EXPERIMENTAL FORMATION OF BED UNDER JET TUBE FLUIDIZATION CONDITIONS USED FOR FOOD COOLING AND FREEZING

Franciszek Kluza¹, Łukasz Stadnik^2
1 Department of Refrigeration and Food Industry Energetics, University of Life Sciences in Lublin, Poland
² Department of Refrigeration and Food Industry Energetics University of Life Sciences in Lublin, Poland

ABSTRACT

The aim of the studies was to assess the influence of nozzle arrangement in plenum on the air flow during impingement jet fluidization. Experiments were carried out using beds of soybean, carrot, sweet corn and potato parallelepipeds. For the present research, circular jets with a 11, 14 16, 20, 22, 35 mm diameter were used. Two different geometrical arrangements of nozzles are studied, a square set-up and a patchwork set-up. The nozzle dimensions are rendered dimensionless by their L/D ratio where L is the distance between standing next to each other nozzles, D is the jet inner dimension. The distance X of the jets from the container wall is characterized by relation the D. During the stabilized bed fluidization the maximal possible bed height was investigated. The most suitable bed boiling conditions were achieved for relations L/D = 4 and X/D = 1. The square set-up of nozzles in plenum in opposite to patchwork set-up effected in more uniform boiling of bed in all of its capacity. The example of the process visualization is included.

Key words: impingement jet fluidization, jet-to-jet arrangement.

INTRODUCTION

One of innovative methods that can be commonly used in cooling treatment of foods is impingement fluidization. It is a method applying the phenomenon of impingement to bring about boiling of the fluidal bed. The phenomenon of impingement consists in proper directing the air, gas or steam flow of proper temperature flowing out vertically from the nozzles with high speed (Fig. 1).

Both short and long nozzles, which can also be arranged in a double setup, can be used in the process of impingement fluidization (Fig. 2).

The strike of the gas on the surface of the product considerably improves the heat and mass transfer in the bed in relation to the classic fluidization, which – through shortened time of the process – affects the product's quality. Impingement fluidization is a combination of impingement phenomenon and classic fluidization (Fig. 3).

In this method, the air stream, after having passed through the bed of the product on a horizontal non-through belt or a working surface, undergoes breaking up and it returns up, passing twice through the bed. This ensures considerable intensification of the heat and mass transfer in the bed in relation to classic fluidization, which makes it possible to shorten the processing time significantly and has a less negative effect on the quality of the final product. In impingement fluidization, the product generally shifts along the marked curved and closing tracks (Fig. 4).

At the moment when the reflected air achieves the speed of fluidization minimum, the uplift force, the air starts to exceed the gravity force of the material, resulting in the material moving upwards. Next, single particles of the bed move outside in relation to earlier tracks as a result of weakened effect of the reflected air and interrelation between the neighbouring particles, and next – under the strike of a new air stream flowing out of the nozzles – they fall down in a turbulent manner in the direction of the bottom of the working container, from where they are again lifted upwards by the stream of the reflected air, and the cycle begins anew [6,7,8,13].

The phenomenon of impingement was for the first time applied in food industry in 1973 by Smith (in ovens). Nowadays it is used in realizing the processes connected with heating, and lately attempts have been made in the field of cooling, freezing and thawing [8,9,12,16,17,18,20]. Among a number of studies on the phenomenon of impingement (e.g. Gordon and Akfirat [4], Martin [10], Kluza and Spiess [6], Huber and Viskanta [5], Kluza and Spiess [7], Kluza [8], San and Lai [15], Salvadori and Mascheroni [14], Sarkar and Singh [16], Sarkar et al. [17, 19], Anderson and Singh [1], Kluza et al. [9], De Bonis et al. [2]), the character of the flow of the air stream coming out of the nozzles was dealt with by Olsson et al. [11]. They studied the effect of the distance between the wall limiting the field of the stream flow and the nozzles on the value of the stream velocity and heat exchange. At present, studies in this field focus on the application of the arrangement of nozzles aiming at increasing the speed of heat and mass transfer [3,9].

Striving at working out the optimum process and measurements conditions of the relations between the working elements of devices is aimed at obtaining the proper boiling of the product in specific processing. However, the fact should be remembered that for geometrically differentiated materials forming the beds, the parameters and dependencies of the dimensions of the working space are universal only within a certain range. Attempts to work out the optimum conditions to obtain the fluidal state of bed boiling equally consider both the effectiveness of the treatment, energy-saving process and the product quality.

The purpose of the present studies was experimental optimization of the bed fluidization in the process of impingement fluidization performed through the choice of proper measurements of the working elements of the device in connection with the basic parameters of the process.

MATERIAL AND METHODS

The studies dealt with the working plenum with nozzles and the system: plenum – container with the treated product. Work on the choice of the basic technical parameters of the plenum and the measurement dependencies in the working space for the cooling treatment of food products using the method of jet tube fluidization was performed using the beds of soybean, maize, carrot and chips. Because of the shape close to the sphere, soybean grains served to observe and form the bed in the boiling state in the initial stage of studies. The materials were characterized by high quality. The studies were conducted on a stand whose basic part was the device enabling realization of impingement fluidization processes in the range of the temperatures of working air (or gas) -30° ÷ +50°C (Fig. 5).

The basic sub-units of the stand include cooling installation with the capacity of 3.9 kW, radiant ventilator (Nyborg-Mawent type WP-20/1,5) with the capacity of 0.6 m³/s (with the pressure of 2600 Pa), high-speed engine with the power of 3 kW with an inverter (type LG SV0751IG5A) serving to power the ventilator. Modifying the frequency of the feeding current led to the desired rotations of the ventilator and then the velocity and pressure of the air flowing out of the plenum nozzles. For the needs of the present study, 11 plenums were built consisting of a sieve bottom with the dimensions of 410×360×18 mm and the nozzles with internal diameters (D), respectively of 11, 14, 16, 20, 35 mm (Fig. 6a, b).

The length (S) of a single nozzle was a multiple of its internal diameter and it was, in case of particular plenums, respectively, within the range S = 6.5D÷20. The distance (L) between the nozzle axes was examined within the range from 19.8 to 175 mm. The distance (Z) between the tip of the nozzles and the container bottom was settled within the range 50÷107 mm. The distance (X) between the symmetry axis of external nozzles and the container wall was within the range from 5.5. to 105 mm. The height (H) of the container walls was examined in the range 100÷250 mm. For all examined plenums the relation of distance L between the symmetry axes of the nozzles and the internal diameter D of the nozzles (L/D) was compared. The studied relation was within the range from 1.8 to 5. The plenums differed from each other with the number of nozzles and the manners of their distribution. A square and square patchwork of nozzles was used 11 containers for the product were prepared which were properly fitted to each plenum. The containers were built of a plate of paramethoxymethamphetamine (PMMA – acrylic glass) with the thickness of 5 mm. Marking the characteristic measurements of the container took place through visual determination of the bed at the walls while approaching or moving away the container wall in relation to the nozzle axis. The examined distance X was a multiple of the internal diameter of the nozzle at the level of 0.5D to 3D. Accuracy of the measurements was ± 0.05 mm. After marking the optimum characteristics of the plenums and the working container, an analysis was made of the work of the bed of particular plant materials. Products prepared in the proper manner were poured into the container so that they would form – after even arrangement – a layer of the same height, and next, using an inverter, the current frequency was modified, which was converted into the ventilator rotations, and hence the value of the pressure and the velocity with which the air flowed out of the nozzles. Next, the material was added successively and the current frequency was increased using the inverter so that the boiling state of the bed could be achieved. The work of the bed was also formed decreasing or increasing the distance (Z) between the container bottom and the end of the nozzles. After the bed of the biggest possible height was obtained, the work of the bed was observed and – in the situation of stable boiling – 10 measurements of its height were made. The measurement was performed in different places of the whole bed by means of a depth gauge (accuracy ± 0.05 mm). Observations also consisted in visual determination of the track along which the bed elements moved, using particular plenums. The course of fluidal boiling of the bed was documented by means a camera JVC GZ-MG21.

RESULTS AND DISCUSSION

The minimum height H of the container walls where the bed boiled each time determined on the basis of observations and measurements was 230 mm. Using containers of lower height of walls is not recommended due to the impossibility of achieving the proper work of the bed.

Examining the spacing of the nozzles in the situation when the relation of the distance of the symmetry axes to their internal diameters was L/D ≤ 1.8, it was found out that the bed boiled in an atypical way. The product moved asymmetrically and was thrown outside the container. In the case of changing the spacing (1.8 ≤ L/D < 4) it was found out that the shifting of the material was disturbed, which resulted in the phenomenon of "bed loosening". On the other hand, in the situation when L/D > 4, dead zones of the product were observed between particular nozzles, which were only slightly affected by the air stream flowing out of the nozzles (Fig. 7).

A detailed analysis of the bed work in the studied conditions showed that the state of fluidal boiling occurred when the distance between the symmetry axed of the nozzles was four times bigger than their internal diameter (L/D = 4). Photos 3A, 3B and 3C present asymmetrical dislocation of the product caused by the use of nozzle spacing of L/D < 4. There are no visible, curvilinear, closing tracks marked by the product. Both in the case of soybean and maize grain as well as carrot cubes and potato chips, the material was partly thrown out of the container, which was caused by too small spacing of the nozzles and the impossibility of the right return passing of the reflected air stream. Further photos (3D, 3E, 3F and 8B) show typical tracks of the product marked by the movement of the air flowing out of the nozzles and reflected from the container bottom. Such character of the bed work (typical of jet tube fluidization) was obtained after marking and using the optimum relation between the diameter of the nozzles and the distance between the nozzles. Using small spacing between the nozzles causes interaction between the streams. As a result, the rectilinear flow of the central stream of the air changes and the effectiveness of the heat exchange lowers (Fig. 8A). Achieving the state of fluidal boiling (Fig. 8B) is possible mainly as a consequence of cooperation and not overlapping of two neighbouring air streams. If the flow of the air streams between the nozzles is intensive enough, then the phenomenon of fluidization takes place. As a result of such a flow of the air streams, an increase of the heat penetration occurs [15]. Using different spacing of the nozzles in the plenum is not recommended due to the irregular effect of the air stream on particular particles of the product and the chaotic character of the bed fluidization.

After the analysis of the bed, 11 plenums of optimum spacing of the nozzles L = 4D were made, with square and square patchwork setups of the nozzles (Fig. 9).

In order to minimize losses of the velocity and pressure of the air flowing out of the nozzles, it was assumed that the relation between the nozzle length S to its diameter D has to be bigger than 6 (S/D > 6) [17]. According to this assumption, all plenums were equipped with nozzles 230 mm in length. Table 1 presents the characteristics of working plenums.

Analyses of distance X between the vessel wall and the axis of the external nozzle showed that with the distance bigger than the internal diameter of the nozzle (X > 1D), the product lay at the wall, while with the smaller distance (X < 1D), the product was blown outside the vessel. The studies found out that achieving the state of fluidization at the container walls is possible when the relation X = D is kept (Fig. 10).

Analyzing fluidization of the maize grain it was found out that with an increased internal diameter of the nozzles, the required rotations of the ventilator necessary to obtain the right boiling of the bed took place (Fig. 11).

Studying fluidization of carrot cubes proved that the biggest heights of the bed at rest next guaranteeing the later fluidal state were obtained when the plenum had the diameter of the internal nozzle equal to 22 mm, both in the square and square patchwork setups (Fig. 12).

It was found out that with decreased internal diameter of the plenum nozzle, the height of the bed (at rest) properly working in the conditions of jet tube fluidization decreased. An exception was the plenum with the internal diameter of the nozzles equal to 14 mm, with a square patchwork arrangement.

On the other hand, the proper fluidal boiling of the bed of potato chips was obtained only using the plenums with the nozzle diameter of 22 mm and 35 mm, distributed in a square setup (Fig. 13).

It was found out that using plenums with nozzles of a bigger internal diameter (D = 35 mm) allows fluidization of twice more potato chips as compared to the plenum with nozzles of the internal diameter equal to 22 mm. The possibility of achieving the state of jet tube fluidization of the bed with a larger quantity of the material can have practical and economic significance, especially while freezing.

Table 2 presents results of measuring the bed layers at rest where the state of jet tube fluidization is possible to obtain.

It was also found out that the use of plenums with nozzles distributed in a square arrangement enables fluidization of the biggest amount of the material. However, both in the case of maize and carrot cubes the best results were obtained using the nozzle with the internal diameter of 22 mm.

Studying the formation of the distance between the nozzle tips and the container bottom where the state of fluidal boiling was obtained, it was found out that in the case of nozzles distributed in a square set-up the studied distance varied for different plenums and the applied material (Fig. 14). Using the nozzles of higher internal diameters affected increased distance (Z) between the nozzle tips and the container bottom. A similar relation was shown using plenums with a square set-up of nozzles (Fig. 15).

Using the plenums with higher internal nozzles diameter had a direct effect on the much more turbulent character of the air flow. Fig. 16 presents examples of jet tube fluidization of some investigated food beds.

CONCLUSIONS

1. On the basis of observations of fluidization of the soybean bed using plenums with the nozzle spacing of L = 1.8÷5D, it was found out that the closer the relation of the distance between the nozzles L and the diameter of the nozzles D was to 4, the fuller fluidal boiling was achieved by the bed.

2. In order to prevent the negative phenomenon of bed particles being thrown outside the working vessel and the formation of particle deposits directly at the walls, the optimum distance X between the axis of the external nozzles and the vessel wall should be equal to the internal diameter of the nozzles.

3. Results of studies prove that the more the shape of bed particles differs from the spherical shape and the bigger dimensions the particles have, the bigger internal diameters of the plenum, nozzles should be if the proper phenomenon is to be achieved.

4. Plenums with a series-wound setup of nozzles ensure jet tube fluidization of the beds with the maximum height in given conditions.

5. Nozzles with the internal diameter of 35 mm and 22 mm cause an almost double increase of the turbulent character of the air stream as compared to the nozzles with the internal diameters of 11 mm.

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Accepted for print: 26.06.2009

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