Laminar-flow Liquid-to-air Heat Exchangers - Energy-efficient Display Cabinet Applications
Abstract: Provisions are stored and displayed in supermarkets and grocery stores, at a temperature lower than the ambient, in display cabinets, which are responsible for a significant amount of the energy use in this sector. During the 1990s, major changes in the regulations governing the use of synthetic refrigerants took place in Sweden. This resulted in many refrigeration systems being converted to systems with indirect cooling by means of a liquid secondary refrigerant. The cooling coil is an important component in a display cabinet and traditionally, different kinds of tube-coils, with aluminium fins on expanded circular copper tubes, have been used. Many secondary heat transfer media have relatively high viscosities at low tem¬peratures, and so the flow regime is often laminar, which may lead to poor heat transfer. However, it is possible to achieve high heat transfer coefficients even for laminar flows if an ap¬propriate design of the heat exchanger is applied. Flat-tube heat exchangers have been used for a long time in automotive applications, where compactness is important. The display cabinet application involves low air velocities in combination with condensation of water vapour, and sometimes even frosting, and is therefore different from other applications where flat-tube heat exchangers have been used earlier. The objective of this research work has been to find a suitable, more energy-efficient, heat exchanger design for indirectly cooled display cabinets. An efficiency that would mean that temperature differences would be so small that frosting could be avoided was aimed for. This research work has sandwiched experimental investigations with theoretical modelling and parameter studies. Initially, the heat transfer and pressure drop performance of conventional cooling coils operating with liquid secondary refrigerants was studied in full-scale experiments. Thereafter, the liquid-side (or tube-side) heat transfer was studied experimentally in small-scale experiments on three single multiport extruded flat tubes with different cross-sections. In both studies, the best agreement was found with predictions using the Gnielinski correlation for thermally developing laminar flows with the constant temperature boundary condition. After this, two different heat exchangers having flat tubes and plain fins on the air side were evaluated experimentally - one with continuous plate fins and one with serpentine fins. The results show that, for conditions similar to those of display cabinets, the heat transfer and pressure drop performance is affected to little or no extent by the occurrence of condensate water. Models for different types of flat-tube heat exchangers were thereafter created using the results from the experimental studies. In order to find appropriate optimising criteria for the display cabinet heat exchangers, the energy efficiency of a complete cooling system of an imagined supermarket has been studied. Finally, different designs of flat-tube heat exchangers with plain fins have been evaluated theoretically in a parameter study. The results show that considerable savings in the required electric drive power can be obtained in comparison with the traditional cooling coil. The savings can be up to 15 %. In addition, the flat-tube heat exchangers could operate with a minimum temperature difference of around 1 K. This makes frost-free operation possible for many display cabinet applications and thereby the savings potential even larger.
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