Research of Influence of Steam Contact Heating on Thermal Stability of a Wall of

Abstract: One of ways of intensification of process of manufacture of heterogeneous tinned products of fruits and vegetables is the use of sated water steam (Patent of USSR No. 500792). This process at application as consumer glass container for such products is especially effective. However, there are some questions which are connected with thermal influence on glass container at realization of such heating. The given research is devoted as the decision of practical questions arising at process engineers.

Key words: Fruits, vegetables, glass canning jar, heat, water steam.

1. Introduction

In the production technology, tinned foodstuff heating by the sated water steam is very often used. This reception is especially effective for products which consist of firm and liquid phases. Such decision for products which are in glass canning container is especially necessary. The values of temperature pressure arising in glass jar walls can exceed admissible size and lead to its destruction. As shown in the Refs. [1, 2], maximum permissible size of pressure for a glass jar makes σ = 12.2 MPа.

At realization of steam contact heating two cases of processing are possible:

(1) Heating of an empty glass can;

(2) Heating of the glass jar filled with fruits.

Let’s consider each case separately, and it can define the extent of the thermal pressure arising in a wall of a glass can. Compared these values to the maximum admissible pressure, the authors will define the possibility of use of steam contact heating.

2. Methods and Data

For the experiments, specially prepared glass jar was used. The internal and external surfaces have been strengthened by six chromyl-koppel thermocouples. The arrangement of thermocouples on glass jar surfaces is presented in Fig. 1.

The choice of points to install the thermocouples on surfaces of jar is caused by that all of them are in various initial and thermal conditions. So, points 1 and 2, 5 and 6 are located in the places subject to a tension already at manufacturing of jar, and pressure in these sections are various because of a difference of radiuses of curvature and a various thickness of a wall of jar. The section in which points 3 and 4 are located is free from initial pressure. At intake of steam in all points of an external and internal surface of a glass can, the various temperatures are established. In view of that studied process is quickly variable, its registration oscillograph use is expedient. In Fig. 2 the experimental scheme of the research stand is presented.

The thermocouples fixed on glass jar (1) through switchboard were connected on an input of the oscillograph (2) electric food was carried out by means of the source of a direct current (3) for calibration test systems of measurement and check its work, the potentiometer has been included in the scheme (4) High speed of record of indications of an oscillograph has allowed receiving indications of thermocouples with high step-type behavior of time (0.1 sec.). Contact heating of a glass can by water steam was carried out in experimental installation (5).

3. Results and Discussion

The received experimental data are presented on Figs. 3-5. Analyzing dependence of temperature in a wall of empty glass canning container on time of contact it with heating water vapor, it is possible to note temperature of water vapor of 98-100 °C of an external surface of glass canning container (a curve of 1), reaching 65 °C within 15 seconds of heating and further (till 60 seconds) this temperature actually doesn’t change. It is connected by that though the reflector and directs steam on an external surface of glass canning container, its thermal potential is already considerably lower. In that case when the reflector completely blocks an internal surface of glass canning container, change of temperature of its external surface is characterized by curve 4. As appearing from the received experimental curves, slow change of temperature of an external surface in this case is observed. In the first 12-15 seconds of processing, speed of change of temperature from 0.333 degree per second in section 1-2 to 0.75 degree per second in sections 3-4 and 5-6. It is obviously connected by that warming up of a wall can occurs at the expense of heat conductivity of glass. Curve 2 shows change of temperature of an internal surface can in the chosen

sections. The analysis of curves shows that the most dangerous section from the point of view is the temperature tension, which is the section of a mouth of a glass can and its ground part. In these sections there is a maximum gradient of temperature between internal and external surfaces banks and, besides, in these sections there are concentrators of pressure. The average on height section can (3-4) is more advantage as there are no concentrators of temperature tension and the temperature gradient here goes down. Temperature tension can be determined by expression:

of the heating environment, reference temperature of glass can and the final average volume temperature of jar.

The FIRST CASE: Heating an empty glass can sate water steam. However, calculation on the given expression allows to receive average temperature of a glass can and not give information on difference of temperatures on its external and internal surfaces. These distinctions—the major factor is causing temperature tension in the case of glass canning container. For defining temperature difference on internal and external surfaces of a glass can when heating by steam, a series of experiments was put. The executed calculations by both options allowed to define compliances between values of arising temperature tension in the chosen sections of glass canning container and define their maximum permissible values. Results are presented in the form of dependence of a gradient of temperature between surfaces from duration of heating by steam (Fig. 5). Such processing allows to establish the absolute size of gradients of temperature in the chosen sections of a glass jar. As seen from the constructed dependences, the maximum differences of

A divergence which were noted, obviously connected with experimental values of sizes εθ and εz were entered into Eq. (1), received for glass pipes. Results of calculation in Eq. (2) and their comparison to the maximum admissible size for glass for glass canning container is equal to 12.2 MPA.

The SECOND CASE: Contact heating by the sated water steam of the glass filled with fruits.

In this case, fruits in glass canning container can protect its internal surface from direct influence of water vapor. The external surface of a glass can thus remains under the same thermal conditions, as the steam heating of empty canning container. In this regard form of curves of heating of an internal surface of glass konsevny container (curves 3 in Figs. 3-5) changes and, therefore, distinction of temperatures between internal and external surfaces of glass canning container. These changes lead to other values of sizes of the temperature pressure arising in various sections of can. Changing temperature in time in the chosen sections in glass canning container when filling with its fruit, the same but absolute sizes of a gradient of temperatures between surfaces in this case approximately is 2.5 times lower. Arising temperature tension in the chosen sections of glass canning container shows as the following:

? the section of a ground part glass can—9.00 МPа;

? the average on height glass can section—8.64 MPa;

? the section of a nimbus of a mouth glass can—10.10 МPа.

Comparison these sizes of temperature pressures with maximum permissible value, it shows that in case of realization of contact heating of fruits in glass jar, the sufficient safety factor providing absence of thermal fight cans exists the sated water steam. Averagely, maximum permissible pressure on a glass jar surface is 1.2-1.4 times more than actual. As shown by the skilled data, the jar in various sections tests sign-variable temperature pressure. The first 2-3 seconds of processing by water vapor the glass canning

4. Conclusions

As a result of the researches, it is shown that the realization of contact heating by the sated water steam of system “a glass container—fruits” in various sections of glass canning jar, it should arise tangential and axial pressure. The sizes of these pressures remain less than maximum permissible values and won’t lead to can destruction.

References

[1] B.L. Flaumenbaum, V.V. Tchernikova, Criteria of durability of glass container at cooling, Journal of Canning Industry 11 (1976) 40-43.

[2] G. Parcus, Unsteady Temperature Pressure, M. Fizmatgiz, 1963, p. 80.

[3] M. Maydannik, N. Kiselev, B. Rubchik, T. Kosinov, Research of a tension of glass pipes at temperature differences, Journal of Glass and Ceramics 7 (1974) 33-35.