Students new to air compressors and compressors often harbor a misconception that an air compressor is the same as a compressor. In reality, air compressors and compressors are two distinct mechanical products, although they share some similarities and connections.
The Difference Between the Two Concepts
Air compressors, specifically, are designed to compress air and are widely used in various applications. A common function of air compressors is to compress air for use as a power source, especially in the electricity industry. They are also used in refrigeration, gas separation, and gas transportation, playing a vital role in everyday life and work.
Compressors, on the other hand, are a broader category of machines that increase the pressure of low-pressure gases to high-pressure gases. This category includes several types, such as refrigerant compressors, hydrogen compressors, and air compressors, with air compressors being just one subtype.
Many people mistakenly believe that air pumps and air compressors are the same. In the past, air pumps referred to piston-type air compressors. However, modern air compressors come in various types, including screw compressors, oil-free compressors, and energy-efficient variable frequency air compressors. The operating principles of air pumps and air compressors differ.
The Working Principle of Air Compressors
Taking screw air compressors as an example, their operating cycle includes three processes: intake (1), compression (2, 3), and exhaust (4). As the rotors turn, each pair of meshing gears completes the same cycle in succession.
1. Intake process:
When the rotors turn, the maximum space in the tooth grooves of the male and female rotors is at the intake end wall opening. At this point, the tooth groove space communicates with the intake port. Since the gas in the tooth groove is completely expelled during exhaust, the tooth groove is in a vacuum state at the end of exhaust. When the rotor turns to the intake port, external gas is drawn in, filling the tooth grooves along the axial direction. Once the gas fills the entire tooth groove, the gas is sealed as the rotor’s intake side end face turns away from the casing intake port.
2. Compression process:
When intake ends, the tips of the male and female rotor teeth seal against the casing, preventing external gas flow. The meshing surface gradually moves towards the exhaust end. The space in the tooth groove between the meshing surface and the exhaust port progressively decreases, compressing the gas and increasing pressure.
3. Exhaust process:
As the meshing end face of the rotor aligns with the casing exhaust port, the compressed gas starts to be expelled. This continues until the meshing surface between the tooth tip and the tooth groove moves to the exhaust end face. At this point, the space in the tooth groove between the meshing surface of the rotors and the casing exhaust port is zero, completing the exhaust process. Simultaneously, the length of the tooth groove between the meshing surface of the rotor and the casing intake port reaches its maximum, and the intake process begins again.
The Working Principle of Process Compressors
For instance, in common centrifugal compressors, a steam turbine (or electric motor) drives the compressor’s main shaft impeller to rotate. Under the action of centrifugal force, gas is thrown into the diffuser behind the impeller. A low-pressure area forms in the middle of the working wheel, allowing the gas to enter the impeller from the steam inlet at the center of the working wheel.
As the working wheel rotates continuously, the gas is consistently expelled, maintaining continuous gas flow in the compressor. The centrifugal force increases the gas pressure, which then leaves the working wheel at high speed. The gas’s velocity decreases in the diffuser, converting kinetic energy into static pressure energy, further increasing the pressure.
If the pressure achieved by a single working impeller is insufficient, multiple impellers can be connected in series to achieve the desired outlet pressure. This connection is accomplished through bends and return flow devices, explaining the working principle of centrifugal compressors.