Introduction

The SINCR frequency converter is used in the frequency conversion transformation of the waste heat power generation project of a steel group in Hebei. It is mainly applied to the induced draft fan of the boiler, and the inverter of 5.5-220kw different power sections is applied. The induced draft fan is one of the important auxiliary equipments of the thermal power plant. The boiler induced draft fan is the main part of the boiler combustion. It is the power equipment that removes the high temperature flue gas generated by the boiler through water grinding and dust removal, electrostatic precipitator and then exhausted through the chimney. When the boiler is burning, the load changes. In order to ensure the negative pressure of the furnace, the oxygen content of the flue gas and the corresponding temperature and pressure are relatively stable, it is necessary to adjust the suction volume of the induced draft fan in time, and adjust the air volume by the opening of the baffle. size.

1. Analysis of the status quo of boiler induced draft fans

In the early thermal power plants, when selecting the boiler supporting fan, it is usually necessary to consider the short-term overload capacity and make appropriate margins to determine the model. When the boiler is selected, the boiler capacity is determined according to the maximum load of the process and the appropriate margin. Therefore, when the induced draft fan is selected, it is limited by the product specification. The final use of the induced draft fan is often too large, and the adjustment of the boiler induced draft fan is done by adjusting the baffle. Therefore, when the air volume changes, the induced draft fan The system wastes a lot of power. There are many problems with the use of baffles for air volume control:

When the baffle is used to control the flow of the wind, a large amount of energy loss is wasted in the throttling process of the baffle, and a large amount of energy is wasted.

The dust in the flue gas has a relatively large impact on the baffle and is prone to failure. Since the dust in the flue gas often fouls on the baffle, the baffle is stuck, which increases the maintenance and repair of the baffle, and the failure rate is high.

Due to the slow movement of the baffle, the actuator will have an increasingly large idle travel for a long time. The actuator of the flapper valve has a large torque, which is prone to problems, and can not be adjusted frequently for a long time. The linearity is not good and the dynamics are not good. It is difficult to form a closed loop automatic control. The actuator for adjusting the opening degree of the boiler adjusts the maintenance amount of the actuator because the baffle is stuck and the arm or the base is often broken, and the switch cannot be in place. In order to avoid the above problems, the waste heat plant adopts frequency conversion speed regulation on the boiler induced draft fan.

2. Energy-saving principle of speed regulation of induced draft fan

The frequency converter is mainly composed of three parts: rectifier, filter system and inverter. When working, the three-phase alternating current is first converted into direct current through the bridge rectifier device. The pulsating DC voltage is smoothed and filtered, and under the control of the microprocessor, the inverse is used. The transformer re-inverts the DC power into a three-phase AC power source with adjustable voltage and frequency, and outputs it to the motor that needs speed regulation. The principle of motor learning: the motor speed formula is n=60/p(1-S), where n, f, d, p represent the motor speed, input frequency, motor slip ratio, and motor magnetic logarithm.

When the speed of the fan is changed from n1 to n2, the approximate relationship of Q, H, and P is:

Q2=Q1(n2/n1)

H2=H2(n2/n1)2

P2=P1(n2/n1)3

Where: Q-wind volume; H-wind pressure; P-fan power.

It can be seen from the above formula that the fan flow rate is proportional to the primary of 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. The ideal situation is as follows:

It can be seen from the above table that when the flow rate needs to be reduced, adjusting the speed can save a lot of energy. For example, when the flow demand is reduced by half, such as by frequency conversion, in theory, the required power of 12.5% can save 87.5. energy. If the traditional baffle method is used to adjust the air volume, although the energy consumption may be reduced accordingly, the saving effect is quite different from the frequency conversion.

3, the advantages of the inverter

(1) Easy to adjust. For boilers that require frequent adjustments, it is very convenient to use inverter speed control.

(2) Energy saving benefits. By comparing the flow change with the power change, it can be seen that the energy-saving efficiency of the frequency conversion speed regulation is very obvious.

(3) Benefits to electrical equipment and the power grid.

1 Due to the excellent soft start/stop function of the inverter (may start at zero speed), the starting current can be limited within the rated current to achieve soft start of the motor, which greatly reduces the impact of the starting surge current on the motor and the grid; Smooth, thereby reducing motor faults, thus greatly extending the maintenance cycle and service life of the motor. 2 Using the frequency converter can increase the power factor and reduce the reactive power loss.

(4) Benefits to mechanical equipment and pipe networks. 1 Due to the full opening of the damper, the possibility of surge, turbulence and damper failure is eliminated. 2 Due to the unique smooth adjustment of the inverter, the mechanical wear of the fan is greatly reduced, the temperature of the bearing and the bearing bush is lowered, and the maintenance cost is effectively reduced. Extends the life of the fan. 3 After using the frequency conversion adjustment, the negative pressure regulation becomes fast and sensitive, which further improves the automation level of equipment operation control and system operation management.

4 Conclusion

Through the modification of the energy-saving control system of the boiler induced draft fan, the problem of the jamming of the baffle and the breaking of the actuator arm can be solved, but the main effect is that the energy saving effect is obvious. Although the investment in the early stage of the transformation is relatively large, in the long run, the cost of electricity saved is far greater than the input. The operation of the boiler induced draft fan system has also been effectively and reasonably controlled. The SINCR frequency converter is stable after operation and has excellent performance and is highly recognized by customers.