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Electrical energy is one of the main forms of energy for fine chemical companies. In addition to the pumps and agitators used for material handling and mixing in the process, high-power equipment is mainly concentrated in public works stations such as compressors, refrigerators, and cooling water pumps. Based on the analysis of the production characteristics of fine chemical enterprises, this paper proposes that the energy supply from the utility supply should be saved, and the operation of the air compressor unit, the refrigeration unit and the cooling water pump should be analyzed, and the inefficient reasons of the conventional control method should be analyzed. The frequency conversion speed optimization control scheme is proposed to improve energy efficiency.
1 Improve the energy efficiency of fine chemical companies from utility supply
Fine chemical industry is based on high and new technology, oriented to market demand, and features a chemical industry with specific functions, high added value, small batch size, multiple varieties and serialization. Small batches and multiple varieties require enterprises to produce multi-variety integrated production processes and multi-purpose and multi-purpose production equipment. High technology density and high added value mean extremely high development costs and profit margins.
The versatile and versatile equipment characteristics determine that it is not possible to design and determine optimal equipment parameters and optimal operating parameters from a particular product, making it difficult to improve energy efficiency from the process (user side). In the practice of energy saving work, it is found that the energy supply end, that is, the utility generating device, has great potential for energy saving. Due to the variety of products and intermittent production of fine chemical processes, the demand for utilities varies with products and operating procedures and is inherently unstable. In order to meet the maximum demand that may occur in production, the utility equipment and the conveying device often use the maximum demand as the design value. In addition, in the design process, from process design to equipment design to equipment manufacturing, each link uses a safety factor, often the final installed equipment capacity is much larger than the actual demand. These two factors lead to the long-term state of the utility equipment in the "large horse-drawn carriage" state, and the motor efficiency is low.
Public utility equipment serves as the public works supply of the whole plant, and the total power consumption accounts for more than half of the total electricity consumption of the whole plant. If you solve the "big horse car" phenomenon on these large devices and optimize their control methods, you can achieve considerable power savings.
2 Principles and benefits of frequency conversion speed regulation
The frequency converter is a device that can adjust the output voltage frequency arbitrarily, so that the three-phase asynchronous motor realizes stepless speed regulation. When the inverter is working, the three-phase AC power of the power supply is first rectified into DC power through the rectifier bridge, and then “inverted†by the inverter bridge into three-phase AC power whose frequency is arbitrarily adjustable.
Chemical companies generally use centrifugal fans and pumps to transport fluids. These loads conform to the quadratic law, that is, the torque of the load is proportional to the square of the rotational speed, and the load power is proportional to the cube of the rotational speed. According to the characteristics of the centrifugal pump or fan "fluid volume flow is proportional to the rotational speed", if the motor speed is adjusted to control the fluid delivery flow, the load power is proportional to the cube of the fluid flow rate, and the load power decreases sharply as the flow rate decreases. The traditional throttling control method reduces the flow rate by increasing the resistance of the pipeline, adding extra power to the throttling element.
Another advantage of variable frequency control is that it can maintain the load operating at the rated efficiency point. In the case of throttling control, the pump efficiency of a centrifugal pump is the most efficient at the rated point, and it is continuously decreasing on both sides, especially in the low speed zone. As the pump efficiency decreases, the shaft power of the drag motor decreases linearly with the flow rate. In addition to energy saving, variable frequency speed regulation can also reduce equipment failure rate, extend equipment life and improve control accuracy, thus generating additional economic benefits.
3 Variable frequency constant pressure control of cooling water transfer pump
In the chemical production, the amount of cooling water is large, and there are many users, forming a cooling water pipe network. In order to avoid or reduce the interference of the user's pipeline valve opening and closing on the flow of other users, it is necessary to keep the pressure of the water delivery pipe constant, that is, keep the outlet pressure of the cooling water delivery pump constant. According to the characteristic curve of the pump, if the flow rate changes at a certain speed, the head of the pump also changes. In order to keep the head of the pump constant, the bypass is usually used to control the outlet pressure of the pump. Bypass adjustment uses a bypass to balance the total flow of the pump to maintain head stability. From the point of view of energy efficiency, the cooling water flowing from the bypass to the pump inlet has no effect on the production. The work consumed by the pump to transport this part of the water is useless and uneconomical.
The variable frequency speed control constant pressure water supply directly compares the pressure value transmitted from the pressure transmitter with the set value, and obtains the inverter output frequency through the PID control algorithm, thereby changing the pump speed and adjusting the pump outlet pressure to form a closed circuit. Control System.
The energy saving of variable frequency constant voltage control is not proportional to the cube of the flow, but only linearly proportional to it. The energy saving amount is significantly lower than the energy saving of the above variable frequency flow control. This is due to the fact that when the user closes the small valve, the main pipe is caused. The increase in road pressure drop leads to additional losses, but this is inevitable to keep the user independent.
4 Variable frequency constant pressure air supply control of air compressor unit
There are many types of air compressors, such as piston air compressors, screw air compressors, and centrifugal air compressors. However, the air supply control methods are almost always controlled by adding and unloading. The addition and unloading control mode causes the pressure of the compressed gas to vary back and forth between p min and p max . p min is the lowest pressure value that can guarantee the user's normal operation, and p max is the set maximum pressure value.
There are great energy wastes in the unloading control mode, which are mainly reflected in the following three parts. 1 The energy consumed by the compressed air pressure exceeding p min. When the air pressure in the gas storage tank reaches the production requirement pmin, the loading and unloading air supply control mode must continue to increase its pressure until pmax. This process requires additional compression work. 2 The energy consumed by the pressure reducing valve. The rated air pressure of the pneumatic component is around p min. Before the gas above p min enters the pneumatic component, it needs to be reduced to p min with a pressure reducing valve, which is an energy loss. 3 The energy consumed by the unreasonable adjustment method when unloading. During the unloading, although the compressor does not need to compress the gas to do work, the motor still needs to consume the electric energy to drive the screw to make a rotary motion. According to estimates, the energy consumption of the air compressor when unloading accounts for about 10% to 35% of the full load operation.
The variable frequency constant voltage control automatically adjusts the motor speed according to the deviation of the gas tank pressure and the set pressure to maintain the constant pressure, which can overcome the above shortcomings of the loading and unloading control, and achieve the purpose of high efficiency, energy saving and precise control of the system. The energy consumption of the variable-frequency constant-pressure gas supply control is more complicated than that of the unloading control. The mathematical expression is often complicated, and the energy-saving quantity is often compared based on the measured data.
Example An enterprise has 4 screw-type air compressors, all of which are 90 kW. The original ones are controlled by loading and unloading. The first three air compressors are in a long-term loading state, while the fourth one is frequently loaded and unloaded. According to statistics, the daily unloading time is 9 h, and the motor power during unloading is 20% at the time of loading. It is a stable pressure, and the fourth air compressor adopts variable frequency PID control. According to the electricity consumption statistics of each month before and after the transformation, the energy saving of the air compressor after the constant voltage control is 30%. If the annual operation is 300 days, the annual energy saving is W s =90×(15+9×20%). ) × 300 × 30% = 136 080 kW? h combined with variable frequency speed regulation, if centralized control of multiple compressors is carried out, reasonable setting of start-stop rules, and optimization of system pressure setting values, more significant energy savings can be achieved.
5 Refrigeration unit frequency conversion control
The compression refrigeration cycle consists of four steps: compression, condensation, throttling, and evaporation. There are two power consumption links in the refrigeration unit: one is the compressor, and the other is the cooling water system. These two links may have the problem of “big horses†in actual operation.
Cooling water system. In design, the capacity of the cooling water pump is determined according to the maximum heat exchange capacity and the worst conditions, that is, the compressor 100% load and the ambient temperature are the highest, and then a certain safety factor is determined. Due to changes in seasonal and daytime temperatures and changes in process cooling load, the actual heat transfer is much smaller than the design value. Therefore, the temperature difference between the inlet and outlet of the cooling water is very small, even about 1 °C. From the point of view of energy saving, as long as the cooling water can take away the heat in the condenser in time to ensure the normal operation of the refrigeration cycle, the smaller the flow rate of the cooling water, the less the pump will do, and the more obvious the energy saving.
Compressor system. The compressor adjusts the evaporator temperature or the amount of cooling by adjusting the amount of refrigerant suction. The guide vane electric regulator automatically controls the opening of the guide vane, and the guide vane can adjust the refrigerant flow between 100% and 15% of the design value to control the refrigeration capacity of the refrigeration unit. The design value of the motor is also 100% of the load, and the phenomenon of “large horse-drawn carriage†appears under partial load.
If the inverter control system is modified by frequency conversion technology, the compressor manufacturer needs to participate in the transformation due to the control of the impeller and the internal modification of the refrigeration unit, and it is not allowed to change it. According to the data provided by the refrigeration unit manufacturer York (YORK), the annual power saving of the inverter controlled cooling capacity is 15% to 25%, and under low load, the energy saving continues up to 75%.
It is simple and easy to use the variable frequency speed control technology to transform the cooling water flow rate. The cooling water system of the refrigeration unit needs to be independent of the workshop cooling water system. Because the head difference is very large, do not share the cooling water pump. When the cooling water system realizes the frequency conversion speed regulation, the temperature difference control of the cooling water inlet and return water is adopted. The large temperature difference indicates that the refrigeration unit has a large refrigeration load. The pump speed should be increased to increase the cooling water flow and increase the cooling capacity. The temperature difference is small, the refrigeration unit load is small, and the cooling water flow is appropriately reduced to increase the energy saving effect. Since the inlet temperature of the cooling water changes with the ambient temperature, the heat exchange effect is affected. Therefore, the set temperature difference should be appropriately adjusted according to the inlet water temperature. When the inlet water temperature is low, the energy-saving effect should be emphasized, and the set temperature difference can be appropriately increased; when the inlet water temperature is high, the cooling effect must be ensured, and the set temperature difference should be reduced. The variable frequency speed control is used to control the cooling water flow of the refrigeration unit. The effective work is proportional to the cube of the flow. The energy saving is very considerable.
6 Conclusion
The uncertainty of the fine chemical production and the conservative design make the phenomenon of “big horse trolley†prevail in production.
Improving energy efficiency from the utility supply side is a reliable and simple way for fine chemical companies to do energy conservation and emission reduction work. Air compressor units, refrigeration units and cooling water transfer pumps are the main power consumers of public utility systems. They use frequency control technology to optimize their operation and improve their operating efficiency, which not only reduces equipment energy consumption, but also improves system stability. Sexuality and control accuracy to achieve considerable economic benefits.
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