I. Introduction
In industrial production, the use of compressed gases is widespread. In a factory, several air compressors installed in one location constitute an air compressor station. Sichuan Zigong Honghe Chemical Industry Co., Ltd. has a hydrogen compressor station equipped with three 110kW explosion-proof reciprocating piston hydrogen compressors for compressing hydrogen. When designing and selecting the capacity of the hydrogen compressor motors, excessive consideration was given to the differences in long-term process requirements before and after construction, resulting in an overly large margin. Furthermore, it is difficult to perform accurate calculations during the design process to consider various problems that may occur during long-term operation. Typically, the hydrogen compressor is operated at full load for extended periods as the basis for selection. However, the range of hydrogen compressor motors is limited, and often, when a suitable motor model cannot be selected, an oversized model is chosen, often exceeding the limit by 20% to 30%. Due to these reasons, the actual operating time of the hydrogen compressors under light load in production increases significantly.
Furthermore, due to the uneven distribution of hydrogen in production, the consumption is dynamic. Sometimes, several hydrogen compressors need to operate simultaneously to supply hydrogen, while at other times, even a single hydrogen compressor may produce a surplus, yet it still runs at full speed. The exhaust pressure regulating device that comes with the hydrogen compressor is a closed-inlet pipe type pressure regulator. Its working principle is that when the air pressure in the gas receiver (air manifold) reaches or exceeds the set pressure (0.82MPa), the disc valve on the compressor inlet pipe automatically closes, and the compressor enters an idling unloading state. When the air pressure in the gas receiver falls below the set pressure (0.77MPa), the disc valve on the compressor inlet pipe automatically opens, and the compressor returns to full-load operation.
The discharge volume and pressure of a hydrogen compressor are not constant during operation, but process requirements change dynamically. Therefore, the hydrogen compressor always operates in a cycle of full-load and unload cycles. The operating current under full load is close to the rated current of the motor, while the idling current under unload is approximately 35-50% of the motor's rated current. This current does not perform useful work but is instead the mechanical loss due to idling at rated speed. Although this mechanical regulating device can achieve pressure regulation, the pressure regulation accuracy is low and pressure fluctuations are large. The hydrogen compressor always operates at rated speed, resulting in high mechanical wear, inefficient operation, and high power consumption.
Variable frequency speed control (VFD) allows for adjusting the motor speed as needed, reducing motor power consumption while maintaining high efficiency and achieving energy savings while meeting production process requirements. From an operational quality perspective, hydrogen compressor systems typically cannot be continuously adjusted based on load. However, VFD enables convenient and effective continuous adjustment, maintaining stable parameters such as pressure and flow rate, thereby significantly improving compressor efficiency and performance.
Based on the above theoretical analysis, under the condition that the cylinder volume of the hydrogen compressor cannot be changed, the only way to change the discharge volume is to adjust the speed of the hydrogen compressor; the hydrogen compressor is a constant torque load, and the compressor shaft power changes proportionally to the first power of the speed; when the total discharge volume of the hydrogen compressor is greater than the gas supply volume, adjusting the gas supply pressure by reducing the compressor speed is an effective way to achieve economical operation of the compressor.
Available speed control methods for AC asynchronous motors include pole-changing speed control, changing the belt pulley transmission ratio, and rotor resistance speed control for wound-rotor motors. Compared with other speed control methods, frequency conversion speed control has the advantages of stepless speed regulation, easy automatic control, no need to change the equipment structure, and minimal installation work. It is highly efficient and energy-saving. In other words, frequency conversion speed control for AC asynchronous motors is a high-efficiency speed control method that consumes no energy.
III. Variable frequency constant pressure gas supply
The frequency converter and pressure transmitter form a pressure closed-loop system, automatically reducing the hydrogen compressor speed and adjusting the gas supply pressure as needed to achieve economical compressor operation. Consider installing a pressure transmitter on the gas storage tank to feed the pressure signal back to the frequency converter terminals, forming a constant pressure gas supply system with a supply pressure set at 0.8 MPa.
The rated current of the hydrogen compressor motor should be equal to or less than the rated current of the constant torque frequency converter. The frequency converter should have a built-in PID controller and a 4-20mA analog signal interface. In this example, a Senlan SB70G132 frequency converter is selected, and the pressure transmission gas is selected from Sennas DG13W=BZ-A, 1.6MPa. The constant pressure gas supply principle diagram is shown in Figure 2.
In the diagram: The pressure sensor PT takes the pressure feedback signal from the gas storage tank and sends it to the input of the PID controller inside the frequency converter. This signal is compared with the preset pressure setpoint signal, and after processing by the PID controller, the operating frequency and speed of the motor are determined. This control method ensures that the hydrogen pressure in the storage tank remains constant while automatically adjusting the motor speed when the gas consumption changes, maintaining high-efficiency operation and achieving energy savings.
The frequency converter controls the first hydrogen compressor. The control keypad on the frequency converter is used for setting the parameters. The frequency converter's multi-function outputs Y1 and Y2 are connected to the start/stop circuits of the autotransformer starters for the second and third hydrogen compressors. This allows the frequency converter's outputs to control the operation or shutdown of the other two hydrogen compressors. In manual operation, the first hydrogen compressor is controlled by the frequency converter, while the second and third compressors can be manually started/stopped using the autotransformer starters. In automatic operation, the first hydrogen compressor initially operates at the frequency converter. When the frequency converter's output frequency reaches 50Hz, if the gas supply is still insufficient, the frequency converter's output nodes Y1 and Y2 activate, starting the second and third hydrogen compressors. If the gas supply exceeds the set value, the system automatically stops the second and third hydrogen compressors. Constant pressure gas supply can be achieved through closed-loop regulation.
Hydrogen is a flammable gas, and the working environment of a hydrogen compressor poses an explosion hazard. The SB70G series frequency converter has an IP20 protection rating, which clearly means it cannot be used in an explosion-hazardous environment. The frequency converter should be installed in a power distribution room where there is no explosion hazard, and operated via a remote control box near the compressor. The remote control box itself must also be intrinsically safe.
IV. Precautions
1. Due to the large rotational inertia of the air compressor, the capacity of the selected frequency converter should be determined according to the specific operating conditions and the on-site operating current.
2. The pressure sensor should be installed in an area where pressure changes are not relatively drastic, preferably on the gas storage tank. To avoid interference, the pressure sensor signal should ideally use a 4–20mA current signal, and the transmission line should be a double-core shielded cable.
3. The inverter control system should have both open-loop and closed-loop control modes to facilitate commissioning and use in special circumstances.
4. Since air compressors cannot operate at low frequencies for extended periods, not only does low-frequency operation reduce stability and increase the risk of surge, but it also worsens cylinder lubrication, accelerating wear. Therefore, a reasonable, effective, and safe lower limit for the operating frequency should be set. Specific settings must be patiently and meticulously adjusted based on different operating conditions, usage requirements, and specific needs.
V. Effects of Energy-Saving Retrofit
The compressor was upgraded in August 2005, and after three months of operation, it achieved the intended purpose. Regardless of changes in the production process or the gas supply volume, the constant pressure hydrogen supply maintained at around 0.8 MPa, and the gas supply quality was significantly improved.
After frequency conversion speed regulation, the air compressor starts smoothly from zero speed, improving production safety. The hydrogen compressor is no longer running at full speed at any time; as the speed decreases, the noise in the working environment also decreases accordingly. Lower speed also reduces mechanical wear, which helps extend the compressor's lifespan and reduce maintenance costs.
In terms of energy saving, as can be seen from the compressor formula, the shaft power PZ (kW) consumed by the compressor is directly proportional to the shaft speed n (r/min), and the compressor's discharge volume QD (m3/min) is also directly proportional to the shaft speed. Therefore, the shaft power PZ (kW) consumed by the compressor is directly proportional to the compressor's discharge volume QD (m3/min). Reducing the speed can save shaft power. The measured energy saving rate reached 26%, achieving good economic benefits.
