Research And Development Of The Heat Pipe Evaporator In Exhaust-source Heat Pump System

The recovery of waste heat has an important significance for the realization ofcomprehensive energy conservation and energy efficiency, especially for low-gradewaste heat recycling. Low-grade waste heat is often discharged without takenseriously because of its low temperature. But the low-grade waste heat has enormouspotential for waste heat recovery for its widespread distribution and large quantity.As a low-grade waste heat, exhaust of gas boiler contains a large amount of latent andsensible heat of vaporization, The depth recovery of exhaust of gas boiler has animportant practical significance for improving the gas utilization and achievingenergy conservation.In this paper, the characteristics and waste heat recovery potential of the exhaustgas were analyzed, then the exhaust-source heat pump system solutions was proposed,and a new type of heat pipe evaporator was developed, which matches theexhaust-source heat pump system. At last, the numerical simulation method isadapted to research the characteristics of heat transfer and gas flow in the new type ofheat pipe evaporator, the specific conclusions are as follows:(1) The actual gas boiler operating conditions were investigated, and the dewpoint of the flue gas and the factors which influence water vapor’s condensation ratewas analyzed. The most basic factor of flue gas dew point is the relative content ofwater vapor in the flue gas. The dew point increases with the increasing moisturecontent of air. Dew point of flue gas decreases with the increase of excess air ratioand decreases with the lower exhaust gas temperature. Water vapor condensation rateraised with the decrease of exhaust gas temperature, and the increasing amplitude ofthe condensation rate is reduced when the exhaust temperature is below35℃.（2）The air source heat pump shows unstable performance because of the seasonaltemperature. The finned tube heat exchanger is not available for low temperature fluegas heat recovery. To solve the problems above, the flue-source heat pump systemwhich uses the low-temperature exhaust heat as low-temperature heat source was proposed, and a new type of heat pipe evaporator was developed, which makes theheat pump system and the boiler exhaust system have a good fit.（3）The effects of different flow rate on heat transfer and flow state were studiednumerically, the Nusselt number and flow resistance increases with the increasinginlet gas flow rate. The most optimal performance is achieved, when the inlet velocityis5m/s. As the pitch increases, the convective heat transfer coefficient increases, withthe drag coefficient reduced. The convective heat transfer capability get weakenedwith the increasing fin hight, and the flow resistance also increases. The heat transferperformance of finned tube heat exchanger can get improved by increasing tubespacing, with the flow resistance reducing at the same time. In the range of fin size,the most optimal thermal performance is with the pitch10mm, fin height5mm, tubepitch60mm. The number of field synergy increased with the increasing fin pitch, thegreater fin height and tube pitch spacing can make the field synergy worse.