosed, fan-cooled motors, preventing debris and contaminants from entering the unit. The fan helps cool the unit, boosting efficiency and equipment life compared with open drip-proof motors.
A coupling connects the main motor to the airend (although some companies use a gearbox or other device between the motor and the airend to avoid use of couplings).
4. Airend
The airend is where a rotary screw air compressor does the main work of pressurizing the air. As the air enters a compression chamber, the two interlocking helical rotors or screws turn in opposite directions, reducing the space between the rotors and casing and compressing the air.
The rotors never touch and don’t wear, so they last a long time, creating long-term reliability that has become one of the hallmarks of rotary screw compressors. An electric motor typically drives the male rotor, which drives the female rotor.
Oil-lubricated rotary screw air compressors rely on oil to lubricate the bearings, seal off the air flow between the rotors and the stator and remove the heat of compression, keeping the compressor and the air flow cooler, reducing wear and tear and prolonging compressor life.
On the other hand, oil-free machines, like our KROF two-stage, oil-free, rotary screw air compressor do not inject oil in the airend. As such users can apply this technology in sensitive applications—such as food processing, semiconductor and electronics—without introducing oil to the pressurized air flow. More on this topic below.
The rotors of our KROF two-stage, oil-free, rotary screw air compressor are carefully machined to micron precision so that they can rotate without touching. And, unlike an oil-flooded unit, they have a coating that reduces corrosion and wear, prevents friction and seals them.
A compressor can also have several stages. A two-stage compressor like Kaishan’s KRSP2 premium rotary screw air compressor splits the workload, building on the pressure generated in the first stage. As a result, the KRSP2 generates up to 15 to 20% more flow than a single-stage compressor of the same size (HP). Payback on energy costs alone may be less than two to three years.
Kaishan’s KRSP2 two-stage rotary screw air compressor is one of the most energy-efficient machines on the market.
Kaishan’s KRSP2 and KRSP premium rotary screw air compressor are backed by a lifetime warranty on the airend.
As the pressure builds up in the airend, a variety of safety switches and pressure devices monitor pressure to ensure the air is at the correct pressure when it leaves the airend.
5. Sensors, Transmitters, Switches, Valves
Compressors use a variety of sensors, transmitters, pressure switches and valves to monitor and control key operational parameters, including pressures, temperatures and flow:
- A minimum pressure valve ensures that the pressure leaving the compressor meets the system requirements.
- Pressure switches track the differential pressure (or ΔP—a comparison of pressures before and after a device) to tell you if a filter is clogged. Most compressor manufacturers measure ΔP on oil filters. The critical inlet air filter mentioned above has a pressure switch to ensure it is not collapsed.
- Internal safety checks and valves measure temperature or pressure to ensure that your compressor is not over-pressurized, preventing injuries to your staff and damage to your equipment.
An oil-flooded screw compressor is monitored for temperature at two or three locations, while oil-free rotary screw compressors monitor additional values for temperature and pressure to protect the oil-free package.
6. Air-Oil Separator
After leaving the airend, compressed air is drawn into an air-oil separator to remove the oil mist the air injected into the air during the compression process in an oil-flooded rotary screw air compressor. The air-oil separator is a specialized device featuring a mesh filter element in a tank called a sump.
The air enters the separator tank or sump, which forces the air into a circular movement. The swirling action in the tank, combined with the filtration action of the separator element, removes oil mist from the air. The oil that falls to the bottom of the sump then goes back to the main oil tank to start the process over again.
All but a tiny fraction of the oil—3 parts per million—is removed. That’s clean enough for most industrial processes. But, as mentioned, it’s not good enough for sensitive applications, including:
The air-oil separator removes nearly all of the oil from the air compressed by a rotary screw machine. With advanced filtration, this air can support many sensitive applications, including some in food and beverage processing.
In those situations, you may be able to get the air quality you need with additional filtration to remove oil or other particles in the air. Or you may want to go with an oil-free rotary screw air compressor, such as Kaishan’s KROF two-stage, oil-free, rotary screw air compressor.
For more on deciding whether you need an oil-free compressor, see our blog post, “Ultimate Guide to Rotary Screw Air Compressor System Design, Part One: 10 Important Questions To Get You Started.”
And for more on air-oil separators, read our blog post, “Everything You Need to Know About Air/Oil Separators.”
Both the air and the oil leaving the air-oil separator need to be cooled.
7. Oil Cooler
Oil-flooded rotary screw air compressors have an oil lubrication system that seals the compression cycle, lubricates components and removes the heat of compression, lowering your system’s operating temperature. That heat is removed in the oil-cooling cycle.
Oil that is sufficiently cool goes back into the airend. Oil that is too hot goes through an oil cooler and is filtered to remove contaminants that could harm the compressor. It is then returned to the airend where the cycle starts again.
And while it’s true that oil-free units do not rely as much on oil, they still use oil to lubricate components and to lower the operating temperatures in the stage cooling jackets (keeping casting temperatures lower).
The compressor also needs to cool the air.
8. Air Cooler
Heat is also a factor for the air. The ambient air that entered your system at 70°F will reach about 175°F−200°F in an oil-lubricated screw compressor or approximately 350°F in an oil-free rotary screw or single-stage piston compressor, according to the Air Compressor Guide.
Air that hot will damage end-use tools and equipment, degrading lubricants and seal materials. It will also contain moisture that will cause corrosion and scale buildup. It may even result in freezing in colder climates, not to mention the quality problems water vapor-laden compressed air would cause in applications like automotive painting, semiconductor and healthcare and medical device manufacturing.
Cooling and drying equipment conditions the air to a more manageable level. To handle the additional heat from oil-free operation, our KROF has oversized stainless-steel precoolers on each stage, as well as intercoolers and aftercoolers.
The critical consideration for coolers is maintaining filters. Clogged filters will impede and eventually block airflow, causing energy loss that is both unnecessary and avoidable. So you’ll need to replace filter elements regularly.
Expert Help Is Close at Hand
If you need help mastering the components of your compressed air system and seeing how the parts fit together, we can help. Kaishan USA works with a nationwide network of independent distributors, who can provide on-site help and consultation as needed. These factory-trained air compression experts have the skills and expertise you need when you need it.
They can help you optimize the operation of your entire compressed air system with fast response times and personalized support tailored to your needs. Working with these compressed air professionals is the best way we can ensure you have the technical help, reliable service and quick access to parts you need.
They’re our partners and can be yours as well—a partner who cares about your business and wants to see it succeed.
Key Takeaways
The important components of a rotary screw air compressor include the following:
1. An air inlet, the gatekeeper allowing air into the compression chamber
2. A control panel, the interface for the control system
3. Motors, which drive the rotors in the airend
4. An airend, where two helical rotors compress the air
5. Sensors, transmitters, switches and valves, which monitor and control key parameters
6. An air-oil separator, where any oil mist is removed from the compressed air
7. An oil cooler, which eliminates the heat of compression from the oil
8. An air cooler, which helps lower the temperature of the compressed air
Let Us Help
Knowing the components of your rotary screw air compressor is an essential step in understanding how it works. And how it can work better. That, in turn, will benefit the operation of your compressed air system and all the processes that rely on that system. If you need help understanding how your compressor works, get in touch with the experts at Kaishan. Contact us today.
