【Original】Energy savings and consumption reduction for air compressors! ! ! ! !
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Energy Saving and Consumption Reduction in Air Compressors Abstract: We have stepped into the new century, yet within it lies a fierce obstacle, with its gaping maw ready to devour all of humanity. This is a century-old issue – the problem of energy conservation and reduction of consumption. It is getting worse, and we must take action to stop it. Keywords: Energy consumption analysis In modern air separation production processes, automation plays an important role. It plays an important role in ensuring safe and reliable production, improving product quality and output, reducing energy consumption and costs, controlling environmental pollution, increasing the operational efficiency of equipment and labor productivity, as well as enabling scientific production control and management. The improvement in the automation level of the air separation industry has also created conditions for the promotion and refinement of new air separation production processes. The air separation production process is complex, with numerous equipment interlocks and various detection signals that influence each other; it is difficult to meet the control requirements using traditional manual control or relay-based instrument control. With distributed control, true decentralization of control and risk, along with centralized management, is achieved. In the event of a malfunction in the equipment, the system can automatically shut down the relevant devices according to pre-programmed procedures, and issue audible and visual alarms to alert operators to take action promptly so as to avoid disruptions to production. For some important parameters and alarms, the system can automatically generate historical records and trends. Operators and technicians can check historical data at any time to understand production status. By statistically analyzing data, new processes and methods are developed to continuously improve the output and quality of products. Air compressors are widely used in industrial production. There are many types of air compressors, including piston air compressors, screw air compressors, and centrifugal air compressors; however, their air supply control methods almost always involve loading and unloading control. Although this gas supply control method is simple in principle and easy to operate, it has many problems such as high energy consumption, prone damage to the intake valve, and unstable gas supply pressure. With the development and progress of society, technologies that are efficient and consume little energy have received increasing attention. Whether variable frequency speed control technology can be applied in the field of air compressor air supply to save electricity, improve the performance of air compressors, and enhance the quality of the supplied air has become a topic of interest for us. 1. Working principle of air compressors: Currently, air compressors use two-point control (upper and lower limit control) or on/off control (for small air compressors). That is, when the pressure in the compression cylinder reaches the set upper limit, the air compressor shuts off the intake valve using its own air pressure or oil pressure; small air compressors simply stop operating. When the pressure drops to the set lower limit, the air compressor opens its intake valve, and the small air compressor starts up again. Traditional control methods can easily cause disturbances to the power grid, and they also pose certain damage to the air compressor itself, especially when there are frequent fluctuations in air demand. Under normal operating conditions, air is compressed into the air storage tank. The monitoring of various parameters of the air compressor (including compressed air temperature and pressure, screw rod temperature, cooling water pressure and temperature, oil pressure and temperature, etc.) as well as overall control are managed by the main control microcomputer. When the outlet pressure of the air compressor reaches the upper set value, the air inlet is closed via the hydraulic bypass valve, while the internal circulation circuit is opened to enable self-circulation operation. At this point, the gas-consuming unit continues to use gas. When the pressure drops to the set lower limit, the hydraulic bypass valve closes the circulation circuit and opens the air inlet, allowing air to be compressed by the filter and stored in the air tank. In static conditions, as well as during the original starting mode (Y-△) and during loading and unloading, it exerts a significant strain on the power supply and distribution equipment as well as the bolts. Especially the severe waste of energy. As the speed of the main motor decreases, the shaft power will drop significantly. The energy-saving potential is quite significant. ) The energy-saving effect of variable frequency technology is very significant, especially in systems and equipment with a wide range of adjustment possibilities. Practical applications show that even slight changes in speed (frequency) when the flow rate changes can result in substantial variations in shaft power. It is precisely because of this characteristic that variable frequency speed control (for energy savings) has become a trend and is increasingly being applied in various industries and different areas requiring adjustment. 2. Problems with the supply air control methods for loading and unloading 2.1 Energy consumption analysis As we know, the loading control method causes the pressure of the compressed gas to fluctuate between Pmin and Pmax. Pmin is the minimum pressure value, that is, the lowest pressure required to ensure normal operation by the user. Under normal circumstances, the relationship between Pmin and Pmax can be expressed by the following formula: CPmax = (1+δ)Pmin. This is a percentage whose value lies roughly between 10% and 25%. If variable-frequency speed control technology is used to continuously adjust the gas supply volume, the pipeline network pressure can be maintained at a level that meets the required gas supply pressure, namely around Pmin. It can be seen that, under the supply air control methods of loading and unloading, the energy wasted by air compressors is primarily in two areas: (1) the energy consumed when the compressed air pressure exceeds Pmin; after the pressure reaches Pmin, the original control method causes it to continue rising (up to Pmax). This process is also an energy-consuming process. (2) Energy consumed due to unreasonable adjustment methods during unloading. Under normal circumstances, when the pressure reaches Pmax, the air compressor reduces the pressure and unloads in the following way: it closes the intake valve to keep the motor running idly, while releasing the excess compressed air in the separation tank through the vent valve. This adjustment method results in significant energy waste. 2.2 Other shortcomings: (1) The intake valve is adjusted mechanically, which prevents continuous regulation of the air supply volume; as the air demand changes continuously, the air supply pressure inevitably experiences significant fluctuations. The precision of gas usage does not meet the process requirements. Furthermore, frequent adjustment of the intake valve accelerates its wear, increasing the amount of maintenance required and the associated costs. (2) Frequent opening and closing of the vent valve does not ensure its durability. 3. Design of the constant-pressure air supply control scheme: In view of the various problems existing in the original air supply control method, and based on the analysis conducted above, variable-frequency speed control technology is applied to achieve constant-pressure air supply. The actual pressure P, measured by the pressure transmitter, is sent to the PID intelligent speed controller. It is compared with the set pressure value P0, and calculations are performed according to the magnitude of the difference following a predefined PID control pattern; the resulting control signal is then sent to the VVVF frequency converter, which uses it to control the motor’s operating frequency and speed, thereby keeping the actual pressure P as close as possible to the set pressure P0. At the same time, this solution enables frequency conversion between mains frequency and variable frequency, while retaining the existing control and protection systems. Additionally, with this solution, the air compressor motor can be started from a stationary state to operating condition using an inverter, thereby achieving soft starting and avoiding the inrush current during startup as well as the mechanical stress exerted on the air compressor due to startup. The specific control system flowchart is shown in Figure 1, while the circuit for switching between the variable-frequency and mains frequency power supplies is shown in Figure 2 ； The electrical control schematic diagram of the air compressor and the wiring diagram of the variable frequency speed control system will be provided separately. 4. Selection of system components and system installation 4.1 Component selection (1) Frequency converters: The cement plant of Yungang Group has two air compressors with power ratings of 85KW and 130KW respectively. Since the air compressor is a load with a large moment of inertia, it was decided to use an inverter of a higher capacity; therefore, we selected two inverters of 90KW and 132KW each. (2) PID intelligent controller (3) Pressure transmitter 4.2 System installation The control cabinet is installed in the air compressor room, separate from the original control cabinet, but the main wiring between it and the compressors should not exceed 30 meters in length. The wiring for the control circuit uses shielded twisted pair, with a twist pitch of less than 15 mm. In addition, the control cabinet is equipped with a ventilation system, and the grounding terminals of the frequency converter are not used in conjunction with the power grounding as required; these measures enhance the stability and reliability of the system. 5. Reform effects: (1) The entire reforming system does not change the original control principle of the air compressor; in other words, the protective devices of the original air compressor system remain effective. Furthermore, the switching between power frequency and variable frequency is achieved through both electrical and mechanical interlocks, thereby **improving the safety and reliability of the system. (2) After the installation of the air compressor renovation project was completed, the initial trial run was successful with stable operation, and the vibration and noise levels of the air compressor were significantly reduced. (3) Except that the buffer cylinder pressure increases by 0.2 kg at certain frequencies, the oil pressure, oil temperature, and measurement values at all points are optimized to remain within safe ranges. (4) After the frequency conversion upgrade, starting is performed in a soft-start manner; there are no inrush currents due to unloading or loading during operation, and the mechanical shocks associated with the air compressor itself are reduced. (5) While ensuring gas supply to the pipeline network, the current **decreases**, and full-load operation hardly occurs; it is generally around 40Hz. Compared with before, the power savings amount to over 30%, and the investment can be recovered in about 10 months. (6) Due to stable air supply, the maintenance volume of air compressors, power supply and distribution equipment, as well as mechanical equipment, **decreases**, resulting in significant overall benefits. (7) The modified air compressor operates safely and reliably, while also meeting the process requirements for air usage.
It was written by our own company; could it possibly be fabricated?
The original poster has written very well. I have already started using variable-frequency motors in my current designs; since the systems have not yet been put into operation, I am unable to indicate how much electricity can be saved each year. I will share the actual figures with everyone once they are available.
The cost is relatively high. In my opinion, if there are few units, this method of starting and operating can be used to reduce the impact of current loads and pressure fluctuations. If the pressure load on the rear side remains relatively constant, then such modifications aren’t very useful. However, if the load varies significantly and there are many units, it’s possible to use pressure values to predict when to start the system, though the results may not be ideal
Our air compressor system consists of 4 40-cubic-foot compressors connected in parallel, but the individual end devices consume a large amount of air, which causes significant fluctuations in the pressure within the pipeline network when these devices start or stop operating. As a result, the total load on the compressors varies between 2.1 and 3.7 units. We modified one of them with variable-frequency control, but due to the issue of oil injection associated with variable-frequency operation of air compressors, it can only operate within the range of 30HZ to 50HZ. Other air compressors will still experience loading and unloading cycles. Moreover, when end devices start up, the load increases too rapidly, which often results in the inverter not responding in time and an inability to maintain the required pressure. Currently, two 10-cubic-meter air compressors have been made available by a sister company; we would like to acquire them and add variable frequency control – will this solve our problem? Brothers, could you give some advice on what to do?
5# zhangbomb: The two small air compressors should be installed as close as possible to the end of the main pipe connection, or at the locations where there are the greatest fluctuations in air demand. Reduce pipe losses. Install a gas storage tank at the user site where the gas pressure fluctuations are most severe to provide buffering. The gas consumption fluctuates within the range of 84–168 m3/h, which is quite problematic. No larger frequency converters will be used anymore; the original frequency converters will still be employed. Please design an electrical digital logic control circuit that includes a pressure sensor. The logic control function should be such that if the pressure does not increase after reaching a preset low value for N minutes (this value can be set according to your process requirements), another large air compressor is activated to take over operation; it is also possible to control the continuous operation of these compressors. After the pressure reaches the preset high limit for N minutes, the large air compressor shuts down automatically (continuous shutdown can be controlled). For the two small air compressors, only one inverter is used, and a digital logic control circuit is implemented as well, following the same principle. This provides two levels of frequency conversion control, thereby increasing the pressure adjustment range. Such control circuits have been applied in parallel water pump systems; by using logic circuits to determine whether the motor needs to be activated, it is possible to make full use of the voltage regulation function of frequency converters for automatically adjusting the motor speed, with relatively low investment. (If you’re interested, you can search online carefully; there should be ready-made control circuits available.) I haven’t done the calculations thoroughly, but if the fluctuations in gas pressure are too large, this approach won’t work either. PS: Couldn’t they call your air compressor station before turning the gas on or off? ?
After reading it, I thought it was great! ! But our company’s air compressors feature heat recovery! ! I can’t upload the files due to level issues! !
We have a total of four air compressors operating in parallel, with a normal supply pressure of 0.7 MPA. Three of these are screw compressors of foreign brands, while one is a screw compressor equipped with its own frequency converter. By adjusting the parameters, when there are changes in the air demand, the screw compressor with the built-in frequency converter can adjust its output, resulting in quite noticeable effects.
In our factory, the previous workshops did not have a dedicated air compression system; each workshop had its own air compressor (with a total capacity of 5×45KW). The air compressors were not used around the clock either. Eventually, a unified air compression system was installed, along with a large buffer tank, so that the various workshops could use the compressors at different times. This way, just 2 compressors with a capacity of 45KW were sufficient!
The energy efficiency of air compressors is also closely related to environmental temperature and the quality of the air. If the density of the incoming air is low (i.e., the air temperature is high), the air compressor can handle only a small amount of air per unit time, resulting in low efficiency. Poor air quality (high dust levels) increases the difficulty of treatment.