Six Major System Modules of Screw Air Compressors – Structure of Screw Air Compressors
Oil-injected screw air compressors generally consist of at least the following systems:
Power System
Air End
Air Intake System
Cooling System
Oil-Air Separation System
Control System
Auxiliaries including silencing, vibration damping and ventilation components
1. Power System
The power system of an air compressor comprises the prime mover and transmission device. The prime movers for air compressors are mainly electric motors and diesel engines.
Stationary air compressors are usually driven by electric motors, while mobile units adopt both electric motors and diesel engines.
Electric motors offer absolute advantages in procurement cost, cost-effectiveness and maintenance operation. Therefore, diesel engine drives are only adopted for field construction, mines and other sites without power supply, and are generally used to drive pneumatic tools and equipment.
Screw air compressors feature multiple transmission modes: belt drive, gear drive, direct coupling drive and integrated shaft drive.
Belt drive: Normally applied to small units. It was widely used for units below 90 kW in early years, but nowadays it is mostly adopted only for models under 45 kW.
Advantages: Simple, economical and convenient for modification, with low requirements on manufacturers’ overall machine design and integration capability.
Disadvantages: High noise, belts prone to wear and slipping, lower transmission efficiency compared with other solutions.
Gear drive: Generally used for high-power air compressors (above 110 kW) requiring speed increase or reduction. Similar to belt drive, it is suitable for applications with a specified transmission ratio, where the motor speed differs from the speed of the air end (male rotor). Gears and the air end are usually installed inside one housing.
Direct coupling drive: The motor shaft and air end shaft are connected via a coupling. Flexible couplings are universally adopted in the industry, allowing a certain tolerance for shaft alignment deviation. Multiple types are available, such as jaw couplings and tyre couplings.
The term "1:1 direct drive" is a commercial marketing phrase within the industry, representing a transmission ratio of 1. It means the motor shaft is connected directly to the male rotor shaft of the air end via a coupling. The air end speed equals the motor speed. This description indicates a low-speed, large-size air end instead of a small air end running at excessive speed with short service life, highlighting high cost performance.
However, some units combine gear transmission and direct coupling. Since the gearbox and air end are integrated into one assembly, the unit appears to have direct connection between motor and air end. Such configuration cannot be defined as 1:1 direct drive.
Integrated shaft drive: A transmission solution emerging alongside permanent magnet integrated machines in recent years. The motor shaft and air end shaft share one single shaft; the output shaft of the air end serves directly as the motor rotor, achieving complete direct transmission.
Compared with conventional direct coupling drive, it features a much shorter overall length and facilitates compact layout of the whole unit (eliminating space for center bracket and coupling).
2. Air End
The air end of an oil-injected screw air compressor is the core of the whole unit, consisting of the compression main body and related accessories such as oil stop valves and check valves.
In terms of working principles, screw air ends available on the market fall into two categories: single-stage compression and two-stage compression. Various innovatively integrated models such as integrated machines do not change the fundamental operating principle.
The most prominent structural difference between single-stage and two-stage compression:
A single-stage compression air end contains only one pair of male and female rotors, whereas a two-stage compression air end has two pairs of male and female rotors.
Principle difference:
Single-stage compression completes the whole gas compression process from suction to discharge via one pair of rotors.
For two-stage compression, gas is compressed by the first-stage air end, cooled, and then delivered to the second-stage air end for further compression.
Currently, two mainstream structures are adopted for two-stage air ends on the market: vertical stacked layout and front-rear layout. The vertical stacked design is most common. The so-called split structure equipped with two permanent magnet motors driving two separate air ends still requires further market verification.
3. Air Intake System
The air intake system of an air compressor mainly refers to components for atmospheric air suction and associated control parts. It generally consists of an air filter assembly and an intake valve assembly.
The air filter assembly has a relatively simple structure, composed of an air filter and intake pipelines (both flexible and rigid pipes). Some small units omit intake pipelines; even air filter housings can be removed, with the air filter cartridge mounted directly onto the intake valve. Larger flow-rate units may adopt multiple air filters to achieve better filtration performance and reduce suction resistance.
As the name suggests, the air filter cleans air drawn into the compressor, mainly removing solid particles such as dust and debris.
The air filter cartridge is a consumable part. The well-known maintenance term "three filters and one oil" for screw air compressors refers to air filter cartridge, oil filter cartridge, oil separator cartridge and compressor lubricating oil.
The intake valve assembly is installed on the air inlet port of the compressor air end. It controls the intake air volume to realize different operating modes including loading, unloading and proportional regulation of the air compressor.
An intake valve assembly usually includes a valve body, actuators such as air cylinders, and control components such as solenoid valves. Two mainstream types are widely used according to valve body structure: piston type and butterfly type.
Piston intake valve: Controls air intake through reciprocating movement of the internal piston. It normally only has two states: fully open or fully closed, and is used for low-flow applications.
Butterfly intake valve: The main component is a butterfly valve with a valve plate as the moving part. Driven by the push rod of a servo cylinder, the valve plate adjusts opening and opening degree to control inlet flow. It is commonly applied to large-flow units.
4. Cooling System
Air compressors adopt two cooling methods: air cooling and water cooling. Neither is universally superior; suitability depends on application conditions.
Water-cooled units require the customer to have a circulating cooling water system. Air-cooled units offer more flexible layout but demand good ventilation at the installation site.
The cooling effect of water-cooled compressors fully depends on the cooling water system. Stable and reliable cooling performance can be achieved if the customer’s water system operates normally. Air-cooled performance is greatly affected by ambient temperature and installation environment. Nevertheless, manufacturers take extreme working conditions into consideration during design, so satisfactory cooling performance can be obtained with standard installation.
Water-cooled compressors do not require additional electric equipment for cooling, consuming no extra power from the compressor itself (power consumption for the customer’s circulating cooling water system is excluded). Air-cooled compressors are equipped with cooling fans, which consume power from the unit.
Water cooling features high heat exchange efficiency and compact structure. In addition, air-cooled units generate obvious fan noise during operation, so water-cooled machines deliver lower overall noise levels.
Two media need cooling inside the air compressor: compressed air and cooling oil (also known as compressor oil or lubricant; all terms refer to the same medium). Lubricating oil is the most critical factor for continuous and stable unit operation.
Compressor oil circulates continuously inside the unit acting as a heat transfer medium. It carries heat generated by air compression inside the air end and other waste heat to the cooler, where heat is removed by cooling medium (air or water).
In engineering practice, almost all air-cooled screw compressors adopt plate-fin heat exchangers, integrating air cooling and oil cooling into one unit. One section cools compressed air, and the other cools lubricating oil. Water-cooled compressors mostly adopt shell-and-tube heat exchangers, with separate oil coolers and air coolers.
Air-cooled plate-fin heat exchangers are manufactured by brazing aluminum or aluminum alloy. High-temperature oil and compressed air flow inside the exchanger, and heat is dissipated by forced convection via cooling fans.
For shell-and-tube water coolers, hot media are high-temperature oil and compressed air, and cold medium is circulating cooling water. Heat is transferred and carried away by cooling water through heat exchange.
5. Oil-Air Separation System
Oil is injected into the compression chamber during operation of oil-injected screw air compressors. Therefore, a dedicated system is required to separate oil from compressed air, retaining oil inside the unit for cyclic reuse and discharging clean compressed air. This is the function of the oil-air separation system.
The wide application of oil-injected screw compressors is enabled by the acceptable separation efficiency achieved by oil-air separation systems.
The oil-air separation system of an air compressor mainly consists of an oil storage tank and oil-air separators.
Working process:
The oil-air mixture discharged from the air end enters the oil tank. Most oil droplets aggregate at the tank bottom through airflow collision and gravity sedimentation, and then flow into the oil cooler for cooling. Compressed air carrying trace lubricating oil passes through the oil separator cartridge to recover residual oil sufficiently. The recovered oil flows back to the low-pressure area of the air end via a throttling check valve.
6. Control System
The control system of an air compressor includes logic controllers, various sensors, electrical control assemblies and other control components. Figuratively speaking, the control system serves as the brain, nerve network and driving muscles of the compressor. An air compressor without a control system is merely a collection of spare parts.
Two main types of controllers are adopted for screw compressors (pure instrument control is nearly obsolete): single-chip microcomputers and PLCs.
Single-chip controllers are specially manufactured by dedicated compressor controller suppliers to satisfy general compressor control requirements, with non-modifiable logic programs.
PLC (Programmable Logic Controller) is a universal industrial controller. Custom programming is required to realize compressor control functions and meet unique control demands for proprietary products (for compressor OEMs).
Pressure sensors and temperature sensors are the main sensors used on air compressors; differential pressure switches are also installed at several positions.
Motor electrical control constitutes the core part of compressor control. Simply put, the whole unit is controlled by switching the motor on and off. Related components include AC contactors, circuit breakers, current transformers and relays. The electrical assembly for the main motor is generally called the starter panel.
Variable frequency technology is increasingly widely adopted in the industry. Apart from motor start-stop control, frequency regulation realizes another core function: motor speed control. This function is implemented by frequency converters or embedded frequency conversion modules. For most variable-frequency air compressors, the frequency converter also controls motor startup and shutdown, except for dual-mode fixed/frequency conversion units.
Beyond electrical control, air compressors also contain multiple mechanical control components, such as thermostatic valves and minimum pressure valves.
Power System
Air End
Air Intake System
Cooling System
Oil-Air Separation System
Control System
Auxiliaries including silencing, vibration damping and ventilation components
1. Power System
The power system of an air compressor comprises the prime mover and transmission device. The prime movers for air compressors are mainly electric motors and diesel engines.
Stationary air compressors are usually driven by electric motors, while mobile units adopt both electric motors and diesel engines.
Electric motors offer absolute advantages in procurement cost, cost-effectiveness and maintenance operation. Therefore, diesel engine drives are only adopted for field construction, mines and other sites without power supply, and are generally used to drive pneumatic tools and equipment.
Screw air compressors feature multiple transmission modes: belt drive, gear drive, direct coupling drive and integrated shaft drive.
Belt drive: Normally applied to small units. It was widely used for units below 90 kW in early years, but nowadays it is mostly adopted only for models under 45 kW.
Advantages: Simple, economical and convenient for modification, with low requirements on manufacturers’ overall machine design and integration capability.
Disadvantages: High noise, belts prone to wear and slipping, lower transmission efficiency compared with other solutions.
Gear drive: Generally used for high-power air compressors (above 110 kW) requiring speed increase or reduction. Similar to belt drive, it is suitable for applications with a specified transmission ratio, where the motor speed differs from the speed of the air end (male rotor). Gears and the air end are usually installed inside one housing.
Direct coupling drive: The motor shaft and air end shaft are connected via a coupling. Flexible couplings are universally adopted in the industry, allowing a certain tolerance for shaft alignment deviation. Multiple types are available, such as jaw couplings and tyre couplings.
The term "1:1 direct drive" is a commercial marketing phrase within the industry, representing a transmission ratio of 1. It means the motor shaft is connected directly to the male rotor shaft of the air end via a coupling. The air end speed equals the motor speed. This description indicates a low-speed, large-size air end instead of a small air end running at excessive speed with short service life, highlighting high cost performance.
However, some units combine gear transmission and direct coupling. Since the gearbox and air end are integrated into one assembly, the unit appears to have direct connection between motor and air end. Such configuration cannot be defined as 1:1 direct drive.
Integrated shaft drive: A transmission solution emerging alongside permanent magnet integrated machines in recent years. The motor shaft and air end shaft share one single shaft; the output shaft of the air end serves directly as the motor rotor, achieving complete direct transmission.
Compared with conventional direct coupling drive, it features a much shorter overall length and facilitates compact layout of the whole unit (eliminating space for center bracket and coupling).
2. Air End
The air end of an oil-injected screw air compressor is the core of the whole unit, consisting of the compression main body and related accessories such as oil stop valves and check valves.
In terms of working principles, screw air ends available on the market fall into two categories: single-stage compression and two-stage compression. Various innovatively integrated models such as integrated machines do not change the fundamental operating principle.
The most prominent structural difference between single-stage and two-stage compression:
A single-stage compression air end contains only one pair of male and female rotors, whereas a two-stage compression air end has two pairs of male and female rotors.
Principle difference:
Single-stage compression completes the whole gas compression process from suction to discharge via one pair of rotors.
For two-stage compression, gas is compressed by the first-stage air end, cooled, and then delivered to the second-stage air end for further compression.
Currently, two mainstream structures are adopted for two-stage air ends on the market: vertical stacked layout and front-rear layout. The vertical stacked design is most common. The so-called split structure equipped with two permanent magnet motors driving two separate air ends still requires further market verification.
3. Air Intake System
The air intake system of an air compressor mainly refers to components for atmospheric air suction and associated control parts. It generally consists of an air filter assembly and an intake valve assembly.
The air filter assembly has a relatively simple structure, composed of an air filter and intake pipelines (both flexible and rigid pipes). Some small units omit intake pipelines; even air filter housings can be removed, with the air filter cartridge mounted directly onto the intake valve. Larger flow-rate units may adopt multiple air filters to achieve better filtration performance and reduce suction resistance.
As the name suggests, the air filter cleans air drawn into the compressor, mainly removing solid particles such as dust and debris.
The air filter cartridge is a consumable part. The well-known maintenance term "three filters and one oil" for screw air compressors refers to air filter cartridge, oil filter cartridge, oil separator cartridge and compressor lubricating oil.
The intake valve assembly is installed on the air inlet port of the compressor air end. It controls the intake air volume to realize different operating modes including loading, unloading and proportional regulation of the air compressor.
An intake valve assembly usually includes a valve body, actuators such as air cylinders, and control components such as solenoid valves. Two mainstream types are widely used according to valve body structure: piston type and butterfly type.
Piston intake valve: Controls air intake through reciprocating movement of the internal piston. It normally only has two states: fully open or fully closed, and is used for low-flow applications.
Butterfly intake valve: The main component is a butterfly valve with a valve plate as the moving part. Driven by the push rod of a servo cylinder, the valve plate adjusts opening and opening degree to control inlet flow. It is commonly applied to large-flow units.
4. Cooling System
Air compressors adopt two cooling methods: air cooling and water cooling. Neither is universally superior; suitability depends on application conditions.
Water-cooled units require the customer to have a circulating cooling water system. Air-cooled units offer more flexible layout but demand good ventilation at the installation site.
The cooling effect of water-cooled compressors fully depends on the cooling water system. Stable and reliable cooling performance can be achieved if the customer’s water system operates normally. Air-cooled performance is greatly affected by ambient temperature and installation environment. Nevertheless, manufacturers take extreme working conditions into consideration during design, so satisfactory cooling performance can be obtained with standard installation.
Water-cooled compressors do not require additional electric equipment for cooling, consuming no extra power from the compressor itself (power consumption for the customer’s circulating cooling water system is excluded). Air-cooled compressors are equipped with cooling fans, which consume power from the unit.
Water cooling features high heat exchange efficiency and compact structure. In addition, air-cooled units generate obvious fan noise during operation, so water-cooled machines deliver lower overall noise levels.
Two media need cooling inside the air compressor: compressed air and cooling oil (also known as compressor oil or lubricant; all terms refer to the same medium). Lubricating oil is the most critical factor for continuous and stable unit operation.
Compressor oil circulates continuously inside the unit acting as a heat transfer medium. It carries heat generated by air compression inside the air end and other waste heat to the cooler, where heat is removed by cooling medium (air or water).
In engineering practice, almost all air-cooled screw compressors adopt plate-fin heat exchangers, integrating air cooling and oil cooling into one unit. One section cools compressed air, and the other cools lubricating oil. Water-cooled compressors mostly adopt shell-and-tube heat exchangers, with separate oil coolers and air coolers.
Air-cooled plate-fin heat exchangers are manufactured by brazing aluminum or aluminum alloy. High-temperature oil and compressed air flow inside the exchanger, and heat is dissipated by forced convection via cooling fans.
For shell-and-tube water coolers, hot media are high-temperature oil and compressed air, and cold medium is circulating cooling water. Heat is transferred and carried away by cooling water through heat exchange.
5. Oil-Air Separation System
Oil is injected into the compression chamber during operation of oil-injected screw air compressors. Therefore, a dedicated system is required to separate oil from compressed air, retaining oil inside the unit for cyclic reuse and discharging clean compressed air. This is the function of the oil-air separation system.
The wide application of oil-injected screw compressors is enabled by the acceptable separation efficiency achieved by oil-air separation systems.
The oil-air separation system of an air compressor mainly consists of an oil storage tank and oil-air separators.
Working process:
The oil-air mixture discharged from the air end enters the oil tank. Most oil droplets aggregate at the tank bottom through airflow collision and gravity sedimentation, and then flow into the oil cooler for cooling. Compressed air carrying trace lubricating oil passes through the oil separator cartridge to recover residual oil sufficiently. The recovered oil flows back to the low-pressure area of the air end via a throttling check valve.
6. Control System
The control system of an air compressor includes logic controllers, various sensors, electrical control assemblies and other control components. Figuratively speaking, the control system serves as the brain, nerve network and driving muscles of the compressor. An air compressor without a control system is merely a collection of spare parts.
Two main types of controllers are adopted for screw compressors (pure instrument control is nearly obsolete): single-chip microcomputers and PLCs.
Single-chip controllers are specially manufactured by dedicated compressor controller suppliers to satisfy general compressor control requirements, with non-modifiable logic programs.
PLC (Programmable Logic Controller) is a universal industrial controller. Custom programming is required to realize compressor control functions and meet unique control demands for proprietary products (for compressor OEMs).
Pressure sensors and temperature sensors are the main sensors used on air compressors; differential pressure switches are also installed at several positions.
Motor electrical control constitutes the core part of compressor control. Simply put, the whole unit is controlled by switching the motor on and off. Related components include AC contactors, circuit breakers, current transformers and relays. The electrical assembly for the main motor is generally called the starter panel.
Variable frequency technology is increasingly widely adopted in the industry. Apart from motor start-stop control, frequency regulation realizes another core function: motor speed control. This function is implemented by frequency converters or embedded frequency conversion modules. For most variable-frequency air compressors, the frequency converter also controls motor startup and shutdown, except for dual-mode fixed/frequency conversion units.
Beyond electrical control, air compressors also contain multiple mechanical control components, such as thermostatic valves and minimum pressure valves.
