Study On Both Heat Transfer Characteristics Of Heat Exchanger And Waste Heat Recovery For Large-scale Low Temperature Air Source Heat Pumps

In this paper,the heat exchanger and waste heat recovery system of large low-temperature air source heat pump are studied.The heat pump introduces a fin-tube three-medium heat exchanger on the basis of an ordinary large-scale air source heat pump,and this type of heat exchanger is employed as the outdoor side heat exchanger.By means of the heat exchanger,the waste heats from oil cooler and condenser are respectively recycled in winter and summer.Firstly the paper established a general heat transfer model for the fin-tube heat exchanger.By introducing graph theory,it is more convenient and intuitive to show the flow path layout of the heat exchanger.On this basis,the effects that two-dimensional wind velocity inhomogeneity have on the heat exchanger’s performance are investigated.The influences which both the physical parameters of the heat sources locating on both sides and the structure parameters of the heat exchanger exert on the heat exchange amount are analyzed.Secondly,optimize the design of the oil cooler and master reasonable structural parameters.Finally,a waste heat recovery system is established for large-scale low-temperature air source heat pump,and the operating modes of the waste heat recovery system are respectively explored in winter and in summer,and therefore the change of heat exchange amount and coefficient of performance(COP)of the system can be revealed.By selecting a screw compressor and using a fin-tube heat exchanger to build a large-scale low-temperature air source heat pump,the analysis of this system shows that:1.The fin-tube heat exchanger is affected by the air side and the low temperature hot water side,the uneven distribution of wind velocity will affect the performance of the heat exchanger,and the attenuation of heat is 20%at most compared with the uniform wind velocity distribution.By adopting multi-branch crossing flow path arrangement,the influence of uneven wind speed on heat exchange can be reduced;When the fin-tube heat exchanger adopts the double heat source mode,the maximum effective compound heat transfer temperature difference needs to be considered;the heat exchanger structure parameters will affect the maximum temperature difference,and this difference is related to both the heat exchange area and convective heat transfer coefficient while the heat transfer between the air side and the low temperature hot water side occurs.With regard to the product of the two side’s heat exchange area and the convective heat transfer coefficient,the smaller the value difference of product,the smaller the maximum temperature difference.The research results can be of reference significance for the design of the fin-tube heat exchanger and the selection of the inlet parameters of the low-temperature hot water side.2.The oil cooler adopts a shell-and-tube heat exchanger with simple structure and reliable operation.In the case of permission of margin,the structural parameters obtained by calculation and selection in summer operating conditions can meet the winter operating conditions.It is of reference significance for the selection of oil cooler of large air source heat pump.It can be selected according to the actual situation whether to use a same oil cooler in winter and summer.3.The operating condition of waste heat recovery system of winter is different with the case of summer.In winter,the waste heat of the oil cooler is first recovered,and the low-temperature glycol aqueous solution is used as the low-temperature heat source to supply heat to the evaporator.In the case of adjusting the water side flow rate,the heat exchange amount can increase by about 50k W on average,and the COP of the system can increase by about 0.07 on average;when the air temperature is very low and only the water side temperature is adjusted,the heat exchange amount can increase by about 80k W on average,and the average COP can be increased by about 0.22.In summer,the waste heat of the condenser is first recovered.When the condensing temperature is low,the waste heat of the oil cooler can be continuously recovered to produce domestic hot water that meets the requirements.If the outdoor air temperature is higher,it can reach above 30℃.At the time,it can produce more than 40m~3of domestic hot water per hour and the temperature reaches45℃,as far as a certain hotel is concerned,it can meet the daily consumption of domestic hot water.The waste heat recovery system I researched can realize waste heat recovery in two heat pump operating states in winter and summer without adding additional heat exchangers.In addition,the waste heat recovery in winter can alleviate the problem that the heat pump heat is greatly reduced as the air temperature decreases.And,it can expand the scope of application of large-scale low-temperature air source heat pumps.