STUDY OF HEAT TRANSFER AND HYDRAULIC RESISTANCE IN A TUBE-IN-TUBE HEAT EXCHANGER

I. P. Chashchin, B. I. Litvinov

(Presented by the scientific seminar of the departments of PMAKhP and OKhT)

In various areas of industry, heat-exchange processes occupy an important place. In the chemical industry, the share of heat-exchange equipment in the total volume of chemical equipment amounts to 15–18%. In chemical technology at present, tubular heat exchangers with smooth tubes are used predominantly (more than 80% of the total output of heat-exchange equipment), and up to 70% of heat exchangers operate in steam–gas and liquid–liquid media at pressures up to 10 kg/cm² and temperatures not exceeding 200°C.

This is a prerequisite for a more detailed study of heat transfer and hydraulic resistances in the above-mentioned designs of heat exchangers.

At present, works devoted to the study of heat transfer and hydraulic resistances in the initial and stabilized sections of heat exchange under laminar flow of a heated or cooled medium are systematically appearing in periodical literature [2, 4]. Interest in this kind of research is due to the fact that in heat-exchange devices operating at different temperatures, the initial section may constitute a significant and, in many cases, the most effective heat-exchange surface.

In convective heat exchange between streams through a separating wall, the measure of the intensity of heat transfer is the heat-transfer coefficient

\[ K = \frac{1}{\frac{1}{\alpha_1} + \frac{1}{\alpha_2} + \frac{\delta}{\lambda}} \tag{1} \]

For the case of heat transfer from saturated water vapor through a metal wall to a gas, taking into account the high values of the heat-transfer coefficients of the condensing steam to the wall \(\alpha_1\) and of the thermal conductance of the wall \(\frac{\lambda}{\delta}\), the heat-transfer coefficient will be determined by the values of the coefficient of heat transfer from the wall to the gas \(\alpha_2\).

To carry out studies of heat transfer and hydraulic resistances in the initial and stabilized sections of the apparatus, we fabricated a tube-in-tube heat exchanger.

The main parameters of the installation are given in Table 1.

The equivalent diameter of the annular space is \(d_{\mathrm{e}} = 9 \cdot 10^{-3}\,m\). Atmospheric air was drawn by a compressor into the annular space of the heat exchanger. Saturated water vapor was obtained in a boiler heated by electric heaters. The steam, at a temperature of \(100^\circ C\), was fed into the inner tube of the heat exchanger. The condensate was collected in a measuring vessel. The steam flow rate was measured by the volumetric method.

Table 1

Main parameters of the heat exchanger

The temperature of the stream and of the wall of the inner tube was measured at 3 points along the height of the channel with an accuracy of up to \(0.5^\circ C\) using chromel-copel thermocouples connected through a switch to a mirror millivoltmeter. The pressure drop during flow in the annular channel was measured with a U-shaped manometer with an accuracy of up to \(1\,kg/m^2\). The air flow rate was measured by a float-type flowmeter (rotameter) with an accuracy of up to \(0.00028\,m^3/sec\).

The experiments were carried out for a laminar character of flow motion within the range of variation of the criterion \(Re = (1.5 \div 3.7)\cdot 10^3\). The results of experiments on the study of heat transfer and hydraulic resistance in the inlet and “clean” sections of the tube are shown in Fig. 1. Lines 1 in Fig. 1 are constructed from the generally accepted dependences: \(Nu = f(Re)\) and \(Eu = f(Re)\) [3] within the above-indicated limits of variation of the criterion \(Re\).

Fig. 1. Heat transfer and hydraulic resistances in the inlet and “clean” sections of the tube

From the graphs (Fig. 1) it is seen that the experimental data on heat transfer and hydraulic resistance in the clean section of the tube agree well with the theoretical ones.

In the inlet section of the tube, the experimental data on heat transfer and hydraulic resistance lie somewhat above the theoretical values (Fig. 1, line 2).

This deviation is explained by the conditions of flow entry into the apparatus, where, owing to additional turbulization of the flow, somewhat elevated data on heat transfer and hydraulic resistance were obtained.

Taking into account that in industrial heat-exchange devices the initial section may constitute a significant part of the heat-transfer surface, the experimental data obtained are of definite theoretical and practical interest.

REFERENCES

1. A. G. Kasatkin. Basic Processes and Apparatuses of Chemical Technology. Goskhimizdat, Moscow, 1960.

2. M. A. Mikheev. Fundamentals of Heat Transfer. Gosenergoizdat, Leningrad, 1956.

3. P. G. Romankov, K. F. Pavlov, A. A. Noskov. Examples and Problems in the Course on Processes and Apparatuses of Chemical Technology. “Khimiya,” Moscow, 1960.

4. B. S. Petukhov, L. I. Roizen. Heat exchange in tubes and annular sections. IFZh, vol. 6, no. 3, p. 40.