BASIC KNOWLEDGE OF AIR COMPRESSORS

1. Basic information about air compressors

Screw air compressors have become the dominant trend in the development of modern air compression systems. Compared to piston air compressors of the same capacity, screw compressors offer superior advantages in terms of stable performance and long-lasting durability. They operate with low vibration, low noise, high efficiency, and do not include fragile components commonly found in other types of compressors.

Having a basic knowledge of screw air compressors is essential for understanding why they have become so widely adopted in industrial applications. This knowledge helps users grasp the core advantages of screw-type designs over traditional models. The precise alignment between the screw rotors and the housing minimizes air leakage and enhances overall operating efficiency. Unlike piston compressors that rely on the reciprocating motion of pistons, screw compressors only involve the meshing of the rotors, significantly reducing sources of vibration and noise. The specialized lubrication method used in screw compressors also supports smoother operating conditions and contributes to greater reliability.

Screw air compressors: A screw air compressor is a type of rotary positive displacement machine in which two helical rotors mesh together, compressing air by reducing the volume at the meshing point and then pushing it out. The screw components in these compressors are manufactured using CNC grinding and advanced laser technology, ensuring extremely high precision during production. Thanks to their reliability and efficiency, screw compressors help reduce operating costs throughout their service life.

The entire air compression process includes multiple systems: the air intake system, the compressor core operation, the oil-air separation system, the lubrication oil cooling cycle, and the air delivery system (see air compressor system diagram).

2. Operation process of the compressor core

The compressor core mainly consists of a male and female screw rotor along with the housing (see Figure 4). Its working principle is similar to that of a piston compressor, as it is also a type of positive displacement compressor. However, instead of using the reciprocating motion of pistons, the screw compressor utilizes the rotary motion of screw rotors.

If the gap between the female rotor teeth and the housing is considered the cylinder (like in a piston compressor), then the spiral movement of the male rotor within the grooves of the female rotor can be likened to the reciprocating motion of a piston.

The operation of the compressor core can be divided into three stages:

1. Suction Stage
   

    When the teeth of the screws mesh together at the contact point with the grooves on the surface, air begins to be drawn in. As the screws rotate, this contact point gradually extends towards the air discharge end, increasing the intake of air. The suction stage ends when the teeth of the male screw move away from the air intake point.

   

2. Compression Stage

   Không gian kín giữa trục vít âm, trục vít dương và vỏ máy sẽ tiếp tục di chuyển về phía đầu thoát khí khi trục vít quay, làm giảm thể tích và nén khí. Trong quá trình này, dầu bôi trơn sẽ được phun vào không gian kín này để giúp làm mát và bôi trơn các bộ phận trong máy.

   

3. Discharge Stage

   Khi răng của trục vít dương đến gần cửa xả khí, thể tích không gian kín đạt đến mức nhỏ nhất và khí nén cùng dầu bôi trơn sẽ được xả ra ngoài. Dầu khí sẽ được dẫn qua van chống đảo chiều vào hệ thống tách dầu, nơi dầu sẽ được tách khỏi khí và đưa trở lại hệ thống tuần hoàn dầu, còn khí nén sẽ được thoát ra ngoài qua cửa xả. 

   

Through this process, it is clear that screw compressors have a simple design without linear motion. Since the screws rotate continuously at high speed, the air discharge remains stable without fluctuations, resulting in very low noise and vibration. However, it is important to note that screw compressors have a specially designed compression process (Stage 2), and the discharge pressure is preset. The compressor must operate within the designated discharge pressure range; exceeding this range will significantly reduce its performance.

3. Discharge Process

The compressed air and oil are expelled from the compressor head and pass through the anti-reverse valve into the oil separation system. This process is referred to as the discharge process, and the discharge system includes the anti-reverse valve and air pipelines. The anti-reverse valve prevents compressed air in the oil separation system from flowing back into the compressor when the machine stops, preventing the screws from rotating in reverse and the lubricating oil from being expelled from the air intake port. Common issues related to the anti-reverse valve include the valve getting stuck, a damaged spring, or aging sealing materials.

4. Oil and gas separation system

The compressed air passes through the oil separation system, where the air is separated from the oil and delivered for use, while the oil is routed to the cooling system. The oil separation system consists of the casing, oil separation components, oil pipelines, and a minimum pressure valve.

• Compressor Casing: Stores both the oil and the compressed air.
  
• Oil Separation Components: As the compressed air passes through the oil separation components, larger oil droplets adhere to the surface and fall to the bottom of the casing, while smaller oil droplets continue to be separated and fall to the bottom of the oil separation components.
  
• Oil Pipeline: The separated oil flows through the oil pipeline back to the compressor intake. If the oil pipeline becomes obstructed, the air supplied will contain a higher oil content, leading to increased oil consumption 
• Minimum Pressure Valve:  The oil circulation system includes components such as the oil separator, oil cooler, temperature control valve, and oil filter.

5. Oil Circulation System

• This air compressor does not use an oil pump; instead, the circulation of lubricating oil is driven by the pressure difference between the oil separator tank and the oil injection port at the compressor head. Basic knowledge of this pressure-driven operating mechanism is essential for understanding how the system functions without a mechanical pump. Lubricating oil plays a crucial role in the operating cycle of the compressor, and its quality must remain stable across a wide temperature range. The oil must not only have low viscosity for smooth flow but also provide effective lubrication, cooling, and sealing functions.
• Lubrication function: The oil lubricates the gears, bearings, rotors, and mechanical seals.
• Cooling function:  It cools the compression process and moving parts such as gears and bearings.
• Sealing function: It seals the gaps between the rotors and between the rotors and housing, reducing internal leakage and improving volumetric efficiency.
• A minimum pressure valve is installed at the discharge port of the oil separator tank. Its primary function is to ensure a minimum pressure inside the tank, which is necessary for forming a reliable oil circulation loop. Understanding the basic design and function of this valve contributes to essential knowledge of compressor operation. Additionally, the valve prevents backflow, stopping compressed air from the pipeline system from flowing back into the separator tank. When the compressor starts to load, the valve maintains the pressure inside the separator tank at a level higher than atmospheric pressure. This helps slow down the airflow and improves oil separation efficiency. The valve is typically set to open at around 4 bar.

5.1. Temperature Control Valve

• The temperature control valve regulates the minimum oil injection temperature into the compressor head. A basic knowledge of this component is essential for understanding its role in protecting the system. If the oil injection temperature is too low, the discharge air temperature will also drop, leading to condensation of water vapor in the oil separator tank. This degrades the oil quality and shortens its lifespan. By keeping the oil temperature above a certain threshold, the valve ensures that the compressed air and oil mixture stays above the dew point, preventing condensation. 

• The valve adjusts the oil flow by bypassing the oil cooler to keep the oil temperature within an optimal range. This operating mechanism allows the system to adapt effectively to changing thermal conditions. When the compressor is first started and still cold, a portion of the lubricating oil bypasses the cooler. As the system warms up and exceeds the valve’s set temperature, all the oil is routed through the cooler. In high ambient temperatures, the entire flow of lubricating oil passes through the oil cooler.

5.2.  Oil Cooler

 The oil cooler is an essential part of the compressor’s cooling system. Most of the heat generated during compressor operation is absorbed by the lubricating oil and released into the environment through forced convection in the air-cooled oil cooler.

5.3. Oil Filter

The oil filter is installed downstream of the oil cooler and the temperature control valve. Its job is to remove dust, impurities, and residues from the circulating oil, ensuring stable rotor operation and preventing deposits in the oil cooling pipes. The filter has a precision rating of 10μm, and the filter element can be replaced independently.

When the element becomes clogged, the pressure difference between the inlet and outlet increases. If this difference exceeds 1.80 bar (the maximum threshold), a warning signal is triggered on the control panel.

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