Heat Exchangers E-Book

Abdelhanine Benallou

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 399

Veröffentlichungsjahr: 2024

Beschreibung

The last few decades have seen huge developments in the use of concentrated solar power plants, communications technologies (mobile telephony and 5G networks), the nuclear sector with its small modular reactors and concentrated solar power stations. These developments have called for a new generation of heat exchangers. As well as presenting conventional heat exchangers (shell-and-tube and plate heat exchangers), their design techniques and calculation algorithms, Heat Exchangers introduces new-generation compact heat exchangers, including printed circuit heat exchangers, plate-fin heat exchangers, spiral heat exchangers, cross-flow tube-fin heat exchangers, phase-change micro-exchangers, spray coolers, heat pipe heat exchangers and evaporation chambers. This new generation of heat exchangers is currently undergoing a boom, with applications in on-board equipment in aircraft, locomotives, space shuttles and mobile phones, where the volume of the equipment is one of the most important design parameters.

Leseprobe

Energy Engineering Set

coordinated byAbdelhanine Benallou

Volume 6

Heat Exchangers

Design and Sizing Algorithms

Abdelhanine Benallou

First published 2024 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA

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© ISTE Ltd 2024The rights of Abdelhanine Benallou to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s), contributor(s) or editor(s) and do not necessarily reflect the views of ISTE Group.

Library of Congress Control Number: 2023952195

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78630-286-1

Introduction

Recent technological developments relate to different areas of industry: telecommunications, computing, avionics, electric cars, railways, new energies, home automation, etc. These developments have led to enormous progress involving the increasing use of ever-more-powerful embedded electronics and computing systems. With the advent of insulated gate bipolar (IGB) transistor technology, the electric car, or the “more electric” aircraft (Boeing 787 Dreamliner), and more generally next-generation transport equipment, are becoming extremely demanding in terms of energy performance, ergonomics and dimensions: the ultimate performance needs, from now on, to adapt to the smallest of sizes.

However, the operation of power electronics, increasingly used in this type of equipment, releases a great amount of heat. As a matter of fact, the heat density released by the operation of a computer chip can reach 450 to 550 kW/m2. For comparison, the average power of a semi-trailer truck is 300 kW and the heat density released at the nose of a space shuttle upon entering the atmosphere is in the order of 500 kW/m2. This means that the heat released by an electronic circuit in operation can be enormous.

However, operation of all electronic components is temperature sensitive. Their performance is defined within very precise temperature ranges. This performance becomes poor outside the prescribed temperature limits. Furthermore, excessively high temperatures can even lead to their deterioration. This constraint means that the cooling efficiency becomes important to maintain the integrity of electronic equipment; importance stressed by an ever-increasing search for small-size designs, compared with the significant power released during operation.

For these particular uses, a specific category of heat exchangers is used to cool electronic boards in computers or power equipment such as those used in new generations of transport vehicles, in mobile telephony or in home automation systems.

Heat exchangers also continue to be present in various industrial processes that require heating or cooling of the products in order to obtain the desired characteristics (chemical reactions, space heating or cooling, etc.). At times, there is even a requirement to vaporize a liquid or condense a vapor (for example, in distillation). All of these types of thermal transformations are carried out in heat exchangers. Indeed, heat exchangers are present almost everywhere; even in the domestic field and long before home automation, where they are the centerpieces of boiler rooms, refrigerators, air conditioners and sanitary water heating systems, whether electric, gas or solar.

However, although several works deal with the conceptual aspects of heat exchangers, sizing and calculation techniques remain the least documented in the literature related to energy engineering. In most cases, specialized books and heat exchanger manufacturers’ instruction manuals, which exist in different languages and with different degrees of detail and precision, are either complex, or present different methodologies, linked to a type of manufacture. They are then beyond the reach of users wishing to produce a rapid and independent design.

This book introduces the different technologies available in the industry of heat exchangers in the broadest sense. It presents the most conventional exchangers, but also new-generation exchangers, more specific to the fields of the automotive, avionics or embedded electronics industry.

In addition to illustrating the fundamental concepts governing each of the available technologies, this book specifies the advantages and disadvantages of each type of exchanger, as well as their preferred areas of use. The criteria that need to govern the choice of the exchanger type to be used, according to the task to be carried out and according to the fluids treated, are thus highlighted.

We have made the choice to start with a detailed description of the available technologies. Subsequently, the general design principles that apply to all heat exchanger types are outlined. Nevertheless, technology diversities impose different sizing methods. For this reason, we have reserved Chapter 6 for the presentation of calculation algorithms for each type of exchanger. These algorithms are implemented in the various practical illustrations that accompany Chapter 6.

1Available 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

Advantages

Disadvantages

Usage

Ease of manufactureRobustnessFew breakdowns

Prone to tube fouling

High pressure vapor/waterSuperheated water/waterThermal fluid/waterFumes/water

Good thermal performanceVery good overall heat transfer coefficient

Not sufficiently compact

Liquid/LiquidLiquid/GasGas/Gas

Good price/performance ratioReliable and simple

Difficult to clean

Evaporation

Withstands high pressuresAccepts large temperature differencesCan be used under boiling conditions

Limited power

Can be used for large flowrates, but must undergo pressure inspection if the volume exceeds 100 liters

Wide range of applicationsCan be used in partial condensation

Sensitive to vibrations

OilsLiquid/fumesWater/thermal fluidRefrigerants

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

Advantages

Disadvantages

Usage

Ease of productionLow cost

Low thermal efficienciesHeat loss through the walls of the tankHeat loss by evaporation if the tank is not covered

Good solution to be performed rapidly to meet an immediate need

Can be built with recovery equipment

Thermal tasks to be completed rapidly for process modifications or to conduct tests

Wide range of applications

Liquid/LiquidWater/low-pressure vapor

Ease of tank cleaning

Inside of the coil difficult to clean

Limit circulation in the helical tube to fluids that are clean

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

Advantages

Disadvantages

Usage

Good performanceCan take on precise shapes

Sensitive to shocks

Water/airOil/airSolid/air

1.5.3. Plate-fin heat exchangers

Commonly used in gas/liquid applications, this type of exchanger consists of a stack of modules, each comprising of two metal plates between which fins of various shapes have been fixed. Figure 1.8 shows an exploded view of a module, while Figure 1.9 indicates that fluids can circulate counter-flow or cross-flow.

Figure 1.8.Plate-fin heat exchanger.

Plate-fin heat exchangers can reach compactnesses beyond 5,500 m2/m3.

Figure 1.9.Common flow configurations in a plate-fin heat exchanger.

Table 1.4 shows the advantages and disadvantages, as well as the common uses, of this type of exchanger.

Table 1.4.Advantages and disadvantages of plate-fin heat exchangers

Advantages

Disadvantages

Usage

Good compactness

Difficult to cleanApplications limited to relatively clean fluids

Gas/gas exchangesGas/liquid exchanges

Low weightHeat transfer area per kg more than 25 times that of shell-and-tube heat exchangers

Rigid structure

Embedded exchangersCommercial aircraftAerospaceLocomotives

Good thermal efficiency

Limited to low pressures

P

≤ 1 kPa

HeatingSuitable for gas/gas and gas/liquid energy recovery techniques

Good circuit sealing

Relatively high pressure drops

Heat exchanges where there must be no fluid contamination

Good value for money

Possibility of poor flow distributionRelative fragility

Protected areas

1.5.4. Plate heat exchangers

Another type of compact heat exchanger has received particular attention in recent decades, as it provides very large transfer areas in very small volumes: the plate heat exchanger.

Figure 1.10 shows an exploded diagram of this type of exchanger. It is possible to visualize how the fluids circulate in juxtaposed plates arranged alternately: “hot” plates, shown in red, and “cold” plates, shown in blue. Each “hot” plate is in this way framed by two “cold” surfaces and vice versa.

Distribution of the fluids over plates is ensured by a head plate, in such a way that the respective fluid circuits do not allow mixing.

Figure 1.10.Plate heat exchanger.

Table 1.5.Advantages and disadvantages of plate heat exchangers

Advantages

Disadvantages

Usage

Good thermal efficiencyVery good heat transfer coefficientsLow heat loss

Relatively large pressure drops

Water/water exchangesWater/fruit juice exchangesWater/sea water exchanges

Good compactnessFully modular: extensions and reductions possible

Relatively limited working pressures

Low-pressure vapor/oilOil/water

Suitable for energy recovery techniques

Rather suitable for small temperature differences

Food industryHeatingUrban heatingCollective networksSolar heating

Good value for money

Poor in gas/gas heat exchangesPoor for high-viscosity fluids

DairiesMarine industryRefrigeration condensationHeat pumps

It should be noted that in addition to its high compactness, this type of exchanger offers very good thermal efficiencies and competitive costs.

1.5.5. Spiral heat exchangers

Spiral heat exchangers form another category of compact exchangers. They are made from rolling two metal sheets one over the other, using studs to separate the different sheets so as to leave a gap between them. These gaps will constitute channels for the circulation of fluids. The exchanger thus obtained is closed at both ends by covers that ensure both sealing of the device and distribution of the fluids.

Figure 1.11.Circulation of fluids inside a spiral3 heat exchanger.

Figure 1.11 shows the spiral heat exchanger obtained, in which the “hot” (shown in red) and “cold” (shown in blue) fluids circulate, in counter-flow, in the spiral channels formed by the sheets. This gives an alternation of “hot” and “cold” cylindrical plates inside the exchanger. A spiral heat exchanger could thus be viewed as a specific plate heat exchanger; instead of its plates being flat, they are cylindrical. This type of geometry offers several advantages, in addition to compactness. In particular, spiral heat exchangers can be used for charged fluids, with small temperature differences, and pressures as high as 16 bar.

Moreover, the fact that the channels are accessible (after opening of the covers) offers a definite advantage for easy cleaning of the device. Note that the studs placed between the rolled plates not only make it possible to maintain a gap for fluid passage, but also play an important role in heat transfer efficiency. Indeed, their presence in the fluid flow path promotes turbulent flow in each channel. They therefore make it possible to obtain larger heat transfer coefficients (see Volume 3).

Note that spiral heat exchangers are very efficient in the case of charged solutions. Indeed, the fact that the fluid flowrate is not shared (as in the case of multitube heat exchangers where the flowrate is divided between the tubes) means that it is forced through the available gap in its entirety. This specific feature imparts a self-cleaning effect on this type of exchanger, since any fouling deposits on the sheets lead to a decrease in the gap available for fluid flow, and therefore an increase in the pressure and flow velocity, which eventually clears these deposits away.

Table 1.6.Advantages and disadvantages of spiral heat exchangers

Advantages

Disadvantages

Usage

Good thermal efficiency

Heat recoveryPaper industryMining industryWater treatmentPharmaceutical industry

Good compactness

AerospaceMaritime

Good heat transfer coefficients thanks to the presence of studs, which increase turbulence

Pressure drops can be significant

SteelworksPetrochemical industriesRefining

Suitable for energy recovery techniques

All sectors

Suitable for very high-viscosity charged fluids containing solid particles or fibers.

Excellent solution for heat exchange between fouling and dirty fluidsWaste heat recoverySuitable for waste-water heat recovery

Good value for money

Low return times for energy-recovery investments

Easy to maintain and clean without the use of special tools

1.5.6. Printed-circuit heat exchangers

Printed-Circuit Heat Exchangers (PCHEs) are manufactured using a process that enables etching (printing) on the inside of solid plates, channels or circuits intended for fluid flow. These channels are created, in a similar way to electronic printed circuits, by chemical etching (Deng et al. 2020). They can also be created by diffusion bonding.

In cross-section, a flow channel generally takes the shape of a semicircle having a diameter of 1 to 2 mm (see Figure 1.12). Other channel shapes (triangles, waves, etc.) can also be encountered, however, depending on the model and the manufacturer; the objective being to obtain the maximum heat transfer area in each channel. The flow configuration may be counter-flow (see Figure 1.12) or cross-flow (see Figure 1.13).

Figure 1.12.Diagram of a counter-flow PCHE core.

Figure 1.13.Diagram of a cross-flow PCHE core.

The fact that the flow channels are etched in the mass enables gasketless and weldless heat exchangers, called “exchanger cores”, which have high resistance and integrity.

This technology provides ultracompact heat exchangers that offer very high performance (Oh and Kim 2008): up to 85 % smaller and lighter than traditional heat exchangers (see comparison, Table 1.7). It also enables the development of heat-exchanger cores that are in line with the constraints and detailed specifications of the customer. Figure 1.14 shows an image of a PCHE core custom-made to occupy a specific space.

Table 1.7.Characteristics of the PCHE compared to other heat exchangers

Type of heat exchanger

Channel diameter (mm)

Compactness (m²/m

3

)

Shell-and-tube

10–50

100

Plates

5

200

Plate-fins

2

1,000

PCHE

1

2,000

Figure 1.14.Image of a custom-made4 PCHE core.

Unless otherwise specified for embedded exchangers, PCHE cores are usually mounted in bodies made of stainless steel or composite materials (see Figures 1.15 and 1.16).

Figure 1.15.Mounting of a PCHE core5.

Figure 1.16.Commercial PCHE6 models.

Developed only recently but presenting great potential, printed-circuit heat exchangers are on course to occupy an important place in the heat-transfer equipment industry. However, the poor availability of thermohydraulic data can make sizing difficult. Recent studies (Ma et al. 2021) make it possible to fill this gap in certain situations.

Table 1.8.Advantages and disadvantages of printed-circuit heat exchangers

Advantages

Disadvantages

Usage

Excellent thermal efficiency and great robustness

Fluids must be very cleanDifficult to cleanChemical cleaning required

AerospaceMaritime equipment: LNG regasification on ships

Exceptional compactness Up to 650 to 1,300 m

2

/m

3

Transients can lead to material fatigue

High-pressure vaporization in fuel gas supply systems

Suitable for very low and very high pressures

Small channel sizePressure drops can be significant for low- and medium-pressure applications

Wide pressure interval: from vacuum to 1,000 barPetrochemical industries: offshore gas compression systemsPharmaceutical industry

Suitable for very low and very high temperatures

Low availability of thermo-hydraulic data

Wide temperature interval: Up to 1,200°C

Energy recovery

Combustion gas treatment systemsElectricity generation: preheating of turbine fuel gas

Various materials, as required

Carbon steels not suitable

Stainless steel, chrome, Cu-Ni-Ti alloys

1.6. Rotary regenerative heat exchangers

Rotary heat exchangers are mainly used in gas/gas applications.

A rotary heat exchanger (see Figure 1.17) consists of a carousel of large metal fins that store heat as the hot gas passes through. The carousel is driven by a rotational motion that allows the fins to be in contact, sometimes with the “hot” gas, and sometimes with the “cold” gas. This makes it possible to transport the heat stored during passage through the hot compartment, to the compartment through which the gas to be heated passes. The rotational velocity of the carousel is usually low (<1 rpm) to allow heat to transfer from the fins to the gases.

Figure 1.17.Rotary regenerative heat exchanger.

Table 1.9.Advantages and disadvantages of rotary heat exchangers

Advantages

Disadvantages

Usage

Vast transfer areas

Low heat transfer coefficients

Energy recovery on gas streams

Exploitation of latent heat

Contamination risks between the two flows

Gas/gas exchanges

Ordinary construction materials

Low price/efficiency ratio

Waste heat cooling

1.7. Phase-change heat exchangers

1.7.1. Condensers

Condensers are specific heat exchangers where the fluid to be cooled undergoes a phase change: it enters the exchanger in the vapor state and exits it in the liquid state.

There are several types of condensers; the most widely used in industry are the shell-and-tube type. Generally, the fluid to be condensed circulates in the shell, while the cooling liquid circulates in the tubes (see Figure 1.18).

Figure 1.18.Multitube condenser.

Other categories of condensers take the form of a coil tube fitted with fins. This type of condenser is mainly used in refrigeration. It is air-cooled (see Figure 1.19), generally by natural convection.

Figure 1.19.Finned-tube condenser

In addition, another type of condenser is also often used in refrigeration, simply consisting of a coaxial single-tube exchanger, usually water-cooled (see Figure 1.20). The water generally circulates in the annulus, while the refrigerant circulates in the central tube.

Figure 1.20.Single-tube condenser.

1.7.2. Reboilers and evaporators

These exchangers make it possible to boil or evaporate a fluid, the objective being to concentrate a solution or produce vapor. Therefore, they are heat exchangers where a phase change occurs. They are widely used in the refining industry (reboilers) and in sugar factories (evaporators). The most widespread technology is that of multitube heat exchangers, but plate heat exchangers are increasingly used.

1.7.2.1. Reboilers

Reboilers are generally the heat sources for distillation columns.

Figure 1.21.Example of a reboiler.

These are composed of several tubes placed in a shell with an enlarged part providing space for the vapor produced (see Figure 1.21). The liquid to be boiled is heated by the vapor flowing through the tubes, or (although this is less common) by an electrical resistor dipping inside the reboiler.

By releasing its latent heat, the heating vapor leaves the tubes in the liquid state.

1.7.2.2. Single-effect evaporators

A single-effect evaporator is composed of a multitube heat exchanger (see Figure 1.22). This type of exchanger is used for concentrating solutions in different industries: sugar factories, manufacture of concentrated fruit juice, etc.

Figure 1.22.Single-effect evaporator.

The evaporator is usually heated using a heating vapor that condenses in the tubes, releasing its latent heat. The concentrated juice is recovered at the evaporator outlet.

1.7.2.3. Multiple-effect evaporators

Multiple-effect evaporators consist of a set of multitube exchangers connected in series. The concentrated solution leaving the first effect is fed to the second effect to be concentrated further, and so on. Figure 1.23 shows a three-effect evaporator.

The use of multiple effects is generally dictated by energy saving requirements. In fact, the heating vapor, V0, makes it possible to produce the concentrated juice, J1, but also produces vapor V1, which is used in the second effect. Likewise, vapor V1 produces J2 and V2, etc. Therefore, in a battery of n multiple-effect evaporators, the consumed vapor, V0, makes it possible to produce n times V0 and therefore makes it possible for all other effects beyond the first to operate.

Multiple-effect evaporators are widely used in the sugar industry to concentrate sugar juices.

Figure 1.23.Multiple-effect evaporators.

1.7.2.4. Refrigeration evaporators

Refrigeration evaporators are used in cooling cycles to evaporate a refrigerant. Cooling is generally provided by air in natural convection; hence the name natural circulation evaporators (also known as static evaporators).