Xie Shuiying 1 Han Chengjiang 1 Xu Weijun 2
1. Zhejiang Industrial Vocational and Technical College, Shaoxing, Zhejiang 312000 2. Shaoxing Litie Pelletizing Plant, Shaoxing, Zhejiang 312041
Abstract: This article describes the frequency conversion principle, frequency conversion control system composition and working process of the screw air compressor for shaft furnace dust removal of the Shaoxing Litie Group Company pelletizing plant, analyzes the advantages and disadvantages before and after the frequency conversion, and calculates the energy saving effect after the frequency conversion. The energy-saving transformation of the air compressor is not only a positive response to the national energy-saving and emission-reduction policy, but also provides a successful experience for the technological transformation of other enterprises.
Keywords: air compressor; frequency conversion speed regulation; frequency conversion energy saving; energy saving transformation
I. Overview
Air compressor is short for air compressor. Its main uses are: compressed gas is used as power, such as pneumatic wrenches, pneumatic machinery; for automatic control, such as vehicle braking, stirring in the brewing industry; for refrigeration and gas separation, such as the refrigeration industry and the use of pure Oxygen pure nitrogen industry, etc.
Compressors are widely used in many fields of industrial agricultural production, such as mining, petroleum, chemical, refrigeration, power, and metallurgy. According to statistics, in a large smelter, the power of motors for compressors throughout the year often reaches tens of thousands of kilowatts; in the metallurgical industry and machinery manufacturing industry, the power consumed by oxygen compressors and air compressor power stations for smelting , Accounting for more than 30% of the plant's electrical energy consumption. How to reduce the energy consumed by the operation of the air compressor is of great practical significance for responding to the national energy conservation and consumption reduction policies and improving the economic efficiency of the enterprise.
2. Analysis of energy saving transformation of air compressor
1. Working principle of air compressor
Air compressors are classified into speed type (turbine type) and volume type according to working principle. The function principle of the speed type air compressor is to accelerate the gas flow speed in some way, and then rapidly decelerate in the pressure expansion device to convert the gas kinetic energy into gas static pressure energy to increase the gas pressure. The principle of the volumetric air compressor is to increase the gas pressure by compressing the volume of air.
Screw air compressors are the most common type of volumetric air compressors, and their working principle is: a pair of yin and yang rotors (or screws) that are parallel to each other rotate in the cylinder, so that the air between the rotor slots is continuously generated With periodic volume changes, air is transported from the suction side to the output side along the rotor axis, thereby achieving the entire process of air compressor suction, compression, and exhaust. The air inlet and air outlet of the air compressor are located at both ends of the casing, respectively, and the slots of the female rotor and the teeth of the male rotor are driven and rotated by the main motor.
2. Control process of air compressor
The control process of the air compressor air supply system is generally: press the start button, the control system turns on the starter coil and opens the fuel cut valve, the air compressor starts in the unload mode, then the intake valve is in the closed position, and put The gas valve opens to discharge the pressure in the oil-gas separator. After the pressure is reduced for 2 seconds, the air compressor starts to load and the system pressure starts to rise. If the system pressure rises to the upper limit value of the pressure switch, that is, the take-off pressure, the controller closes the intake valve, the oil and gas separator deflates, and the compressor runs without load. When the system pressure drops to the lower limit value of the pressure switch, that is, the rebound pressure, the controller opens the intake valve, the air release valve of the oil and gas separator closes, and the compressor runs at full load.
3. The original problems of the shaft furnace dust-removing air compressor of Litie Pelletizing Plant
The air compressor for the shaft furnace dust removal of this plant is screw type, consisting of two large and small units, the larger one is model SA60-7, the exhaust volume is 10.3m3 / min, the rated / maximum exhaust pressure is 0.7 / 0.75MPa, the motor The power is 55Kw and the motor speed is 2970r / min. The smaller model is OG-50W, the shaft power is 37KW, the volume flow is 6.1m3 / min, the rated voltage is 380V, and the air supply pressure is 0.7MPa.
The function of the air compressor is to generate high-pressure gas, which is used to transport the ball powder removed from the shaft furnace to the ball-making raw material warehouse for recycling. Due to the unbalanced air volume in the dust removal and transportation system, the air supply volume of the air compressor is generally greater than the actual air volume. In order to maintain the gas storage pressure unchanged, the adjustment method must be adopted. Among them, the large compressors are often in full-load operation. When the compressed air output cannot meet the working conditions, the small compressors are turned on; when the compressed air is excessive, it is vented, which is a waste of energy. In addition, the air compressor unit uses star-delta starting before frequency conversion. Although it is controlled by PLC, the air compressor uses intermittent control operation mode, and the operating state is very uneconomical. keeping it:
(1) Although the main motors of large and small start with a star-delta voltage reduction, the starting current is still large, which will cause certain impact and impact on the power grid and other equipment.
(2) Large motors are often operated with no load, and electric energy is wasted more seriously.
(3) Small motors often start and stop frequently, affecting the service life of the motor
(4) The power frequency operation of the main motor causes a lot of noise during the operation of the air compressor.
In view of the lack of control methods of dust-removing air compressors and large energy consumption, the factory carried out energy-saving transformation of air compressors in 2009.
3. Selection of air compressor energy-saving scheme
1. Selection of energy saving scheme for air compressor
The energy-saving transformation of the air compressor can be carried out from the mechanical system and the control and regulation system. The transformation of the mechanical system is mainly to improve and optimize the pipeline system, etc., in order to reduce the resistance of the pipeline network and reduce power consumption. The transformation of the control and regulation system is mainly to choose a reasonable flow regulation method. At present, the flow adjustment methods of the air compressor generally include intermittent control operation of the compressor, suction adjustment, cylinder unloading, and stepless speed adjustment.
The intermittent operation of the air compressor will cause the compressor to start and stop frequently, increase power consumption, increase the fluctuation of the power grid, and also affect the life of the equipment; while the cylinder unloading not only wastes energy, but also aggravates equipment wear and increases operating costs.
At present, more and more adopt the stepless speed regulation, namely the frequency conversion speed regulation technology. The frequency of the motor input voltage is changed by the inverter to control the speed of the motor, and then the output power of the motor and the input power of the air compressor are controlled to match the air volume of the air compressor with the actual air volume. This adjustment method can realize the continuous adjustment of the motor speed, make the air compressor operate under the design, greatly improve the working efficiency of the air compressor during light load operation, reduce the energy consumption of the air compressor, and create a better Economic benefits are of great significance to respond to the current national call for energy saving and consumption reduction.
For the screw air compressor, the frequency converter can be used to change the displacement of the screw rotor by changing the speed of the screw rotor. When the air consumption changes, the frequency converter changes the speed of the air compressor to adjust the exhaust volume to achieve the exhaust The pressure is constant and the purpose of saving energy.
In response to the problems in the control system of the shaft furnace dust air compressor, Shaoxing Litie Group Pellet Plant carried out frequency conversion energy-saving transformation in 2009.
2. Frequency conversion energy saving analysis
(1) Frequency control principle
According to the asynchronous motor speed formula: n = (1-s) 60f / p
In the formula, f is the power frequency, S is the motor slip, and P is the number of pole pairs of the motor.
When P and S are determined, the motor speed is proportional to the power frequency, so changing the power frequency can change the motor speed n, thereby achieving frequency conversion speed regulation.
(2) Frequency conversion energy saving principle
According to the operating characteristics of the air compressor, the air compressor basically belongs to a constant torque load. The frequency converter is used to adjust the motor speed according to the amount of air supply, which can make the output power of the motor basically equal to the speed (air supply). direct ratio. When the air consumption decreases, the pressure at the exhaust port rises, and the closed-loop feedback to the inverter reduces the motor speed, which reduces the shaft output power; when the air consumption increases, the exhaust pressure decreases, and the closed-loop feedback to the inverter, Increase the motor speed and increase the shaft output power.
Therefore, the variable frequency air compressor system can track the load change of the air supply system in real time through the pressure closed loop, adjust the speed of the air compressor motor, ensure the constant pressure of the exhaust port, and make the compressor motor work in the most economical operating state. It avoids the frequent loading and unloading of the air compressor in the original control mode, and frequently starts and stops, which greatly reduces the electric energy absorbed from the power grid.
(3) Frequency conversion control process
The variable frequency speed control system takes the output pressure of the compressed air of the air compressor as the control object. The frequency converter, pressure sensor, PLC, and motor form a closed-loop constant pressure control system. The working pressure value is set directly by the operation panel, and the on-site pressure is detected by the sensor. After being converted into 4 ～ 20mA current signal, it is fed back to the inverter. The inverter performs comparison calculation through the built-in PID, so as to adjust its output frequency, to achieve the purpose of adjusting the motor speed, and then adjusting the motor output power.
It can be seen that as long as the speed of the motor is adjusted, the output power of the motor shaft can be adjusted, that is, the input power of the air compressor is adjusted, so that the air volume of the air compressor is matched with the air volume, and the purpose of constant pressure air supply and energy saving is achieved .
Four, air compressor frequency conversion energy-saving transformation
1. Energy-saving retrofit design requirements
According to the problems existing in the control system of the original air compressor unit and combined with the production process requirements, the system should meet the following requirements after the frequency conversion of the unit control system is transformed:
(1) The system should have two sets of control loops of frequency conversion and power frequency.
(2) Keep the outlet pressure of the gas storage tank stable. When the motor is operated by frequency conversion, the outlet pressure fluctuation range cannot exceed ± 0.02MPa.
(3) One inverter can control two air compressor units, which can be switched by a changeover switch.
(4) According to the industrial control requirements of the air compressor, the control system should make the motor have constant torque running characteristics.
(5) The control system is still controlled by PLC.
2. Block diagram of the control system
The entire system consists of a frequency converter, PLC, pressure transmitter, power frequency contactor, motor, air compressor, etc., a pressure closed-loop control system, which automatically adjusts the speed of the motor, thereby adjusting the air intake of the air compressor, so that the air storage tank The air pressure is stable within the set range and constant pressure control is performed. The feedback pressure is compared with the set pressure, and the output of the inverter is controlled in real time through the PLC, thereby adjusting the motor speed to achieve the purpose of energy saving. The functional block diagram of the variable frequency speed regulation system of the air compressor unit is shown in Figure 1.
3. Working process
The schematic diagram of frequency conversion control wiring is shown in Figure 1. The sampling point of the pressure sensor is installed at the exhaust outlet of the air compressor, and the required current signal is obtained through the pressure sensor. When the system gas consumption is small, the pressure of the air compressor exhaust port increases and the resistance value of the pressure sensor decreases. The 24V DC power supply converts the resistance value of the pressure sensor into a current signal, and the current signal input to the inverter is increased by the PLC. After the inverter compares with the set value, the deviation ε is obtained. After PID adjustment, ε makes the frequency of the output power supply It gradually becomes smaller, the motor speed decreases, and the air compressor motor speed also decreases, and the pressure decreases. When the system uses a large amount of gas, the system pressure gradually decreases, the frequency converter output power frequency increases, the motor speed increases, and the gas production increases, so that the pressure at the outlet of the air compressor increases to meet the system gas requirement. When the air supply and air consumption are basically the same, the inverter will run at the frequency of this point, so that the system pressure is constant, and the motor and air compressor have been running in the most economical operation state.
For the sake of safety, the original power frequency control circuit is retained when the inverter device is installed, and the power frequency / frequency conversion switch is provided on the frequency conversion cabinet. When the frequency conversion device fails, it can be easily converted to the power frequency operating state. Normal gas supply does not affect production (as shown in Figure 2).
Fifth, the advantages after frequency conversion 1. The starting current is small, no impact on the power grid
The inverter can make the current of the motor start up and load rise gently without any impact; the motor can be stopped softly to avoid the harm caused by the regenerative current, which is beneficial to prolong the service life of the equipment
2. Stable output pressure
After the frequency conversion control system is adopted, the pressure of the gas in the gas supply pipeline can be monitored in real time, so that the pressure of the gas in the gas supply pipeline is kept constant, and the production efficiency and product quality are improved.
3. Small equipment maintenance
The frequency conversion starting current of the air compressor is small, less than 2 times the rated current, and the loading and unloading valve does not need to be repeated. The frequency conversion air compressor automatically adjusts the motor speed according to the gas consumption, the operating frequency is low, the speed is slow, the bearing wear is small, the service life of the equipment is extended, and maintenance The workload becomes smaller.
4. Low noise
The frequency conversion provides energy according to the gas demand, without too much energy loss, the motor runs at a low frequency, and the mechanical rotation noise is reduced. Because the frequency conversion adjusts the motor speed, there is no need to repeatedly load and unload, and the noise of frequent loading and unloading is gone. , Continue to pressurize, the noise produced by unstable air pressure also disappeared.
5. Obvious energy saving effect
The energy-saving effect of the inverter transformation is very obvious. After the plant experiment, a 55KW air compressor has a loading time of 15 seconds and a loading current of 110A; an unloading time of 20 seconds, an unloading current of about 80A, and an upper and lower working pressure limit of 0.75MPa / 0.62MPa. According to the loading and unloading time and current, The power consumption of the air compressor before energy-saving transformation can be calculated at 45 degrees per hour. After the frequency conversion, the power saving rate is about 30%. Based on the calculation of 0.75 yuan per kilowatt hour, the electricity cost can be saved for one year:
45 × 0.75 × 24 × 365 × 30% = 88695 (yuan) = 8.8695 (ten thousand yuan)
The cost of air compressor frequency conversion transformation is about 100,000 yuan (10 / 8.870) × 12 = 13.52
The investment can be recovered in about 14 months. In the future, it will save more than 80,000 yuan in electricity costs.
Therefore, the frequency conversion energy-saving system is connected to the gas supply system, and the frequency conversion technology is used to change the speed of the air compressor to adjust the flow rate in the pipeline, instead of adjusting the valve, which can not only achieve the purpose of constant pressure gas supply, but also achieve significant energy saving effect. In addition, the inverter's soft start function and smooth adjustment feature can realize the balance adjustment of the flow, reduce the starting shock, and also extend the service life of the compressor and the unit tube.
Six, matters needing attention
1. The air compressor belongs to a large rotational inertia load. This starting feature can easily cause the V / F control mode inverter to skip current protection when starting. It is recommended to select a speed sensorless vector inverter with high starting torque to ensure the continuity of constant pressure gas supply and the reliable and stable operation of the equipment.
2. The air compressor is not allowed to run at low frequency for a long time. If the speed of the air compressor is too low, on the one hand, it will deteriorate the working stability of the air compressor, on the other hand, it will deteriorate the lubricity of the cylinder, and will accelerate wear. Therefore, the lower frequency limit should not be lower than 20Hz.
3. In order to effectively filter out the higher-order harmonic components in the current at the output end of the inverter and reduce the electromagnetic interference caused by the higher-order harmonics, it is recommended to use the output AC reactor, which can also reduce the motor operating noise and temperature rise, and improve the motor operating stability .
4. It is recommended that the power use an inverter that is one level higher than the power of the air compressor to avoid frequent trips when the air compressor starts.
References [1] Gao Xiangjia, Chen Fang. Economic analysis and technical scheme of frequency conversion energy-saving transformation of screw air compressors [J]. Technical Transformation, 2009, 3
[2] Li Qiang, Qu Baocun, Ge Zhiqiang. The application of PLC and frequency converter in the energy-saving transformation of air compressor [J]. Programmable Controller and Factory Automation, 2006,7
[3] Li Chuanhuai. Application of PLC and frequency converter in energy saving transformation of air compressor [J]. Science and Technology Information, 2007, 2
* This article is one of the research achievements of the high-level platform project (Compressor Energy-Saving Control Strategy Research, Project Number: KY2010144) of Zhejiang Industrial Polytechnic.
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