Application of variable frequency speed regulation technology in energy saving analysis of fans and pumps

1. Introduction
In industrial production and product processing manufacturing, fans and pumps are widely used. Their power consumption and throttling losses of valves, baffles and related equipment, as well as maintenance and repair costs account for 7% to 25% of the production cost, which is a considerable production cost. With the continuous deepening of economic reform and the continuous intensification of market competition, energy saving and consumption reduction have become one of the important means to reduce production costs and improve product quality.
The variable frequency speed regulation technology developed in the early 1980s conforms to the requirements of the development of industrial production automation and has ushered in a new era of intelligent motors. It has changed the old mode that ordinary motors can only run at a fixed speed, so that the motor and its drag load can adjust the speed output according to the production process requirements without any modification, thereby reducing the power consumption of the motor and achieving the purpose of efficient operation of the system.
In the late 1980s, this technology was introduced into China and promoted. It has now been applied in motor transmission equipment in various industries such as electricity , metallurgy, petroleum, chemical industry, papermaking, food, and textiles. At present, variable frequency speed regulation technology has become a major development direction of modern electric power transmission technology. Excellent speed regulation performance, significant power saving effect, improvement of the operating conditions of existing equipment, improvement of system safety and reliability and equipment utilization rate, and extension of equipment service life are fully reflected with the continuous expansion of application fields.
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. OverviewUsually
in industrial production and product processing and manufacturing industries, fan equipment is mainly used in boiler combustion systems, drying systems, cooling systems, ventilation systems and other occasions. According to production needs, furnace pressure, wind speed, air volume, temperature and other indicators are controlled and adjusted to meet process requirements and operating conditions. The most commonly used control method is to adjust the size of the damper and baffle opening to adjust the controlled object. In this way, regardless of the size of production demand, the fan must run at full speed, and the change of operating conditions causes energy to be consumed by the throttling loss of the damper and baffle. In the production process, not only is the control accuracy limited, but it also causes a lot of energy waste and equipment loss. This leads to increased production costs, shortened equipment service life, and high equipment maintenance and repair costs.
Pump equipment also has a wide range of applications in the production field. Water pumping stations, water tank supply and drainage systems, industrial water (oil) circulation systems, and heat exchange systems all use centrifugal pumps, axial flow pumps, gear pumps, plunger pumps and other equipment. Moreover, according to different production needs, throttling devices such as regulating valves, reflux valves, and stop valves are often used to control signals such as flow, pressure, and water level. In this way, not only a large amount of energy is wasted, and the sealing performance of pipelines and valves is damaged; it also accelerates the wear and cavitation of the pump cavity and valve body, which in severe cases damages the equipment, affects production, and endangers product quality.
Most fans and pump equipment are directly driven by asynchronous motors , which have the disadvantages of large starting current, mechanical shock, and poor electrical protection characteristics. Not only does it affect the service life of the equipment, but it also cannot instantly protect the equipment when the load has a mechanical failure. It is often the case that the pump is damaged and the motor is also burned.
In recent years, due to the urgent need for energy saving and the requirement for continuous improvement of product quality, the use of variable frequency speed regulators (referred to as inverters) is easy to operate, maintenance-free, high in control accuracy, and can achieve high functionality; therefore, the inverter-driven solution has begun to gradually replace the control solution of dampers, baffles, and valves.
The basic principle of variable frequency speed regulation technology is based on the relationship between the motor speed and the working power input frequency: n = 60 f (1-s) / p, (where n, f, s, and p represent the speed, input frequency, motor slip rate, and motor pole pair number respectively); the purpose of changing the motor speed is achieved by changing the working power frequency of the motor. The inverter is a comprehensive electrical product based on the above principle, using AC-DC-AC power conversion technology, power electronics
, microcomputer control and other technologies. 3
. Energy-saving analysisAccording
to the basic laws of fluid mechanics, fans and pumps are all square torque loads, and their speed n has the following relationship with flow rate Q, pressure H and shaft power P: Q∝n, H∝n2, P∝n3; that is, flow rate is proportional to speed, pressure is proportional to the square of speed, and shaft power is proportional to the cube of speed.
Take a water pump as an example, its outlet pressure head is H0 (the outlet pressure head is the static pressure difference between the pump inlet and the pipeline outlet), the rated speed is n0, the pipe resistance characteristic when the valve is fully open is r0, the corresponding pressure under rated working conditions is H1, and the outlet flow rate is Q1. The flow-speed-pressure relationship curve is shown in the figure below.
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In field control, the outlet valve of the water pump is usually used to control the flow rate at a constant speed. When the flow rate decreases by 50% from Q1 to Q2, the valve opening decreases, causing the pipe network resistance characteristic to change from r0 to r1, and the system operating point moves from the original point A to point B along direction I; under its throttling effect, the pressure H1 changes to H2. The actual value of the pump shaft power (kW) can be obtained by the formula: P = Q?H／(η c?η b)×10-3. Among them, P, Q, H, η c, η b represent power, flow, pressure, pump efficiency, and transmission efficiency respectively, and direct transmission is 1. Assuming that the total efficiency (η c?η b) is 1, when the pump moves from point A to point B, the power consumption saved by the motor is the area difference between AQ1OH1 and BQ2OH2. If the speed control method is used to change the speed n of the pump, when the flow rate decreases by 50% from Q1 to Q2, the pipe network resistance characteristic is the same curve r0, and the system operating point will move from the original point A to point C along direction II, and the operation of the pump will also be more reasonable. When the valve is fully open and there is only pipe network resistance, the system meets the flow requirements of the site, and the energy consumption is bound to be reduced. At this time, the power consumption saved by the motor is the area difference between AQ1OH1 and CQ2OH3. Comparing the use of valve opening adjustment and water pump speed control, it is obvious that the use of water pump speed control is more effective and reasonable, and has a significant energy-saving effect.
In addition, it can be seen from the figure that when the valve is adjusted, the system pressure H will increase, which will threaten and damage the sealing performance of the pipeline and valve; while when the speed is adjusted, the system pressure H will decrease with the decrease of the pump speed n, so it will not have a negative impact on the system.
From the above comparison, it is not difficult to conclude that when the demand for water pump flow on site drops from 100% to 50%, the use of speed adjustment will save the power corresponding to BCH3H2 compared with the original valve adjustment, and the energy saving rate is more than 75%.
Similarly, if variable frequency speed regulation technology is used to change the speed of pumps and fans to control other process control parameters such as on-site pressure, temperature, water level, etc., the relationship curve can also be drawn based on the system control characteristics to obtain the above comparison results. In other words, the method of using variable frequency speed regulation technology to change the motor speed is more energy-saving and economical than using valves and baffles to adjust, and the equipment operating conditions will also be significantly improved.
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. Energy-saving calculationFor
the energy-saving effect of fans and pumps after variable frequency speed regulation, the following two methods are usually used for calculation:
1. Calculate based on the known flow-load relationship curves of fans and pumps under different control modes and the load changes in field operation.
Take an IS150-125-400 centrifugal pump as an example, with a rated flow of 200.16m3/h and a head of 50m; equipped with a Y225M-4 motor with a rated power of 45kW. The flow-load curve of the pump during valve adjustment and speed adjustment is shown in the figure below. According to the operating requirements, the water pump runs continuously for 24 hours, of which 11 hours a day runs at 90% load and 13 hours a day runs at 50% load; the annual operating time is 300 days.
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The annual power saving is: W1=45×11×(100%-69%)×300=46035kW?h
W2=45×13×(95%-20%)×300 =131625kW?h
W = W1＋W2=46035＋131625=177660kW?h
Calculated at 0.5 yuan per kilowatt-hour, the annual electricity bill can be saved by 88,830 yuan.
2. According to the square torque load relationship of fans and pumps: P / P0=（n / n0）3, P0 is the power at rated speed n0; P is the power at speed n.
Take a 22 kW blower used in an industrial boiler as an example. The operating condition is still 24 hours continuous operation, including 11 hours of operation at 90% load (frequency calculated at 46Hz, motor power consumption calculated at 98% when the baffle is adjusted), and 13 hours of operation at 50% load (frequency calculated at 20Hz, motor power consumption calculated at 70% when the baffle is adjusted); the annual operating time is 300 days as the basis for calculation.
Then the annual power saving when frequency conversion speed regulation is: W1=22×11×[1－（46/50）3]×300=16067kW?h
W2=22×13×[1－（20/50）3]×300=80309kW?h
Wb = W1＋W2=16067＋80309=96376 kW?h
The power saving when the damper is open is: W1=22×（1－98%)×
11×300=1452kW?h W2=22×（1－70%)×11×300=21780kW?h
Wd = W1＋W2=1452＋21780=23232 kW?h
Compared with the electricity saving: W = Wb-Wd = 96376-23232 = 73144 kW?h
Calculated at 0.5 yuan per kWh, the use of variable frequency speed regulation can save 36,570 yuan in electricity costs each year.
The parameters of a factory's centrifugal water pump are: centrifugal pump model 6SA-8, rated flow 53.5 L/s, head 50m; equipped with motor Y200L2-2 type 37 kW. The measured data of the water pump under valve throttling control and motor speed control are recorded as follows:
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Flow rate L/s Time (h) Power consumption of power grid output (kW?h)
 Valve throttling adjustment Motor frequency conversion speed regulation
47 2 33.2×2=66.4 28.39×2=56.8
40 8 30×8=240 21.16×8=169.3
30 4 27×4=108 13.88×4=55.5
20 10 23.9×10=239 9.67×10=96.7
Total 24 653.4 378.3
In comparison, variable frequency speed regulation can save 275.1 kW?h of electricity in one day compared to valve throttling control, with a power saving rate of 42.1%.
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. ConclusionThe
use of variable frequency speed regulation technology to achieve energy-saving operation in equipment such as fans and pumps is a key technology for energy conservation in China and has received widespread attention from the national government. Article 39 of the Energy Conservation Law of the People's Republic of China lists it as a general technology for promotion. Practice has proved that the use of frequency converters in fan and pump equipment drive control occasions has achieved significant power saving effects and is an ideal speed control method. It not only improves equipment efficiency, but also meets production process requirements, and thus greatly reduces equipment maintenance and repair costs, and also reduces downtime cycles. The direct and indirect economic benefits are very obvious, and the one-time investment in equipment can usually be fully recovered in 9 to 16 months of production.