Modelling of temperature fluctuations within frozen foods stored in cylindrical containers

A mathematical model is presented for the prediction of temperature response in cylindrically shaped frozen foods when exposed to a periodically varying environmental storage temperature. The one-dimensional conduction equation in cylindrical co-ordinates was solved including the effect of surface resistances. The prediction model was verified against experimental temperatures obtained from frozen ice-cream samples exposed to two different storage regimes. The experiments were conducted with and without commercial packaging cardboard. Experimental and predicted temperatures presented excellent agreement in both cases. Packaging materials and plastic overwraps of small cylindrical containers can provide an effective resistance against ambient temperature variations.     

Calculation of thermodynamic properties of R407C and R410A by the Martin–Hou equation of state — part I: theoretical development

A theoretical development of the thermodynamic properties of two mixtures of hydrofluorocarbon (HFC) refrigerants, i.e. R407C and R410A (in the superheated vapour state), is carried out. The modelling is based on the Martin-Hou equation of state, which has long been used for pure hydrofluorocarbons (e.g. R134a) with good results. Since R407C and R410A are very well investigated refrigerants, the analytical procedure here derived concerns with those thermodynamic properties of R407C and R410A (in the superheated state) that are not published in the current specialised literature. They are: compressibility factor, isentropic and isothermal compressibility, volume expansivity, isentropic and isothermal exponent, speed of sound and Joule–Thomson coefficient. These properties may be used as a theoretical basis for research into the optimal HFC-mixture for compressor efficiency and for performing cycle calculations in the vapour-phase region for systems working with R407C and R410A.     

Multi-bed regenerative adsorption chiller — improving the utilization of waste heat and reducing the chilled water outlet temperature fluctuation

A multi-bed regenerative adsorption chiller design is proposed. The concept aims to extract the most enthalpy from the low-grade waste heat before it is purged into the drain. It is also able to minimise the chilled water temperature fluctuation so that downstream temperature smoothing device may be downsized or even eliminated in applications where tighter temperature control may be required. The design also avoids a master-and-slave configuration so that materials invested are not under-utilised. Because of the nature of low-grade waste heat utilization, the performance of adsorption chillers is measured in terms of the recovery efficiency, η instead of the conventional COP. For the same waste heat source flowrate and inlet temperature, a four-bed chiller generates 70% more cooling capacity than a typical two-bed chiller. A six-bed chiller in turn generates 40% more than that of a four-bed chiller. Since the beds can be triggered into operation sequentially during start-up, the risk of ice formation in the evaporator during start-up is greatly reduced compared with that of a two-bed chiller.     

Freezing of a water droplet due to evaporation—heat transfer dominating the evaporation–freezing phenomena and the effect of boiling on freezing characteristics

One of the authors has proposed a novel transport/storage system for the waste cold from the gasification process of liquefied natural gas (LNG), which consists of an evaporator, a cold trap, and a pipeline. In order to estimate the performance of this system, one should know the pressure in the evaporator, in which evaporation–freezing of a PCM occurs, and in the cold trap, as well as the pressure drop of the pipeline due to the flow of low pressure vapor of the PCM. In this paper, the cooling/freezing phenomena of a water droplet due to evaporation in an evacuated chamber was experimentally examined, and the heat transfer dominating the evaporation-freezing phenomena was investigated in order to estimate the pressure in the evaporator. From the results, it was shown that the water droplet in the evacuated cell is effectively cooled by the evaporation of water itself, and is frozen within a few seconds through a remarkable supercooling state, and that the cooling rate of the water droplets were dominated by heat transfer within the droplet under the abrupt evacuation condition. The later result means that, in order to obtain an ice particle by evaporation–freezing, the surroundings of the water droplet should be evacuated at the pressure as low as the saturate pressure of water at the maximum supercooling temperature of the droplet.     

Condensation inside and outside smooth and enhanced tubes — a review of recent research

Condensation heat transfer, both inside and outside horizontal tubes, plays a key role in refrigeration, air conditioning and heat pump applications. In the recent years the science of condensation heat transfer has been severely challenged by the adoption of substitute working fluids and new enhanced surfaces for heat exchangers. Well-known and widely established semiempirical correlations to predict heat transfer during condensation may show to be quite inaccurate in some new applications, and consequently a renewed effort is now being dedicated to the characterisation of flow conditions and associated predictive procedures for heat transfer and pressure drop of condensing vapours, even in the form of zeotropic mixtures. This paper critically reviews the most recent results appeared in the open literature and pertinent to thermal design of condensers for the air conditioning and refrigeration industry; both in-tube and bundle condensation are considered, related to the use of plain and enhanced surfaces.     

Equipment design and installation features to disperse refrigerant releases in rooms—part II: determination of procedures

There is always a risk of leakage of refrigerant into a room that refrigeration and air conditioning equipment occupies. Mitigation of build-up of flammable concentrations from leakage through appropriate equipment construction and installation criteria minimises the potential for ignition. This paper is the first part of an investigation into design and installation measures to disperse leaked flammable refrigerant. It mainly describes the experiments and provides an analysis of the data. The paper describes a purpose built test facility, which was used to carry out experiments to study the dispersion of carbon dioxide to simulate leaked refrigerant. By measuring carbon dioxide concentrations and making flow visualisation, the effects of parameters such as equipment airflow and installation height were observed. The observed trends provide guidance for designing refrigeration and air conditioning equipment, which helps to ensure rapid dispersion of flammable concentrations in the event of a leak of flammable refrigerant. A second paper (Part II) discussed the development of numerical correlations, which are used in the resulting design procedure.     

Evaluation of the mean ice ratio as a function of temperature in a heterogeneous food: Application to the determination of the target temperature at the end of freezing

The freezing process is widely used in the food industry. In the 70s, French regulation authorities have created in collaboration with the food industry the concept of «surgélation» process with the objective of improving the image of high quality frozen foods. The process of “surgélation” which could be translated as “super freezing” corresponds to a freezing process for which a final temperature of −18 °C must be reached “as fast as possible”. This concept was proposed in opposition to a conventionally “freezing” process for which no specific freezing rate is expected and the final storage temperature can be of −12 °C only. The objective of this work is to propose a methodology to evaluate the mean amount of frozen ice in a complex food as a function of temperature and to deduce a target temperature that must be considered as the temperature for which the food may be considered as “frozen”. Based on the definition proposed by the IIF-IIR red book, this target temperature has been defined as the temperature for which 80% of the freezable water is frozen. A case study is proposed with a model food made of two constituents.     

Numerical modelling of the temperature increase in frozen food packaged in pallets in the distribution chainModélisation numérique de l'augmentation de température des produits alimentaires congelés palettisés dans la chaîne de distribution

Frozen food can undergo temperature rise essentially during handling processes which are the weakest links of the cold chain: delivery, loading or unloading operations and temporary storage where pallets are generally handled in an ambience above 0°C. In this study, the temperature rise in a pallet is investigated numerically and experimentally. A three-dimensional finite-volume heat transfer model is developed using Phoenics Computational Fluid Dynamics (CFD) software. Food temperature within the pallet is predicted as a function of time of exposure, ambient conditions, product initial temperature, palletization and thermal characteristics of products and packaging. The experiments are carried out with packaged frozen fish pallets placed on a closed or open dock. The temperatures are recorded, at different levels in pallets over 25- to 85-min periods. The model shows good agreement with experimental results.     

A DDC-based capacity controller of a direct expansion (DX) air conditioning (A/C) unit for simultaneous indoor air temperature and humidity control – Part II: Further development of the controller to improve control sensitivity

The development of the novel direct digital control (DDC)-based capacity controller for a direct expansion (DX) air conditioning (A/C) unit having variable-speed compressor and supply fan to simultaneously control indoor air temperature and relative humidity (RH) in a conditioned space served by the DX A/C unit has been reported in Part I of the two-part series. The results of preliminary controllability tests for the novel capacity controller presented in Part I, however, suggested that the controller developed was operational, with acceptable control accuracy but rooms for improvement with respect to control sensitivity. This paper, the second part of the two-part series, reports on the further development of the controller to improve its control sensitivity and the associated controllability test results. Both control accuracy and reasonable control sensitivity were achieved by incorporating a traditional Proportional–integral (PI) controller into the DDC-based capacity controller.     

Computation of the airflow in a pilot scale clean room using K- turbulence models

This work deals with the assessment of the airflow in a food-processing clean room. The flow pattern inside the working area of a pilot scale clean room was numerically investigated using a computational fluid dynamics code based on a finite volume formulation. Two versions of the k- turbulence model were tested: the standard and the RNG version. The analysis of the velocity magnitude does not reveal sensitive differences between them. Moreover, both models well predict the main features of the flow and numerical results agree with experimental measurements. However, a further examination shows that the RNG k- turbulence model predicts more swirls and more complex trajectories. As the standard k- model overestimates the turbulent diffusion, the RNG version seems to be more suitable to calculate the airflow in clean rooms. The influence of initial turbulence intensity is also pointed out. Finally, the study of the airflow below a laminar flow unit confirms that the design of clean rooms can benefit from the numerical approach.     

Contribution to the gas chromatographic analysis for both refrigerants composition and cell gas in insulating foams – Part II: Aging of insulating foams

In part I, a simple analysis protocol for refrigerant substances has been proposed for use in refrigerating plants or as blowing agents for insulating foams. The present work proposes original results obtained with that method to study the aging of insulating foams. The characteristics of insulating foam panels have been studied including the initial composition and heterogeneity of cell gases, as well as the aging of both unprotected and protected insulating panels. A special focus has been laid on aging at the junctions in refrigerated trucks panels. Indeed, insulation boxes for refrigerated trucks are usually made of various foam panels covered by a single gas-tight facing. A quicker aging was observed under the facing in the foam panels' junction areas, and we could assert that the global aging of the refrigerated trucks box mainly results from the aging of the assemblies (sides and junctions).     

Simulation and measurement on the full-load performance of a refrigeration system in a shipping containerSimulation et mesures de la performance d'un système frigorifique d'un conteneur maritime entièrement chargé

A mathematical model of a refrigeration system in a shipping container has been developed to allow for full-load simulation of its thermal performance. Sub-models are created on the key components: compressor, evaporator, condenser, and thermostatic expansion valve. The sub-models are then coupled by appropriate mass and energy transfer relations to form the full model. Comparison with a series of cooling capacity tests conducted on a 2.2 m (40 ft) fullscale container housed in a temperature-controlled environmental test chamber indicates good agreement, with simulation results being within ±10% uncertainty of measurements.