Application of frequency converter in energy saving retrofit of air compressor

2017-01-20

A group of companies for the steel industry, refractory products supplier, the transformation of the air compressor station consists of 4 air compressors, respectively for one of the 250kW mountains and three 110kW mountains of screw type air compressor.

Before the transformation of the equipment, four sets of air compressor work in the power frequency. The two-point control (upper and lower control), which is the pressure when the air compressor cylinder reaches the set value upper limit, the air compressor itself through the pressure off the intake valve, when the pressure drops to set the lower limit, the air compressor intake valve open. This will lead to frequent air compressor unloading and loading, is often late with less gas, and air compressor running at full speed, which is a waste of energy. Therefore, it is necessary to transform the screw air compressor to save energy, improve equipment stability, reduce maintenance costs and reduce noise pollution.

Design requirements of frequency conversion energy saving reconstruction scheme

According to the existing problems and combined with the requirements of the production process, the system needs to meet the following requirements:

1 can be adjusted according to the actual production of air volume air compressor production, not only to meet the production requirements, while reducing energy consumption.

2 system with frequency conversion and power frequency two sets of control loop, and in the air compressor failure, can effectively control the air compressor oil leakage problem.

3 the original system uses the frequency converter for speed regulation, can not change the original main circuit and the two circuit control mode.

4 reduce running noise, especially in the startup and full load operation, the noise should be significantly reduced.

Implementation scheme of frequency conversion of air compressor

The main motor of the air compressor is 250kW motor rated current 450A, technical personnel according to the actual situation, select the appropriate air compressor to control the inverter, feedback to the pressure of the gas tank through the remote pressure gauge, when the feedback pressure is higher than the production demand pressure, inverter frequency reduction operation, until the lower limit frequency setting, when the lower limit frequency continuous operation of 300S, frequency of dormancy; when the feedback pressure is lower than the demand pressure, frequency increase, air compressor load operation. The design of the converter is used in the electrical system. When the frequency converter is in trouble, it can be switched to the original power frequency operation by the rotary converter bypass switch to improve the stability of the system.

Before the renovation of the main motor has been running at full speed, in the process of operation, pressure is always rising (even if the production process does not need so much pressure), when the air tank pressure has been increased to maximum pressure, the air compressor will stop loading. Usually when setting this parameter value, need to use rate and production process generally considering equipment, the upper limit will set than the actual value need 2bar pressure as the upper limit, the increase of the discharge pressure will affect the power consumption, the pressure per 1bar increase about 6% increase in power consumption.

In the PID inverter constant pressure control system, variable frequency speed regulation system of pipeline pressure as the control object, installed in the tank pressure transmitter will change the pressure of the gas tank into electrical signals sent to a controller inside the PID regulator, compared with the given pressure value, and according to the difference of the value according to the established PID control mode operation, control signals are generated to control the inverter output voltage and inverter frequency, adjust the motor speed, so that the actual pressure maintained at the set pressure, resulting in constant pressure.

For the main inverter when the motor is started, the impact on the current equipment, using the optimized SPWM algorithm, make the motor starting smoothly, while increasing the starting torque, but also reduce the interference current impact on other equipment; in the acceleration process using advanced multi-point acceleration curve, reduce the wear of the die and the quick start bearing main screw, you can greatly increase the service life of mechanical equipment and equipment maintenance cycle. In addition, if the inverter when failure occurs, production process does not allow air compressor shut down, so the system has set up frequency and frequency switching function, so that when the inverter fault occurs, the power supply by direct frequency power supply contactor, the air compressor can work as usual.

Energy-saving effect

250kW ordinary units, running 8759 hours a year, including loading time of 2442 hours, unloading time of 6317 hours, electricity to $0.7 / degree calculation.

1, no-load loss:

The no-load current loss generated during unloading unloading time x (1.732 x 0.38 x 175 x 0.85) x 6317 hours / year =618442 of electricity; but considering to ensure no leakage of lubrication at low screw air compressor, generally takes the frequency lower limit frequency is 30Hz, the frequency of power conservation is: 618442 * 40%=247377.

2, pressure loss:

Loading time * higher than 1bar pressure loss caused by 0.6 * 7% * 250kW * 2442 hours / year =25641 degrees.

To sum up, the frequency conversion unit than the ordinary unit of energy saving 247377+25641=273018 degrees.

Annual electricity saving 273018 x 0.7 yuan / degree =191111 yuan.

Problems should be paid attention to in system reformation

1, the screw can not reverse operation, the transformation must pay attention to the direction of motor operation.

2, the pressure will be transferred to the set pressure and the same (because the frequency converter has a lower limit frequency), so as to prevent the state of the inverter in the acceleration to do no work.

3, attention should be paid to the heat dissipation of the motor, the motor frequency conversion speed, the speed is reduced, the cooling effect of the motor fan should be reduced.

4, attention should be paid to the air compressor lubrication problem, the lower the air compressor speed, the smaller the amount of oil consumption, the worse the lubrication effect.

5, the pressure of the system to meet the requirements of the production process, the lower the pressure set the better, because the higher the air compressor exhaust pressure,