Closed cooling tower assisted crankshaft forging line circulating cooling water waste heat utilization

The design and initial investment analysis of the waste heat utilization system of circulating cooling water for the crankshaft forging production line on the site of a customer show that the waste heat utilization system of circulating cooling water is energy-saving and feasible, which is worthy of further research and promotion. The global energy shortage and the serious pollution caused by fossil fuel to the environment are the two major problems troubling the world. As one of the industrial waste heat, the circulating cooling water of the process equipment releases a huge amount of waste heat energy to the environment during the cooling process. We must pay attention to the recycling and utilization of waste heat of circulating water, which is related to energy conservation, protection of the ecological environment and comprehensive utilization of resources. 

With the rapid development of China's automobile industry, there will be more and more automobile parts manufacturing projects. The intermediate frequency induction heating furnace of crankshaft forging production line adopts cooling tower for direct cooling, which will generate a large amount of residual heat, and the recovery and utilization of this part of energy is very considerable.

1. Introduction to cooling water system of crankshaft forging production line

   The middle frequency induction heating furnace of a crankshaft forging production line is directly cooled by a cooling tower, and the process flow is as follows:

   1.1 The basic structure of the system: this system adopts a closed circulating cooling water system, and the hot water from the forging line is filtered and transferred to the cooling tower for cooling, and then transferred to the forging line by a circulating pump. When the system is not in operation in winter, the anti-freezing pump and electric heater shall be in operation to avoid the pipe freezing crack in winter. Under the design condition, the circulating cooling water system of this project will generate 6740kW residual heat.

   1.2 Control description of circulating cooling water system:

   (1) The circulating cooling water system is composed of 4 cooling towers, each of which can be independently controlled.

   (2) A circulating pump with a standby, circulating pump for variable frequency pump.

   (3) All control measures must ensure that the temperature sensor T1 temperature value is not lower than 38℃.

   (4) When T2≥60℃, 4 cooling tower at the same time, the cooling tower water inlet electric valves are open.

   (5) When 53℃≤T2 < 60℃, three cooling towers are in operation. The electric valve on the inlet water pipe of the running cooling tower is opened, while the electric valve on the inlet water pipe of the running cooling tower is not closed.

   (6) When 46℃≤T2 < 53℃, two cooling tower operation, running cooling tower on the water inlet electric valve open, do not run the cooling tower on the water inlet electric valve closed.

   (7) When 38℃≤T2 < 46℃, a cooling tower runs, running cooling tower on the water inlet electric valve open, do not run the cooling tower on the water inlet electric valve closed.
   

   (8) In winter, the system does not operate, when T1, T2 temperature value below 2℃, all cooling tower water inlet electric valve are open, electric heater into operation, anti-freeze pump start, when T1, T2 temperature value higher than 10℃ when the electric heater heating, anti-freeze pump stop running.

From the above control notes, it can be seen that the residual heat of the circulating cooling water system is the variable value. Under special circumstances, when the production capacity is not up to the normal requirements, there will be insufficient water temperature in the control requirements.

      2. Design of residual heat utilization system of cooling water system

          Hot air heating system and domestic hot water system need to use waste heat as heat source in the project. The hot air heating system consumes 3500kW, and the domestic hot water system consumes 850kW. The total load is 4350kW<6740kW. Therefore, under the design condition, the thermal medium of hot air heating system and domestic hot water system can fully utilize the residual heat.

        2.1 Design principles. The residual heat utilization of the cooling water system should not damage the service conditions of the original cooling water system and not affect the use of the original control system. Because the residual heat generated by the system is uncertain, the design conditions can meet the requirements of hot air heating and domestic heat. In the design of the waste heat utilization system, it is necessary to consider that steam is used as the heat source to make up for the system.

        2.2 System design. The design adopts the method of series cooling water system pipe network of plate heat exchanger unit. The hot water from the forging line is first cooled by heat exchange unit, and then cooled by cooling tower and then transferred to the forging line by pump.

              2.2.1 control description of waste heat utilization system for hot air heating:

                       (1) When hot air heating with plate heat exchanger unit running, electric regulator V5 closed.

                       (2) In winter, when the temperature of the temperature sensor T4 drops to 46℃, the electric regulating valve V5 is opened, and the temperature of T4 is controlled by adjusting the water flow into the heat exchanger unit. When the electric regulating valve V5 is fully open, the plate heat exchanger unit is closed; When the temperature of the temperature sensor T4 rises to 53 ° c (adjustable), the plate heat exchanger is re-opened. By adjusting the water flow into the plate heat exchanger, the temperature of T4 is controlled to be no less than 46 ° c.

              2.2.2 control description of waste heat utilization system for domestic hot water:

                       (1) When the temperature of the temperature sensor T3 drops to 60℃, the plate heat exchanger unit shuts down; When the temperature of the temperature sensor T3 rises to 60℃ (adjustable), the plate heat exchanger unit starts.

                       (2) When the plate heat exchanger unit is closed, the domestic hot water is provided by direct steam heating.

      3. Energy saving pre-analysis of waste heat utilization system

          3.1 Increased initial investment as can be seen from the above figure of waste heat utilization, both hot air heating system and domestic hot water system are designed with steam as an auxiliary heat source, so the use of waste heat utilization system will increase the following initial investment.

          3.2 Operation cost analysis the hot air heating system operates for 3 months a year, in 83 days and 16 hours in 2 shifts in the processing workshop; Domestic hot water system runs for 2 hours a day for 250 days a year.

                3.2.1 Annual heat consumption of hot air heating: QV=ZNQ (ti-ta)/(ti-tw) [1]

                         Where, QV: annual heating consumption, kJ;

                                        Z: operating hours of hot air heating device per day, h;

                                        N: heating period days;

                                        Q: heat load, kW;

                                        Ti: indoor calculated temperature, ℃;

                                       Ta: average outdoor temperature in heating period, ℃;

                                       Tw: calculated temperature of outdoor ventilation in winter, ℃;

                                      QV=0.036×16×83×3500× (15-3.2) ÷ ((16- (-1.6)) =3116MW

                         Steam consumption: G=3.6QV/r[2]

                         Where, G: steam consumption, kg/h;

                                    QV: annual heat consumption for heating, W;

                                       R: latent heat of vaporization of steam under condensation pressure, kJ/kg.

                                      G = 3.6 QV/r = 3.6 x 3116 x 106 present 2164 = 5184 t

                         The consumption of hot air heating is calculated by 70% of the waste heat supply, and the consumption is calculated by 180 yuan /t of steam, which saves 650,000 yuan per year.

                3.2.2 Annual heat consumption of domestic hot water:

                     Q annual = 831.2×250×2 = 415.6MW, about steam 692t/ year, the hot air heating consumption is calculated by 70% from the waste heat supply, and the steam consumption is calculated by 180 yuan /t, so 88,000 yuan is saved every year.

                3.2.3 electricity cost of plate heat exchanger unit for hot air heating: the electricity consumption of plate heat exchanger unit is 33kW per hour, and the electricity consumption during heating period is 33×16×83=43824kW. The electricity charge is 1.2 yuan/KWH, and the total cost is 43824×1.2= 52,600 yuan.

                3.2.4 Electricity cost of plate heat exchanger unit for domestic hot water:

                         The energy consumption of the plate heat exchanger unit is 3kW per hour, and the annual energy consumption is 3×250×2=1500kW. The electricity charge is 1.2 yuan/KWH, and the total cost is 1500×1.2=0.18 million yuan.

                 3.2.5 Payback period: the annual operating savings are 65+8.8-5.26-0.18= 683,600 yuan, and the payback period is 92.5÷68.36=1.35 years.

      4. Conclusion: 

          Through the above analysis and comparison, we fully realize the importance of recycling cooling water waste heat utilization system, and it also shows that the recycling cooling water waste heat utilization system is feasible and energy-saving.