Absorption Heat Transfer Performance of Ammonia-Water Mixture in 116 Tube Mini-Channel Heat Exchanger

摘要

It is known that decreasing the channel size in heat exchangers increases its heat and mass transfer performance. Using such heat exchangers offers the opportunity to reduce the size and cost of industrial heat pumps, what should lead to better market acceptance. In the past, many experiments have been done for air/water mixtures (adiabatic) and refrigerants like CO_2, R134a and water (diabatic). A variety of models predicting heat transfer coefficients for these refrigerants are available in literature, but for certain systems they are not in agreement with each other. Currently data for ammonia/water mixtures, a fluid used in absorption and compression-resorption heat pumps is missing. A novel mini channel shell and tube heat exchanger with 116 tubes with an inner diameter of 0.5 mm, an outer diameter of 1.0 mm and a length of 0.655 m has been developed to increase the heat transfer performance in industrial compression-resorption heat pumps working with ammonia-water as refrigerant. In the current research the tube side heat transfer performance is investigated using the ammonia-water mixture, while water is used in the shell side of the heat exchanger. The influence of mass flow rate, heat load and vapor quality on the heat transfer performance and pressure drop are investigated. The heat load was varied between 200 and 1800 W, with the refrigerant mass flux varied between 20 and 75 kg m~(-2) s~(-1) with the average vapor quality ranging between 0.2 and 0.6 kg kg~(-1) and operating pressures between 5 and 13 bar. Overall heat transfer coefficients, based on the outer diameter of the tubes, between 70 and 700 W m~(-2) s~(-1) have been obtained. The approach temperature at the absorber inlet, after calibrating the PT-100 elements, ranged between 0.3 and 4 K and the average temperature driving force is determined to be between 8 and 25 K. The measured pressure drop ranges between 0.02 and 0.2 bar. Trends show an increasing pressure drop and heat transfer coefficient with increasing mass flux and vapor quality. The heat transfer coefficient on the shell side appears to be the limiting factor at higher measured mass fluxes. The heat load was limited by the maximum flow of the water pumps on the shell side as well as the maximum available heating power of 3.5 kW.