For a long time, air compressors have been using a step-down start, working in the power frequency constant speed mode, because the air compressor is selected according to the maximum amount of compressed air. In order to ensure that the gas pressure is within a certain range, the air compressor should frequently change the operation mode according to the gas demand.
A common method of using the intake valve control is that when the line pressure reaches the upper limit of the set pressure, the intake valve is closed, and the air compressor is in no-load operation. Although it is operated by the motor, it does not output compressed air. The road pressure no longer rises; when the line pressure reaches the set lower limit, the intake valve opens, the compressed air is output, and the line pressure rises.
In this way, the motor keeps running during the whole running process, but the air compressor enters the no-load operation from time to time, and frequent starting occurs, resulting in waste of energy. Due to the design time air compressor, the possibility of long-term operation under full load cannot be ruled out. Therefore, the motor can only be selected according to the maximum demand, so the motor capacity is generally large. In actual operation, the time of light load operation often accounts for a very high proportion, which results in a large energy waste.
Optimize the design of air compression systems
The implementation of centralized air supply at the air compressor station not only facilitates management, saves investment, but also improves the utilization rate of equipment, and more conveniently adjusts the supply pressure and gas supply to meet the production needs.
When designing, firstly, according to the distribution of gas-using devices, the gas time and gas pressure of each device should avoid the large resistance loss and leakage loss caused by the long pipeline, and design one or two. Or even more air compressor stations, which implement split air supply or fragmented network supply, and the position of the air compressor station should be located in the center of the gas supply system, and try to be more energy-consuming or use high gas pressure. The device is closer.
Secondly, the design of the pipeline should be taken seriously, and the pipeline should be as short as possible without obstacles. In order to minimize the leakage rate, the pipeline layout should be designed with radial and less dendritic. Compressed air; optimize the supply pressure and supply of the critical equipment. Finally, the design of the air compressor should be considered in the design environment, to minimize the suction temperature, to ensure the cleanliness of the suction, and a good cooling system.
Frequency control mode
1, the working principle of frequency conversion speed regulation
The frequency conversion speed regulation technology is a technology to change the motor frequency and change the voltage to achieve the purpose of click speed regulation. The motor used in the air compressor is usually an AC asynchronous motor, and its output speed is:
n=60f (1-s)/p ...............(1)
N—motor speed
F—the frequency of the power supply, Hz
P—the number of pole pairs of the motor
S—the slip of the motor, (0-6%)
From the formula (1) of the AC asynchronous motor, it can be seen that the motor speed n can be changed very conveniently as long as the power supply frequency f of the three-phase AC motor is changed. When the slip rate s does not change much, the speed n of the AC motor is basically proportional to f, that is, the speed of the asynchronous motor can be adjusted when the power supply frequency is adjusted, which is the theoretical basis of the frequency conversion. Therefore, the use of the frequency converter to change the power supply frequency of the motor, also changes the motor speed, but also changes the air frequency of the air compressor cylinder, thereby changing the gas pressure of the air compressor output, to adjust the output gas of the air compressor The purpose of stress.
According to the principle of electromagnetic induction, in the frequency conversion speed regulation, as long as the magnetic flux Φ remains unchanged, the power supply frequency f is proportional to the stator voltage of the motor, and then the rotational speed and output power of the motor change with the change of the power frequency. When the power frequency is reduced, the motor speed is also reduced. By changing the input voltage on the stator side, the frequency of the motor can be adjusted. This is the constant voltage ratio (v/f) control method. The method is simple to implement, and is convenient for controlling in a situation where the speed regulation performance requirement is not very high, and is also a widely used speed regulation method.
When the air compressor adopts the frequency conversion speed regulation technology for constant pressure air supply control, the system principle is shown in Figure 1.
Description of the system: It takes the output pressure as the control object, and the feedback signal taken by the pressure transmitter is connected to the inverter with its own PID adjustment function, compared with the preset pressure given signal, and integrated by PID adjustment. The signal is connected to the input end of the inverter, so that the operating frequency and the speed of the motor M are determined according to the variation of the pressure, and the frequency conversion adjustment mode is realized.
2, frequency conversion speed regulation energy saving analysis
(1) Constant torque load application
The constant torque load is constant regardless of the rotational speed. Its formula is as follows:
P=K×T×n..................(2)
Where: P is the shaft power, K is the coefficient, T is the load torque, and n is the speed.
As can be seen from equation (2), the shaft power is proportional to the speed of the motor. After applying the variable frequency speed control technology, if the motor speed n is reduced to 80% of the rated speed due to the needs of the process, the corresponding power consumption is reduced to 80%.
(2) Variable torque load application
According to the basic laws of fluid mechanics, fans, pumps and compressors are all variable torque loads. The speed n is related to the flow rate Q, the pressure H and the shaft power P as follows: Q∝n, H∝n2, P∝ N3; that is, the flow rate is proportional to the rotational speed, the pressure is proportional to the square of the rotational speed, and the axial power is proportional to the cube of the rotational speed. When the motor speed drops slightly, the shaft power will drop significantly. For example, when the flow rate is 80% of the rated flow rate, the Pn is only 0.51Pe, that is, the motor shaft power is only 51% of the rated power, which is reduced by nearly half. Therefore, when the frequency converter is applied to loads such as fans, pumps, compressors, etc., when the flow rate changes, energy can be saved, and the energy saving effect is very remarkable.
Principle of waste heat recovery
For air compressors, about 80% of the input energy will be converted into heat energy. If the air compressor is properly modified according to the structure and principle of the corresponding type of air compressor, the heat can be recycled, and the waste can be turned into a treasure. The heat collected into the environment is used to reduce the fuel consumption for heating for other purposes. The principle of heat recovery is shown in Figure 2.
Based on the principle of waste heat recovery, we collect all or part of the heat originally taken away by the oil cooler exhaust fan by adding heat exchangers and corresponding control devices in the oil circuit of the air compressor to heat the cold water, thereby reducing The energy needed to heat it. The inlet temperature of the cooler is about 85-100 °C. By installing the corresponding heat exchanger, using countercurrent heat transfer and appropriate oil temperature and water temperature control measures, the outlet water temperature is raised to 65-80 ° C, and the compressor is guaranteed to operate normally. . The specific heat recovery system flow is shown in Figure 3.
Improve the efficiency of air compressors
The key to improving the operating efficiency of the air compressor is to increase the displacement of the air compressor. There are many factors affecting the air compressor exhaust volume. Through the operation practice, we can start from the following aspects and achieve very obvious results.
1. Ensure a good inhalation environment and cooling conditions. Strengthen the ventilation of the suction point, regularly clean and replace the suction filter, which can reduce the resistance and effectively prevent the dust in the atmosphere from entering the cylinder, preventing the dust from aggravating the wear of the piston ring and the cylinder and destroying the airtightness of the valve. The inner leak. Because the suction temperature of any one level is increased by 1 °C, the displacement is reduced by about 2%, and the power is not reduced. Therefore, it is better to use circulating or demineralized water that has been treated with chlorination or dosing to ensure heat exchange. Cooling effect and ensure the amount of cooling water and water pressure required by the air compressor.
2. Do a good job of maintenance, reasonable determination of the maintenance cycle, strict requirements for maintenance quality. Regularly clean the air filter and air compressor cooling system, regularly repair or replace the air valve, piston ring and packing ring; and carry out maintenance quality inspection by professional technicians. After confirming the repair, the cylinder clearance volume is controlled within a reasonable range to ensure the piston. Assembly quality of the ring and maintenance quality of the valve.
3. Strengthen operational management and implement inspections. The operator requires timely drainage, oil discharge and dust discharge to the condensers and auxiliary equipment at all levels to ensure that the blow-off valves are not leaking; the water pressure and water volume of the cooling water should be checked and adjusted in time to ensure that the temperature difference between the inlet and outlet of the cooling water is greater than 10 °C, the intercooler outlet temperature is less than 40 °C; carefully check the working status of each valve, and find problems in time to deal with or report.
The energy saving and consumption reduction of the air compression system directly affects the production cost and investment benefit of the enterprise. By optimizing the design of the air compression system, using the frequency conversion speed regulation technology and the waste heat recovery technology, the operation management of the air compression system is strengthened, and the operation efficiency of the air compressor is improved. And by strengthening the maintenance of air compressors, we can achieve very significant energy savings.
