Modern freezing facilities

This paper gives a monograph of freezing and quick-freezing equipment. Following a brief summary of the theory of freezing time, the author reviews the principal industrial processes and equipment in use today, classifying them according to their technological characteristics.     

Measurement and prediction of freezing times of vacuum canned Pacific shrimp

Freshly cooked Pacific shrimp, vacuum packed in cans fitted with monitoring instruments, were frozen under industrial conditions. Surface heat transfer coefficients were determined experimentally using aluminium cyclinders. Freezing times were measured and also calculated using Plank's equation and the formulae of Cleland and Earle. The investigation indicated that the vacuum level had little effect on the freezing time and that freezing time essentially doubled when cans were enclosed in a commercial carton. For the prediction of freezing times, Cleland and Earle's method was found to be more accurate than Plank's for the tests performed, with predicted values within ± 10% of measured values.     

Freezing of strawberry pulp in large containers: experimental determination and prediction of freezing times

Strawberry pulp packed in bulk in containers of various shapes and of large sizes (10–2001) was frozen in a tunnel at −35°C. Time-temperature curves and freezing times for heat transfer in one, two and three dimensions were obtained. Experimental freezing times were compared with predicted values given by approximate methods based on two different equations for the calculation of freezing times in slabs, which in turn used three different methods for the calculation of shape factors. The predicted results obtained in this work confirmed that both the simplified prediction methods of freezing times and the shape factors used were sufficiently accurate for the design of freezing equipments for these types of product and working conditions.     

Theoretical and experimental freezing times of strawberries

Strawberries were frozen at different air velocities in an air blast freezer at −30°C. The freezing time was taken as the time required to lower the temperature at the geometric centre of the samples to −10°C. The freezing times measured in the experiments were compared with the values calculated using Plank's equation. The freezing times calculated by Plank's equation were fround to be higher than those found experimentally at any given air velocity. Freezing time decreased with increasing air velocity. This is attributed to the increase in heat transfer coefficient at increased air velocities.     

Freezing of foodstuffs with variations in environmental conditions

Methods previously proposed to calculate the effect of changes in environmental conditions were reexamined. A theoretical basis was provided for the method of adding freezing time fractions. Testing against numerical calculations was carried out over a wide range of parameters. The results showed that the method based on following the movement of the freezing front performed reasonably well, especially when Pham's equation was used to calculate freezing times. However, inaccuracies may arise when the heat transfer coefficient decreases sharply, especially for very low cooling medium temperatures or for shapes other than infinite slabs. The method of adding freezing time fractions performed well for changes in cooling medium temperature but not for changes in heat transfer coefficient. A graphical method is also provided for the general case.     

Prediction of freezing and thawing times for foods — a review

This communication presents a review of methods to predict the freezing and thawing times of foods. A bibliography of over 400 references was used.     

Prediction of freezing and thawing times for foods of two-dimensional irregular shape by using a semi-analytical geometric factor

Semi-analytical solutions for conduction and convection heat transfer problems with phase change were used to derive formulae for a geometric factor that describes the effect of product shape on freezing and thawing times for two-dimensional irregular products. These formulae depend only on the Biot number and three geometric parameters: the characteristic dimension, the volume and the surface area of the product. The accuracy of these formulae is demonstrated by comparisons with numerically calculated data and experimental freezing and thawing data for irregular objects. The new formulae out-perform all geometric factors previously proposed.     

Prediction of freezing and thawing times for multi-dimensional shapes by numerical methods

Testing the accuracy of freezing and thawing time prediction methods requires accurate experimental data. To complement existing data, 175 experimental measurements, 68 for thawing of rectangular bricks and 107 for both freezing and thawing of 12 different multi-dimensional irregular shapes, were made using Tylose, a food analogue, over a wide range of conditions. Twelve additional experiments were conducted using an actual food material, minched lean beef. Details of all the experimental conditions are reported.     

Prediction of thawing times for foods of simple shape

A set of 104 experimental measurements of thawing time were made over a wide range of conditions for slab, infinite cylinder and sphere shapes of a food analogue material. These results were used to assess existing thawing time prediction methods. Versions of both the finite difference and the finite element numerical methods that accounted for continuously temperature-variable thermal properties gave accurate predictions. No previously published simple prediction formula was found that was both sufficiently accurate and expressed in a form suitable for it to be adopted as a general thawing time prediction method. Four accurate, but simple, empirical formulae based on Plank's equation were developed. These formulae predicted thawing times that were both highly correlated with those predicted by the numerical methods and agreed with the experimental data to within ±10% at the 95% level of confidence. The agreement was more limited by uncertainties in the experimental and thermal property data than by inaccuracy in the prediction formulae. Significantly more accurate simple formulae are unlikely to be developed unless more accurate experimental data are available.     

Freezing time predictions for regular shaped foods: a simplified graphical method

A graphical method is proposed for the estimation of freezing times of foods been developed from the predictions of a numerical model which solves the heat balance for a food undergoing freezing. The method is valid for foods with different shapes (flat plate, cylinder and sphere) and covers a wide range of working conditions for industrial freezers. It also enables the prediction to be made for any given end temperature in the thermal centre of the food. Freezing times predicted by this method have been compared with published experimental data, giving an average error in the predicted values of only 4%.     

Prediction of freezing and thawing times for multi-dimensional shapes by simple formulae Part 1: regular shapes

Numerical prediction methods were used to generate data to assess and develop geometric factors taking account of the effect of product geometry on freezing and thawing time. Improved empirical formulae for two existing geometric factors were developed; these depend only on the Biot number and parameters that describe object shape. The new formulae are accurate for both freezing and thawing of an extended range of regular multi-dimensional shapes and for a wider range of conditions than the original formulae. Used in conjunction with accurate slab freezing and thawing time prediction formulae, the improved geometric factors accurately predicted a large set of experimental freezing and thawing times for various shapes. As the improved geometric factors are both accurate and generally applicable there is no need for shape-specific freezing and thawing time prediction formulae to be developed.     

Thermal design calculations for food freezing equipment--past, present and future

The literature on methods for thermal design of food freezing equipment is reviewed with emphasis on two questions: what do those who design, build and commission freezers most need from researchers in terms of improved design calculation methods, and what are the most limiting factors in determining whether a particular freezer design will satisfactorily freeze the product at the required throughput rate? Freezing time prediction methods have been significantly improved over the last two decades and are now infrequently the factor most limiting accurate design. There is a much greater need for more accurate thermophysical properties and better information on heat transfer coefficients for a variety of practical situations. The failure of many industrial freezers to deliver the design conditions to product in all parts of the freezer is also important. Future research should address the full range of factors limiting accurate freezer design, which may mean less emphasis on freezing time prediction.

Résumé

La littérature concernant la conception thermique des congélateurs industriels est passée en revue et l'accent est mis sur deux aspects: qu'est ce que les concepteurs, les fabricants et les installateurs attendent le plus des chercheurs en termes d'amélioration des méthodes de calculs de conception, et quels sont les facteurs dans la conception d'un congélateur qui exercent la plus grande influence sur la performance, c'est à dire qui déterminent si une conception donnée va permettre de congeler un produit à la vitesse voulue? Pendant les deux dernières décennies, on a amélioré de façon significative les méthodes qui prédisent le temps de congélation, et de nos jours, ces méthodes sont rarement le facteur qui limite le plus la conception précise. On a nettement plus besoin de précision sur les propriétés thermophysiques et les coefficients de transfert de chaleur pour différentes situations. Il est important de savoir que beaucoup de congélateurs industriels n'arrivent pas à reproduire les performances prévues lors de leur conception en tout point de l'appareil. Dans l'avenir, la recherche devrait être axée sur tous les facteurs limitant la conception précise des congélateurs, quitte à mettre moins l'accent sur la prédiction du temps de congélation.     

Freezing processes in high-pressure domains

Experimental freezing of water in high-pressure domain is studied considering temperature reduction (TRF) as well as high-pressure-assisted freezing (HPAF). The most important advantage of HPAF is that the whole volume of the sample is subcooled when an expansion is made, so a rapid and uniform nucleation and growth of ice crystals are produced. In this work through mathematical modelling the amount of ice appearing instantaneously in the latter freezing, is predicted.     

Freezing of foods under surface boiling boundary conditions

The surface boiling boundary condition is encountered in the freezing of foods when foods are immersed in boiling freezants such as R12. This phenomenon was incorporated in a mathematical model of the freezing process as a surface temperature dependent convective boundary condition. A finite difference numerical scheme was formulated to solve the model for one- and two-dimensional geometries. The pool boiling characteristic for R12 was obtained by inverse heat transfer analysis of the experimental quenching curves of a transient calorimeter. The model was used to simulate the experimental freezing processes with reasonable agreement.     

Numerical simulation of individual quick freezing of spherical foods

A one-dimensional three time level implicit finite difference computer program was developed to predict temperature profiles during the individual quick freezing of spherically shaped foods. Temperature varying thermal properties necessary for these predictions were calculated based upon properties of the unfrozen product. The centre temperatures of individual peas within a bed of peas being frozen under various conditions of fluidization were recorded. Comparison of experimental results and published data with predicted freezing curves showed good agreement.     

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.     

On reducing the size of liquid separators for injector circulation plate freezers

Liquid separators for injector plate freezers are large because the liquid level rises towards the end of the freezing process. To calculate the volume of liquid being collected in the separator during freezing the freezing time and the heat removed must be evaluated. A simple method of freezing time estimation based on the progression of a phase change front is proposed. The size of separators can be reduced considerably by letting part of the liquid feed by-pass the injector during initial freezing. With this arrangement the injector dimensions are based upon a refrigerating capacity lower than the maximum.     

Modelling the freezing of butter

Butter is a water-in-oil emulsion so its behaviour during freezing is very different from that of most food products, for which water forms a continuous phase. The release of latent heat during freezing is controlled as much by the rate of crystallization of water in each of the water droplets as by the rate of heat transfer. Measurements of the freezing of butter show that the release of latent heat from the freezing water depends on the degree of supercooling, which, in turn, depends on the cooling medium temperature, the size of the butter item, the packaging and the type of butter. Four modelling approaches were tested against the experimental data collected for a 25 kg block of butter. A “sensible heat only model” accurately predicted the butter temperature until temperatures at which water freezing becomes significant were reached. An “equilibrium thermal properties model” predicted a temperature plateau near the initial freezing point of the butter in a manner that was inconsistent with the measured data. A third model used a stochastic approach to ice nucleation based on supercooling using classical homogeneous nucleation theory. The predicted temperatures showed that supercooling-driven nucleation alone is not sufficient to predict the freezing behaviour of butter. A fourth approach took account of time-dependent nucleation and ice crystal growth kinetics using classical Avrami crystallization theory. The relationship between the ice crystal growth rate and the supersaturation was assumed to be linear. The model predicted the experimental data accurately, particularly by predicting the slow rebound in the temperature following supercooling that is found when freezing butter under some conditions.     

Prediction of freezing and thawing times for multi-dimensional shapes by simple formulae part 2: irregular shapes

Calculated and experimental data for multi-dimensional irregular shapes wer used to assess various methodologies to include the effect of shape in empirical freezing and thawing time prediction methods. The principles underlying two existing geometric factors, EHTD and MCP, were found to be valid; so there seems to be no need for other approaches. Used in conjunction with accurate slab freezing and thawing time prediction methods, the proposed empirical formulae for EHTD and MCP gave accurate predictions for all of the two-dimensional shapes and most of the three-dimensional shapes tested, except those with oval cross-sections in the third dimension. This was attributed to the lack of data for this group of shapes.     

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.