Design of ammonia-water condenser with a fluted tube

This paper develops a design model of an ammonia-water condenser in absorption heat pump systems. An ammonia-water mixture enters the condenser through a helically coiled fluted tube as a superheated vapor, cools, and then condenses into a liquid, which is further subcooled. The hydronic fluid flows in the annular channel around the coiled tube in confined cross-flow. The helically coiled fluted tube was used to enhance heat and mass transfer in the mixture and heat transfer in the hydronic side. The ammonia-water condenser was divided into four different regions according to the characteristics of the heat transfer mechanism-superheated vapor, rectification, condensation, and subcooled liquid regions. This paper studies the effects of heat transfer coefficients in each region, the inlet concentration of vapor, and the geometric details of the fluted tube on the heat exchanger size. The design results show that the molar concentration of ammonia in rectifying vapor is between bulk vapor concentration and liquid concentration, i.e., x₁<z<x_vb, and that the molar concentration should be calculated by combined heat and mass transfer analysis. The heat transfer rate per unit length in the rectification region was the highest due to the effect of mass transfer. As the inlet concentration of ammonia in the vapor decreases, the length of the condenser decreases. The heat transfer coefficient in coolant flow, h_c, has the dominant effect on the condenser size, while h_v has the most significant effect on the rectifier size. It also was found that the condenser size decreases with increasing D_bi, decreasing D_eo, D_c, N_s, and pitch of the fluted tube.