Heat Transfer Enhancement In Air-Cooling Zone Of Power Plant Condenser Based On Heat Pump Technology

Because of high pressure of condenser, economical operating of power plant is seriously affected. The output of power station unit will be insufficient,station service power consumption rate for power plant will arise, coal consumption rate of unit electricity output will increase. The function of air cooling zone is continuously condensing of steam from main condensing zone of condenser, together condensing the mixed air to reduce load of vacuum pump. The aim is to realize better extraction effect, which is helpful to low condenser pressure. During actual operation of power plant condenser, especially in summer, cooling capacity of circulating cooling water will drop due to high temperature of outdoor air. Cooling capacity of air cooling zone is also become poor, steam-air mixture with large amount of uncondensed steam is extracted though extraction opening of air cooling zone which connected with vacuum pump. Load of vacuum pump increases, and air leakage into condenser will not be extracted in time. Heat transfer resistance becomes larger, heat transfer coefficient becomes smaller due to existence of air, which will lead to decrease of condenser vacuum value. If heat transfer of air cooling zone is enhanced, more steam among steam-air mixture extracted by vacuum pump will be condensed and the air is also be cool down. Extraction load of vacuum pump is largely lessened and condenser vacuum value is easily maintained. Heat pump is used to enhance heat transfer in air cooling zone. Heat pump’s influence on condenser vacuum value is discussed. Law of steam condenses with large amount of non-condensable gas under vacuum condition is also discussed. Interactions between heat pump and condenser are revealed. All these have important practical engineering meanings and expand new applications for heat pump.This paper gives study on applying heat pump to main condensing zone and air cooling zone by experiment, numerical computation, numerical simulation and economical analysis. Total mean heat transfer coefficient of heat pump evaporator tube bundles under the condition of "double side phase change" heat transfer. Mathematic model for film condensation of steam with large amount of non-condensable gas on horizontal pipe bundles is also given. Mathematic model for heat pump using for air cooling zone is provided. Main research work of this paper is given as follows: (1) Experiment platforms are set up to study heat transfer enhancement of applying heat pump to air cooling zone of condenser. There are two experiment platforms, one for overall enhancement and the other for partial enhancement of air cooling zone. Performances of condenser and heat pump are discussed under overall enhancement first, then partial enhancement with circulating cooling water is discussed.(2) Evaporator of heat pump is combined with steam condenser. The refrigerants in evaporator pipe bundles evaporate by absorb heat from steam outside the pipe bundles. Pure steam undergoes film condensation outside evaporator pipe bundles in main condensing zone while steam with large amount of non-condensable gas undergoes film condensation in air cooling zone. In either case the heat transfer will be "double side phase change". It is important to ensure overall heat transfer coefficient of this heat transfer question. This paper gives formulation to this question and it is precondition for mathematic simulation of heat pump evaporator.(3)Steady mathematic models for the main four parts of heat pump:compressor, condenser, thermal expansion valve, evaporator are established. Results of steady simulation of heat pump match well with the test results. Steady simulation of heat pump laid foundation for optimization of heat pump later.(4) Economical analysis on project is carried out by taking one300MW power plant unit for instance. The overall investment of the project, including investment of heat pump system and investment of steam condenser reform, is calculated. Annual operation cost of heat pump and annual earnings of coal saving are confirmed. Payback period is also calculated according to results above. Economical analysis results show that it is worthwhile to apply heat pump in air cooling zone. The project has small investment, short payback period and high earnings and it is suitable for promotion.(5) Experiments for applying heat pump in main condensing zone show heat pump has best performance when used in middle and rear parts. The performs of heat pump and steam condenser are acquired with variation of circulating water temperature and flow rate, together with variation cooling water temperature and flow rate. COP of heat pump, condenser pressure, heat transfer of condenser are obtained by experiments.(6) Experiments for applying heat pump in air cooling zone are carried out. The performs of heat pump and steam condenser are acquired with variation of circulating water temperature and flow rate, together with variation cooling water temperature and flow rate. Three situations:without "quench area", three "quench area", middle and rear "quench area",are used for comparison. COP of heat pump, condenser pressure, heat transfer of condenser, extraction parameters of vacuum pump are obtained by experiments.From discussion above, the technology of applying heat pump in air cooling zone to enhance heat transfer is feasible and economical. The results of the paper can provide guidance for actual project and lay foundation for later research. The results have theoretical meanings and engineering application value.