Frequency Conversion Energy-Saving Retrofit Program of Air Compressor Solution

I. Brief Introduction to Working Principle of Air Compressor

A pair of Yin and Yang rotors (or screws) parallel to each other rotate in the cylinder, so that the air between the rotor teeth continues to produce periodic volume changes, and the air along the rotor axis is transported from the suction side to the delivery side, to achieve the whole process of suction, compression and exhaust of screw air compressor. The air inlet and the outlet of the air compressor are respectively located at the both ends of the shell, and the slot of the Yin rotor and the teeth of the Yang rotor are rotated by the main motor drive. The main motor power of the original air compressor is 45KW, with a total of three units. Operation mode: It adopts operation mode of △-Y decompression and then runs with full pressure. Specific operating procedures: after pressing the start button, the control system turns on the starter coil and opens the oil trap. The air compressor starts in the unloading mode, where the intake valve is in the closed position and the bleed valve is open to discharge the pressure in the oil and gas separator. And after stepping down for 2 seconds, the air compressor begins to run with load and the system pressure begins to rise. If the pressure of an air compressor is not enough, please start the second air compressor, and if the pressure of the second air compressor is still not enough then please start the third one. If the system pressure rises to the upper limit of the pressure switch, i.e., the take-off pressure, the controller closes the valve, and the oil and gas is separated and the gas blows off. The air compressor runs without load until the system pressure falls below the lower limit of the pressure switch, i.e., the rebound pressure. The controller opens the intake valve, and bleed valve of the oil and gas separator is closed. The air compressor is opened, and the bleed valve of the oil and gas separator is closed. The air compressor runs with full load. 

II. Existing Problems in the Original System

1. Although the main motor adopts starting mode of △-Y decompression, the current is still large when starting, which will affect the stability of the grid and running safety of other electrical equipment.

2. The main motor often runs without load, which leads to serious energy wasting.

3. The main motor running at power frequency leads to large noise in the operation of air compressor.

4. The main motor runs at power frequency, resulting in big impact on equipment and high mechanical wear of motor, which leads to large amount of large mechanical maintenance work.

III. Frequency Conversion Retrofit Program

1. Principle of retrofit program

Pressure closed-loop control system composed of frequency inverter, pressure transmitter, motor and spiral rotor automatically adjusts the speed of motor, so that the air pressure inside the tank stabilizes within the set range to achieve constant pressure control. The feedback pressure is compared with the set pressure to control the output of the frequency inverter in real time to adjust the motor speed so that the air pressure in the tank can be stabilized at the set pressure.

2. Design principles of frequency conversion retrofit program

According to existing problems in the original working condition and combined with the production process requirements, air compressor system after frequency conversion retrofit has the following functions:

(1) The tank output is stable when the motor is driven by frequency inverter, and the pressure fluctuation range can not exceed ± 0.02Mpa;

(2) The system should have two sets of control loop including varaible frequency and power frequency;

(3) The system has two sets of control loop including open loop and closed loop;

(4) An air compressor inverter can control three pieces of air compressor;

(5) According to the requirements of air compressor operating conditions, the system should ensure that the motor has running characteristics of constant torque ;

(6) In order to prevent the non-sinusoidal wave interference in air compressor controller, the input of frequency inverter should have effective measures to suppress electromagnetic interference;

(7) In the case of a small amount of gas, both motor winding temperature and motor noise do not exceed allowable range when frequency inverter runs at low frequency.

(8) Taking future expansion problem of the system into account, 55KW frequency inverter is selected to meet future requirements of future expansion;

3. System debugging

Debugging work is divided into two parts:

Firstly, preset frequency converter parameters according to the process requirements, motor parameters and load characteristics.

Secondly, the system linkage debugging.

Conduct system linkage debugging after the completion of setting parameters and running without load of the frequency inverter.

(1) Connect the drive to the system.

(2) Conduct the running of power frequency loop mode.

(3) Conduct the running of the frequency conversion loop mode, including two parts debugging of open-loop and closed-loop control:

Open loop: mainly observe frequency rising situation of the frequency inverter. Whether operation sound of the equipment is normal? Whether the pressure rising of the air compressor is stable? Whether the pressure transformer display is normal? Whether the equipment is shut down properly, etc? If everything is normal, you can perform closed-loop debugging.

Closed loop: mainly achieve that frequency rising speed and falling speed of the frequency inverter match with pressure movements of air compressor. Do not produce pressure oscillations. Besides, carefully observe the mechanical resonance point and skip those frequencies near resonance point.

IV. Benefit of Frequency Conversion Retrofit of Air Compressor;

1. Save energy

Compared with traditional compressors, the energy saving of compressors driven by frequency inverter is the most practical. Adjust the compressor output according to air demand.

2. Lower operating costs

The operating costs of traditional compressors consist of three items: initial procurement costs, maintenance costs and energy costs. Energy costs account for about 70% of the total cost. By reducing energy costs by more than 30%, coupled with reduced impact on equipment after the start of frequency inverter and thus reduced maintenance times, the operating costs will be greatly reduced.

3. Improve the accuracy of pressure control

The frequency conversion control system has precise pressure control capability, so the air pressure output of compressor matches with the gas amount required for user's air system. The output gas’s amount of compressor controlled by frequency inverter changes with speed of the motor.

As the frequency control electromechanical speed accuracy increases, it can make the pipe network system pressure changes remain within the required range, which effectively improves the quality of air supply.

4. Extend the service life of the compressor

The drive starts the compressor from OHZ and its start-up acceleration time can be adjusted, which reduces the impact on the electrical components and mechanical components of air compressor when motor starts and enhances the reliability of the system, so that the service life of the compressor is extended. In addition, the frequency conversion control can reduce current fluctuation that will will affect the power grid and the electricity of other equipment when the unit starts. The frequency inverter can effectively reduce the peak of the starting current to a minimum.

5. Lower air compressor noise. According to the compressor operating conditions, after frequency adjustment and transformation, the motor speed significantly slows down, which effectively reduces the running noise of air compressor. Compared with the original system, the noise drops by about 3 to 7 dB according to the past experience.

V. Equipment Investment

Special Note:

1. The control cabinet has a VEICHI AC70-T3-55K Frequency Inverter;

2. The control system is the core control system of the air compressor. It adopts VEICHI AC70K series frequency inverter one VFD drive three motor control mode, which greatly reduces the investment cost. The investment is affordable and effect is large. When the system used in various types of speed control system, its energy saving can reach up to 20-50% and generally can reach the upper limit without influence of other factors.

VI. Payback Period

1. Electricity saving of air compressor

Take 45KW fan running at 40Hz as an example: When running at the 80% of the operating speed, motor shaft power consumption: Pz=0.83Pn=0.512Pn. As known, there are 12 months in a year, 30 days in a month and 24 hours in a day. After considering all kinds of looses, and suppose energy savings account for 30% of electricity: W = 45*12*30*24*30% = 116640 kwh, and assume 0.5 RMB per kilowatt hour, the annual electricity savings: 116640*30.5 = 58320 RMB.

2. Save maintenance costs;

3. Investment return cycle = 12 (month) * 41500/58320 = 12 ÷ 152% = 8.5 (month). The service life of frequency inverters can reach more than 10 years. Only considering energy saving of motor, it takes 8 months or so to recover all the investment, which means “one investment, long-term benefit”. Frequency conversion speed control technology is a new technology which has perfect energy-saving effect in motor drag field. It can vigorously promote technological progress of enterprises and improve the comprehensive benefits of enterprises.