Research and Application of Energy-saving Technology of Two-Stage Permanent Magnet Frequency Conversion Air Compressor

Compressed air is an indispensable power source for beer production. In the beer production process, the compressed air system is responsible for providing the air source for the beer process, production equipment and various pneumatic components. However, the high energy consumption of the compressed air system has also been the focus of beer production enterprises, according to statistics, the compressed air system accounts for about 15% of the total electricity consumption of the beer production enterprises; of which, the air compressor, as the core equipment in the compressed air system, its energy consumption accounts for more than 70% of the entire compressed air system. So the key to energy-saving modification of the compressed air system is to effectively reduce the level of energy consumption of the air compressor. In recent years, in the face of increasingly severe external environment and the intensification of competition in the beer market, energy-saving technical transformation of the compressed air system, especially the use of lower unit energy consumption of air compressors instead of higher unit energy consumption of air compressors, has a certain degree of feasibility, which is not only conducive to energy saving and consumption reduction of the compressed air system, to improve the service life of the equipment, but also to achieve the goal of the beer producers to reduce costs and increase efficiency. Read about Two-Stage Permanent Magnet

1. Analysis of the Current Situation of Air Compressors in the Beer Industry

At present, by the enterprise scale, production capacity and capital and other factors, the air compressors of many small and medium-sized beer producers are generally industrial screw air compressors. During operation, the industrial screw compressor is adjusted according to the exhaust port pressure and the dynamic change of gas consumption in the pipeline network; when the amount of gas consumption changes constantly, the compressor will continuously cycle between startup, loading, pressure preservation, unloading, and shutdown to stabilize the air pressure. Frequent loading and unloading of the air compressor not only makes the air compressor operating efficiency is low, resulting in serious energy loss at the same time, but also further reduces the service life of the air compressor, so that the air compressor maintenance costs increase year by year with increased usage time; in addition, “High pressure, low use”, irrational matching and too wide a pressure band setting are also common problems in the current use of air compressors in the compressed air system of beer production enterprise.

As the demand for low energy consumption and energy saving becomes more and more urgent, some large-scale beer production enterprises have started to gradually use two-stage permanent magnet frequency conversion screw air compressor to replace the traditional industrial frequency screw air compressor, to promote the enterprise’s cost-effective work and energy saving and emission reduction goals.

2. Operation Analysis of Two-stage Permanent Magnet Frequency Conversion Screw Air Compressor

2.1 Motor Analysis of Two-stage Permanent Magnet Frequency Conversion Screw Air Compressor

Generally speaking, the motor of two-stage permanent magnet frequency conversion screw air compressor is permanent magnet synchronous motor Permanent magnet synchronous motor has no excitation coil, the DC excitation winding of rotor is replaced by permanent magnet, which significantly improves the power factor and reduces the stator current and copper consumption; at the same time, the rotor permanent magnet and the stator rotating magnetic field run in synchronization, which does not produce rotor induced current and reduces the rotor loss. Compared with Y series asynchronous motors, permanent magnet synchronous motors are close to the maximum efficiency at the initial stage of rotational speed, permanent magnet synchronous motors have greater energy-saving advantages in energy-saving effect compared with industrial frequency motors.

In addition, compared with the soft start method ,the permanent magnet synchronous motor with variable frequency drive (Variable Frequency Drive, VFD), permanent magnet frequency conversion starting method is smoother, smaller starting current, lower energy consumption, start completely avoid the current peak, which effectively improves the reliability and stability of the compressor, and completely eliminates the impact of the industrial frequency start on the grid and equipment (Figure 1). When the unit is operated under frequency conversion, the permanent magnet synchronous motor will maintain extremely high efficiency, so that the motor speed, torque, and operating characteristics can be adapted to the load of the compressor, and the peak efficiency and power factor can be fully utilized under full load.

Compared with the frequent loading and unloading of the industrial frequency screw air compressor, after the two-stage permanent magnet variable frequency screw air compressor starts to run at the rated pressure, its frequency converter can adjust the motor speed through the real-time air consumption and carry out stepless speed tracking of the air production, controlling the air compressor to output the actual amount of air needed under the condition of keeping the pressure unchanged. As shown in Figure 2, the pressure of an ordinary industrial frequency screw compressor fluctuates within 0.07MPa above and below the rated pressure, while the pressure of a two-stage permanent magnet frequency conversion compressor with a VFD can be stabilized within 0.01MPa above and below the rated pressure, thus realizing a constant pressure supply of air, which not only reduces the energy consumption of the compressor, but also improves the reliability and service life of the unit and the equipment that uses air.

2.2 Compression Technical Analysis of Two-stage Permanent Magnet Frequency Conversion Screw Air Compressor

Most of the industrial screw air compressors are single-stage screw air compressors, mainly with a set of parallel meshing yin and yang rotors rotating relative to each other in the body. According to the engineering thermodynamics theory, in the compression process, the isothermal compression of the compressor saves the most power. Figure 3 shows pressure P vs. volume V for isothermal compression and single-stage compression of the gas, which, area 0-1-2′-3-0 for isothermal compression of the power required, area 0-1-2-3-0 for single-stage compression of the power consumed. As can be seen from Figure 3, the power consumption of isothermal compression is smaller compared to that of single-stage compression, and isothermal compression of an air compressor is not only conducive to reducing the power consumed, but also reduces the temperature of the compressed gas. Therefore, for air compressors, in the compression process, isothermal compression is the most ideal compression process for air compressors. However, isothermal compression of air compressors is almost difficult to realize in actual operation, which is due to the temperature of the compressed gas can’t be completely equal to the outside temperature. Therefore, to reduce the temperature of the compressed gas and improve the efficiency of the compressor, the compression process of the compressor should be as close as possible to isothermal compression .

The two-stage permanent magnet frequency conversion screw air compressor, on the other hand, adopts two sets of yin and yang screw rotors that are equally meshed parallel to each other, realizing reasonable pressure distribution through two-stage compression and reducing the compression ratio of each compression. In addition, after one stage of compression, the compressed gas is cooled by adding intermediate cooling. The coolant in the compressor passes through a few injection holes in the body of the compressor, forming a mist-like spray curtain that cools the hot compressed air almost perfectly, greatly reducing the temperature of the compressed air before it enters the second-stage compression stage, which is close to the isothermal compression process and saves the energy required for compression. Figure 4 shows pressure P vs. volume V for two-stage compression of the gas, from the above, area 0-1-2′-3-0 and area 0-1-2-3-0 are the work consumed in isothermal compression and single-stage compression, respectively; when two-stage compression is used, the work consumed in compression is the sum of the regions included 0-1-4-5-0 and 5-4′-2′ ‘-3-5. By comparing the work consumed by single stage compression (0-1-2-3-0), the power saved by two-stage compression is area 4′-4-2-2”-4’, thus it can be seen that two-stage compression has a better energy saving advantage as compared to single stage compression .

3. Energy Saving Retrofit Cases

A beer producer uses compressed air piping to blow the lees in three saccharification lees sections (shown in Figure 5). Among them, the flow demand of compressed air for the first line of lees blowing is 17.1m³/min, with a daily average use of 9 hours; the second line is 16.91m³/min, with a daily average use of 6 hours; the third line is 18.8m³/min, with a daily average use of 4 hours. The compressed air usage of the exhaust accounted for 43.55% of the total air consumption of the whole plant.

 Currently, the beer producer is equipped with three industrial frequency screw air compressors for pipeline blowing and lees removal (shown in Table 1). As can be seen from Table 1, the three air compressors are two micro-oil air compressors with rated pressure of 0.7MPa and flow rate of 10m³/min and one micro-oil air compressor with rated pressure of 0.8MPa and flow rate of 20m³/min, while the pipeline air blowing to remove the lees actually only 3.5-4.5bar, this is the phenomenon of “big marat small car”. 

 The energy-saving reform is carried out at the site, and three two-stage permanent magnet frequency conversion screw air compressors with an exhaust capacity of 18m³/min, pressure of 4bar and specific power of 4.5kwh/(m³/min) are used to replace the original three industrial frequency screw air compressors. Before the transformation, the average annual flow rate of the three industrial frequency screw air compressors is about 16350596m³, and the average annual power consumption is 1791208kwh; after the transformation, the annual power consumption of the three two-stage permanent magnet frequency conversion screw air compressors is about 1226295kwh, and compared with the 1791208kwh before the transformation, the power saving is about 564913kwh, and the energy-saving rate reaches 31.5%. Compared with the industrial frequency screw air compressor before the transformation, the adoption of two-stage permanent magnet frequency conversion air compressor has an obvious energy saving advantage.

4. Concluding Remarks 

Our company analyzed the current situation of air compressors in the compressed air system of the beer industry, compared the characteristics of industrial frequency screw air compressors and two-stage permanent magnet frequency conversion air compressors from the technical point of view of the problem of high energy consumption of air compressors and verified that the two-stage permanent magnet frequency conversion air compressors have more obvious energy saving effect than the industrial frequency screw air compressor system through the relevant successful cases. However, many small and medium-sized beer producers in the beer industry prefer to use industrial screw air compressors for capital cost considerations, and the popularity of two-stage permanent magnet frequency conversion screw air compressors is still relatively low. As a new type of energy-saving equipment for compressed air system, the two-stage permanent magnet frequency conversion screw air compressor is more energy-saving, longer life, and has been vigorously developed in recent years, and has been widely used in other traditional industries.