Application  | 

Frequency AC Drives in Air Compressors In The Chemical Industry

  I. Introduction

    In industrial production, the use of compressed gas is very common. In a factory, several air compressors are installed in one place to form an air compressor station. 110kW explosion-proof reciprocating piston hydrogen compressor, used to compress hydrogen. When designing and selecting the capacity of the hydrogen compressor motor, too much consideration was given to the difference in long-term process requirements before and after construction, resulting in an excessive margin. In addition, it is difficult to make accurate calculations during the design process. Considering various problems that may occur during long-term operation, the hydrogen compressor is usually operated at full load for a long time as the basis for selection. However, the series of hydrogen compression motors is limited. Yes, if you can't choose a suitable motor model, you often rely on it. 20% to 30% larger ones are more common. Due to the above reasons, the light-load operation time of hydrogen compressors will increase significantly in actual production.

   In addition, due to the uneven use of hydrogen in production, the gas consumption changes dynamically. Sometimes it is necessary to run several hydrogen compressors at the same time to supply gas, and sometimes the gas produced by one hydrogen compressor is surplus, but the hydrogen compressor Still running at full speed. The exhaust pressure regulating device equipped with the hydrogen compressor when leaving the factory is a closed air inlet pipe pressure regulator. Its working principle is that when the air pressure in the gas storage tank (air bag) reaches or exceeds the set pressure (0.82MPa), the compression The butterfly valve on the air inlet pipe of the machine automatically closes, and the compressor enters the idling unloading state. When the air pressure in the gas storage tank is lower than the set pressure (0.77MPa), the butterfly valve of the compressor air inlet pipe automatically opens, and the compressor enters full load operation again.

   The exhaust volume and pressure of the hydrogen compressor are not constant during operation, but the process requirements change dynamically, so the hydrogen compressor always repeats the full load-unload mode when working. The operating current at full load is close to the rated current of the motor, while the idling current when unloaded is about 35~50% of the motor's rated current. This part of the current is not useful work, but the idling loss of the machine at the rated speed. Although this kind of mechanical adjustment device can also play a role in pressure adjustment, the pressure adjustment accuracy is low and the pressure fluctuation is large. The hydrogen compressor always works at the rated speed, has large mechanical wear, operates in an inefficient state, and consumes high power.

If variable frequency speed regulation is used, the speed of the motor can be adjusted as needed to reduce the operating power of the motor in a highly efficient state, thereby achieving energy saving while meeting the requirements of the production process. From the perspective of operating quality, most hydrogen compressor systems cannot continuously adjust according to the severity of the load. After adopting variable frequency speed regulation, continuous adjustment can be carried out very conveniently and effectively to maintain the stability of pressure, flow and other parameters, thus greatly improving the working efficiency and performance of the compressor.

   According to the above theoretical analysis, under the condition that the cylinder volume of the hydrogen compressor cannot be changed, the exhaust volume can only be changed by adjusting the rotation speed of the hydrogen compressor; the hydrogen compressor is a constant torque load, and the compressor shaft power is proportional to the first power of the rotation speed. Proportional change; when the total exhaust volume of the hydrogen compressor is greater than the air supply volume, adjusting the air supply pressure by reducing the compressor speed is an effective method to achieve economical operation of the compressor.

  The AC asynchronous motor speed regulation methods that can be used include pole-changing speed regulation, changing the pulley transmission ratio, winding motor rotor string resistance speed regulation, etc. Compared with other speed regulation methods, frequency conversion speed regulation has the characteristics of stepless speed regulation, easy automatic control, no need to change the equipment structure and small installation workload, efficient speed regulation and energy saving. That is, AC asynchronous motor frequency conversion speed regulation is an efficient speed regulation method that does not consume energy.

   2. Frequency conversion constant pressure air supply

   The frequency converter and pressure transmitter form a pressure closed-loop system, which automatically reduces the hydrogen compressor speed and adjusts the air supply pressure as needed to achieve economical operation of the compressor. Consider installing a pressure transmitter on the gas storage tank to feed back the pressure signal to the terminals of the frequency converter to form a constant pressure air supply system, with the air supply pressure set to 0.8MPa.

The rated current of the hydrogen compressor motor should be equal to or less than the rated current of the constant torque inverter. The frequency converter must have a built-in PID regulator and a 4-20mA analog signal interface. Hydrogen is a flammable gas, and there is a danger of explosion in the workplace of the hydrogen compressor. The protection level of the inverter is IP2O, so it obviously cannot be used in places where there is a danger of explosion. Install the frequency converter in a power distribution room with no risk of explosion, and use a remote control box to operate it next to the compressor. The remote control box must also be considered intrinsically safe.

   3. Precautions

   1. Due to the large moment of inertia of the air compressor, when selecting a frequency converter, the capacity of the selected frequency converter should be determined based on the specific working conditions and the operating current on site.

   2. The pressure sensor should be installed in a place where the pressure changes are not relatively severe, and it is best to install it on the gas storage tank. In order to avoid interference, the pressure sensor signal should use a 4-20mA current signal as much as possible, and the transmission line should use a two-core shielded wire.

   3. The control system of the frequency converter should be equipped with two control modes, open loop and closed loop, to facilitate debugging and use under special circumstances.

   4. Since the air compressor is not allowed to work at low frequency for a long time, running at low frequency will not only worsen the stability, but also easily cause surge. On the other hand, it will also worsen the lubrication of the cylinder and accelerate wear, so it should be set Set a reasonable, effective and safe lower limit of operating frequency. Specific settings must be patiently and carefully adjusted according to different working conditions, different usage conditions and requirements.

   5. Effects after energy-saving renovation

  After three months of operation, the compressor has achieved the expected purpose. No matter how the production process changes or the gas supply volume changes, the hydrogen pressure of constant pressure gas supply is maintained at about 0.8MPa, and the gas supply quality is greatly improved.

After variable frequency speed regulation, the air compressor starts smoothly at zero speed, improving production safety. The hydrogen compressor no longer operates at full speed at any time. When the rotational speed is reduced, the noise in the working environment will also decrease accordingly. When the rotational speed is reduced, the mechanical wear will be reduced, which will help extend the life of the compressor and reduce maintenance costs.

In terms of energy saving, it can be seen from the formula of the compressor that the shaft power PZ (kW) consumed by the compressor is proportional to the shaft speed n (r/min), and the displacement QD (m3/min) of the compressor is proportional to the shaft speed Proportional, then the shaft power PZ (kW) consumed by the compressor is proportional to the displacement QD (m3/min) of the compressor. Reducing the speed can save shaft power. The measured power saving rate reaches 26%, which is a good result. economic benefits.