Application of Frequency Control Technology in Reciprocating Compressor
Frequency conversion speed regulation technology is a motor drive speed regulation and energy saving technology developed rapidly in the 1980s. It has been widely used in water pumps, fans, elevators, etc., and has the characteristics of improving control levels and energy saving. In the gas heating industry, a large number of gas compressors or blowers are used. Usually, the machine selection and configuration are always based on the maximum demand. In actual operation, the user's requirements often change due to various reasons. How to make it not only meet the requirements of users, but also achieve economy, energy saving, safe use, and easy operation has always been the goal that people seek. The gas pressure delivery section of our company's gas production plant has two Roots blowers with a single displacement of 2 400 /h, which can provide 4 reciprocating gas compressors with a pressure of 50 kPa. The single displacement is in the early stage of commissioning. , The air supply pressure requirement is relatively small, only about 30 kPa, and the air supply volume is 2 000-2 500 /h. The Cairo Roots blower can basically meet the air supply pressure and flow requirements. When the air supply situation fluctuates greatly, manual adjustment of the valve is used to control the return flow. When the error is large, the outlet pressure can fluctuate by ±5 kPa, and the air supply quality cannot be guaranteed. With the production specifications, it is required to increase the pressure, large flow rate and stable gas supply, and reciprocating gas compressors must be used. However, it is difficult to fully balance the air supply volume and the compressor displacement, and manual adjustment of the return flow is difficult to ensure the stability of the demand pressure and flow, which makes the system's following shortcomings obvious: the air supply is not large, making the gas repeated Pressure delivery, low operating efficiency, consumes a lot of electric energy, and increases the cost. Use a bypass valve to manually adjust the air supply. The workload is large, the air supply pressure is not easy to control, the air supply quality is poor, and the system safety is poor. Therefore, the reciprocating compressor system adopts frequency conversion speed regulation technology transformation.
2 Energy-saving principle and technical solution of frequency conversion speed regulation When the air supply volume needs to be changed within the rated displacement of a single compressor, certain measures must be taken to adjust the compressor displacement to balance the supply and demand and the balanced operation of the transmission and distribution system. For reciprocating compressors, there are many ways to adjust the displacement, but the method of changing the speed is the most economical and practical.
Adopting reflux adjustment can not change the suction volume, so the compressor can always run under full load, which can not save energy. Changing the speed of the compressor changes the air intake of the compressor, and its power consumption also changes accordingly.
The use of frequency conversion speed regulation technology makes it possible for the compressor to achieve variable speed adjustment. By changing the compressor motor speed to change the compressor speed, the goal of energy saving is finally achieved.
According to the production status, two of the compressors were changed to a power frequency and a variable frequency, and one used and one standby.
In terms of electrical control circuit design, taking into account the special requirements of reciprocating compressors for lubrication and cooling, all the process interlocking and accident shutdown control circuits of the original system are retained, and they are added to the variable frequency electrical system to ensure the safety of the compressor Stable operation. Since the compressor equipped with the motor is a 10-pole 280 kW asynchronous motor, the large-scale frequency converter that matches it is not capable of domestic production. Therefore, the FRN280P94 inverter is used. According to the needs of the system, an ST 200 programmable controller PLC is additionally equipped. And the EPROM memory used by this controller can store 2000 steps of programs, comments, parameters, etc., with a large storage capacity. A pressure transmitter is installed at the entrance of the gas supply pipe network, and a PID closed-loop adjustment link is installed in the control system for gas and heat to automatically adjust the flow rate.
The system adjusts the set pressure through a potentiometer, and sends it to the PLC for PID calculation together with the outlet pressure signal (4-20 mA) from the remote pressure transmitter, and the comparison result is output to the inverter to make it automatically frequency conversion according to the instruction Adjust the motor speed: when the outlet pressure is higher than the set pressure, the frequency drops, when the motor decelerates and the outlet pressure is lower than the set pressure, the frequency rises and the motor accelerates. Thereby adjusting the outlet pressure to automatically track the set pressure, realize automatic constant pressure control, and ensure the stability of the system and the quality of gas supply.
Using variable frequency speed regulation technology, the machinery and pipelines in the original system do not need to be modified, only the step-down starting cabinet of one of the machines in the power supply circuit needs to be replaced with a frequency converter cabinet, and two machines with variable frequency power frequency are added to the main circuit. Interlocking can be completed, so the workload of the transformation process is not large.
It is worth noting in the system debugging that the compressor load belongs to the category of constant torque load. When the motor speed is too low and the load is large (that is, the pressure of the pipe network is high), the motor output will be insufficient and the motor will lock up. Prone to equipment accidents. Therefore, through many experiments, it is found that setting the lower limit frequency to 15 Hz can work stably. In addition, in order to ensure the safety of the pipe network system, the return valve of the pipe network should be opened appropriately to ensure that the pressure of the pipe network will not become too high when the gas consumption drops sharply.
3 Economic indicators of system energy saving effect The system transformation investment totals 350,000 yuan, and it has been used for more than one year.
According to the needs of users, the company first tested different pressure conditions such as 30 kPa, 70 kPa, 100 kPa, etc., and the work was very stable. After working at 100 kPa pressure for a year, the frequency can be guaranteed below 18 Hz, which not only meets the pressure and air supply requirements, but also fully reflects the energy-saving effect.
Practice has proved that the smaller the air supply, the more obvious the power saving effect. According to the current working conditions, when the return valve is slightly opened, the air supply volume is 1 200～1 300 h, and the outlet pressure is 100 kPa. In order to ensure the stable and safe operation of the system, the frequency is slightly higher than the minimum frequency limit by 15 Hz and kept at about 18 Hz. The current is only 100 ~ A, and the no-load current of the power frequency operation of the system is about 400 A, which is very effective in energy saving in comparison.
The power during industrial frequency operation is about 235.55 kW, and the power during variable frequency operation is about 65.47 kW. According to the current company’s production status, the daily supply is calculated at the Wuhan average electricity price of 0.60 yuan/kWh, which saves an annual electricity cost of 496 thousand yuan. Yuan. The cost recovery time is 8.5 months.
4 Conclusion (1) The variable frequency speed controller can realize the wide range stepless speed regulation of the motor by simply setting the parameters, making it run stably at 15-50 Hz, and realize the soft start of the motor, which can also improve the power factor.
(2) The closed-loop control of variable frequency speed regulation can make the output power of the compressor motor change according to the shaft power required by the compressor, and the energy saving effect is obvious.
(3) After frequency conversion speed regulation, the compressor speed usually always runs below the rated speed (the current working condition speed is only 1/3 of the rated speed), so by reducing the compressor displacement, the compressor can pass The pressure loss of the valve and pipeline is reduced, and the cooling effect of the gas in the cylinder is strengthened, so that the power consumption is reduced.
(4) The reduction of speed reduces mechanical friction, reduces frictional power consumption, prolongs the service life of the equipment, while reducing noise and improving the working environment.
(5) Realize the fully automatic control of pressure and flow, make the system control simple and reliable, which not only greatly improves the gas supply quality and the safety of pipe network operation, but also makes the operation extremely simple and reduces the labor intensity of the operators.
(6) Due to the use of programmable controller closed-loop control speed, the control accuracy can reach ±1, the reliability of safe and stable operation is greatly enhanced, and it is more convenient to settle accounts with users.