Experimental Study On A Flue Gas Heat Recovery System By Direct Expansion Heat Pump

China has become the world's largest fossil energy consumer.With the rapid development of the economy,it is inevitable that the consumption of resources is excessive.Environmental damage caused by unreasonable energy use patterns has emerged.Therefore,the Chinese government has taken active measures to reduce carbon emissions and advocate the development concept of“green economic growth”.All provinces in China began to accelerate the implementation of the"coal to gas"project,especially in the Beijing-Tianjin-Hebei region.The purpose is to"control the total consumption of coal and promote the clean use of energy".Gas boilers have been widely promoted and applied.By the end of 2016,Beijing's gas boilers reached 9,635,accounting for more than 80%of the city's heating boilers.The conventional gas-fired hot water boiler has a exhaust gas temperature of about 150 to 200 ~oC,and its heating efficiency is about 90%,which has a large heat recovery space,especially the recovery of latent heat of condensation.Therefore,it is of great significance for the research of waste heat recovery technology of gas boilers.In order to use the hot water pipe network to carry out deep flue gas waste heat recovery for small and medium-sized flue gas boilers(10 kW-10 MW),this paper proposes a flue gas heat recovery system by direct expansion heat pump.Through the experiment,the residual heat recovery effect of the system is explored and its influencing factors are analyzed.The direct expansion heat pump is applied to the flue gas,which has three characteristics:high temperature of flue,small space available,and high return water temperature of the heat network.According to the above characteristics,the design method of the direct expansion heat pump system is introduced in detail from four aspects:working fluid,compressor,direct expansion evaporator and heat exchanger.The working principle,core device design,experimental scheme and test system of the system test bench are introduced.Determine the system performance evaluation standard combining the flue gas waste heat recovery effect and the direct expansion heat pump operation energy efficiency,and evaluate the actual operation effect of the flue gas heat recovery system by direct expansion heat pump.Analyze the effect of the inlet water temperature of the waste heat recovery system and the water flow rate of the heat recovery system on the residual heat recovery of the system and the COP of the direct expansion heat pump.The results show that the flue gas heat recovery system by direct expansion heat pump has better residual heat recovery effect.Under the different working conditions in this experiment,the gas utilization efficiency of the system is between 13%and 18%,the lowest outlet exhaust gas temperature can reach 19 ~oC,and the waste heat recovery outlet water temperature is up to 77 ~oC.Compared with ordinary boilers,the thermal efficiency of the direct expansion heat pump type flue gas condensing boiler has increased by 15.8%,and the energy saving rate has increased by 7.2%.Compared with the conventional heat pump type flue gas condensing boiler,the thermal efficiency of the direct expansion heat pump type flue gas condensing boiler is increased by 5.4%,and the energy saving rate is increased by 2.8%.At the same time,the investment recovery period of the direct expansion heat pump type flue gas condensing boiler is 1.42 years,and produces about 97.69 tons of condensed water per year,which has obvious advantages in energy saving,economy and environmental protection.