With the promulgation and implementation of the National Energy Law and the increasing global energy shortage, energy conservation efforts by enterprises have become increasingly important. In a typical thermal power plant, the overall thermal efficiency is only 30%–40%, with the remaining heat being wasted. The largest contributor to this loss is the condenser's cold source loss, accounting for approximately 60% of the total loss. How to reduce cold source loss, improve overall plant thermal efficiency, and achieve energy conservation is an urgent problem to be solved. A certain thermal power plant is a small-scale combined heat and power (CHP) plant primarily for heating. Its units are small and have low thermal efficiency. In 2001, it achieved good results and gained considerable experience by using a low-vacuum circulating water heating system. The following is a detailed description. [b]1 Feasibility Analysis of Circulating Water Heating[/b] The thermal power plant has a configuration of 9 boilers and 6 turbines, with a total steam production capacity of 530 t/h and a power generation capacity of 36 MW. Utilizing circulating water for heating requires implementation within the extraction condensing unit. The plant has three extraction condensing turbine units, one of which, a 6 MW unit, is cooled by three fiberglass cooling towers. Due to the initial design location, the water collection pool and cooling area are too small, resulting in insufficient cooling performance to meet design requirements. Furthermore, the plant is located in an area with very hard water, situated next to a street. Soon after operation, large amounts of dust and sludge accumulate inside the towers, severely clogging the gaps in the packing material and hindering water flow. This necessitates continuous forced ventilation using three fans, consuming significant amounts of electricity. Even so, the temperature difference between the circulating water inlet and outlet is typically only 3-5°C. Additionally, due to the limited water collection pool, the sludge and impurities deposited inside the towers do not have time to settle before returning to the circulating water. This sludge adheres to the inner wall of the condenser copper tubes, causing scaling, poor heat exchange, and increased exhaust steam temperature (in severe cases, exceeding 60°C), creating a vicious cycle in heat exchange. To solve this problem, the plant must clean the condenser copper tubes and cooling tower packing annually, increasing production costs. If this unit were to utilize circulating water for heating, it would offer several advantages: firstly, it would solve the problem of poor cooling tower performance; secondly, the use of cleaner, softened water would prevent scaling on the inner walls of the condenser copper tubes; and thirdly, given the unit's already high exhaust steam temperature, using circulating water for heating would result in a smaller increase in exhaust steam temperature compared to other units, thus minimizing the impact on the unit. Therefore, modifying this 6 MW extraction condensing turbine unit is necessary. 1.16 MW extraction condensing unit technical parameters Model: CN 6—35/9 Manufacturer: Hangzhou Steam Turbine Generator Factory Design exhaust temperature: 36 ℃ Design exhaust pressure: 0.005 9 MPa Design vacuum value: -0.094 MPa Circulating water flow rate: 1400 t/h Heat network supply water temperature: tg≈60 ℃ Enthalpy of supply water: hg=251.5kJ/kg Heat network return water temperature: th≈50 ℃ Enthalpy of return water: hh=209.3kJ/kg Circulating pump motor: 37 kW 3 units Cooling tower fan: 30 kW 3 units 1.2 Calculation data The calculation data after reducing the condenser vacuum and increasing the circulating water temperature is shown in Table 1. It can be seen that if the exhaust steam temperature of the unit is increased to 70 ℃, the power generation of the unit will decrease by 8.0%, and the circulating water temperature can be heated to above 60 ℃. Although the supply water temperature is not high, the low temperature and high flow rate method can meet the heating needs in winter. 1.3 Calculation of heating area that can be provided by circulating water heating The heat required per unit heating area in Shijiazhuang City is 0.064 kW/m2; the heat released by the circulating water is 16,411 kW; the heating area is 256,000 m2. According to theoretical calculations, this scheme is feasible. 2 Safety analysis of the unit and pipeline Because the unit increases the exhaust steam temperature and reduces the condenser vacuum, it changes the design operating parameters of the unit, which will inevitably have a certain impact on the unit. In order to ensure the safety of the unit, the following problems are solved. 2.1 Condenser pressure problem The heat users of this plant are located in a plain area, and the required pressure of the circulating water is not high. The return water pressure is generally 0.2 MPa. The condenser's pressure resistance is 0.6 MPa, which is sufficient. However, to prevent special circumstances such as sudden disconnection of the heating network, the following measures were taken: a. Two hot water circulation pumps were used as backups for each other and interlocked to ensure normal circulation of the heating network. b. Safety valves were installed on the return water pipes of heat users to ensure that the return water pressure does not exceed 0.2 MPa. c. Check valves were installed on the heating circulating water circuit. 2.2 Copper Pipe Scaling Problem Although the increased exhaust steam temperature can easily cause copper pipe scaling, the heating network circulating water uses chemically treated softened water, which reduces hardness, and the return water pipe has a dirt remover, greatly improving water quality. Compared to the previous circulating water condition of this unit, the situation has been greatly improved, and scaling problems have been reduced. In addition, the condenser is regularly cleaned using a rubber ball cleaning device. 2.3 Heating Circulating Water Makeup Issues The heating circulating water uses softened water. A softened water treatment device, one condensate tank, and two makeup water pumps need to be installed in the heat exchange station specifically for making up the circulating water. The makeup water pumps use variable frequency control to maintain a constant makeup water pressure. [b]3 Selection and Renovation of Heating Areas[/b] Currently, the heating area supplied by this plant is mainly in the southwest region of Shijiazhuang City, relatively close to the plant. The areas mainly used for heating and relatively concentrated are as follows: the Second Prison of Hebei Province to the west of the plant, 1000 m away, with a heating area of ​​70,000 m2; the 375 heat exchange station in the southwest corner of the plant, with a heating area of ​​80,000 m2; and the Zhuoda residential area to the west of the plant, 1500 m away, with a heating area of ​​100,000 m2. The 375 heat exchange station belongs to the plant, making engineering modifications relatively simple. It has a short heating distance, low pressure loss, and convenient operation and management. The plant's personnel can directly handle heat network switching, and the heat exchange station can be retained as a supplementary heat source in emergencies. In summary, the above three areas should be selected for this circulating water heating project, with a heating area of ​​250,000 m². [b]4 Remedial Measures for Circulating Water Heating System Failures[/b] Circulating water heating using condensing turbine units requires stable unit operation. If the unit shuts down for any reason, the exhaust steam heat source required for circulating water heating disappears, and the circulating water heating cannot meet the heating requirements. Therefore, remedial measures must be in place for circulating water heating system failures. a. Expand the heating equipment at the 375 heat exchange station to a large station with a heating capacity of 300,000 m². The circulating water heating and heat exchange station heating equipment should be connected in parallel, serving as backups for each other and allowing for switching. Increase the flow rate of the circulating water pumps, increasing the power from 37 kW to 250 kW and raising the head to 50 m. b. During the start-up and shutdown of the unit, in order to ensure the stability of the heating supply, it is necessary to switch between two systems. Before the unit starts up, the heat exchange station heating system is used for heating; after the unit is running normally under load, it is gradually switched to the circulating water heating system. c. When the circulating water temperature rise decreases during low-load operation of the unit and cannot guarantee the heating demand, it is necessary to use the heat exchange equipment in the heat exchange station to supplement the system with secondary heating to meet the temperature requirements of the heating water network. d. When the outside temperature rises and the return water temperature rises, and cannot meet the unit's condensation needs, the method of switching standby heat users is adopted to switch the users of the original heat exchange station to the circulating water heating system; after the temperature drops, these users are switched back to the original heat exchange station to ensure the unit output. At the same time, the original cooling tower system is retained, and some circulating water can also enter the cooling tower circulation loop for cooling. [b]5 Economic Benefit Calculation[/b] 5.1 The annual heating fee can be increased by RMB 3.3/(m2·month) × 4 months/a × 250,000 m2 = RMB 3.3 million/a. 5.2 The impact of using circulating water heating on electricity consumption each year: a. Reduced power generation per heating season: 0.6 million kWh × 8.0 × 24 × 120 = 1,382,400 kWh. b. Reduced power consumption due to discontinuing the original 3 circulating water pumps and 3 cooling tower fans: (30 × 3 + 37 × 3) × 24 × 120 = 579,000 kWh. c. Reduced power consumption due to discontinuing the original heat exchange station's hot water pumps: 37 × 24 × 120 = 107,000 kWh. d. Increased power consumption due to the new circulating pump motors: 250 × 24 × 120 = 720,000 kWh. The total annual electricity loss is 138.24 - 57.9 - 10.7 + 72 = 141.64 million kWh. At a price of 0.365 yuan per kWh, this equates to 141.64 × 0.365 = 517,000 yuan. 5.3 Difference in turbine circulating water makeup: The original system's makeup water volume was 1400 × 4% = 56 t/h, while the new system's makeup water volume is 8 t/h, saving 48 t per hour. With 120 days of operation per heating season and a water price of 1 yuan/t, the annual savings are 48 × 24 × 120 × 1 = 138,000 yuan. 5.4 Considering all factors, the annual additional benefit is 330 - 51.7 + 13.8 = 292.1 million yuan. 5.5 The estimated investment for this renovation project is 560 million yuan. 5.6 The payback period for this renovation project is 2 years. [b]6 Summary[/b] The plant's approach of reducing the vacuum of the condenser unit, increasing the exhaust steam temperature, and utilizing circulating water for heating to reduce cold source losses is very successful. The modification is relatively simple, the equipment can operate safely and stably, and the energy-saving effect is particularly significant, resulting in considerable economic benefits.