An Experimental Study on the Effect of a new Defrosting Strategy on the Energy Consumption of Household Refrigerators

In a previous study an experimental testing apparatus was constructed and used to measure the defrosting efficiency of different heaters working under different control strategies. Based on the results obtained two defrosting heaters (one calrod and one distributed) and two control strategies (constant power and steps mode) were selected for this study. In the original mode the power was held at 160 W for 17.0 and 19.3 min for the distributed and the calrod heaters, respectively. In the steps mode the power was gradually reduced from 160 to 20 W, and the defrost length was set to 28.5 and 22.3 min, respectively, for the distributed and calrod heaters. Energy consumption tests were carried out following the ISO and AHAM standards, using the same top-mount frost-free refrigerator. Changes were implemented in the electronic board in order to vary the power and length of the steps. A glass window was installed in the rear wall of the refrigerator to visually monitor the defrosting process. A 4.3% drop in the energy consumptions obtained applying the two standards was achieved by replacing the calrod heater and the constant power mode with the distributed heater and the steps mode. Clogging tests were also carried out, using an automatic door-opening device to check the robustness of the different defrosting systems

An Experimental Study of HC-600a Flow Through Variable Expansion Devices for Household Refrigerators

This work focuses on the HC-600a flow through small capacity fixed and variable expansion devices. To this end three electronic expansion valves of the same innovative design but with distinct orifices, were firstly tested in a dry nitrogen flow testing apparatus by varying the opening ratio an the operating conditions. Secondly, equivalent capillary tubes were defined based on the measured nitrogen mass flow rates through an updated version of the well know Kipp Schmidt’s equation. Thirdly, simulations were carried out with an in-house validated capillary tube model to estimate the HC-600a mass flow rate, using the length and inner diameter of the equivalent capillary tubes as input data. A test rig was also designed, constructed and used to measure the valve HC-600a mass flow rates under different operation conditions and opening ratios. Comparative analyses between the valve and capillary tube mass flow rates were then carried out under different conditions. In-depth knowledge about the two-phase capillary tube and valve flows were then used to explain the observed differences

Um Estudo in-situ da formação de geada em evaporadores No-Frost

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2010A formação de geada em evaporadores é um fenômeno inevitável e indesejável, uma vez que degenera a capacidade de refrigeração e o desempenho do sistema. A perda de desempenho ocorre porque a camada de geada aumenta não só a resistência térmica entre o ar e os tubos aletados, mas também a restrição ao escoamento de ar, o que faz com que tanto a vazão deslocada pelo ventilador como a condutância térmica do evaporador diminuam. Com a redução na taxa de transferência de calor no evaporador, as temperaturas internas do refrigerador tendem a aumentar e isso exige que o compressor permaneça mais tempo ligado. Por essa razão, a geada deve ser periodicamente removida pela ação de uma resistência elétrica, o que eleva o consumo de energia. A crescente busca por um melhor entendimento desses fenômenos, bem como de alternativas para minimizar o crescimento da geada em evaporadores, deu origem a diversos procedimentos experimentais e a modelos matemáticos voltados à formação de geada em evaporadores. Entretanto, na ampla maioria dos trabalhos, os experimentos foram conduzidos em túnel de vento, onde o evaporador é submetido a condições uniformes de temperatura, umidade e vazão em sua entrada, o que não ocorre em aplicações reais típicas. Para preencher esta lacuna, foram realizados experimentos com o evaporador montado em sua posição normal, dentro de um refrigerador frost-free do tipo top-mount. O sistema de refrigeração do produto foi substituído por um calorímetro especialmente construído para tal finalidade, capaz de controlar não só a pressão de evaporação, mas também o grau de superaquecimento no evaporador. Além disso, desenvolveu-se um modelo computacional para prever o acúmulo de geada em cada fileira do evaporador. O modelo foi validado contra dados experimentais, quando se observaram desvios da ordem de 10% na estimativa da massa de geada acumulada. Finalmente, foram realizados ensaios experimentais para investigar oportunidades de melhoria do sistema de degelo do evaporador, quando se observou um aumento da eficiência de degelo de 25 para 60%

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

Non-Condensable Gases and their Effect on the Dynamic Behavior of Domestic Refrigerators

The aim of this work was to experimentally investigate the effect of non-condensable gases on the thermal acoustic behavior of domestic refrigerators under dynamic conditions. To this end, an acrylic made filter dryer was installed in the system for a proper visualization of the refrigerant flow at the capillary inlet. An accelerometer was also installed at the evaporator inlet to capture vibrations/noise signals, caused by the refrigerant pattern at the inlet of the expansion device. During the experiments the energy consumption was measured with the system systematically doped with precise amounts of nitrogen. It has been found that the energy consumption increases by 13%, when a mass fraction of 0.43% of N2 is added to the system. Moreover, it has been found that the energy consumption increases almost linearly with the N2 mass fraction. Pull-down tests were also carried out, showing a clear relationship between the N2 amount and the power and mass flow rate oscillations, soon after the compressor start-up, caused by the unbalance between the compressor and capillary tube mass flow rates. It was also observed that the noise level is higher when liquid refrigerant enters the capillary, thus increasing the corresponding mass flow rate

The Influence of Non-Condensable Gases on the Thermal-Acoustic Behavior of Household Refrigerators

In refrigeration systems with evaporating pressure below atmospheric pressure, air from the external environment can infiltrate into the circuit through small leaks in the suction line. Additionally, if a problem occurs during the evacuation process on the production line, residual air might be left inside the circuit. This paper reports an experimental study on the influence of non-condensable gases on the thermal-acoustic behavior of a household refrigeration system. Controlled amounts of nitrogen were injected into the system through a purpose-built device. Steady-state energy consumption tests were carried out for each situation. Simultaneously, acceleration signals were monitored by an accelerometer installed at the evaporator inlet and videos of the flow pattern at the capillary tube inlet were recorded. The results show that, with very small amounts of non-condensable gases, the system performance was slightly improved. However, with large amounts, a worse performance was observed combined with large fluctuations in the flow pattern and the acceleration readings. In addition, for all cases it was noted that the sub-cooling degree increased with the amount of non-condensable gases

Pros & Cons of Using Hot-Wall Condensers in Household Refrigerators

This work focuses on the pros and cons of using hot-wall condensers in household refrigerators, based both in numerical and experimental approaches. To this end, eight different refrigerator samples of the same model were manufactured with distinct designs of hot-wall condensers. The design parameters were the following: (i) adhesive tape (aluminum or polyethylene), (ii) tube outer diameter (4 or 4.76 m), (iii) total length (10 or 11.5 m) and (iv) lay-out. An in-house mathematical model for hot-wall condensers was added to an in-house system simulation tool to predict the samples performance. Experiments were also conducted in a climate-controlled test chamber with each sample. Is has been found that the model predictions are close to the energy consumption measurements with deviations of the order of ±10%. It has also been found that the heat load is increased by 7.7% when a hot-wall condenser is added to the system. An extensive sensitivity analysis was also carried out, showing that the hot-wall condenser and thus the refrigerator performance is very much affected by the outer sheet thermal conductivity and thickness, but mainly by the tape thermal conductivity. The contact area between tape and outer sheet also plays a significant role in the heat transfer, meaning that a cheaper polymeric tape might be used if enough contact area is provided. Additionally, it has been found that there is always a tube pitch which minimizes the energy consumption in despite of the condenser geometry

An Experimental Study on the Use of Vaccum Insulation Panels in Household Refrigerators

The Vacuum Insulation Panels, commonly called VIPs, are a good alternative for energy savings since their thermal conductivities are about five times smaller than those of the currently used polyurethane foams. However, their widespread use is limited by their cost and by the extra handling, application and storage cares. The aim of this work is to study the thermal performance of a specific refrigerator model based on experiments carried out with 16 samples, assembled with different amounts of VIPs, placed in different positions around the refrigerator envelope. Cyclic energy consumption tests were carried out showing, as expected, that the energy consumption decreases with the VIP coverage area, but not in a linear fashion. It was found that samples with equal VIP coverage area might show different energy consumption, depending on the panel positions. It was also found that the addition of vacuum panels in the doors and in the rear wall of the refrigerator dropped the energy consumption by approximately 6% and 11%, respectively. In general, a 60% VIP coverage area dropped the energy consumption by 21%

A Numerical and Experimental Study on Skin Condensers Applied to Household Refrigerators

The aim of this work is to numerically and experimentally investigate the performance characteristics of skin condensers applied to a specific household refrigerator model. To this end a mathematical model which takes into account both the heat transfer to the ambient and to the refrigerated compartments was developed. The model predicts the condenser performance from the internal and external air temperatures, geometry, mass flow rate, pressure and temperature at the heat exchanger inlet. Steady-state energy consumption tests – compressor running continuously with electric heaters being used to create artificial thermal load – were carried out in different operating conditions with the product maintained within a climate-controlled chamber. Tests were carried out at 25°C and 32°C ambient temperatures, 5°C to 10°C fresh-food compartment temperatures, -20°C to -15°C freezer compartment temperatures and compressor speeds of 3000 rpm and 4500 rpm. Thermo graphic images of the outer steel shell temperature fields were also recorded during the experiments. The model predictions were compared with the experimental database with the deviations falling inside a ±5% error band. It has been found that, independently of the operating conditions, 60% of the condenser heat released rate is transferred to the ambient while the remaining 40% is transferred to the refrigerated compartments. It has also been found that radiation plays an important role, being responsible by over 80% of the heat rejected to the ambient

Aplicabilidade de dispositivos de expansão de ação variável em refrigeradores domésticos

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2019.O presente trabalho teve como objetivo investigar a aplicabilidade de válvulas de expansão eletrônicas (EEVs) em sistemas domésticos de refrigeração. Primeiramente, testes com nitrogênio foram realizados para avaliar o potencial de três válvulas de orifícios distintos quando se verificou que elas se comportam de forma equivalente a tubos capilares. Tal avaliação deu origem a uma bancada desenvolvida para estudar o escoamento de HC-600a através das EEVs em função das pressões de evaporação e condensação, e do sub-resfriamento na entrada. No total, 138 testes foram realizados com isobutano, os quais permitiram a obtenção de dados de vazão para condições de escoamento blocado e não-blocado, além de correlações para o coeficiente de vazão, validadas dentro de uma faixa de ±10% de erro. Na sequência, a válvula mais promissora foi montada num refrigerador doméstico para correlacionar a área de passagem com o consumo de energia sob diferentes condições de funcionamento. Ensaios foram feitos também com o mesmo refrigerador, mas montado com um tubo capilar. Em paralelo, um modelo matemático foi desenvolvido para simular o efeito da carga de refrigerante e da restrição sobre o desempenho do refrigerador, apresentando erros inferiores a 15%. Mais de mil casos foram simulados e os resultados mostraram que o superaquecimento nulo nem sempre é o melhor dos cenários. Além disso, notou-se que os menores consumos são atingidos para as situações de maior carga e maior restrição. Observou-se também que é possível ajustar o par carga-restrição de maneira independente da rotação do compressor. Por fim, constatou-se que quando a temperatura ambiente ou a rotação do compressor são modificadas, pode-se reduzir o consumo de energia alterando-se a restrição do dispositivo de expansão, algo que só é possível com EEVs. O sistema montado com a EEV, no entanto, apresentou consumos superiores aos observados com o sistema montado com o tubo capilar para a maioria das condições, o que foi atribuído à arquitetura do ciclo e à carga térmica gerada pela EEV. Porém, ao aumentar o comprimento do trocador de calor interno e minimizar o calor gerado pela EEV, verificou-se que o sistema montado com ela consumiu entre 4 e 9% menos energia que o sistema montado com o tubo capilar em condições de operação diferentes daquela para a qual o tubo capilar foi dimensionado.Abstract: The aim of this work was to investigate the applicability of electronic expansion valves (EEVs) in domestic refrigerators. Firstly, tests with nitrogen were carried out to preliminarily evaluate the potential of three EEVs with distinct orifices where it was observed that they behave like capillary tubes typically found in practice. Such an evaluation gave birth to another test bench that was assembled to study the HC-600a flow through the EEVs as a function of the evaporation and condensation pressures, and the sub-cooling degree at the inlet. In total, 138 tests were conducted with isobutane and mass flow rate data under choked and non-choked conditions were gathered. Based on these results, correlations for the flow coefficient were proposed and validated within a ±10% error band. After that, the most promising valve was installed in a domestic refrigerator in order to correlate the flow area with the overall energy consumption under different operating conditions. Tests with the same cabinet, but mounted with a capillary tube, were also performed. In parallel, a mathematical model was devised to simulate the effect of the refrigerant charge and the expansion device restriction on the system performance and it was validated with errors of less than 15%. More than a thousand runs were simulated and the results revealed that zero superheating is not always the best-case scenario. In addition, it was noticed that the lowest energy consumptions were achieved for the highest refrigerant charges and expansion device restrictions. It was also verified that it is possible to adjust the pair charge-restriction independently of the compressor speed. Finally, it was found that when the ambient temperature or the compressor speed are changed, the energy consumption can be reduced by varying the restriction of the expansion device, which is only possible with EEVs. The system mounted with the EEV, however, presented higher energy consumptions than the system with the capillary tube for most conditions, which was related to the cycle architecture and the thermal load generated by the valve. Nevertheless, by increasing the length of the internal heat exchanger and minimizing the heat generated by the EEV, it was found that the system assembled with the EEV consumed between 4 and 9% less energy than the system mounted with the capillary tube under operating conditions different from that the capillary tube has been sized