Principle and application of absorption large temperature difference heat exchange unit
Due to the continuous expansion of the heating area in China and the increasing demand for heat load, the existing pipe network in the central heating system has been difficult to meet the requirements due to the flow and temperature restrictions. In order to meet the requirements of heat supply, we must start from increasing the flow and temperature difference. However, for cities with dense traffic, it is necessary to increase the temperature difference between supply and return water. Therefore, only by increasing the temperature difference between supply and return water can the transmission and distribution capacity of the pipe network be improved to meet the requirements of thermal load. In the central heating system, due to the limited bearing capacity of the pipe network, the water supply temperature can not be significantly increased, so reducing the return water temperature substantially is the main problem to solve the current bottleneck of the pipe network transmission and distribution.
At present, the conventional heat exchange station of our company mainly realizes the heat exchange between the primary network and the secondary network through the water-water plate heat exchanger. 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 heat exchange station heating mode, this part of irreversible losses should be reduced. The absorption and heat exchange system can effectively utilize the work capacity of absorption heat pump water supply without increasing the water supply temperature of the primary network and the flow of the network, reduce the irreversible loss caused by partial direct heat exchange, and at the same time, reduce the return water temperature of the main network, and greatly improve the transmission capacity of the heating network.
1、 Principle and application of absorption heat exchange unit.
This study adopts the research method of Fu Lin and others from Tsinghua University, taking the primary network temperature of 120 ℃/25 ℃ and the secondary network temperature of 45 ℃/60 ℃ as an example. 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 ℃ heat supply network water first enters the absorption heat pump generator as a high-temperature heat source; First, fully heat it with lithium bromide dilute solution, then cool it to about 90 ℃, then cool it with high temperature water, and then enter the heat exchanger. In the heat exchanger, indirectly exchange with part of the secondary network water, so that the high temperature rise is reduced from 90 ℃ to about 55 ℃, and then enter it into the evaporator of the absorption heat pump as a low temperature heat source. With the change of the working temperature, the working medium state in the evaporator changes to about 25 ℃, After heat exchange, 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 ℃. Some of the water entering the heat exchanger and the primary network water of the absorption heat pump generator directly exchange heat and heat up to 85 ℃. The rest of the water is heated up to 56 ℃ through the absorption heat pump absorber and condenser in turn. When the two parts of water are mixed to reach the design temperature of 60 ℃, the heat is heated to the user.
The combined use of absorption heat pump unit and water-water heat exchanger can effectively reduce the energy waste in the heat exchange process of the primary network, so that the final return water temperature of the primary network water can be significantly lower than the secondary network water temperature, thus greatly reducing the heat exchange temperature of the primary network, thus greatly reducing the heat exchange temperature of the primary network. When the primary network flow is maintained unchanged, the temperature difference between supply and return water increases, and the heat supply network heats up and increases the heat transmission. When the temperature of the secondary network is constant, the temperature of the primary network decreases, the irreversible loss during the heat exchange of the primary and secondary networks is reduced, and the heating capacity is improved, so that the goal of reducing the initial investment of the heating network and the operating cost of the water pump can be achieved.
The essence of absorption heat transfer is to use the heat of a network of water in a cascade, and use the high temperature section of the network of water as the driving heat source, while the low temperature network of water after heat exchange continues to exchange heat as the low temperature heat source of the heat pump. It is due to making full use of the work capacity of the primary network of water to improve its overall absorption and heat exchange efficiency, thus increasing the temperature difference between the supply and return water of the primary network of water. Lithium bromide absorption heat pump is the main equipment in the absorption heat exchange system.
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. The absorption heat exchange system is adopted to make the return water temperature of the primary network of the whole thermal system lower than 30 ℃. The return water at this temperature can directly recover the heat of condensate of the steam turbine unit through heat exchange, reducing the load of 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 unit.
At present, the provincial capital cities headed by Taiyuan in China, such as Yinchuan, Shijiazhuang, Xi'an, Jinan, Zhengzhou, Beijing, Chifeng in Inner Mongolia, Datong in Shanxi, Zhangjiakou in Hebei, are promoting or demonstrating the use of absorption large temperature difference heat pump technology, which has the following advantages:
1. The use of large temperature difference absorption heat pump unit can make the primary network temperature change from 110 ℃/60 ℃ to 110 ℃/30 ℃, and the temperature difference increase from 50 ℃ to 80 ℃, which means that the heat transfer capacity of the network has doubled. Compared with the traditional plate heat exchanger, the new pipe diameter is reduced and the pipe network construction investment is saved.
2. The heat added by the large temperature difference absorption device can provide heat source for the new construction project or be added to other systems needed to improve the adjustability of the heat network.
3. The traditional plate heat exchanger is connected with the primary and secondary networks, and its heat transfer temperature difference is large, and the irreversible loss is serious. In contrast, the absorption heat exchange unit effectively uses the available potential energy between the primary and secondary heat networks to drive the absorption heat exchange device and make full use of energy.
4. Since the return water temperature of the primary network is reduced to 30 ℃, the temperature difference between the primary network and the heat exchanger is increased, which is beneficial to the recovery of the waste heat of the condenser and the improvement of the energy utilization rate.
To sum up, the absorption heat exchange technology with large temperature difference 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. Taking 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. With this method, under the condition that the primary flow is constant, the heat exchange of the heating station can be significantly increased, so that the existing primary pipe network can be used to meet the larger heat load demand.