**3. Other selective complementary sources**

A very general review on advances in heat transfer enhancements was performed by Siddique et al. [3]. They addressed most usual heat transfer enhancers: fins and microfins, porous media, large particles suspensions, nanofluids, phase‐change devices, flexible seals, flexible complex seals, vortex generators, protrusions, and ultra‐high thermal conductivity composite materials. In addition, theoretical enhancement factors along with numerous heat transfer correlations were presented in this review for each heat transfer enhancer.

#### **3.1. Materials**

The conventional heat exchangers are manufactured in metal (such as stainless steel, copper and aluminum) and have disadvantages in terms of weight and cost. In addition, specially treated metal heat exchangers are needed if the working fluids are corrosive. Alternative materials have been used for heat exchangers that can overcome some of the disadvantages of the conventional ones like weight, cost, and chemical resistance, be more adequate for partic‐ ular applications, and also have comparable heat exchange efficiency and be easily fabricated.

Due to their low cost, lightweight, and corrosive resistant features, *polymer* heat exchangers are receiving growing interest from researchers, engineers, and other industry related players. Thus, as a better alternative to metallic heat exchangers in a wide range of applications, these particular heat exchangers have extensively been investigated in recent years. This can be evi‐ denced from a recent review paper by Chen et al. [4] who reported developments including theoretical modeling, experimental findings, heat transfer enhancement methods of polymer materials, and a wide range of applications of polymer heat exchangers. Another interesting review work by T'Joen et al. [5] discussed the use of polymer matrix composites in HVAC&R applications, showing how a careful material selection and modification of the design allows to fully exploit the material properties.

Lin et al. [6] proposed a new compact *graphite foam* heat exchanger for vehicle cooling appli‐ cation that allows to match the increasing cooling power and space limitation in vehicles. Their simulation results show that the wavy corrugated foam presents high thermal perfor‐ mance and low pressure drop. A comparative study between the wavy corrugated foam heat exchanger and a conventional aluminum louver fin heat exchanger was also carried out to evaluate the performance of graphite foam heat exchangers. Furthermore, several recommen‐ dations were made about the further development of the application of graphite foam heat exchangers in vehicles.

For heat exchanger applications needing extreme operation temperatures such as in the field of power generation or heat recovery, *ceramics* and ceramic matrix composites are suitable to design heat exchangers and particularly adequate to attain optimal overall efficiency, cost, and size of the system. The review by Sommers et al. [7] provides the current state‐of‐the‐art of ceramic materials for use in a variety of heat transfer systems.

### **3.2. Nanofluids**

to introduce closely related industrial practices for cleaning and green technology mainte‐ nance of heat exchangers. For an industry, the proper cleaning method and control play an important role to reduce the production costs. Production cost significantly increases due to chemical usage, maintenance work and downtime loss, and water wastage. Therefore, the authors underlined the importance of corrosion control, fouling cleaning, and enforcement of specific standards for cleaning procedures in the industries. They also proposed the applica‐

A very general review on advances in heat transfer enhancements was performed by Siddique et al. [3]. They addressed most usual heat transfer enhancers: fins and microfins, porous media, large particles suspensions, nanofluids, phase‐change devices, flexible seals, flexible complex seals, vortex generators, protrusions, and ultra‐high thermal conductivity composite materials. In addition, theoretical enhancement factors along with numerous heat transfer

The conventional heat exchangers are manufactured in metal (such as stainless steel, copper and aluminum) and have disadvantages in terms of weight and cost. In addition, specially treated metal heat exchangers are needed if the working fluids are corrosive. Alternative materials have been used for heat exchangers that can overcome some of the disadvantages of the conventional ones like weight, cost, and chemical resistance, be more adequate for partic‐ ular applications, and also have comparable heat exchange efficiency and be easily fabricated. Due to their low cost, lightweight, and corrosive resistant features, *polymer* heat exchangers are receiving growing interest from researchers, engineers, and other industry related players. Thus, as a better alternative to metallic heat exchangers in a wide range of applications, these particular heat exchangers have extensively been investigated in recent years. This can be evi‐ denced from a recent review paper by Chen et al. [4] who reported developments including theoretical modeling, experimental findings, heat transfer enhancement methods of polymer materials, and a wide range of applications of polymer heat exchangers. Another interesting review work by T'Joen et al. [5] discussed the use of polymer matrix composites in HVAC&R applications, showing how a careful material selection and modification of the design allows

Lin et al. [6] proposed a new compact *graphite foam* heat exchanger for vehicle cooling appli‐ cation that allows to match the increasing cooling power and space limitation in vehicles. Their simulation results show that the wavy corrugated foam presents high thermal perfor‐ mance and low pressure drop. A comparative study between the wavy corrugated foam heat exchanger and a conventional aluminum louver fin heat exchanger was also carried out to evaluate the performance of graphite foam heat exchangers. Furthermore, several recommen‐ dations were made about the further development of the application of graphite foam heat

tion of a mitigation approach to deal with fouling and corrosion.

correlations were presented in this review for each heat transfer enhancer.

**3. Other selective complementary sources**

4 Heat Exchangers– Advanced Features and Applications

**3.1. Materials**

to fully exploit the material properties.

exchangers in vehicles.

Recent advances in nanotechnology have allowed the development of a new category of fluids termed "nanofluids" [8]. Among many other applications, nanofluids can be used as thermal fluids and are considered heat transfer enhancers. The review by Huminic and Huminic [9] summarized the important available publications on the enhancement of the convection heat transfer in heat exchangers using nanofluids. They also presented the theo‐ retical and experimental results for the effective thermal conductivity, viscosity, and the Nusselt number reported in the literature. It also focused on the application of nanofluids in various types of heat exchangers: plate, shell and tube, compact, and double‐pipe heat exchangers.

A special type of nanofluids is ionanofluids that can be defined as suspensions of nanoma‐ terials in ionic liquids. Ionic liquids possess promising thermophysical properties and great potential for numerous applications, particularly as new heat transfer fluids. Since ionic liquids are the base fluids in ionanofluids, the above‐mentioned heat transfer enhancement obtained with nanomaterials suspensions can be potentiated by the thermophysical proper‐ ties of ionic liquids. Nieto de Castro et al. [10] presented some pioneering researches that indicate that ionanofluids show great promises to be used as innovative heat transfer fluids in heat exchangers and novel media for many green energy‐based applications.

## **3.3. Special design**

The efficient design of heat exchangers is more critical in some applications, requiring devices having superior performance and reliable mechanical characteristics at high pressure and high temperature and complying with geometric constraints. Therefore, design of heat exchangers is one of the aspects where continuous advances are registered.

The review by Li et al. [11] reported the performances of *compact* heat exchangers, including well‐established devices, some relative newcomers to the market and also designs still being tested in the laboratory. The structures, heat transfer enhancement mechanisms, advantages, and limitations are summarized, and an example of an application as a solar receiver is given. It also referred available correlations for heat transfer and friction factor developed by various researchers.

*Microchannels* represent the next step in heat exchanger development due to their high heat transfer performance and reduced weight as well as their space, energy and materials savings potential. Khan and Fartaj [12] made a survey of the published literature on the status and potential of microchannels, identifying research needs. They also developed an experimental infrastructure to investigate the heat transfer and fluid flow for a variety of working fluids in different microchannel test specimens.

The study by Abu‐Khader [13] presented the advances in *plate* heat exchangers both in theory and applications. The selected areas discussed in this review are the ones that attracted more attention recently, namely compactness and downsizing without the loss of performance, which is crucial for the industry; theoretical developments; reducing fouling and corrosion of plates in sever processes, with a direct impact on operational cost; and using nanofluids.

In the paper by Wang et al. [14], a general review was provided on developments and improvements of *shell‐and‐tube* heat exchangers with helical baffles of different improved designs. Extensive results from experiments and numerical simulations indicated that these heat exchangers have better flow and heat transfer performance than the conventional baffled heat exchangers, therefore allowing to save energy, reduce cost, and prolong service life and operation time in industrial applications.

The heat pipes are accepted as an excellent way of saving energy due to the high heat recov‐ ery effectiveness of these devices. A brief literature review was performed by Srimuang and Amatachaya [15] on the applications of *heat pipes* heat exchangers for waste heat recovery in both commercial and industrial applications. The authors also summarized the experimental studies on the conventional heat pipe, two‐phase closed thermosiphon, and oscillating heat pipe heat exchangers.

#### **3.4. Applications**

There are review studies available on other types of heat exchangers used in particular cases of applications where innovative and state‐of‐the‐art equipment must be developed and used, such as exemplified with the cases of geothermal processes with ground heat exchangers [16] and biotechnological industries [17].
