Preface

As motive force of processes, heat must be transferred from one fluid to other, task that is performed by means of heat exchangers. From this point of view, heat exchangers represent an important element of thermal facilities that has substantially contributed to technical development of the society. Today it is impossible to imagine any branch of process engineering and energy technology without involvement of heat exchangers. Advanced models of these apparatus were proposed in the middle of the 18th century, while theoretical backgrounds have been completed a century later.

Corresponding to practical importance of heat exchangers, innumerable studies and treatises are devoted to processes taking place in these devices and their constructive shaping. The actual development trend in this field is guided by the ideas of reduction of thermal transport resistances and the raise of energy conversion efficiency. These ideas have also guided the conception of the present book. It is a collection of contributions prepared by the specialists. It consists of 21 Chapter that are arranged in 6 Sections:

Section 1: General Aspects,

Section 2: Micro-Channels and Compact Heat Exchangers,

Section 3: Plate Heat Exchangers,

Section 4: Helical Coils and Finned Surfaces,

Section 5: Energy Storage, Heat Pumps and Geothermal Energy,

Section 6: Fouling of Heat Exchangers.

#### **Section 1 - General Aspects**

This part comprises 7 Chapters dealing mainly with the questions of fluid flow and heat transfer in heat exchangers. Chapter 1 by Awad and Muzychka addresses the entropy generation arising from heat transfer and fluid flow and provides a basis for thermodynamic optimisation of heat exchangers. Gvozdenac gives in Chapter 2 a detailed analysis of convective heat transfer in heat exchangers at different flow arrangements under transient conditions. Kansha et al. present in Chapter 3 a self-heat recuperation technology for transport of latent and sensible heat of the process streams without heat addition and introduce a theoretical analysis of this technology. Carvajal-Mariscal et al. recommend in Chapter 4 combinations of process parameters that give high efficiency of two-phase thermosyphons. These devices are used for transport of high heat flow rates from heat source to heat sink by connecting the evaporator and

#### X Preface

the condenser. The heat flow rate is reduced, if the elements of the heat exchangers are not evenly supplied with the fluid. Bury examined the issue in Chapter 5 and reports an average deterioration factor of 15% for a cross-flow heat exchanger. In Chapter 6, Minato et al. deal with the LNG pyrolysis in connection with regenerative cooling of rocket engines. The processes occurring are exceedingly complex, not only because of high process temperatures, which usually cause large temperature gradients. Thermal stresses thus induced may impair the structural stability of constructions, as is exemplified by A. Chudzik in Chapter 7 for a shell-and-tube heat exchanger.

Preface XI

Plate heat exchangers consist of a number of plates assembled in parallel next to one another thus forming flow channels such that each plate separates hot from cold fluid stream. Plates are provided with macro structures and the neighbouring plates touch each other on the crests of the structures. The channels are gasketed or brazed on the circumference. In Chapter 15 Muthuraman presents results of the experimental condensation studies of R410A in brazed plate heat exchangers for various plate

Part 5 bundles the contributions dealing with the storage and conversion of thermal energy, focussing on alternative energy sources and clean energy. Thermal radiation of the Sun counts to the cleanest energies; being availably mainly seasonally, its storage is becoming increasingly important. If stored as internal energy of a substance, the substance should undergo an endothermic phase transition during storing while exothermic when releasing the heat. Chapter 16 by Dolado et al. deals with the heat exchangers comprising phase change materials, illustrating the processes mainly in form of temperature history diagrams. Thermal energy stored in the ground may be utilized by means of heat pumps. Chapter 17 by Kharseh illustrates an example with surface geothermal energy, while Ledésert and Hébert give in Chapter 18 an overview on the exploitation of deep geothermal energy. The high temperature of energy source

in later case allows transformation of geothermal energy in other energy forms.

Dissolved solid substances and impurities contained in process streams interact with the heat transfer surfaces, attractively or repulsively. In case of attraction, the concentration of the dissolved may reach the solubility boundary and initiate crystallisation. Starting from this initial state, a solid layer grows on the surface during operation; it diminishes the thermal capacity of heat exchanger, if its thermal conductivity is low. This is referred to as fouling. The contribution devoted to fouling of heat exchangers are grouped in this Part. Chapter 19 by Kazi addresses general questions of fouling, thereby analysing its impacts on heat transfer. Some practical insights into fouling in plate heat exchangers are provided in Chapter 20 by Bani Kananeh and Peschel, while in Chapter 21 Klaren and de Boer report on the self-

**Prof. Dr. Ing. Jovan Mitrovic**

Germany

Thermodynamics and Thermal Process Engineering

structures and provides correlations for heat transfer and pressure drop.

**Section 5 - Energy Storage, Heat Pumps and Geothermal Energy** 

**Section 4 - Helical Coils and Finned Surfaces** 

**Section 6 - Fouling of Heat Exchangers** 

cleaning fluidised bed heat exchangers.

### **Section 2 - Micro-Channels and Compact Heat Exchangers**

Section 2 clusters the contributions dealing with the processes occurring in microchannels and apparatus composed of such elements. By using micro-channels, one pursues the idea of shortening the heat transfer paths, thereby trying to copy solutions evolved in the nature. In Chapter 8, Asgari analyses numerically the heat transfer in a heat exchanger, consisting of a number of rectangular micro-channels, connected in parallel, at low Re numbers with fully developed flow. The applied heat flux is orthogonal and uniform on the bottom plane of the apparatus. Zhang et al., in Chapter 9, simulate dynamically a compact heat exchange reformer for high temperature fuel cell systems under catalytic conditions. The modelling technique is suited for quick and real time calculations. With single phase flow, Dang et al. show in Chapter 10 the hydraulic diameter of the micro channel to be the chief parameter governing the thermo-fluid characteristic of the apparatus. Both experimental and numerical treatments confirm advantages of counter-current fluid flow arrangement. The contribution in Chapter 11 deals with the thermoelectric devices, which are used either to generate electric potential in a temperature gradient (Seebeck effect) or to generate a temperature difference by means of electric current (Peltier effect). Astrain and Martínez analysed there the efficiency of the device, mainly focussing on heat transfer. The thermal performances of heat transfer modules are demonstrated to decisively affect the efficiency of the whole system.

#### **Section 3 - Plate Heat Exchangers**

Section 3 comprises the contributions dealing with the heat exchangers consisting of helical coils and finned surfaces. Helical coils provide the simplest construction of heat exchangers while fining of surfaces should compensate for the low heat transfer. In Chapter 12 Jayakumar presents a detailed analysis of hydrodynamic and heat transfer of single-phase and two-phase water flow in helical pipes, for various coil parameters and boundary conditions. Basing on the results, correlations for the average and local Nusselt numbers were developed. Wais pursues in Chapter 13 the possibilities of finding the fin shape that should maximize the heat transfer and reduce the fin mass. The results of numerical experiments are used for developing of heat transfer correlations. Chapter 14 by Khamis Mansour deals with the thermal design of cooling and dehumidifying coils. The used row-by-row calculation method provides a better reliability than the common averaging method. This is of particular importance when local data are required as in case of air dehumidifiers.

#### **Section 4 - Helical Coils and Finned Surfaces**

X Preface

the condenser. The heat flow rate is reduced, if the elements of the heat exchangers are not evenly supplied with the fluid. Bury examined the issue in Chapter 5 and reports an average deterioration factor of 15% for a cross-flow heat exchanger. In Chapter 6, Minato et al. deal with the LNG pyrolysis in connection with regenerative cooling of rocket engines. The processes occurring are exceedingly complex, not only because of high process temperatures, which usually cause large temperature gradients. Thermal stresses thus induced may impair the structural stability of constructions, as is

Section 2 clusters the contributions dealing with the processes occurring in microchannels and apparatus composed of such elements. By using micro-channels, one pursues the idea of shortening the heat transfer paths, thereby trying to copy solutions evolved in the nature. In Chapter 8, Asgari analyses numerically the heat transfer in a heat exchanger, consisting of a number of rectangular micro-channels, connected in parallel, at low Re numbers with fully developed flow. The applied heat flux is orthogonal and uniform on the bottom plane of the apparatus. Zhang et al., in Chapter 9, simulate dynamically a compact heat exchange reformer for high temperature fuel cell systems under catalytic conditions. The modelling technique is suited for quick and real time calculations. With single phase flow, Dang et al. show in Chapter 10 the hydraulic diameter of the micro channel to be the chief parameter governing the thermo-fluid characteristic of the apparatus. Both experimental and numerical treatments confirm advantages of counter-current fluid flow arrangement. The contribution in Chapter 11 deals with the thermoelectric devices, which are used either to generate electric potential in a temperature gradient (Seebeck effect) or to generate a temperature difference by means of electric current (Peltier effect). Astrain and Martínez analysed there the efficiency of the device, mainly focussing on heat transfer. The thermal performances of heat transfer modules are demonstrated to decisively

Section 3 comprises the contributions dealing with the heat exchangers consisting of helical coils and finned surfaces. Helical coils provide the simplest construction of heat exchangers while fining of surfaces should compensate for the low heat transfer. In Chapter 12 Jayakumar presents a detailed analysis of hydrodynamic and heat transfer of single-phase and two-phase water flow in helical pipes, for various coil parameters and boundary conditions. Basing on the results, correlations for the average and local Nusselt numbers were developed. Wais pursues in Chapter 13 the possibilities of finding the fin shape that should maximize the heat transfer and reduce the fin mass. The results of numerical experiments are used for developing of heat transfer correlations. Chapter 14 by Khamis Mansour deals with the thermal design of cooling and dehumidifying coils. The used row-by-row calculation method provides a better reliability than the common averaging method. This is of particular importance when

exemplified by A. Chudzik in Chapter 7 for a shell-and-tube heat exchanger.

**Section 2 - Micro-Channels and Compact Heat Exchangers** 

affect the efficiency of the whole system.

local data are required as in case of air dehumidifiers.

**Section 3 - Plate Heat Exchangers** 

Plate heat exchangers consist of a number of plates assembled in parallel next to one another thus forming flow channels such that each plate separates hot from cold fluid stream. Plates are provided with macro structures and the neighbouring plates touch each other on the crests of the structures. The channels are gasketed or brazed on the circumference. In Chapter 15 Muthuraman presents results of the experimental condensation studies of R410A in brazed plate heat exchangers for various plate structures and provides correlations for heat transfer and pressure drop.

#### **Section 5 - Energy Storage, Heat Pumps and Geothermal Energy**

Part 5 bundles the contributions dealing with the storage and conversion of thermal energy, focussing on alternative energy sources and clean energy. Thermal radiation of the Sun counts to the cleanest energies; being availably mainly seasonally, its storage is becoming increasingly important. If stored as internal energy of a substance, the substance should undergo an endothermic phase transition during storing while exothermic when releasing the heat. Chapter 16 by Dolado et al. deals with the heat exchangers comprising phase change materials, illustrating the processes mainly in form of temperature history diagrams. Thermal energy stored in the ground may be utilized by means of heat pumps. Chapter 17 by Kharseh illustrates an example with surface geothermal energy, while Ledésert and Hébert give in Chapter 18 an overview on the exploitation of deep geothermal energy. The high temperature of energy source in later case allows transformation of geothermal energy in other energy forms.

#### **Section 6 - Fouling of Heat Exchangers**

Dissolved solid substances and impurities contained in process streams interact with the heat transfer surfaces, attractively or repulsively. In case of attraction, the concentration of the dissolved may reach the solubility boundary and initiate crystallisation. Starting from this initial state, a solid layer grows on the surface during operation; it diminishes the thermal capacity of heat exchanger, if its thermal conductivity is low. This is referred to as fouling. The contribution devoted to fouling of heat exchangers are grouped in this Part. Chapter 19 by Kazi addresses general questions of fouling, thereby analysing its impacts on heat transfer. Some practical insights into fouling in plate heat exchangers are provided in Chapter 20 by Bani Kananeh and Peschel, while in Chapter 21 Klaren and de Boer report on the selfcleaning fluidised bed heat exchangers.

> **Prof. Dr. Ing. Jovan Mitrovic** Thermodynamics and Thermal Process Engineering Germany

**Part 1** 

**General Aspects** 

**Part 1** 

**General Aspects** 

**1** 

*1Egypt 2Canada* 

**Thermodynamic Optimization\***

*2Faculty of Engineering and Applied Science, St. John's, NL,* 

Second law analysis in the design of thermal and chemical processes has received considerable attention since 1970s. For example, Gaggioli and Petit (1977) reviewed the first and second laws of thermodynamics as an introduction to an explanation of the thesis that energy analyses of plants, components, and processes should be made by application of the second law that deals with the availability of energy or the potential energy. They illustrated their methodology suggested by applying it to an analysis of the Koppers-Totzek gasification system. Optimization of heat exchangers based on second-law rather than firstlaw considerations ensures that the most efficient use of available energy is being made.

Second-law analysis has affected the design methodology of different heat and mass transfer systems to minimize the entropy generation rate, and so to maximize system available work. Many researchers considered these processes in terms of one of two entities: exergy (available energy) and irreversibility (entropy production). For instance, McClintock (1951) described irreversibility analysis of heat exchangers, designed to transfer a specified amount of heat between the fluid streams. He gave explicit equations for the local optimum design of fluid passages for either side of a heat exchanger. To the knowledge of authors, McClintock (1951) was the first researcher who employed the irreversibility concept for estimating and minimizing the usable energy wasted in heat exchangers design. Bejan (1977) introduced the concept of designing heat exchangers for specified irreversibility rather than specified amount of heat transferred. Many authors used this technique in the field of cryogenic engineering (Bejan and Smith (1974, 1976), Bejan (1975), and Hilal and Boom

One of the first examinations of entropy generation in convective heat transfer was conducted by Bejan (1979) for a number of fundamental applications. Much of the early

 The part of this chapter was presented by Y. S. Muzychka in fall 2005 as Part III during the short course: Adrian Bejan, Sylvie Lorente, and Yuri Muzychka, Constructal Design of Porous and Complex Flow Structures, Memorial University of Newfoundland, Faculty of Engineering and Applied Science,

**1. Introduction** 

(1976)).

St. John's, NL, Canada, September 21-23, 2005.

 \*

M.M. Awad1 and Y.S. Muzychka2 *1Mechanical Power Engineering Department, Faculty of Engineering, Mansoura University,* 

*Memorial University of Newfoundland,* 
