Application of FD100 frequency inverter in heat exchange station

1. Overview：

In order to save energy and reduce urban pollution, the waste heat of the steam turbine after the power generation of the thermal power plant is fully utilized to centrally heat the northern cities in winter. When the hot water sent from the power plant goes to the heat exchange station in the city, the temperature of the primary water supply is more than 90 degrees. After the heat ex-changer, the temperature of the primary backwater hot water drops to more than 60 degrees, and then flows back to the power plant. The hot water sent to the homes of urban residents, the secondary backwater temperature into the heat ex-changer of the heat exchange station is more than 50 degrees, and the secondary water supply temperature is more than 60 degrees. There are many such heat exchange stations in Baoji, Shaanxi province, among which one heat exchange station has four heat ex-changers, a circulating pump group composed of four 37kW pipeline pumps, and a 3.7kW supplementary water pump. The circulating pump and filling pump use manual opening and closing valves to control the flow, which increases the damping of the pipeline and causes power waste.

2.Implementation plans：

1. Frequency conversion speed control of supplementary water pump

In order to further save energy, the automatic transformation of the heat exchange station is implemented, the circulating pump and supplementary pump are adjusted by frequency conversion, and the whole city heating system is monitored by computer, realizing the unattended heat exchange station. Make the hot water run in the heating system through the circulating pump, and the leakage of pipes and valves causes the reduction of the water pressure of the circulating water. If the water is not replenished in time, the abnormal operation of the heating system will be caused. The water supply mode of the frequency conversion pump is relatively simple. The constant pressure water supply mode is adopted, and the set pressure is 4kg. In this example, FGI company's FD 100 inverter is selected, and frequency conversion speed regulation system is selected as shown in the figure:

2. Frequency  control of circulating pump

The ultimate goal of the heating system is to keep the indoor temperature of the thermal user stable, but because the thermal user does not have a room temperature regulator, it is impossible to form a closed loop control for the room temperature of many thermal users. In order to achieve economic operation and ensure the quality of heating, the most effective method is to control the secondary water supply temperature of the heat exchange station. Under the steady state conditions, the heat supply of the system, the heat dissipation of the radiator and the heat consumption of the user under the steady state conditions can be obtained：

The Formula (1) is modified and considers that the indoor temperature, the ratio of the actual flow rate of the secondary pipe network to the design flow rate and the backwater temperature are approximately constant, then:

Where a, b and c are the relevant parameters of the meteorology in the area where the pipe network is located.

Formula (2) is the calculation method of the given value of the secondary water supply temperature. The t2g determined by formula (2) can track the change of outdoor temperature tw, so that the indoor temperature of thermal users is not affected by the change of tW, and realize stable heating.

If the outdoor temperature changes, to make the indoor temperature basically constant, a control strategy is to use the temperature difference of the secondary inlet and the backwater to control the circulation pump inverter speed, set the temperature difference between the secondary inlet and the backwater to 12 degrees Celsius. When the temperature difference between the secondary inlet and backwater is greater than 12 degrees Celsius, the circulating pump converter accelerates; when the temperature difference between the secondary inlet and backwater is less than 12 degrees Celsius, the circulating pump converter slows down.

In the figure, FD 100 frequency converter, BU-soft starter (self-coupling and decompression starter), the system adopts the cycle switching mode, the temperature difference signal into the PLC, after PLC treatment, to the frequency converter as the speed control signal. The operation information of the whole system is sent to the computer by the PLC.

3. Application advantage

After the frequency conversion speed regulation of the circulating pump, all valves have the opening is the largest and the resistance of the system is the smallest. When the average flow is 80% of the design flow, the power saving rate can be calculated according to the calculation formula in the implementation supervision guide of the mandatory national standard of three-phase asynchronous motor:

The power saving rate is 36%, which shows that the benefit of saving electricity is very considerable

China is one of the countries with in energy, energy saving and consumption reduction is our national policy. There are thousands of heat exchange stations in cities across the country. If all of them are energy-saving transformation, the electricity saved can not be underestimated. Moreover, the system operation is stable and reliable, to achieve the unattended, economic benefits and social benefits are obvious.