Principle and application of absorption type large temperature difference heat exchange unit

Principle and application of absorption type large temperature difference heat exchange unit
Due to the continuous expansion of heating area in our country, the demand for heat load is increasing day by day. The existing pipeline network in the centralized heating system is unable to meet the requirements due to flow and temperature limitations. To meet the requirements of heating, it is necessary to start from two aspects: increasing flow rate and increasing temperature difference. However, for cities with dense transportation, increasing the temperature difference between supply and return water is inevitable. Therefore, only by increasing the temperature difference between heating supply and return water can the transmission and distribution capacity of the pipeline network be improved to meet the requirements of thermal load. In centralized heating systems, due to the limited capacity of the pipeline network, the water supply temperature generally cannot be significantly increased. Therefore, significantly reducing the return water temperature is the main problem to solve the current bottleneck of pipeline transmission and distribution.
At present, our company's conventional heat exchange station mainly achieves heat exchange between the primary and secondary networks through water water plate heat exchangers. Due to the large temperature difference between the primary and secondary heat exchange ends during the heat exchange process, irreversible losses are caused. Therefore, in the heating mode of the heat exchange station, it is necessary to reduce this part of irreversible losses. The absorption heat exchange system can effectively utilize the power output of the absorption heat pump water supply without increasing the temperature of the primary network water supply and the flow rate of the pipeline network, reducing irreversible losses caused by partial direct heat exchange. At the same time, it can reduce the return water temperature of the main network and greatly improve the transportation capacity of the thermal pipeline network.
1、 The principle and application of absorption heat exchange units.
This study adopted the research method of Fu Lin et al. from Tsinghua University, using a primary network temperature of 120 ℃/25 ℃ and a secondary network temperature of 45 ℃/60 ℃ as examples. Among them, the absorption heat exchange process mainly includes two parts: one is the heat exchange process of the absorption heat pump, and the other is the direct heat exchange process. The 120 ℃ heating network water first enters the absorption heat pump generator as a high-temperature heat source; First, fully heat it with a dilute solution of lithium bromide, then cool it to around 90 ℃, then cool it with high-temperature water, and then enter the heat exchanger. In the heat exchanger, it indirectly exchanges with some of the secondary network water, reducing the high temperature rise from 90 ℃ to around 55 ℃. Then, it is used as a low-temperature heat source to enter the evaporator of the absorption heat pump. With the change of working temperature, the working fluid state in the evaporator changes to around 25 ℃. After the equipment exchanges heat, all the return water is sent back to the thermal power plant. On the secondary side, the return water temperature of the secondary network is 45 ℃. Part of the water enters the heat exchanger and directly exchanges heat with the absorption heat pump generator's primary network water, heating it up to 85 ℃, while the rest is sequentially heated up to 56 ℃ through the absorption heat pump absorber and condenser. When the two parts of water are mixed and reach the design temperature of 60 ℃, heat them up to the user.
The combination of absorption heat pump units and water water heat exchangers can effectively reduce energy waste during the primary network heat exchange process, allowing the final return water temperature of the primary network water to be significantly lower than that of the secondary network water, thereby greatly reducing the primary network heat exchange temperature and significantly lowering the primary network heat exchange temperature. When maintaining a constant flow rate in the primary network, the temperature difference between supply and return water increases, causing the heating network to provide heating and increase heat transfer. When the temperature of the secondary network remains constant, the temperature of the primary network decreases, and the irreversible losses during the heat exchange period of the primary and secondary networks are reduced. The heating capacity is improved, which enables the goal of reducing the initial investment of the heating network and the operating cost of the water pump to be achieved.
The essence of absorption heat transfer is to utilize the heat of a network of water in a cascade manner, using the high-temperature section of the network of water as the driving heat source, and the low-temperature network of water after heat exchange as the low-temperature heat source of the heat pump to continue heat exchange. It is due to the full utilization of the work capacity of a network of water, which improves its overall absorption and heat transfer efficiency, thereby increasing the temperature difference between the supply and return water of a network of water. Lithium bromide absorption heat pump is the main equipment in absorption heat exchange systems.
If the absorption heat exchange system is applied to the heat exchange station, the raw water water plate heat exchanger can continue to be used, and only one absorption heat pump with corresponding capacity needs to be added. By adopting an absorption heat exchange system, the primary network return water temperature of the entire thermal system is lower than 30 ℃. The return water at this temperature can directly recover the condensation heat of the steam turbine unit through heat exchange, reducing the load on the cooling tower of the power plant. By reducing the energy consumption and water consumption rate of the cooling tower, the overall energy efficiency level of the steam turbine unit can also be improved, laying a foundation for improving the energy utilization efficiency of the system.
2、 Advantages of absorption heat units.
Currently, provincial capital cities in China, led by Taiyuan, such as Yinchuan, Shijiazhuang, Xi'an, Jinan, Zhengzhou, Beijing, Chifeng in Inner Mongolia, Datong in Shanxi, and Zhangjiakou in Hebei, are promoting or demonstrating the use of absorption type large temperature difference heat pump technology. This technology has the following advantages:
1. By using a large temperature difference absorption heat pump unit, the primary network temperature can be increased from 110 ℃/60 ℃ to 110 ℃/30 ℃, and the temperature difference can be increased from 50 ℃ to 80 ℃. This means that the heat transfer capacity of the network has doubled. Compared to traditional plate heat exchangers, it reduces the new pipe diameter and saves investment in pipeline construction.
2. Due to the use of large temperature difference absorption devices, the increased heat can be provided as a heat source for new construction projects or added to other required systems, thereby improving the adjustability of the heat network.
3. Traditional plate heat exchangers are connected to the main and secondary networks, resulting in large heat transfer temperature differences and severe irreversible losses. Compared to this, the absorption heat exchange unit effectively utilizes the available potential energy between the primary and secondary heat networks, driving the absorption heat exchange device to fully utilize energy.
4. Due to the temperature drop of the return water in the primary network to 30 ℃, the temperature difference between the water and the heat exchanger has increased, which is beneficial for the recovery of waste heat in the condenser and improves energy utilization efficiency.
In summary, the large temperature difference absorption heat exchange technology is a new thermal energy heating technology developed to coordinate the existing heat capacity load and the insufficient energy supply of the existing heating network. Using the useful energy generated by the large temperature difference between the primary and secondary heating networks as the driving force, the return water temperature of the primary network can be greatly reduced without changing the supply and return water temperature of the heating station. By using this method, the heat exchange of the heating station can be significantly increased while the primary flow rate remains constant, thereby utilizing the existing primary pipeline network to meet larger heat load demands.