General view with explanations of pipe-in-pipe heat exchanger

Contents

Contents

Summary

Explanatory Information

Introduction

Typical constructions

Calculations

General part

Option

Option

Conclusions

Literature

Summary

The purpose of this course design is to calculate and select the optimal normalized heat exchanger. To achieve this goal, two types of apparatus are described in comparison: pipe-in-pipe type and shell-and-tube heat exchangers (differing in the hydrodynamic mode of heat carriers).

The essence and purpose of the heat transfer process in chemical technology are briefly described. When choosing a heat exchanger, various parameters were adopted as optimal criteria: the size of the devices, their cost, weight, which made it possible to choose the best option among the devices of various designs.

Explanatory Information

Introduction

Heat exchange processes are widespread in chemical technology and many fields of technology. Heating, cooling, evaporation, condensation have a general pattern: their intensity is determined by the laws of heat transfer.

Consideration of any process includes balance sheet (thermal - for thermal processes) and kinetic calculations.

The balance calculation solves the issue of the amount of heat that should go from one medium to another in the apparatus. The kinetic calculation determines the required heat transfer surface, i.e. the size of the apparatus. Kinetic calculation is impossible without a preliminary balance sheet, and a balance sheet without kinetic remains only an unrealized task.

In heat exchangers, heat transfer from one medium to another through the wall separating them is due to a number of factors and is a complex process that is usually divided into three elementary types of heat exchange: thermal conductivity, convection and thermal radiation. In practice, these phenomena are not isolated, are in some combination and occur simultaneously. For heat exchangers, convective heat exchange or heat transfer, which is carried out with the combined and simultaneous effect of thermal conductivity and convection, is of greatest importance.

The dependence of the heat transfer coefficient on the nature and speed of the working media, their physical properties, the size and shape of the heat exchange surface and other factors is very complex and cannot be theoretically established. Therefore, experimental studies and generalization of experimental data using similarity theory are used to determine the heat transfer coefficient. The heat transfer process is determined for different cases by the corresponding criteria: the Nusselt criterion, the Pecle criterion, the Prandtl criterion, the Reynolds criterion, the Grasghoff criterion. Based on similarity theory, any relationship between variables characterizing any phenomena can be represented as a generalized relationship between similarity criteria. The specific type of dependencies is established as a result of processing experimental data; generally, power equations are obtained.

Heat transfer during transverse washing of pipes, and especially pipe bundles, is characteristic in heat exchange equipment. In this case, the washing conditions of the pipes with liquid change around the circumference of the pipe, which leads to a sharp change in the values ​ ​ of local heat transfer coefficients. The generalized criterion equations obtained from the processing of numerous experimental data allow us to calculate the average values ​ ​ of α, which are usually used in calculations .

As heat carriers in industry, water vapor, gaseous products of fuel combustion and chemical reactions are most common. As cooling agents, water, air and aqueous solutions of certain salts (NaCl, CaCl2) are most often used. The heat transfer coefficient depends on the physical properties of the heat transfer agents, so the selection or calculation of the physical parameters of the heat transfer agents depending on temperature and pressure makes up the calculation element of the heat exchange equipment.

Typical constructions

Heat exchange processes are carried out in devices of various types and structures .

According to heat transfer method heat exchangers are divided into surface and mixing ones. In surface apparatuses, working media exchange heat through walls made of thermally conductive material, and in mixing apparatuses, heat is transferred with direct mixing of working media (used when mixing of working media is permissible according to production conditions).

Surface heat exchangers are divided recuperative and regenerative.

Shell and tube heat exchangers

These heat exchangers are common due to the compact placement of a large transfer surface in a unit of volume of the apparatus. Heat exchange surface in it (Fig.2.2.1.) is formed by bundle of tubes 5, ends of which are fixed in two tube grids 4. The tubes are enclosed in a cylindrical casing 1.

Distribution heads (3) are bolted to pipe grids. For supply and removal of heat-exchanging media there are nozzles 2 in the apparatus. Tubes are most often fixed in grids by flaring, welding and less often with the help of glands.

To increase the rate of movement of heat carriers in order to intensify heat exchange, partitions are often installed in both tube and tube spaces.

Shell and tube heat exchangers can be horizontal, vertical, inclined. Depending on the value of the temperature extensions of the tubes and the housing, shell-and-tube heat exchangers with a rigid, semi-rigid and non-rigid structure are used.

Due to the increasing scale of production, the installation of a battery of shell and tube heat exchangers with a large heat transfer surface is increasingly used. Such an installation is profitable, as it saves production space.

Pipe-in-pipe heat exchangers

Heat exchangers of this type consist of a number of serially connected links. Each link represents two coaxial pipes. Pipes are usually connected to each other by "rockers" or knees.

Double-tube heat exchangers having a significant heating surface consist of a number of sections connected in parallel by headers. By selecting the diameters of the inner and outer tubes, it is possible to provide both working media with the necessary speed to achieve high heat exchange intensity.

Advantages of a double-tube heat exchanger: high heat transfer coefficient, suitability for heating or cooling media at high pressure, ease of manufacture, installation and maintenance. Disadvantages - bulkiness, high cost due to the high consumption of metal on external pipes that are not involved in heat exchange, the difficulty of cleaning the annular space .

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