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Available Technologies

1.1. Introduction

A heat exchanger is a device designed to transfer energy from one fluid to another, without any mixing between the fluids. When these energy transfers occur without any phase change, these devices are referred to as heat exchangers. When, on the other hand, heat transfer is accompanied by a phase change (liquid to vapor or vice versa), they are known as evaporators, reboilers or condensers, as appropriate.

Heat exchangers are frequently used in industry to heat or cool gases, liquids and even solids. In the automotive industry, heat exchangers are used for engine cooling; in this case, we refer to radiators. However, in the embedded electronics field, we know them as heat sinks, which are essential for cooling power components in circuit boards.

This diversity in designations also covers a diversity of technologies and above all a variety of calculation methods. As a result, several heat-exchanger design and production technologies exist. The purpose of Chapter 1 is to present the different technologies available on the market and to identify the parameters used to define each type of device and the corresponding fields of use.

1.2. The simplest form of heat exchanger: single-tube or coaxial

The simplest device is the single-tube heat exchanger, consisting of two coaxial tubes: one of the fluids circulates in the central tube, and the second fluid circulates in the annulus between the two coaxial tubes. Figure 1.1 shows a laboratory embodiment where the outer tube (the sleeve or shell) is made of glass, enabling the inner tube to be viewed. In practice, however, the two concentric tubes are metallic (usually made of steel or copper).

Figure 1.1. “Single-tube” exchanger1.

This exchanger belongs to the category of tubular heat exchangers also known as “shell-tube heat exchangers”. By its simplicity, it offers high reliability and relatively low costs.

It is, however, suitable only for limited power applications, and presents risks of hammering for small diameters (<50 mm). It did, however, recently gain new interest for use as a recuperative heat exchanger in the liquid-hydrogen zero-boil-off storage system. (Qiu et al. 2023)

1.3. Shell-and-tube heat exchangers

The slightly more elaborate shape of shell-tube exchangers makes it possible to have larger transfer areas in a given volume: the volume of the sleeve. Instead of the heat exchanger consisting of a central tube placed in a sleeve (the shell), a bundle of parallel tubes is placed in the shell (see Figure 1.2). One of the fluids (generally, the hot fluid) circulates inside the tubes, while the other flows shell-side. Heat exchange then takes place between the fluid passing through the shell and that circulating in the tubes.

The transfer area depends on the number of tubes integrated into the shell.

This type of heat exchanger is by far the most widely used in the industry, thanks to its relative ease of production and, above all, thanks to its robustness and low cost. Table 1.1 summarizes the advantages and disadvantages of this type of exchanger.

Figure 1.2. Shell-and-tube heat exchanger.

Table 1.1. Advantages and disadvantages of shell-and-tube heat exchangers

1.4. Coil-in-tank heat exchangers

By far the easiest to produce, coil-in-tank heat exchangers are widely used to perform tasks of preheating by heat recovery on available fluids.

They belong to the category of shell-tube heat exchangers.

A coil-in-tank heat exchanger consists of a coil immersed in a tank. Usually, the heating medium passes through the coil, while the fluid to be heated is in the tank, which is generally stirred and thermally insulated (see Figure 1.3).

Figure 1.3. Coil-in-tank heat exchanger.

Table 1.2. Advantages and disadvantages of coil-in-tank heat exchangers

1.5. Compact heat exchangers

Exchangers presenting a large heat transfer surface area per unit volume are generally referred to as compact heat exchangers2. The interest placed in this category stems from the fact that, in practice, the aim is always to use devices presenting the most m² available for heat exchange per m3 of device.

This type of heat exchanger includes cross-flow exchangers, finned exchangers used as car radiators or refrigeration evaporators (which generally have surface densities of around 1,000 m2/m3), or plate exchangers, plate-fin exchangers, spiral or lamella exchangers and printed-circuit exchangers.

1.5.1. Finned-tube cross-flow heat exchangers

These exchangers are generally used to transfer heat between a liquid and a gas. To compensate for the low transfer coefficients on the gas side, the transfer area in contact with the gas flow is increased by attaching fins to the tubes (see Figure 1.4).

Figure 1.4. Finned tube.

Thus, cross-flow exchangers are most often composed of a tube or a set of tubes that pass through metal plates that act as fins, always with a view to increasing the transfer area (see Figure 1.5). The liquid generally circulates inside the tubes and the gas circulates fin-side. The directions of the two fluids are perpendicular, hence the name “cross-flow”. Note that the fluids cross each other inside the exchanger, but without mixing.

Figure 1.5. Finned-tube cross-flow heat exchanger.

1.5.2. Car radiators

The car radiator is a common version of the finned-tube cross-flow heat exchanger. It consists of tubes passing through a honeycomb of fins with a density between 450 and 1,100 fins/m, enabling compactnesses of between 950 and 1,700 m2/m3.

Car radiators are used to cool the water circulating to remove the heat produced by the engine (see Figure 1.6). The cooling water flows through the tubes and the air passes fin-side.

Figure 1.6. Principle of the car engine cooling system.

Therefore, the heat is first transferred by convection from the hot fluid circulating in the tubes toward the tube walls. It is then conveyed by conduction toward the metal fins, then by convection between these fins and the air. In this way, heat is extracted from the cooling liquid (which passes through the tubes) to the outside air crossing fin-side (see Figure 1.7).

Figure 1.7. Car radiator.

Table 1.3. Advantages and disadvantages of finned-tube heat exchangers

1.5.3. Plate-fin heat exchangers

Commonly used in gas/liquid applications, this type of exchanger consists...