1. Electric Motor
Air compressors only use two types of asynchronous motors: 2-pole and 4-pole.
The synchronous speed of a 2-pole motor is 3000 rpm; for asynchronous motors used in air compressors, the 2-pole speed reaches ≥ 3000 × 97% = 2910 rpm, and it is usually presented to customers as 2960 rpm, with a 3% deviation.
The synchronous speed of a 4-pole motor is 1500 rpm; for asynchronous motors used in air compressors, the 4-pole speed reaches ≥ 1500 × 97% = 1455 rpm, and it is usually presented to customers as 1480 rpm, also with a 3% deviation.
The motors used in air compressors are only 2-pole and 4-pole types, and their speeds comply with national standards and can be considered constant (1480 rpm and 2960 rpm).
Service Factor: In the air compressor industry, motors are often non-standard, with a tendency toward “small machines labeled with large power ratings.” The typical service factor ranges from 1.1 to 1.2. For example, a 200kW air compressor with a service factor of 1.15 can actually reach a maximum power of 200 × 1.15 = 230kW. When explaining this to customers, it can be stated that an extra 15% of power has been reserved – this is a way to “add value.”
Conventional motors tend to be the opposite – “large machines labeled with small power ratings.” For instance, a motor labeled as 100kW may only deliver 80% of that power, which is already considered good. The usual explanation is a power factor cos∮=0.8 – which can be seen as material cutting.
Protection Class: This is the standard for a motor’s resistance to water and dust. IP23 is sufficient, but in the air compressor industry, 380V motors are basically all IP55 or IP54. As for 6KV and 10KV motors, IP23 is mainly used, though some customers may require IP55 or IP54.
Insulation Class: This refers to the motor’s heat resistance and fire resistance. Class F is commonly used, with temperature rise tests required according to Class B standards – that is, one level stricter than Class F.
Starting Method: The star-delta starting method is used.
Motors are divided into conventional types and inverter-compatible types. Variable frequency air compressors use inverter motors, which adjust the motor speed by changing the electrical frequency, thereby regulating the air output of the compressor.
2. Structure and Principle of the Screw Air Compressor
An air compressor is a type of mechanical equipment used to increase the pressure of air. In a screw air compressor, the most important component is the main compressor unit, commonly referred to as the compression element or compressor head. The technological heart of the compressor head is the pair of screw rotors, consisting of a female rotor and a male rotor. Among them, the male rotor is larger in size, while the female rotor is smaller.
The main structure of the compressor head includes the screw rotors, the casing (also known as the cylinder), bearings, and shaft seals. Specifically, the two screw rotors – a pair of male and female rotors – are supported by bearings at both ends and are fixed inside the casing. During operation, air is drawn in from one end, then through the mutual rotary motion of the two screw rotors, combined with the meshing of the tooth crests and grooves, the air is compressed, pressure increases, and the air is pushed out at the other end.
During the air compression process, the compressor head requires an input power source and must also have a cooling, sealing, and lubrication system. Therefore, the compressor head can only operate normally when it is integrated into a complete air compressor system.
Screw air compressors are classified as high-tech products mainly due to this compressor head.
The reason why the compressor head is considered a high-tech product stems from two main factors:
- The machining precision is extremely high, which cannot be achieved by conventional equipment.
- The shape of the screw rotor is a curved surface in three-dimensional space, and the design of its contour (called the profile) is currently a proprietary technology held by only a very few foreign companies.
In terms of structure, there is no direct contact between the male and female rotors; the gap between them is only about 2–3 strands of hair (i.e., a few tenths of a millimeter). The clearance between the rotors and the casing is also similar – they do not touch, and there is no friction. The front faces of the rotors and the casing also maintain such a small gap. Because there is no contact or friction, the service life of the compressor head mainly depends on the bearings and the shaft seals.
The lifespan – or in other words, the replacement cycle – of the bearings and shaft seals is closely related to the rotational speed and the forces acting on them. Therefore, direct-drive compressor heads, with low rotation speeds and bearings not subjected to extra forces, will have the longest service life. Conversely, in compressors using belts, the compressor head usually has a high rotation speed, and the bearings must withstand greater forces, leading to shorter service life.
Installing bearings in the compressor head is an extremely sophisticated technical process, carried out in a specialized room with stable temperature and humidity conditions, using specialized tools. If the bearings are damaged – especially for large compressor heads – they must be sent back to a specialized repair factory for replacement.
The transport and repair process takes a lot of time and causes many inconveniences for users. Once the air compressor stops working, the entire production line must also halt, workers are suspended, and the loss can reach tens of thousands of yuan per day.
Therefore, from a sense of responsibility toward the user, we always prioritize recommending the use of direct-drive air compressors that do not use gearboxes and operate at low rotation speeds.
3. Structure and Principle of the Oil Separator
The oil separator tank is a container used to perform phase separation between the air-oil mixture. Typically, the oil separator tank has a cylindrical shape and is welded from 45# carbon steel to form a shell made of steel plates. The use of stainless steel to manufacture the oil separator tank is very rare and generally unnecessary under normal conditions. In addition to its oil separation function, the tank also serves to store lubricating oil.
Inside the tank, there is an oil separator element—commonly referred to as the oil separator core—which is usually made from about 23 layers of imported fiberglass. Some cost-saving manufacturers may use only around 18 layers, which negatively affects oil separation efficiency.
The working principle of the oil separator core is that when the air-oil mixture passes through the fiberglass layers at a certain speed, the small oil droplets are captured by the physical filtering layer, gradually coalescing into larger droplets.
Due to gravity, these droplets fall to the bottom of the filter core. From there, through the oil return pipe, this oil is directed back into the compression head to continue participating in the next compression cycle. There is a check valve on the oil return pipe, which ensures that the oil can only flow from the bottom of the separator core into the compression chamber.
In practice, before the air-oil mixture even passes through the filter core, about 99% of the oil has already been separated and settled at the bottom of the oil separator tank.
The high-temperature, high-pressure (around 80℃) air-oil mixture exiting the compression head enters the oil separator tank tangentially to the inner wall of the tank. Under the effect of centrifugal force, most of the oil in the mixture is thrown against the inner wall of the tank and then flows downward to the bottom of the tank by gravity. A small portion of the remaining oil droplets continue to collide, coalesce into larger droplets, and also fall to the bottom of the tank for reuse in the next cycle.
The air, after being filtered through the separator core, passes through a minimum pressure valve (referred to as the minimum pressure valve) and is then directed to the aftercooler to reduce its temperature before being discharged through the compressor system.
The minimum pressure valve is also a type of check valve, also known as the air discharge valve on the oil separator tank. It is installed at the outlet of the oil separator, on the upper part of the tank. The valve opening pressure is usually set to around 0.45 MPa, and it serves three main functions:
(1) When starting the machine, the valve first establishes the necessary oil circulation pressure to ensure the system is fully lubricated from the beginning.
(2) The valve only opens when the air pressure inside the tank exceeds 0.45 MPa. This helps reduce the airflow speed through the oil separator core, thereby not only improving the oil separation efficiency but also protecting the core from damage caused by excessive pressure differences.
(3) Check function: When the air and oil pressure in the tank decreases after the machine stops, it prevents compressed air in the pipeline from flowing back.
The end cap of the oil separator tank is also equipped with a safety valve. When the internal pressure exceeds 1.1 times the set pressure, the safety valve automatically opens to release excess air, preventing the pressure from rising too high and causing danger. The way to check the safety valve is to lightly pull the valve’s release lever while the compressor is running under full load; if air is released, the valve is working properly.
There is also a pressure gauge on the oil separator tank, used to measure the pressure before the separator core. At the bottom of the tank, there is a drain valve that must be opened periodically to discharge water and impurities accumulated during operation.
Next to the oil separator tank, there is a transparent oil sight glass for observing the oil level inside. The correct oil level is when the compressor is running normally, and the oil lies between the high and low marks of the sight glass. If the oil level is too high, it will cause oil overflow; if it is too low, it will affect the overall safety of the equipment.
The oil separator tank is a pressure vessel and must be manufactured by factories certified with the appropriate production qualifications. Each oil separator tank must be accompanied by a quality certificate.
4. Cooling System
In air-cooled screw air compressors, the oil cooler and the aftercooler for compressed air are typically designed as a single integrated unit. These are made in a plate-fin structure using aluminum, sealed through vacuum brazing technology. Once an oil leakage occurs, repairs are nearly impossible—it usually requires complete replacement.
Operating principle: Hot oil and high-temperature compressed air (around 80℃) flow separately through internal pipelines within the cooler. A fan, driven by a motor, blows cool air perpendicularly across the tubes carrying the hot oil and air, thereby enabling heat exchange. If you touch the air stream exiting the cooler, you’ll clearly feel a burning heat.
For water-cooled screw compressors, tube-type coolers are generally used. In these systems, water flows inside the tubes while hot oil flows outside. Through heat exchange, the cold water is heated up. However, to reduce cost, many manufacturers replace copper tubes with steel ones.
Water-cooled compressors require the addition of a cooling tower to cool down the heated water for reuse in the next cycle. Water quality has a major impact on the cooling performance, and the installation of a cooling tower significantly increases capital investment.
As a result, water-cooled units are less commonly used than air-cooled ones. That said, in dusty environments—such as cement plants or powder coating workshops prone to flammable dust—water-cooled compressors are strongly recommended.
Once scaling occurs inside the water cooler, cleaning becomes extremely difficult. It typically requires chemical soaking for several days, followed by flushing with high-pressure water or air. The amount of cooling water required for a water-cooled compressor is clearly defined in the JB/T6430-2002 standard. On average, compressing one cubic meter of air consumes approximately 4 liters of water.
For air-cooled radiators, the hot air discharged after cooling must be effectively exhausted via air ducts or other proper heat dissipation methods, such as installing industrial fans to draw the hot air out of the compressor room.
If the hot air is not properly exhausted and instead circulates back into the compressor’s air intake, the consequences can be severe: the machine may shut down due to overheating.
After passing through the aftercooler, the compressed air still contains a significant amount of water vapor, and most of this moisture can be separated out in the air receiver tank.
Compressed air outlet temperature:
- For water-cooled machines, the air temperature is typically about 10℃ higher than the ambient temperature.
- For air-cooled machines, the air temperature is about 15℃ higher than the ambient temperature
5. Temperature Control Valve
The main function of the thermostatic valve is to regulate the temperature of the lubricating oil sprayed into the compressor element, thereby adjusting the discharge air temperature at the outlet of the compressor block. If the discharge temperature is too low, water vapor in the compressed air may easily condense inside the oil separator tank, leading to emulsification of the lubricating oil.
- When the oil temperature is ≤ 70℃, the thermostatic valve prevents the lubricating oil from flowing into the oil cooler.
- When the oil temperature is > 70℃, the valve allows part of the hot oil to pass through the cooler to be cooled. The cooled oil is then mixed with the remaining uncooled hot oil, and this mixture flows into the compressor head to continue the compression cycle.
- When the oil temperature reaches ≥ 76℃, the valve fully opens the passage to the oil cooler, and the entire amount of hot oil flows through the cooler to be cooled before returning to the compressor head.
In simple terms, the thermostatic valve functions as a temperature regulator for the lubricating oil
6. PLC and Display Screen
The PLC (Programmable Logic Controller) can be thought of as the central processing unit (CPU) of a computer, while the LCD screen of the air compressor acts like the monitor. The PLC handles functions such as receiving input signals, sending output signals (to the screen), performing calculations, and storing data.
Thanks to the PLC, the screw air compressor becomes an intelligent “point-and-shoot” machine. Any malfunction in any part of the compressor will be detected and displayed on the screen through the PLC, making maintenance easier.
During operation, parameters such as current, air pressure, and temperature (high or low) are all processed by the PLC and clearly shown on the screen.
When it’s time to replace components like the air filter, oil filter, oil separator element, or lubricating oil, the PLC will automatically issue a warning on the screen to remind the operator to perform timely maintenance.
The screen also has buttons that function similarly to a computer, allowing the user to adjust the discharge air pressure to suit the downstream equipment.
7. Other Components of the Screw Air Compressor
1. Air Filter
The air filter element is a dry-type paper filter, which plays a crucial role in filtering the intake air. Its pleated design increases the surface area for better airflow. The pore size of the filter is about 3μm. Its main function is to filter out dust and solid particles larger than 3μm, thereby preventing premature wear of the screw rotors and avoiding early clogging of the oil filter and oil separator.
Normally, after every 500 hours of operation (or sooner depending on the working environment), the filter should be removed and cleaned by blowing compressed air ≤ 0.3 MPa from the inside out to unclog the pores. Excessive air pressure may tear or enlarge the pores, reducing the filter’s effectiveness. Therefore, when using remaining compressed air from the air tank for cleaning, always check the pressure gauge to ensure it does not exceed 0.3 MPa, as this
2. Air Inlet Valve (also called Intake Control Valve)
The air inlet valve controls the amount of air entering the compressor head, thereby regulating the air discharge flow rate.
This is a linear control valve operated by a proportional valve and a pneumatic servo cylinder. The push rod of the servo cylinder controls a valve plate inside the inlet valve to regulate the intake airflow, allowing smooth adjustment from 0% to 100%.
Control air is drawn from the oil separator tank and split into two branches after passing through the proportional valve:
- Branch A leads to the servo cylinder to control the valve plate;
- Branch B passes through the vent solenoid valve and into the air inlet valve.
3. Proportional Valve and Servo Cylinder
The “proportional” here refers to the division ratio between airflow in Branch A and Branch B. “Reverse” means the less air enters the servo cylinder, the wider the valve plate opens, and vice versa – more air into the cylinder closes the valve more.
4. Vent Solenoid Valve
Installed next to the air inlet valve, it functions to release the air inside the oil separator and compressor head through the air filter when the compressor shuts down. This prevents residual oil from remaining in the compressor head upon restart, which could cause a heavy load start, high inrush current, and potentially burn out the motor.
5. Temperature Sensor
Installed at the discharge port of the compressor head, this sensor measures the temperature of the compressed oil–air mixture. The other end is connected to the PLC and displays the temperature on the LCD screen.
6. Pressure Sensor
Located at the air outlet, after the oil separator element. The pressure before the separator is called the pre-filter pressure. If the pressure differential between the pre- and post-separator reaches ≥ 0.1 MPa, the oil separator element needs replacement.
One end of the sensor connects to the PLC to display pressure on the LCD screen. Additionally, a mechanical gauge on the oil separator tank displays the pre-filter pressure, while the digital screen shows the post-filter pressure.
7. Oil Filter
This is a paper filter with a filtration precision of about 10 to 15 microns. Its purpose is to remove tiny metal particles, dirt, oxidation byproducts, and gum from the oil, helping protect the bearings and screw rotors for smooth operation.
If the oil filter is clogged, it will reduce oil supply to the compressor head, affecting bearing life, causing high discharge temperature, and leading to localized carbon buildup.
8. Oil Return Check Valve
Separated oil accumulates at the bottom of the oil separator element and is returned to the compressor head via the oil return line. To prevent oil from flowing back into the separator, the return line is fitted with a check valve. If oil consumption suddenly increases during operation, check if the orifice inside the check valve is clogged.
9. Oil Piping System in the Compressor
These pipes are responsible for circulating the lubricating oil. The high-pressure, high-temperature oil–air mixture exiting the compressor head passes through these pipes. To prevent bursting, the pipes are usually wrapped in spiral protective sleeves. The oil return pipe connecting the separator tank to the compressor head is typically made of metal (iron pipes).
10. Cooling Fan for Radiator
Generally, an axial fan driven by a small motor is used to blow cooling air perpendicular across the radiator.
In some compressor models without a thermostatic valve, oil temperature is regulated by switching the cooling fan on or off. When the exhaust temperature reaches 85℃, the fan starts; when the temperature drops below 75℃, it stops—maintaining a stable temperature range. In models with a thermostatic valve, the fan operates in the same way.
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