OVERVIEW OF AIR COMPRESSORS: CONCEPT, CLASSIFICATION, OPERATING PRINCIPLE, AND POPULAR BRANDS

I. Concept of Air Compressors

An air compressor is a vital piece of equipment in compressed air supply systems. Its main function is to convert the mechanical energy generated by an engine (typically an electric motor) into compressed air energy. It is a device that creates air pressure by compressing air and supplying it to various industrial systems and other applications.
Air compressors belong to the category of power machinery, capable of reducing the volume of air while simultaneously increasing its pressure. The kinetic energy stored in compressed air can be utilized for mechanical drive systems or adapted for other purposes depending on the requirements. Understanding their operating principle is essential for selecting the right type of compressor for specific industrial or commercial needs.

II. Classification of Air Compressors

Classification by structure:

• Positive displacement compressors: These compressors reduce the volume of air while increasing its pressure.
• Piston compressors: In this type, the working volume changes cyclically, but the spatial position remains constant.
• Rotary compressors: The working volume changes cyclically, and the spatial position also changes.
• Dynamic compressors: These increase the molecular velocity of the air, converting the kinetic energy of air molecules into air pressure energy.
• Centrifugal compressors: A type of high-speed compressor that uses one or more rotating impellers to accelerate the air, causing it to move in a radial direction.

Classification by compressed air pressure:

• Blowers: 0.01 – 0.1 MPa
• Low pressure: 0.2 – 1.0 MPa
• Medium pressure: 1 – 10 MPa
• High pressure: 10 – 100 MPa
• Ultra-high pressure: > 100 MPa

Classification by compressed air flow rate:

• Micro air compressors: < 1 m³/min
• Small air compressors: 1 – 10 m³/min
• Medium air compressors: 10 – 100 m³/min
• Large air compressors: > 100 m³/min

Classification by Cooling Method:

• Air-cooled: Uses air as the cooling medium, with an automatic fan that cools the air within the system.
• Water-cooled: Uses water as the cooling medium, typically combined with a cooling tower or a water circulation system.

Classification by Lubrication Method:

• Oil-lubricated: Includes oil-injected compressors or oil-limited compressors.
• Oil-free lubrication:
  • Water-lubricated
  • Dry screw type
  • Oil-free piston
  • Oil-free centrifugal

III. Operating Principle of Air Compressors

1. Piston Air Compressor

When the piston moves from the top dead center (TDC) to the bottom dead center (BDC), the volume inside the cylinder increases and the pressure inside the cylinder decreases. Once the internal pressure is lower than the external atmospheric pressure, outside air—driven by this pressure difference—overcomes the spring force and pushes the intake valve open, allowing air to enter the cylinder (at this point, the exhaust valve remains closed).
When the piston reaches the BDC, the cylinder is fully charged with air, and the internal pressure is equal to the atmospheric pressure. Since the pressure inside and outside the cylinder has equalized, the spring pushes the intake valve closed. This marks the beginning of the operating cycle.

As the piston then moves from BDC back to TDC, both the intake and exhaust valves remain closed. The air trapped inside the cylinder starts to get compressed. The higher the piston moves, the smaller the cylinder volume becomes, and the pressure of the compressed air increases. This is the compression process—one of the key stages in the operating principle of piston air compressors.

When the pressure of the compressed air exceeds the total of the spring tension of the exhaust valve plus the pressure in the discharge pipe, the exhaust valve is pushed open and the compressed air is released through the exhaust pipe. This discharge continues until the piston reaches TDC again. At this point, most of the air in the cylinder has been discharged, the pressure drops sharply, and the exhaust valve closes due to the spring force. This is the discharge process.

When the piston once again moves from TDC down to BDC, fresh air is drawn into the cylinder, and a new intake cycle begins. The piston air compressor continuously repeats this intake–compression–discharge cycle to perform the function of compressing air. This repetitive operating sequence defines the mechanical rhythm of piston-type compressors.

Note: In today’s low-pressure air compressor market, piston-type compressors are gradually being replaced by screw-type compressors. Therefore, it is sufficient for us to focus on understanding the working principle of the piston compressor.

2. Screw-type Air Compressor

Screw-type compressors are divided into two main categories: twin-screw compressors and single-screw compressors.

2.1) Working Principle and Introduction to Twin-Screw Air Compressors:

Twin-screw air compressors consist of a pair of intermeshing rotors (commonly referred to as the male and female rotors or screws). These two screws rotate in parallel within a cylinder chamber, cyclically changing the volume between the screw grooves. Air enters from the intake port, is drawn into the chambers between the screw teeth and the cylinder casing, and is then transported along the screw axis toward the discharge port. During this process, air is continuously sucked in, compressed, and expelled. The intake and discharge ports are located at either end of the compression unit. The male and female rotors are driven by the main motor, ensuring a continuous process of intake–compression–discharge.

2.2) Working Principle and Introduction to Single-Screw Air Compressors:

Single-screw air compressors are characterized by the use of only one screw. However, in practice, the mechanism contains three rotating shafts: one main screw and two planetary gears positioned perpendicularly to the screw. Although classified in the same family as twin-screw compressors and sharing some similar advantages, single-screw compressors have not been widely adopted in industrial applications due to several unresolved technical challenges:

• Numerous moving components: Single-screw compressors involve three rotating elements. The significant stiffness disparity between the screw and planetary gears often leads to uneven deformation during operation, resulting in poor engagement accuracy. This causes a low volumetric efficiency. Moreover, in currently available single-screw compressor models, planetary gears are mostly made from non-metallic materials, which have low wear resistance.When the compressor is operating at high speeds, heavy wear leads to greater internal leakage, and a noticeable reduction in airflow capacity typically occurs after just 3,000 to 4,000 hours of operation (with an average flow decrease of 5–10%). The uneven deformation also undermines the mechanical stability of the entire machine, increasing failure rates and maintenance frequency—significantly limiting its industrial application potential.

• Planetary gear material needs improvement: The planetary gear is one of the core components in a single-screw air compressor, primarily serving the sealing function. If steel is used as the gear material, its high thermal expansion coefficient necessitates a large clearance between the gear and the screw, leading to gas leakage, low efficiency, and a higher risk of jamming, which could cause serious failures. On the other hand, if composite materials are used to address this issue, problems arise in terms of low strength and poor wear resistance. During operating conditions, under cutting forces and friction, these materials wear quickly, resulting in internal leaks, reduced efficiency, more frequent maintenance, and higher repair costs. Therefore, developing a material with high strength, low expansion, and good wear resistance remains a key challenge. However, given the current state of materials science, a comprehensive solution is unlikely in the short term.

• Screw profile needs optimization: Due to the lack of satisfactory solutions for the aforementioned problems, the development of single-screw compressors remains limited. Research institutions and major manufacturers have not invested significantly in improving screw profile designs, resulting in few breakthroughs in this area. Discovering an optimal screw profile is a prerequisite for large-scale commercialization of the product. However, because of the limited market potential, major brands show little interest in investing, making significant short-term improvements unlikely.

In conclusion, from both technical and practical perspectives, twin-screw air compressors currently represent a mature and advanced technology that has been widely proven in terms of stability and performance. Compared to their single-screw counterparts, twin-screw compressors offer a more refined operating principle and greater reliability, making them the preferred choice in most industrial settings discussed in the general overview of air compressors.

2.3) Centrifugal Air Compressors

Centrifugal air compressors use an impeller with rotating blades mounted on a shaft. As air enters the impeller, the spinning blades impart kinetic energy to the air, increasing its velocity and partially its pressure. The air then enters a diffuser, where the velocity decreases and the kinetic energy is converted into pressure. The compressed air continues through ducts and a return channel to the next stage (if any), and the compression process continues until the desired pressure is achieved.

2.4) Scroll Air Compressors

Scroll air compressors consist of two spiral-shaped scroll disks—one fixed and one orbiting—designed based on double-curve equations. The fixed scroll is mounted to the machine frame, while the orbiting scroll is driven by an eccentric shaft and is constrained by an anti-rotation mechanism that allows only orbital motion around the center of the fixed scroll. Air enters from the outer edge of the fixed scroll through a filter and becomes trapped in the crescent-shaped chambers between the scrolls. As the orbiting scroll moves, the trapped air is gradually compressed toward the center and finally discharged through a central port in the fixed scroll. The entire intake–compression–discharge cycle occurs smoothly and continuously.

2.5) Sliding Vane Air Compressors

Sliding vane air compressors have a rotor mounted eccentrically within a cylindrical chamber. The rotor has 4 to 6 vanes that slide radially in and out of slots. Springs beneath the vanes ensure constant contact with the chamber walls. As the rotor spins, the volume between the vanes changes, drawing in, compressing, and discharging air in a continuous manner. This mechanism is simple, highly efficient, and operates with low noise, making it suitable for many small to medium compressed air applications.

IV. Introduction to Oil-Free and Oil-Injected Screw Air Compressors

1. Introduction to Oil-Free Screw Air Compressors

2. Internal Structure of the Screw Element in Oil-Free Compressors

3. Schematic Diagram of an Oil-Free Screw Air Compressor

1- Air intake port
2- Sound-absorbing chamber
3- Air filter
4- Low-pressure compression screw
5- Intercooler
6- Water separator and drain pipe
7- High-pressure compression screw
8- Silencer
9- Check valve
10- Aftercooler
11- Compressed air outlet
12- Drain valve and silencer
13- Water inlet
14- Water outlet
15- Oil cooler
16- Oil filter
17- Oil bypass valve
18- Oil pump

4. Operating Principle of Oil-Injected Screw Air Compressors

V. Key Technical Parameters of Air Compressors

1. Discharge Pressure

Definition — The pressure of the air finally discharged from the compressor. For piston compressors, this pressure is measured at the air storage tank. For screw compressors, it is measured at the oil-air separator.
Rated discharge pressure — This is the design pressure under standard operating conditions and is typically indicated on the compressor’s nameplate.
Compressors can operate at any pressure below the rated pressure. If the compressor’s structure can tolerate the associated forces and exhaust temperatures, it may also operate temporarily above the rated pressure.

Common units of measurement: bar or MPa

2. Volumetric Flow Rate (Air Discharge Flow Rate)

Definition — The volume of air discharged from the final compression stage of the compressor per unit of time. This value is measured after the aftercooler, check valve, and moisture separator, and then converted to the standard pressure and temperature at the inlet of the first compression stage (i.e., initial intake conditions).
Formula:
Volumetric flow rate (discharge flow) = Air intake volume – Leakage volume

Units of measurement:

• m³/min (cubic meters per minute)
• L/min (liters per minute)
• CFM (cubic feet per minute) 
  

Conversion:
1 m³ = 1000 L = 35.315 CFM

In China, the common unit used is m³/min (standard air flow), also referred to as: Nm³/min.
Volumetric flow rate is often referred to as discharge flow rate or nameplate flow rate.

3. Discharge Air Temperature

Definition — The temperature of the compressed air after each stage of compression. Typically, this is measured at the compressed air outlet pipe.
Units of measurement:

• °C (Celsius)
  
• °F (Fahrenheit)
• K (Kelvin – absolute temperature)

4. Pressure Dew Point

When air is cooled to the point where the partial pressure of water vapor equals the saturation pressure, the corresponding temperature is called the dew point temperature, indicating 100% relative humidity.

Pressure dew point is the dew point temperature measured at the working pressure of compressed air.
Atmospheric dew point is the temperature at which moisture in the air begins to condense under normal atmospheric pressure.

Any further cooling beyond the dew point will cause water vapor to condense into liquid, forming condensate.

5. Oil Content in Discharged Air

Unit of measurement: PPM (mg/m³)
This index represents the mass of oil (including oil droplets, aerosols, and vapor) present in each unit volume of compressed air, converted to standard conditions:

• Absolute pressure: 0.1 MPa
• Temperature: 20°C
• Relative humidity: 65%

Typically, PPM refers to weight ratio.
For example: 1 PPM by weight = 1 part per million of 1 kg = 1 mg.

Standard conversion:
1 PPMw = 1.2 mg/m³ (under conditions: P = 0.1 MPa, t = 20°C, φ = 65%)

VI. Popular Air Compressor Brands

International Brands:

• Atlas Copco (Sweden)
• Ingersoll-Rand (USA)
• Compair/Demag (Germany)
• Sullair (USA)
• Fusheng (Taiwan)
• Kaeser (Germany)
• Quincy (USA)
• Boge (Germany)
• ZSCREW (Japan)
• Kobelco (Japan)
• Hitachi (Japan)

Domestic Brands (China):

• Bolite (Shanghai)
• Fuda (Liuzhou)
• Unites (Shanghai)
• Weiken (Shanghai)
• Skerlo (Shanghai)
• Langchao (Shanghai)
  
• Risheng-Schneider (Shanghai)
  
• Dongfang (Shanghai)

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