Thermodynamic Design of Condensers and Evaporators: Thermodynamic Design of Condensers and Evaporators: Formulation and Applications

This paper assesses the approach introduced in a prior publication for the optimum design of condensers and evaporators aimed at balancing the heat transfer and pressure drop tradeoffs. Case studies carried out with different heat exchanger configurations for light commercial and household refrigeration applications are reported. The analysis indicated that a heat exchanger design with a high aspect ratio is preferable to a low aspect ratio one as the former produces a dramatically lower amount of entropy. In addition, since the refrigeration system COP obeys the Te/(Tc–Te) scale, it was found that the heat exchanger design that presents the best local (component-level) performance in terms of minimum entropy generation also leads to the best global (system-level) performance

Thermodynamic Design of a Mesoscale Vapor Compression Cooling Device

This paper presents a thermodynamic methodology for designing a vapor compression refrigeration system aiming at electronics cooling. A cycle simulation model was developed firstly assuming isentropic compression and isenthalpic expansion, whereas the heat exchangers (condenser and evaporator) were modelled following a distributed approach. Whilst a 3-D heat conduction model calculates the heat leakage from the condenser to the evaporator, 2-D heat conduction models provide the temperature distribution (and the heat transfer rates) at the cold and hot ends. The fluid flow was modelled as 1-D considering both the momentum and the energy conservation equations to design the heat exchangers geometry and circuitry considering the heat and fluid flow trade-offs that take place when the system is scaled down. Subsequently, semi-empirical sub-models for variable-speed compressors and fixed-orifice expansion devices were incorporated to the cycle simulation model, which was then used to assess the effect of the components characteristics (expansion orifice size, compressor stroke and speed) on the system COP. When the case where a 5×5 cm heat source at 40°C with the surroundings at 25°C is considered, the optimal design provides a cooling capacity of 110 W with a COP of 1.6. If compared to a thermoelectric device available on the market operating at the same conditions, the thermoelectric cooler provided a COP of 0.3, nearly 5 times lower than that provided by vapor compression system designed by means of the thermodynamic methods presented in this work

Defrost Cycle Optimization for Fan-Supplied Tube-Fin Evaporators Subjected to Frosting Conditions

Frost accretion on a tube-fin evaporator surface raises its thermal and hydraulic resistances, which in turn deteriorates the cooling capacity of the refrigeration system. For this reason, periodic defrost operations are required to recover the initial performance of the system. Most defrost techniques rely on supplying heat to the frost layer. Due to technological and thermodynamic limitations, only part of the heat supply is effectively used to melt the frost built-up on the evaporator, while the rest increases the refrigerator thermal load. Therefore, the amount of heat dissipated, the duration of a defrost operation, and the time between two consecutives defrost operations must be considered for properly designing the defrost system. The present paper assesses, by means of a simulation model that considers the airflow reduction with frost accretion, the effects of evaporator operating and geometric conditions, and defrost cycle parameters on the time-averaged cooling capacity. The results pointed out the existence of an optimum time for the defrost operation, indicating that short intervals between two defrost operations increase the thermal load by frequent heat supply, whilst long intervals promote severe capacity depletion by frost clogging

Das Netz auf der rechten Seite auswerfen. Erfahrungen eines Pfarrers in Europas größtem Business-Netzwerk Xing

Der Beitrag beschreibt den überraschend erfolgreichen Versuch, mit Hilfe der Business-Plattform Xing im Internet ein kirchliches loka- les Netzwerk in einer sehr jungen und dynamischen Großstadtpfarrei aufzubauen. Von Januar 2008 bis Mai 2009 wurden rund 3000 Personen kontaktiert, die sich im örtlichen Umfeld einer katholischen Seelsorgeeinheit in Stuttgart bei Xing registriert hatten. Insgesamt 576 Personen nahmen die Einladung des Pfarrers an. Der Aufbau des Netzwerkes wird technisch und organisatorisch erläutert. Anschließend werden seine Entwicklung beschrieben und die Ziele und Ergebnisse pastoral beurteilt, insbesondere unter den Aspekten der Evangelisierung und der „missionarischen Kirche im Volk“ sowie mit Blick auf die aktuellen religionssoziologischen Problemanzeigen. EnglishChristian Hermes: Casting the Net on the right side. A priest‘s experiences in Europe‘s biggest internet Business Network Xing This article describes a surprisingly successful test to establish a local Church Network in a very young and dynamic city parish by using the internet Business Network Xing. From January 2008 to May 2009 about 3000 persons were contacted who are living in a local Catholic entity in Stuttgart and had registered at Xing. 576 of those accepted an invitation of the local parish priest. The article explains how the network was technically established, organized and developed. The pastoral aims and results are especially explained and assessed under the aspect of evangelising and a „missionary Church among people“ as well as from the view on disclosing actual social religious problems

Thermodynamic Optimization of Tube-Fin Evaporators Operating under Frosting Conditions

In this study, the method of entropy generation minimization (i.e., design aimed at facilitating both heat, mass and fluid flows) is used to improve the evaporator geometry (aspect ratio and fin density) considering the irreversibilities produced by the heat and mass transfer, and viscous flow processes. A fully algebraic semi-empirical model was put forward to simulate the thermal-hydraulic behavior of tube-fin evaporator coils running under frosting conditions. The model predictions were validated against experimental data from Silva (2012), showing a good agreement between calculated and measured counterparts, with errors within a ±5% band for the cooling capacity and the air flow rate. The optimization exercise pointed out that high aspect ratio heat exchanger designs lead to minimum entropy generation in cases of fixed heat duty and air flow rate constrained by the characteristic curve of the fan

Modelling flows of recreational ecosystem services in Germany

While policy interest and research on recreational ecosystem services (RES) has increased substantially, insights into the actual flows of benefits to people have remained scarce, especially at levels beyond local communities. The aim of this paper is to model RES flows and benefits for Germany. We use Germany as a case study due its diversity of landscapes and availability of relevant spatial and empirical data. We develop and apply an assessment approach that considers RES demand and supply based on user preferences. Our results show distinct demand-supply matches and mismatches, for example in south-west Germany, and highest flows near population centres. Monetary benefits are highest in counties with high RES Supply that are close to densely populated areas. Our results can usefully inform planning and decision making, for example to improve and further justify destination management, landscape development and investments in RES at local, regional and national levels.Bundesamt für Naturschutz / Bundesministerium für Umwelt, Naturschutz, Bau und Nukleare Sicherheit/UFOPLAN 2018/3518 81 040B/E

Alternative Energy Test Method for Frost-Free Refrigerators and Freezers

This paper outlines an alternative test method to evaluate the energy consumption of frost-free refrigerators and freezers for residential applications. While the standardized methods require the refrigerating appliance to be kept running according to its onboard control system, which usually drives the refrigerator through an on-off cycling pattern, the proposed approach assesses the refrigerator energy performance in the steady-state regime, being therefore much faster and more reliable. In this procedure, the cooling capacity is matched to the cooling loads by PID-controlled electrical heaters installed within the refrigerated compartments, so that the compartment temperatures are kept at the desired standardized levels. Comparisons between the experimental results obtained using the steady-state energy (SSE) test and the standardized procedures showed that the former follows closely the trends observed for the latter