Experimental investigation of absorption in upward and downward flow of NH₃-CO₂-H₂O in a mini-channel heat exchanger

Abstract

Heat pumps can drastically reduce energy requirements in industry. Operating a compression resorption heat pump with an NH₃-CO₂-H₂O mixture has been identified as a promising option that can have an increased performance compared to only NH₃-H₂O. In this paper an important process of the heat pump cycle is investigated: The absorption process. A mini-channel heat exchanger with 116 tubes of inside diameter of 0.5 mm is used for this purpose. For the NH₃-H₂O experiments overall heat transfer coefficients of 2.7–6 kW/(m²K) were reached for mixture mass flows of 0.71–2.5 kg/h. For the NH₃-CO₂-H₂O mixture pumping instabilities limited the operating range which resulted in higher pressures and higher mixture mass flows compared to NH₃-H₂O. The overall heat transfer coefficients were lower in the case of the added CO₂, with the maximum of 3 kW/(m²K) corresponding to a mixture mass flow of 4.2 kg/h. However, an increase in heat transfer of approximately 5% was reached with the added CO₂ which is beneficial for heat pump applications. Additionally, limited research has been conducted on absorption in upward versus downward flow and, therefore, these two configurations have also been tested in the mini-channel heat exchanger. Even though the pumping instabilities vanished with absorption in upward flow it was confirmed that absorption in downward flow with the mixture on the tube side is the most beneficial configuration for absorption of ammonia in NH₃-CO₂-H₂O or NH₃-H₂O in a mini-channel heat exchanger. The performance increased by approximately 10% with absorption in downward flow.