In recent years, the air compressor industry has been advocating for variable frequency retrofitting for increased energy efficiency. Is it really effective? Our kotech editor shares the principles and advantages of variable frequency retrofitting for screw air compressors!
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Principles of Energy-saving Retrofit for Air Compressors
- Introduction to the Working Principle of Screw Air Compressors: The working principle of an air compressor involves a pair of intermeshing parallel rotors (or screws) rotating inside a cylinder. This rotation leads to periodic changes in the volume between the rotor grooves, causing air to undergo cyclic volume variations. The air is then transported from the suction side to the discharge side along the axis of the rotor, facilitating the entire process of air intake, compression, and exhaust in the air compressor. The inlet and outlet of the air compressor are located at both ends of the casing, and the grooves of the male rotor and the teeth of the female rotor are driven to rotate by the main motor.
- Analysis of Issues in the Original Air Compressor System Operation:
a) Although the main motor starts with a star-delta reduced voltage, the starting current is still significant, impacting the stability of the power grid and the safe operation of other electrical devices.
b) The main motor often operates in an unloaded state, leading to uneconomical operation and severe energy wastage.
c) Frequent running of the main motor at the grid frequency results in high noise levels during air compressor operation.
- Necessity of Energy-saving Retrofit for Air Compressors: Given the principles of air compressors and the analysis of the current operating conditions, we believe that energy-saving retrofitting for air compressors is essential. This not only saves a considerable amount of operational costs and reduces production costs but also lowers the noise generated during air compressor operation, leading to reduced equipment maintenance expenses.
Application Optimization Analysis of Variable Frequency Energy-saving Retrofit
The design requirements for energy-saving retrofitting should address the issues existing in the original operating conditions and consider production process requirements.
After the variable frequency retrofit, the air compressor system should meet the following requirements:
- The motor’s variable frequency operation should maintain stable pressure at the outlet of the air tank, with pressure fluctuations not exceeding ±0.2 bar.
- The system should have two sets of control loops for variable frequency and grid frequency.
- In accordance with the industrial control requirements of the air compressor, the system should ensure that the electric motor exhibits constant torque characteristics.
- To prevent non-sinusoidal wave interference with the air compressor controller, effective measures to suppress electromagnetic interference should be implemented at the output of the variable frequency drive.
- In situations of low electrical consumption, when the variable frequency drive operates at low frequencies, measures should be in place to ensure that motor winding temperatures and motor noise do not exceed allowable limits.
- Considering future system expansion, the variable frequency drive should meet the requirements for future industrial control expansion.
- Installing an input reactor at the upper end of the variable frequency drive effectively suppresses interference of the variable frequency drive with the power grid.
Advantages of Variable Frequency Energy-saving Retrofit for Air Compressors
- Energy-saving Variable Frequency Control: Compared to traditional control methods, energy savings with variable frequency control are the most significant. Operating the compressor based on the air volume demand results in an economically efficient operation. Approximately 20% or more in electricity cost savings can be achieved, with a return on investment in about six months.
- Reduced Operating Costs: The operating costs of traditional compressors comprise initial procurement costs, maintenance costs, and energy costs. Energy costs make up about 77% of compressor operating costs. By reducing energy costs by 44.3%, combined with reduced impact on equipment during variable frequency start-up, maintenance and repair requirements decrease, leading to a significant reduction in overall operating costs.
- Improved Pressure Control Accuracy: Variable frequency control systems have precise pressure control capabilities, ensuring that the compressed air pressure output matches the air volume required by the user’s air system. The output air volume of a variable frequency-controlled compressor changes with motor speed. With the improved accuracy of variable frequency control motor speed, the system’s pressure variation can be kept within the range of ±0.2 bar, effectively enhancing the quality of operating conditions.
- Extended Compressor Lifespan: Starting the compressor from 0Hz with a variable frequency drive allows adjustment of the start acceleration time, reducing the impact on the compressor’s electrical and mechanical components during start-up. This enhances system reliability and extends the compressor’s lifespan.
- Reduced Air Compressor Noise: Following the variable frequency speed control retrofit, the motor’s operating speed significantly decreases, effectively reducing the noise generated during air compressor operation. Field measurements show a reduction in noise of approximately 3 to 7 decibels compared to the original system.
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- Additionally, variable frequency control can reduce the current fluctuations during unit startup. This current fluctuation can affect the power grid and other devices’ power consumption, and the variable frequency drive can effectively minimize the peak current during startup.
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