The effect of radiation shields around the air condenser and compressor of a refrigerator on the temperature distribution inside it

In almost all domestic refrigerators–freezers all components are assembled in the same relative position since several years ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the compressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, part of it is due to thermal radiation that causes an overheating of the refrigerator–freezer surfaces adjacent to those equipments. As a consequence there are more heat gains to the refrigerator–freezer through these surfaces and hence higher air temperatures inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radiation. The improvement is achieved by placing a radiation shield – a sheet of aluminium foil – over the surfaces close to the condenser and the compressor. For validating this technique a refrigerator–freezer was monitored with thermocouples for the measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this practice the average inside air temperatures in the refrigerator–freezer could decrease to about 2 K. An available commercial code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both situations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which validates the experiments carried out.Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations.     

Experimental investigation on R134a vapour ejector refrigeration system

The experimental investigation of the performance of a vapour ejector refrigeration system is described. The system uses R134a as working fluid and has a rated cooling capacity of 0.5 kW. The influence of generator, evaporator and condenser temperatures on the system performance is studied. This kind of system can be operated with low grade thermal energy such as solar energy, waste heat, etc. The operating conditions are chosen accordingly as, generator temperature between 338 K and 363 K, condenser temperature between 299 K and 310.5 K, and evaporator temperature between 275 K and 285.5 K. Six configurations of ejectors of different geometrical dimensions are selected for the parametric study. The performance of the refrigeration system at different operating temperatures is presented.     

Flow boiling heat transfer characteristics of CO2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO₂ at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO₂ at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO₂ is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO₂ in good agreement with the experimental data, suggesting the need for further research in this area.     

Methodology for thermal design of novel combined refrigeration/power binary fluid systems

Refrigeration cogeneration systems which generate power alongside with cooling improve energy utilization significantly, because such systems offer a more reasonable arrangement of energy and exergy “flows” within the system, which results in lower fuel consumption as compared to the separate generation of power and cooling or heating. This paper proposes several novel systems of that type, based on ammonia–water working fluid. Importantly, general principles for integration of refrigeration and power systems to produce better energy and exergy efficiencies are summarized, based primarily on the reduction of exergy destruction. The proposed plants analyzed here operate in a fully-integrated combined cycle mode with ammonia–water Rankine cycle(s) and an ammonia refrigeration cycle, interconnected by absorption, separation and heat transfer processes. It was found that the cogeneration systems have good performance, with energy and exergy efficiencies of 28% and 55–60%, respectively, for the base-case studied (at maximum heat input temperature of 450 °C). That efficiency is, by itself, excellent for cogeneration cycles using heat sources at these temperatures, with the exergy efficiency comparable to that of nuclear power plants. When using exhaust heat from topping gas turbine power plants, the total plant energy efficiency can rise to the remarkable value of about 57%. The hardware proposed for use is conventional and commercially available; no hardware additional to that needed in conventional power and absorption cycles is needed.     

Thermodynamic properties of lithium bromide–water solutions at high temperatures

Emerging triple-effect LiBr–water absorption chillers operate at higher temperatures and pressures than traditional double-effect chillers. However, there is not enough data about thermodynamic properties of LiBr–water solutions at such high temperatures. Using recently measured data of vapor pressure and heat capacity, we have developed the equations which can calculate the vapor pressure, enthalpy and entropy of LiBr solutions at such high temperatures. The developed equations are valid from concentrations of 40–65 wt.% and also from temperatures of 40–210°C. These equations will be very helpful for the modeling and design of triple-effect LiBr–water chillers.     

Influence of fungicide treatment and storage conditions on mould and yeast activity on “Satsuma” mandarin

The activity of mould and yeast population on the surface of ‘Satsuma’ mandarin was studied at four temperatures (2, 4, 10 and 20 °C) and two relative humidity (RH) levels (80 and 93%) on fruits washed and on fruits washed and treated with a fungicide mixture (1200 ppm thiabendazole + 800 guazatine + 400 ppm prochloraz), stored for 20 and 40 days. The lowest final microbial counts (CFU) were for the fruits treated and stored at 2 °C and 93% RH and the highest counts were for fruits stored at 20 °C and 93% RH regardless of fungicide treatment. Throughout the storage period, yeast and Cladosporium spp. were predominant in the microbial counts and generally showed reducing trends. The fungicide treatment was very efficient and persistent. The absence of antifungal treatment and the increase of RH levels caused a greater rise of CFU than the increase of the storage period or temperature. An increase in the CFU values with the increase of RH was observed in untreated fruits. The treatment of ‘Satsuma’ fruits with the fungicide mixture, followed by cold storage at 2 °C and 93% RH, extended the storage period at least 40 days.     

Refrigeration throughout the world

The economic impact of refrigeration technology throughout the world is already very impressive, and more significant than is generally believed. While the yearly investment in machinery and equipment may approach US$100 billion, the value of products treated by refrigeration is perhaps ten times this amount. The importance of refrigeration is bound to increase since it will be an essential factor in solving two major problems of the future: the supply of enough food and the development of new energy sources. The use of low temperatures constitutes a major means of conservation of perishable foods during storage and distribution and it is widely applied in the developed countries. In the Third World, however, the use of low temperatures is mainly limited to food for export and the needs of the ‘well-to-do’ sector of the population. Refrigerated or frozen foods, in their present form, are generally priced too high for the undernourished poor. There is a pressing need for simple and inexpensive cooling methods to enable efficient low cost mass production and distribution under more primitive conditions. The future of man, and his food supply in particular, depends on the availability of sufficient energy. At present, fossil fuels carry the main load, but in the course of the next century new energy sources, such as nuclear fusion, will have to be commissioned. Refrigeration technology will play an important part in this development.     

Influence of cryogenic temperatures on food products and packaging during rail transport

Six commercially packaged frozen food products were subjected to simulated railway shock and vibration conditions and manual handling abuse at both cryogenic (−70°C) and non-cryogenic (−18°C) temperatures. The amount of package and food product damage under different temperatures and levels of abuse were determined. The effects of cryogenic temperatures upon sensory and various analytical quality attributes of the food products were examined. The test results indicated that frozen food can be shipped at cryogenic temperatures in railcars equipped with cushioned draft gears with minimal product or packaging damage, provided there is not excessive manhandling abuse.     

Influence of cooling and temperature maintenance on the quality of California grown stone fruit

Stone fruits require rapid cooling within a few hours of harvest. High temperature and cooling delays can speed fruit deterioration from several causes. Flesh softening is most rapid at temperatures between ≈4 and 38°C, where ethylene sensitivity is greatest. Water loss is related to exposure time and the magnitude of water vapour pressure difference between fruit and environment. Internal breakdown, a low temperature injury problem, is aggravated in California by high temperature delays before cooling. Fruit rot organisms spread most rapidly at warm harvest temperatures and are slowed or stopped at near 0°C. When packing is delayed, then cooling before packing is essential. Because injury susceptibility is least at intermediate temperatures (5–20°C), fruit may be cooled to 5–10°C for next day packing, but should be cooled to near 0°C for longer delays. Regardless of prior cooling, the warming that will occur during packing makes essential final cooling to near 0°C following packing. Methyl bromide fumigation, if needed for quarantine treatment, should occur before cooling because warm fruit is less injured and fumigant doses are lower at warmer temperatures.     

History of cryogenics: the epoch of the pioneers from the beginning to the year 1911

The invention of the thermometer by Galileo in 1592 may be considered as the start of the science of thermodynamics. Guillaume Amontons predicted for the first time the existence of an absolute zero in 1702, which marks the beginning of the science of low temperatures. Around 1780, the liquefaction of a gas was achieved for the first time. It took almost another 100 years before a so-called “permanent” gas, i.e. oxygen, was successfully liquefied. Thereafter Linde and Claude founded the cryogenic industry, which today has annual sales of more than 30 billion US $. Kamerlingh Onnes and his Cryogenic Laboratory in Leiden worked in the field of low temperature physics, which contributed to the experimental proof of the quantum theory.     

The temperature performance of domestic refrigerators

Chilled foods are stored for periods of between a few hours and many days in domestic refrigerators. However, there are little published data on the temperature performance of domestic refrigerators within the home. A survey has been taken in 252 households in the UK and some of the results are presented in this paper. The refrigerators investigated in the survey were found to have an overall mean temperature of approximately 6°C, which ranged from 11.4 to −0.9°C. Temperature ranges over the whole refrigerator varied from 4.5 to 30.5°C with 3.7% of the total being warmer than 20°C. On average 29.9% of refrigerators operated below 5°C and 66.7% operated below 7°C. Few refrigerators (7.3%) ran, on average, above 9°C. No refrigerator characteristic (apart from type) could be related to temperatures or temperature distribution in the refrigerators investigated.     

Thawing of frozen strawberries

Frozen strawberries were thawed under different controlled conditions (natural thawing at room temperature, thawing in circulating air, thawing in a refrigerator, thawing in water and thawing in a convection oven). The temperature rise at the geometric centre of the strawberries was monitored until the temperature reached 6°C and thawing diagrams were drawn. The effects of thawing method on the weight loss in strawberries were determined. Strawberries thawed at higher temperatures showed greater weight loss. During thawing in circulating air, thawing time decreased with increasing air velocity.     

Zeolite heat pump — adsorption of methanol in synthetic zeolites 13X, 4A and 5A

The adsorption isotherms for methanol adsorption in synthetic zeolites 13X, 4A and 5A were measured gravimetrically at various temperatures. The model of Langmuir and the potential theory were used for correlating the equilibrium data. The monolayer capacity was calculated using these two models. A small deviation at elevated temperatures was observed. The differential heat of adsorption was measured calorimetrically for all three zeolites. The isosteric heat of adsorption was calculated for zeolite 13X using measured equilibrium data. The log p, 1/T equilibrium diagram for zeolite 13X was computed using the Langmuir equation.     

Assessment of condensation heat transfer correlations in the modelling of fin and tube heat exchangers

This is the second paper of a series that assesses the performance of a refrigeration system model by means of cycle parameters. In this case, the condensation temperature is the parameter to study and it is focused on fin and tube condensers. It also studies the influence of the heat transfer models on the estimation of this refrigeration cycle parameter and different correlations for the heat transfer coefficients have been implemented in order to characterise the heat transfer in the heat exchangers. The flow inside the heat exchangers is considered one-dimensional as in previous works. In the cycle definition, other submodels for all the cycle component have been taken into account to complete the system of equations that characterises the behaviour of the refrigeration cycle. This global system is solved by means of a Newton–Raphson algorithm and a known technique called SEWTLE is used to model the heat exchangers. Some experimental results are employed to compare the condensation temperatures provided by the numerical procedure and to evaluate the performance of each heat transfer coefficient. These experimental results correspond to an air-to-water heat pump and are obtained by using R-22 and R-290 as refrigerants.     

Analysis of stresses during the freezing of solid spherical foods

An analytical model is presented to calculate thermal stresses and strains during the freezing of a spherical food, taking into account both the expansion during phase change and subsequent thermal contraction due to temperature decrease. The Young modulus and Poisson ratio are assumed to undergo a step change at the freezing point. The expansion due to phase change cause a uniform and virtually constant isotropic tensile stress in the unfrozen core. In the frozen shell, this expansion gives rise to tensile radial stress and compressive tangential stress. The thermal contraction subsequent to phase change causes reverse effects, i.e. uniform compressive stress in the unfrozen core and compressive radial stress in the frozen shell, while tangential stress is tensile on the outside and compressive on the inside of the frozen shell. The effect of thermal contraction is noticeable only at cryogenic temperatures.     

Application of hydrocarbon mixtures in small refrigerating and cryogenic machines

Application of hydrocarbon mixtures enables the creation of simple, reliable and durable refrigerating and cryogenic Joule–Thomson micro coolers for the temperature range of −73 to −183 °C. The temperature, thermal, power and hydraulic performances of a series of prototypes are presented. The results of tests demonstrate that small, single stage, sealed, lubricated compressors can be applied to these purposes. The start up and steady operation hydraulic performance of those machines are quite similar to the performance of domestic refrigerators. The last, together with the fact that in the studied micro coolers the lubricated compressors are used at temperatures down to −183 °C, ensures a large resource of operation. That is just the reason that holds out a hope for prospects in a broad field of applications for the studied prototypes, despite their lower power performances in contrast to gas micro coolers.     

Modelling of the thermodynamic properties of the water–ammonia mixture by three different approaches

This paper deals with the modelling of the thermodynamic properties of the water–ammonia refrigerant mixture. Three different approaches are formulated and compared. The first is an empirical approach based on a free enthalpy model of the mixture considered as the resultant of the properties of its pure components and of an excess term corresponding to the deviation to the ideal solution concept. Secondly, a semi-empirical approach based on the PATEL and TEJA cubic equation of state is considered. Finally, a theoretical approach formulated as PC-SAFT (perturbed chain statistical associating fluid theory) equation of state is treated. A comparison of these three methods proves the superiority of PC-SAFT in predicting and extrapolating the thermodynamic properties of the water–ammonia system up to very high temperatures and pressures.     

Bubble pressures and saturated liquid molar volumes of difluoromonochloromethane—fluorochloroethane binary mixtures: experimental data and modelling

A previously described apparatus (variable volume cell method) was used to obtain bubble pressures and saturated liquid molar volumes at four temperatures for systems involving difluoromonochloromethane with tetrafluorodichloroethane, trifluorotrichloroethane and difluoromonochloroethane. The experimental bubble pressures are well represented by the Peng-Robinson equation of state with one adjusted interaction parameter. Adjusting a second interaction parameter does not significantly improve the representation. Soave and Mathias equations give a quite similar representation. Saturated liquid molar volumes calculated from the different equations of state, with binary interaction parameters adjusted on bubble pressure data, do not agree well with experimental results: the standard deviation between calculated and experimental densities is ≈ 8% with the Peng-Robinson equation of state and ≈ 18% with that of Soave and Mathias.     

Comparison of the performance of different ice slurry types depending on the application temperature

The ice slurry medium type used in a refrigeration application could influence the performance of an ice slurry system. For this reason and depending on the refrigeration application, the user has to usually carry out a judgemental selection of the ice slurry mixture type, which should take into account the solute type and its concentration. This article compares the performance of several commonly used organic and inorganic ice slurry secondary refrigerants. This study was based on thermophysical assessments carried out at different operating temperatures. The calculation method that was used to determine the ice slurry properties is first presented. Then, in order to describe the thermophysical efficiency of mixtures at various operating temperatures (−5, −20 and −35 °C), three performance criteria were compared, namely, the volume enthalpy drop, the temperature at the inlet of the application and the relative viscosity of the ice slurry. The results showed that inorganic mixtures are good selection candidates, except for situations where low temperatures and high ice concentrations are encountered at the inlet of the application. Methyl alcohol came out as a good performance candidate for all refrigeration applications, although NH₃ was the best choice based on the current thermo-physical property assessments.     

Numerical optimisation of a two-stage ejector refrigeration plant

Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.