A brief discussion on the application of frequency converter in thermal production

In thermal production, the water supply, water supply pressure, furnace required air volume, air pressure, etc. are regulated by frequency conversion control. Compared with traditional methods, it has obvious energy-saving effect.

I. Introduction

In Fushun Industrial Development Zone, two 20t/h steam boilers are installed in the heating unit. Each boiler has a 90kW induced draft fan, a 55kW forced draft fan and a 45kW boiler water pump motor . The users are mainly manufacturing enterprises, which operate all year round, but the load is unstable, the boiler starts and stops frequently, and in most cases it is in a low-load operation state. Therefore, the production cost is high and the economic benefits are not good, especially under low-load conditions, the waste of electricity is serious. For this reason, it is planned to carry out technical transformation of the boiler induced draft system and water supply system. By adopting variable frequency speed regulation, good energy-saving effects have been achieved.

2. Air supply system

During the boiler combustion adjustment process, it is required to keep the steam pressure constant under different load conditions. In order to ensure the relative balance between the boiler steam output and heat supply, it is necessary to adjust the amount of furnace coal, air volume and wind pressure to make the furnace heat reach a certain value. The amount of air and the air pressure in the furnace determine the combustion rate and efficiency of the coal layer, and affect the boiler steam output and constant main steam pressure.

Traditionally, air volume and air pressure regulation is achieved by adjusting the throttle opening through an electric actuator, which has the following disadvantages:

1) When the load is large, the damper must be opened to increase the wind pressure and air volume in the furnace, speed up the coal seam combustion, increase the steam production per unit time, and meet the steam supply requirements; when the load is small, the damper must be closed. In both cases, the induced draft fan maintains the rated speed. When the load is small, the huge air volume generated by the induced draft fan is blocked by the damper, and only a small part enters the air duct, increasing the pressure difference loss, wasting electricity, increasing production costs, and reducing economic and social benefits.

2) The induced draft fan has high power and requires a large voltage reduction starting device under the condition of industrial frequency power supply. For this reason, the main circuit needs to be equipped with high-power contactors, reactors and other voltage reduction starting devices, which will increase the number of electrical equipment in the main circuit. In the case of frequent starting and braking, the equipment failure rate will increase, the maintenance workload will increase, and even the boiler will be forced to shut down to deal with the failure, unable to supply steam normally, affecting the normal production of users, not only causing huge economic losses, but also affecting the reputation of the heating unit.

3) In the case of frequent starting and braking, the starting surge current generated will have a certain impact on the power grid and the wind turbine system, directly affecting the power supply quality of the power grid and the safe operation of the wind turbine system.

4) The damper is adjusted through complex operating mechanisms such as electric actuators, servo amplifiers, and operators. However, due to the large number of auxiliary equipment, the equipment failure rate is also increased.

However, the air volume is adjusted by frequency conversion speed regulation. Since frequency conversion starting is a soft start, the impact of the starting process on the power grid and the fan system is reduced, and the power consumption during the starting process is reduced. Therefore, it has the advantages of high efficiency, wide speed regulation range, hard characteristics, high adjustment accuracy, convenient and flexible braking, low energy consumption, low failure rate, and simple operation. In addition, the adjustment of wind pressure and air volume is directly achieved by controlling the speed of the induced draft fan motor through the inverter potentiometer on the operation panel. The damper and its complex operating mechanism are eliminated, so that the air duct is unobstructed, reducing the loss of pressure difference and air volume. At the same time, the original huge step-down equipment has been eliminated, which also reduces

After the induced draft fan speed regulating device is transformed by frequency conversion technology, it has the following characteristics:

1) Automatic energy optimization, which realizes the full application of motor quota, facilitates the speed control of pumps and fans, and can better regulate the process;

2) The soft start function avoids the impact of the startup process on the power grid and wind turbine system;

3) Built-in incoming line reactor, allowing the motor incoming line cable length to be 150m;

4) Complete protection, such as motor overload protection, overheat protection, ground fault protection, short circuit protection, etc.

5) Simple installation and debugging, simple operation, low energy consumption, and low operation and maintenance costs.

3. Water supply system

In the original boiler water supply system, a 45kW motor drives the water pump to run for a long time. The water volume and water supply pressure are regulated by adjusting the opening of the water supply valve with an actuator. Its disadvantage is that the water supply pump always runs at the rated speed, wasting electricity. In addition, the water supply auxiliary equipment is complicated, the actuator moves frequently, and the stability is poor. After the frequency conversion speed regulation transformation, all devices for adjusting the water supply valve are removed, the valve opening is maximized, and the water level automatic control table or pressure controller directly outputs a 4 ~ 20mA current signal to achieve PID closed-loop control, adjust the frequency, change the speed of the water supply motor, change the water supply pressure and flow, and meet the needs of water volume and water pressure under different loads.

By using variable frequency speed regulation technology and the existing pressure sensor and pressure controller on the boiler, the liquid level automatic controller forms a dual closed-loop control system of pressure and liquid level.

4. Effect of Technological Transformation

Since the implementation of the air supply system and water supply system transformed by the water pump inverter, significant energy and water saving effects have been achieved. According to calculations, the power saving after the transformation is about 35%, and the water saving is about 20%. In addition, the equipment failure rate is greatly reduced after the transformation, the hot downtime is reduced, the maintenance personnel workload is reduced, and the maintenance cost is reduced. In short, after the technical transformation, the production cost is significantly reduced, and the economic and social benefits are improved. According to preliminary estimates, the equipment investment will be recovered within one and a half years.