Investigation of air-cooled condensers for waste heat driven absorption heat pumps

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Chakraborty, Subhrajit
Garimella, Srinivas
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Vapor absorption based heating, refrigeration, ventilation, and air-conditioning (HVAC&R) systems can utilize low-grade waste heat streams to provide heating and cooling. These thermally driven absorption systems use environmentally benign working fluids but require more heat and mass exchangers than conventional vapor compression systems. The implementation of these systems in the residential and light commercial market has not been practical for several reasons, including the lack of compact and economically viable heat and mass exchangers. Indirect coupling of the condenser in an absorption system to the ambient through an intermediate fluid loop requires additional electrical input for pumping and lowers the overall coefficient of performance (COP) of the system. This study considers the development of condensers directly coupled to the ambient and aims at improving the understanding of heat and mass transfer processes in heat exchangers used in absorption systems. A detailed experimental and analytical investigation of air-coupled condensers for use in small-scale, ammonia-water absorption systems is conducted. Ammonia-water is the preferred fluid pair in small-scale absorption systems due to its high operating pressure, which allows for compact component design. Customized round-tube corrugated-fin condensers are built for an absorption chiller of 2.71 kW cooling capacity operating at severe ambient temperature conditions. Novel multi-pass tube-array design condensers are also fabricated for the same application and their performance compared with the performance of the conventional condensers. A segmented heat and mass transfer model is developed to simulate the performance of the condensers. 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. The experimental data are used to validate and refine the design models. Performance of the condensers is recorded over a range of air temperatures, refrigerant inlet temperatures, air flow rates, and refrigerant flow rates. Several, operating variables have a large impact on round-tube corrugated-fin condenser performance, although the effect of air flow rate was lower. The novel multi-pass condensers demonstrated a steady increase in performance with increasing air flow rates. Although some fabrication issues in the present study led to the full potential of microchannel condensers not being realized, it appears that such multi-pass microchannel tube-array condensers can out-perform the conventional designs with proper manufacturing techniques. The results from this study can be applied for the development of a variety of condensers of novel configurations.
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