Kompresory
Application of SLANVERT Inverter on Air Compressor of Chemical Industry
I Foreword
The compressed gas is widely used in industrial production. If several air compressors are installed in a place within the plant, a compressed air station is constituted. When selecting the motor capacity of hydrogen compressor, too much consideration is given to the difference required by long-term process before and after construction, which results in too much surplus capacity. Besides, accurate calculation is difficult to be made during design. Considering various problems which may be occurred in long term operation, the long term full load operation of the hydrogen compressor is always taken as the basis for type selection. However, the series of hydrogen compressor motor is limited, no appropriate motor type can be selected usually, and the power which is 20%~30% greater than the rated power always selected. The period of light running of hydrogen compressor will be obviously increased in the actual production due to the above reasons.
Moreover, the gas consumption is dynamically changed due to the irregularity of hydrogen use in production, and several hydrogen compressors are required to be operated at same time, while the gas of a hydrogen compressor is sufficient at some times, while under such condition, the hydrogen compressors are still full speed running. The discharge pressure regulation unit provided with the hydrogen compressor when leaving the factory is intake pipe closed type, and its operating principle is when the air pressure inside the air reservoir (air receiver) reaches or exceeds the setting pressure (0.82MPa), the butterfly valve on the inlet pipe of the compressor will be automatically closed, and the compressor will enter into idle unloading status. When the air pressure inside the air reservoir is less than the setting pressure (0.77MPa), the butterfly valve on the inlet pipe of the compressor will be automatically opened, and the compressor will enter into full load operating status.
The air discharge and pressure of the hydrogen compressor are not always unchanged, but the requirements on the process are dynamically changed, thus the hydrogen compressor is always in the repetitious operation mode of full-load and unloading. The operating current under full load condition is in proximity to the rated current of the motor, while the no-load running current under unloading condition is about 35~50% of the rated current of the motor, and under such condition, the current is not doing useful work but produces idling loss made by the machinery under rated rotation speed. Though this type of mechanical regulation unit can regulate the pressure, its pressure regulation accuracy is low, and the pressure fluctuation is great. If the hydrogen compressor is always working under rated rotation speed, the mechanical wear will be great, the operation efficiency is always in low-level, and the power consumption is high.
If frequency-conversion speed regulation is employed, the rotation speed of the motor can be regulated according to the demand, and the operating power of the motor can be reduced under high efficiency, which can realize the purpose of energy saving when the requirements on production process are met. In view of operating quality, most of hydrogen compressor systems cannot be continuously regulated according to the load. After frequency-conversion speed regulation is employed, continuous regulation can be made conveniently and effectively, and the parameters such as pressure and flow can be maintained stable, thus greatly improving the working efficiency and performance of the compressor.
II Energy Saving Principle of Hydrogen Compressor
According to air compression theory, the shaft power PZ(kW), air discharge QD(m3/min) and shaft speed n(r/min) of the hydrogen compressor meet the following formulae:
Where:Mr—average torque of hydrogen compressor input (N•m)
K—Coefficient relating to volume, pressure, temperature and leakage of the cylinder
n2—rotation speed of the compressor after regulation (r/min)
V—volume of cylinder (m3)
According to the above theoretical analysis, the rotation speed of the hydrogen compressor can only be regulated by changing the air discharge when the cylinder volume of hydrogen compressor cannot be changed. The torque load of the hydrogen compressor is constant, i.e. the shaft power of the compressor is in direct proportion to the first power of the rotation speed; when the total air discharge of the hydrogen compressor is greater than the air supply volume, the air supply pressure can be adjusted by reducing the rotation speed of the compressor, and it is an effective method for economic operation of the compressor.
III Air Supply under Frequency Conversion and Constant Pressure
The inverter and pressure transmitter constitute a pressure closed-loop system, under which the air supply pressure will be adjusted automatically by reducing the rotation speed of hydrogen compressor according to the demand, so as to realize economic operation of the compressor. A pressure transmitter is considered to be installed on the air reservoir to feed back the pressure signal to the inverter terminal, which constitutes a constant pressure air supply system, with the air supply pressure of 0.8MPa.
The rated current of the hydrogen compressor motor is equal to or less than the rated current of the inverter with constant torque. A PID regulator and 4~20mA analog signal interface shall be built in the inverter. A SLANVERT SB61G110KW inverter is selected in this case, and Senex DG13W=BZ-A (1.6MPa) is selected for pressure transmitter. The schematic diagram of air supply under constant pressure is as shown in Fig. 2:
In the figure: the pressure feedback signal is taken from air reservoir by pressure sensor PT and then sent to input end of built-in PID regulator of the inverter, which shall be compared with the preset pressure demand signal, at last the working frequency and rotation speed of the motor shall be determined after processed by PID regulator. Under such control mode, the rotation speed of the motor will be automatically regulated when the air consumption is changed under the premise that the pressure of the hydrogen in the air reservoir is always constant, so as to maintain highly efficient operation and realize energy saving.
Fig. 2 Schematic Diagram of Air Supply under Constant Pressure
The inverter controls the first hydrogen compressor, and the operation keyboard on the inverter is used for the setting control. Multifunctional outputs Y1 and Y2 of the inverter are connected to the startup/stop circuit of auto-transformer starter of the second and third hydrogen compressors, thus the operation or shutdown of other two hydrogen compressors can be controlled by the output of the inverter. Under manual operation, the first hydrogen compressor is controlled by the inverter, and the second and third hydrogen compressors can be manually started/stopped with auto-transformer starter. Under automatic operation, the first hydrogen compressor will be operated under frequency conversion, and when output frequency of the inverter reaches 50Hz, the air supply volume is still insufficient, the output nodes Y1 and Y2 of the inverter will be activated to start the second and third hydrogen compressors; if the air supply volume is greater than the setting value, the system will automatically stop the second and third hydrogen compressors working, so as to realize air supply under constant pressure by means of closed loop regulation.
Hydrogen is combustible gas, thus the work site of hydrogen compressor has explosion hazard. The level of protection of SB61G series inverter is IP21, and obviously, it shall not be used in the site where the explosion hazard exists. The inverter shall be installed in a switching house where no explosion hazard exists, and a remote control box shall be used nearby the compressor for operation. The remote control box shall be intrinsically safe.
IV Precautions
1. Because the moment of inertia of air compressor is great, the inverter capacity shall be determined according to the actual operating conditions and running current of the site when the inverter is selected.
2. The pressure sensor shall be installed at the place where the pressure change is not intense, and it may be installed on the air reservoir for the best. 4~20mA electric current signal shall be used as the signal of pressure sensor to avoid interference, and the transmission line shall select dual core shield line.
3. Open loop and closed loop control modes shall be set in the control system of inverter, so as for commissioning and use under special conditions.
4. Because the air compressor is not allowed to be working under low frequency condition for long term, under which condition, the stability becomes inferior, surging is easy to be occurred, the lubrication of cylinder body becomes inferior, which accelerates abrasion, thus appropriate, effective and safe lower limit of working frequency shall be set. The actual setting shall be subject to aborative regulation according to different operating conditions and use conditions and requirements.
V Effect after Transformation of Energy Saving
The compressor is altered in Aug. 2005 and realizes the anticipated purpose after three months’ operation. The hydrogen pressure is maintained about 0.8MPa no matter how the production or air supply volume is changed, and the gas supply quality is greatly improved.
The air compressor is smoothly started at zero velocity after frequency-conversion speed regulation is employed, which greatly improves the safety in production.
The hydrogen compressor is not full speed running any longer at any time. When the rotation speed is reduced, the noise of working environment is reduced correspondingly; when the rotation speed is reduced, the mechanical wear and tear is reduced correspondingly, which is good for prolonging the life of the compressor and reducing the maintenance cost.
In view of energy saving, it can be seen from the formula of the compressor, the shaft power PZ(kW) consumed by the compressor is in direct proportion to shaft speed n(r/min), while gas discharge of the compressor QD(m3/min) is in direct proportion to shaft speed, thus the shaft power PZ(kW) consumed by the compressor is in direct proportion to gas discharge of the compressor QD(m3/min). The reduction in rotation speed can save shaft power, and the power saving ratio is actually measured to be 26%, thus significant economic benefit is obtained.