Investigation of Air-Cooled Condensers for Ammonia-Water Absorption Chillers

Absorption heat pumps are being considered as alternatives to vapor-compression systems in some applications. These systems utilize environmentally friendly working fluids and can be driven by a variety of heat sources, such as process waste heat, solar, and geothermal energy. In some implementations of absorption systems, the use of direct air coupling of the heat exchangers can reduce the system size, complexity, and inefficiency. However, issues of material incompatibility with the refrigerant mixture and poor heat transfer properties of air pose challenges for these heat exchangers. Prototype air-coupled condensers for use in an ammonia-water absorption chiller driven by waste heat from diesel engine exhaust are investigated experimentally and analytically.  A segmented model is developed to predict the performance of round-tube corrugated-fin and multi-pass tube-array condensers designed for a 2.64-kW cooling capacity absorption system operating at high ambient temperature (51.7°C) conditions. Results from heat transfer and pressure drop correlations based on the two-phase flow regimes for zeotropic mixture condensation are compared at the design conditions and the most appropriate correlation is applied. Air-side heat transfer coefficients and pressure drop are calculated taking the fin-geometry, the fin and tube spacing, and air properties into consideration. Additionally, maldistribution of vapor and liquid in the header affects the performance of the air-cooled condenser significantly. The segmented models quantify maldistribution in the header based on the applicable two-phase flow regimes and account for its effect on heat transfer. Two round-tube corrugated-fin condensers (9.525 mm and 15.875 mm tube OD) and two multi-pass tube-array designs are modeled, fabricated and tested. A single-pressure ammonia-water test facility is constructed and used in conjunction with a temperature- and humidity-controlled air-handling unit to evaluate the condensers at design and off-design operating conditions. Condenser performance is recorded over a range of air temperatures (35-55°C), refrigerant inlet temperatures (57-67°C), air volumetric flow rates (0.33-0.45 m3 s-1), and refrigerant mass flow rates (0.0019-0.0026 kg s-1). Furthermore, surface temperatures of tubes in the condenser are measured to understand vapor- and liquid- phase maldistribution. Measured heat transfer rates and pressure drop values are compared with the predictions of models from the literature. Results from this investigation guide the development of air-coupled zeotropic mixture condensers for compact absorption heat pumps