| There is still a large amount of waste heat in gas condensing boiler exhaust which has not been recovered.If sensible heat and latent heat of water vapor condensation can be deeply recovered,the thermal efficiency of boiler can be further improved,which is of great significance for promoting energy conservation and emission reduction of building heating system and promoting building "carbon peak and carbon neutrality".In order to solve the problem of flue gas residual heat of gas condensing boiler,a heat pump type flue gas residual heat recovery system is proposed in this paper.The system takes flue gas as low temperature heat source,recovers flue gas residual heat through finned tube heat exchanger and low temperature water,and raises low heat energy to high heat energy to heat the building through water source heat pump.According to the theoretical calculation,the residual heat of the system recoverable flue gas is 117.64 k W.According to the theoretical calculation of the heat exchanger,the form and structural parameters of the heat exchanger are determined.The factors affecting the flue gas waste heat recovery system are analyzed theoretically,and the main factors are gas flow rate,evaporator outlet temperature and evaporator side water flow rate.The change of evaporator outlet temperature will change the heat pump circulation and affect the evaporator side heat absorption,that is,the amount of flue gas waste heat recovery.The change of gas flow rate and evaporator water flow rate will affect the heat transfer coefficient of heat exchanger,cause the change of evaporator outlet temperature,and then affect the performance of heat pump.Based on the heating system of a gas condensing boiler of an office building in Qingdao,an experimental platform of flue gas waste heat recovery system of heat pump type was set up.The control variable method was used to conduct experiments to study the effects of gas flow rate,evaporator outlet temperature and evaporator side water flow rate on system performance.The experimental results show that:(1)With the increase of gas flow,the evaporator outlet temperature increases,and the fundamental cause of the change is the imbalance between the heat transfer of flue gas and water and the heat absorption of evaporator side,and the difference between the two acts on the rise and fall of water temperature.(2)With the increase of the evaporator outlet temperature,the flue gas waste heat recovery increases,and the heat pump COP increases.With each increase of 1℃,the recovery of flue gas waste heat increases by2.75%-3.46%,and the COP of heat pump increases by 2.19%-2.86%.(3)With the increase of water flow at the evaporator side,the evaporator outlet temperature increases,the flue gas waste heat recovery increases,and the heat pump COP and EER increase.When the water flow rate increases by 3m3/h,the flue gas waste heat recovery rate increases by 1.33%～5.23%,COP of heat pump increases by 1.99%～4.02%,and EER of heat pump increases by 0.98%～3.12%.The optimal water flow rate of this system is determined to be 29.5m3/h.Under the best working condition,the continuous operation experiment of the system is carried out.The results show that: During continuous operation of the system,the average daily flue gas waste heat recovery was 104.30 k W,the heat pump COP was4.449,and the heat pump EER was 4.141.The boiler thermal efficiency(based on the low heat value of gas)could reach 110.32%～111.34%,and the flue gas waste heat utilization effect was remarkable.The average daily gas saving is 134.34Nm3,energy saving is 33.49 kgce,carbon reduction is 83.48 kg CO2,energy saving and emission reduction effect is remarkable.The system can save 29988.09 yuan per year,the static payback period is 5.46 years,and the economic benefits are remarkable.TRNSYS simulation platform was built,experimental data was imported,and the continuous operation effect of the system was simulated.The simulation results were as follows: the average COP was 4.3,the average flue gas waste heat recovery was103.54 k W,and the average daily gas saving was 133.73Nm3.The simulation results are basically consistent with the experimental results.The heat pump flue gas waste heat recovery system proposed in this paper can effectively absorb the waste heat of low-temperature flue gas condensing boiler,improve the thermal efficiency of gas condensing boiler,save gas consumption,reduce carbon emissions from gas combustion,and provide reference and support for the application of heat pump flue gas waste heat recovery system in engineering. |
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