Application of Sunwoele VFD (Variable-frequency Drive) in comprehensive energy saving of boilers

I. Introduction

The main control indicators for boiler energy saving and consumption reduction are coal consumption and electricity efficiency. Traditional methods often use air dampers, valves and mechanical speed regulation to control the flow of drum (induced) fans, feed water pumps, circulating pumps and the operating speed of the grate. The design does not consider the purpose of energy saving. At the same time, due to different processes and production tasks, when the steam demand changes, the amount of coal feeding (injection drop) needs to be changed to achieve high-efficiency combustion. The traditional control method uses manual operation and consumes high power. , The control accuracy is low, and it is easy to cause environmental pollution (black smoke from the chimney) due to improper operation.

2. Transformation of boiler water supply control system

The working principle of water pump

From the working principle of the water pump, it can be seen that the flow rate Q is proportional to the rotation speed N, the head H is proportional to the square of the rotation speed N, the shaft power P is proportional to the cube of the rotation speed N, and the motor rotation speed N is proportional to the power frequency F, so changing the power frequency , can change the speed of the electric motor, that is, the feed water pump, thereby achieving the purpose of adjusting the feed water flow.

Original equipment working condition

Nowadays, the boiler feed water pump runs continuously at a constant speed, and the flow is controlled by adjusting the regulating valve and branch return flow in the pipeline. When the regulating valve is used to adjust, due to the reduction of the valve opening, the pressure at the water pump outlet increases. , the pressure difference on both sides of the valve will increase, causing a waste of energy and easily damaging the wear of the valve and bearings; when branch return adjustment is used, the return flow of a large amount of water will also cause energy consumption.

Renovation plan

Install a Sunwoele HSF600 dedicated VFD (Variable-frequency Drive) for water pumps and fans to the original system, and retain the power frequency equipment of the original system for backup. The normally open contact of the VFD (Variable-frequency Drive) fault output relay is connected to the delay relay and then connected to the original system. The power frequency main circuit contactor ensures the normal operation of the boiler water supply system in the event of a VFD (Variable-frequency Drive) failure. At the same time, the sound and light alarms in the control room prompt the operator to troubleshoot the problem in time; the VFD (Variable-frequency Drive) sets a lower limit frequency. Ensure that the boiler feed water volume is above the safe value;

VFD (Variable-frequency Drive) working status

In the control room, the operator can adjust the frequency of the VFD (Variable-frequency Drive) in real time according to changes in the drum water level, so as to meet the evaporation volume and water supply volume while striving to operate the VFD (Variable-frequency Drive) in the best economical state.

On-site measurement: Compared with the original regulating raft control, the power saving rate of the water supply pump is 24%, and the wear of the water pump is greatly reduced, extending the service life of the equipment.

3. Renovation of boiler drum (induction) air control system

Original on-site working conditions

The air volume of the boiler's drum (induction) fan often changes with the change of steam volume. The original on-site valve adjustment was used. The distance between the control room of the boiler and the valve is long, making it inconvenient to operate and difficult to control accurately. If the air volume is adjusted too large, the air contains The amount of oxygen exceeds the standard, which wastes heat energy; the air volume is adjusted too small, and the residual carbon content of the cinder exceeds the standard and is not burned, which increases coal consumption; the noise generated by the high-speed operating fan also causes great pollution to the environment; due to long-term high-speed operation, the fan bearings The temperature rise of both the motor and the motor is very high, and the service life of the equipment is reduced.

VFD (Variable-frequency Drive) working status

After installing the Sunwoele HSF600 series VFD (Variable-frequency Drive), the operator can adjust the speed of the drum (induction) fan at any time in the control room according to changes in steam consumption, reducing noise pollution to the environment (the motors all run below the rated speed) , the wind noise is reduced), because the motor and fan bearings are not easily damaged when the induced draft fan is operated at a lower than rated speed for a long time, the service life is extended, the heat generated by the motor is also reduced, and the amount of maintenance is reduced. Downtime is reduced, saving a lot of maintenance costs.

energy saving effect

Generally, the rated air volume and flow rate of the fan and water pump equipment in the design usually exceed the actual required air volume and flow rate, and the air volume and flow rate change in real time during operation. At present, baffles or valves are used to adjust the air volume and flow rate. It is relatively common. Although the method is simple, it actually achieves the purpose of adjustment by artificially increasing resistance. This throttling adjustment method wastes a lot of electric energy. Recycling this part of the electric energy loss will achieve great energy-saving effects.

From the principles of fluid mechanics, we know that the relationship between fan air volume, rotation speed and motor power is expressed by the following relationship:

In the formula, Q - air volume (flow rate) H - wind pressure (lift) P - shaft power n - speed

When the air volume decreases and the fan speed decreases, the motor input power decreases rapidly. For example, when the air volume drops to 80% and the speed (n) also drops to 80%, the shaft power drops to 51% of the rated power; if the air volume drops to 50% , the shaft power will drop to 13% of the rated power, and its power saving potential is very large. Therefore, for fans and water pumps with a large air volume and flow adjustment range, using variable frequency speed control instead of dampers or valve adjustments is an effective way to achieve energy saving. .

The characteristics of variable frequency speed regulation are high efficiency, no additional slip loss caused by speed regulation, large speed regulation range, high precision and stepless speed regulation. It is easy to realize coordinated control and closed-loop control. Since the original squirrel-cage motor can be used, it is especially suitable for the technical transformation of old equipment. It not only maintains the advantages of the original motor's simple structure, reliability, durability, and easy maintenance, but also achieves energy saving. The remarkable effect is the most ideal way to save energy for fans and water pumps.