| In recent years,the production and use of natural gas has increased.The high-temperature flue gas emitted by conventional gas-fired boilers contains a large amount of waste heat resources which are recycled and used to save energy and reduce pollutant emissions.It is in line with the concept of high-quality development in China.In view of this,this paper designs and constructs a direct expansion heat pump unit(DEHP)to recover the waste heat of flue gas,through the way of experiment and simulation to study the unit operation performance.Firstly,an experimental test bench of DEHP for flue gas waste heat recovery is set up.The test bench uses the heat network backwater temperature and flow rate as variables to conduct an experimental study on the unit performance and its influencing factors.The high temperature flue gas produced by the boiler is passed into the waste heat recovery tower based on the principle of cascade heat utilization.First,it enters the heat exchanger and the air humidified water to exchange heat and release sensible heat.Next,it enters the evaporative end of the direct expansion heat pump and the refrigerant to exchange heat,releasing latent heat and discharging low temperature.The waste heat can be used to preheat the heat network backwater through the heat pump system.The experimental results show that the performance index of the DEHP system can be effectively improved by reducing the heat network backwater temperature and increasing the heat network backwater flow.The system can reach the best operating state and the COP can reach at 3.1 when the heat network backwater temperature is 40℃and the heat network backwater flow is 1800L/h.At this time,the DEHP can effectively reduce the emission flue gas temperature to below 25℃,with good waste heat recovery effect.Secondly,DEHP mathematical model is established.The model studied different energy efficiency design variant of the condition of the effect of improving the performance through numerical simulation.Bench actual operating data based on the mathematical model for reliability verification,comparison result indicates an error condition of each simulation and experimental data were less than 11%.The simulation results show that increasing the evaporating temperature and reducing the condensing temperature of the direct expansion heat pump system,controlling the sub-cooling degree at about 7℃,choosing a length of 0.4m,raising the vertical spacing of the pipe sections,reducing the horizontal spacing of the pipe sections and increasing the thickness of the fins can all be Effectively improve the performance index of the system.The compressor can be optimized in the following two forms:a two-stage compression heat pump can increase the COP by up to 23.9%;an additional liquid injection device can increase the COP by 35.1%when the liquid injection volume is increased to 10%.Finally,the visual design and calculation software of DEHP is compiled,and based on the multi-objective optimization method,DEHP with different working conditions and energy-saving and efficiency-enhancing transformation measures were analyzed,and the best operation plan with both economic performance and thermal performance was determined.The application of DEHP on boilers was evaluated through application evaluation indicators such as thermal performance,economic performance and environmental performance.The results showed that the exhaust flue gas temperature of the boiler equipped with DEHP was reduced by more than 85%,and the gas saving could reach up to 1.32×10~6m~3/year.It had better energy saving and environmental protection benefits,and the use effect on larger boilers was more significant. |
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