# Kays and London's Compact Heat Exchangers: A Classic Reference for Engineers

## Compact Heat Exchangers: What They Are and Why They Matter

Heat exchangers are devices that transfer heat between two or more fluids or solid surfaces at different temperatures. They are widely used in various industries, such as power generation, chemical processing, refrigeration, air conditioning, and aerospace. Heat exchangers can improve the efficiency, performance, and safety of many processes and systems by reducing energy consumption, waste heat, and environmental impact.

## Compact Heat Exchangers, Kays And London, Mcgraw Hilll

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However, not all heat exchangers are created equal. Some heat exchangers are more compact than others, meaning that they have a high ratio of heat transfer surface area to volume. Compact heat exchangers can offer significant advantages over conventional heat exchangers, such as higher heat transfer rates, lower pressure drops, smaller sizes, lighter weights, and lower costs. However, they also pose some challenges, such as higher complexity, lower reliability, and higher fouling potential.

In this article, we will explore the basics of heat exchangers, the advantages of compact heat exchangers, the design and analysis of compact heat exchangers, and the book by Kays and London on compact heat exchangers. By the end of this article, you will have a better understanding of what compact heat exchangers are and why they matter.

## The Basics of Heat Exchangers

### What is a heat exchanger?

A heat exchanger is a device that transfers heat between two or more fluids or solid surfaces at different temperatures. The fluids or surfaces can be in direct contact or separated by a solid wall. The direction of heat transfer can be parallel, counterflow, crossflow, or mixed flow. The purpose of a heat exchanger is to either increase or decrease the temperature of one or both fluids or surfaces.

### Types of heat exchangers

There are many types of heat exchangers, depending on the configuration, construction, and operation. Some common types are:

Shell-and-tube: This type consists of a series of tubes enclosed in a cylindrical shell. One fluid flows inside the tubes and another fluid flows outside the tubes. The tubes can be arranged in different patterns to enhance heat transfer.

Plate: This type consists of a stack of thin metal plates separated by narrow gaps. One fluid flows in alternate gaps and another fluid flows in the remaining gaps. The plates can have different shapes and corrugations to enhance heat transfer.

Fin: This type consists of a base surface with extended fins attached to it. One fluid flows over the base surface and another fluid flows over the fins. The fins increase the surface area for heat transfer.

Regenerative: This type consists of a rotating wheel or drum filled with a porous material that absorbs and releases heat. One fluid flows through the wheel or drum and another fluid flows around it. The wheel or drum alternately heats up and cools down as it rotates.

Heat pipe: This type consists of a sealed tube containing a working fluid that evaporates and condenses at different locations. One end of the tube is in contact with a hot fluid or surface and the other end is in contact with a cold fluid or surface. The working fluid transfers heat from the hot end to the cold end.

### Applications of heat exchangers

Heat exchangers are widely used in various industries, such as:

Power generation: Heat exchangers are used to convert thermal energy into mechanical or electrical energy, such as in steam turbines, gas turbines, nuclear reactors, and solar panels.

Chemical processing: Heat exchangers are used to control the temperature and phase of chemical reactions, such as in distillation columns, reactors, and heat recovery systems.

Refrigeration: Heat exchangers are used to transfer heat from a low-temperature medium to a high-temperature medium, such as in refrigerators, freezers, air conditioners, and heat pumps.

Aerospace: Heat exchangers are used to cool or heat the air and fuel in aircraft engines, rockets, satellites, and space vehicles.

## The Advantages of Compact Heat Exchangers

### What is a compact heat exchanger?

A compact heat exchanger is a heat exchanger that has a high ratio of heat transfer surface area to volume. There is no universal definition of what constitutes a compact heat exchanger, but a common criterion is that the ratio should be greater than 700 m/m. Compact heat exchangers can achieve this ratio by using small-diameter tubes, thin plates, or extended fins. Compact heat exchangers can also be classified into two types: primary surface and secondary surface. Primary surface compact heat exchangers have only one type of surface for heat transfer, such as tubes or plates. Secondary surface compact heat exchangers have two types of surfaces for heat transfer, such as fins or louvers.

### Benefits of compact heat exchangers

Compact heat exchangers can offer significant advantages over conventional heat exchangers, such as:

Higher heat transfer rates: Compact heat exchangers have higher surface area per unit volume, which means that they can transfer more heat per unit volume. This also means that they can achieve higher temperature differences between the fluids or surfaces.

Lower pressure drops: Compact heat exchangers have smaller hydraulic diameters, which means that they have lower resistance to fluid flow. This also means that they require less pumping power and reduce the risk of leakage.

Smaller sizes: Compact heat exchangers have smaller volumes, which means that they occupy less space. This also means that they can fit into tighter spaces and reduce the weight and cost of the system.

Lower costs: Compact heat exchangers have lower material and fabrication costs, which means that they are more economical. This also means that they can reduce the operating and maintenance costs of the system.

### Challenges of compact heat exchangers

However, compact heat exchangers also pose some challenges, such as:

Higher complexity: Compact heat exchangers have more complicated geometries and configurations, which means that they are more difficult to design and manufacture. This also means that they require more advanced tools and techniques for analysis and optimization.

Lower reliability: Compact heat exchangers have thinner walls and smaller gaps, which means that they are more prone to failure and damage. This also means that they require more careful inspection and testing for quality assurance.

Higher fouling potential: Compact heat exchangers have smaller passages and higher velocities, which means that they are more susceptible to fouling and corrosion. This also means that they require more frequent cleaning and protection for performance preservation.

## The Design and Analysis of Compact Heat Exchangers

### Design criteria for compact heat exchangers

The design of compact heat exchangers involves selecting the appropriate geometry, configuration, material, and operating conditions for a given application. The design criteria for compact heat exchangers depend on the objectives and constraints of the system, such as:

Heat duty: This is the amount of heat that needs to be transferred between the fluids or surfaces.

Temperature difference: This is the difference between the inlet and outlet temperatures of the fluids or surfaces.

Pressure drop: This is the loss of pressure due to fluid friction and flow resistance in the heat exchanger.

Size and weight: This is the physical dimensions and mass of the heat exchanger.

Cost and performance: This is the trade-off between the initial and operating expenses and the effectiveness and efficiency of the heat exchanger.

The design criteria for compact heat exchangers are often conflicting and require optimization techniques to find the best compromise. For example, increasing the surface area can increase the heat transfer rate but also increase the pressure drop and the cost. Similarly, decreasing the hydraulic diameter can decrease the pressure drop but also decrease the heat transfer coefficient.

### Analysis methods for compact heat exchangers

The analysis of compact heat exchangers involves calculating the thermal and hydraulic performance of a given geometry, configuration, material, and operating conditions. The analysis methods for compact heat exchangers depend on the type of flow, phase change, and reaction involved. Some common methods are:

LMTD method: This method uses the logarithmic mean temperature difference (LMTD) between the hot and cold fluids or surfaces to calculate the overall heat transfer coefficient and the heat duty of the heat exchanger. The LMTD method is simple and intuitive but requires a prior knowledge of the inlet and outlet temperatures of both fluids or surfaces.

ε-NTU method: This method uses the effectiveness (ε) and the number of transfer units (NTU) to calculate the heat duty and the outlet temperatures of both fluids or surfaces. The effectiveness is defined as the ratio of the actual heat transfer rate to the maximum possible heat transfer rate. The NTU is defined as the product of the overall heat transfer coefficient and the surface area divided by the minimum heat capacity rate. The ε-NTU method is more general and flexible than the LMTD method but requires a prior knowledge of either one outlet temperature or one inlet temperature difference.

P-NTU method: This method is a variant of the ε-NTU method that was specifically developed for shell-and-tube heat exchangers. The P-NTU method uses a correction factor (P) to account for the deviation from ideal counterflow or parallel flow due to shell-side leakage, bypass, and crossflow. The P-NTU method can also be applied to other types of heat exchangers with complex flow patterns.

### Examples of compact heat exchanger geometries

There are many possible geometries for compact heat exchangers, depending on the shape, size, and arrangement of the heat transfer surfaces. Some examples are:

Plate-fin: This geometry consists of a stack of flat plates with fins attached to them. The fins can have different shapes, such as rectangular, triangular, wavy, or louvered. The plates and fins can be brazed or diffusion bonded together to form a solid block. The fluids flow in alternate channels formed by the plates and fins.

Printed circuit: This geometry consists of a stack of metal sheets with etched or stamped channels on them. The channels can have different shapes, such as circular, rectangular, or zigzag. The sheets are diffusion bonded together to form a solid block. The fluids flow in alternate channels formed by the sheets.

Formed plate: This geometry consists of a stack of metal plates with formed channels on them. The channels can have different shapes, such as sinusoidal, chevron, or dimpled. The plates are welded or brazed together to form a solid block. The fluids flow in alternate channels formed by the plates.

Microchannel: This geometry consists of a bundle of small-diameter tubes or a matrix of microchannels on a substrate. The tubes or microchannels can have different shapes, such as circular, rectangular, or triangular. The bundle or matrix is enclosed in a shell or a casing. The fluids flow inside and outside the tubes or microchannels.

## The Book by Kays and London on Compact Heat Exchangers

### Who are Kays and London?

Kays and London are two prominent researchers and authors in the field of heat transfer and compact heat exchangers. William Morrow Kays was a professor of mechanical engineering at Stanford University from 1946 to 1987. He was also the director of the Thermosciences Division and the High Temperature Gasdynamics Laboratory at Stanford. He made significant contributions to the theory and practice of heat transfer, fluid mechanics, gas dynamics, and combustion. He received many honors and awards for his work, such as the ASME Heat Transfer Memorial Award and the AIAA Thermophysics Award. He passed away in 2012 at the age of 94.

Alexander Louis London was a professor of mechanical engineering at Stanford University from 1950 to 1989. He was also the associate director of the Thermosciences Division and the High Temperature Gasdynamics Laboratory at Stanford. He made significant contributions to the theory and practice of heat transfer, fluid mechanics, gas dynamics, and thermodynamics. He received many honors and awards for his work, such as the ASME Heat Transfer Memorial Award and the AIAA Thermophysics Award. He passed away in 2008 at the age of 88.

### What is the book about?

The book by Kays and London on compact heat exchangers is titled Compact Heat Exchangers (Third Edition) and was published by Krieger Publishing Company in 1998. It is a comprehensive reference book that compiles experimental data on the basic heat transfer and flow friction characteristics of compact heat exchangers. The data can be applied to space heating, spacecraft heat exchangers, aircraft heat exchangers, and various cooling systems.

The book covers topics such as:

Introduction: This chapter gives an overview of compact heat exchangers and their applications.

Heat Exchanger Thermal and Pressure-Drop Design: This chapter gives an introduction to compact heat exchanger design and analysis using the LMTD method and the ε-NTU method.

The Transient Response of Heat Exchangers: This chapter gives an introduction to compact heat exchanger transient response using lumped parameter models and distributed parameter models.

The Basic Data for Various Types of Compact Heat Exchanger Surfaces: This chapter gives detailed data on the heat transfer coefficients and friction factors for various types of compact heat exchanger surfaces, such as plate-fin surfaces, tube-fin surfaces, plate surfaces, tube surfaces, wire-screen surfaces, etc.

The Use of Basic Data for Compact Heat Exchanger Design: This chapter gives examples of how to use the basic data for compact heat exchanger design and optimization.

Boiling and Condensing in Compact Heat Exchangers: This chapter gives an introduction to compact heat exchanger design and analysis for boiling and condensing flows.

Heat Exchangers with Endothermic or Exothermic Reaction: This chapter gives an introduction to compact heat exchanger design and analysis for endothermic or exothermic reaction.

### Why is the book important?

The book by Kays and London on compact heat exchangers is important because it is one of the first and most authoritative books on the subject. It provides a wealth of experimental data and theoretical solutions for compact heat exchanger design and analysis. It also covers a wide range of applications and geometries for compact heat exchangers. The book is still relevant and useful today, as compact heat exchangers are increasingly used in various industries and systems that require high efficiency, performance, and safety.

## Conclusion

In this article, we have explored the basics of heat exchangers, the advantages of compact heat exchangers, the design and analysis of compact heat exchangers, and the book by Kays and London on compact heat exchangers. We have learned that compact heat exchangers are devices that transfer heat between two or more fluids or solid surfaces at different temperatures with a high ratio of surface area to volume. We have learned that compact heat exchangers can offer significant benefits over conventional heat exchangers, such as higher heat transfer rates, lower pressure drops, smaller sizes, lighter weights, and lower costs. We have also learned that compact heat exchangers pose some challenges, such as higher complexity, lower reliability, and higher fouling potential. We have learned that the design and analysis of compact heat exchangers involve selecting the appropriate geometry, configuration, material, and operating conditions for a given application and calculating the thermal and hydraulic performance using methods such as the LMTD method or the ε-NTU method. We have also learned that Kays and London are two prominent researchers and authors in the field of heat transfer and compact heat exchangers who wrote a comprehensive reference book on the subject in 1998.

We hope that this article has given you a better understanding of what compact heat exchangers are and why they matter. If you are interested in learning more about compact heat exchangers, we recommend that you read the book by Kays and London or consult other sources listed in the references section.

## References

Kays, W.M., London, A.L., Compact Heat Exchangers (Third Edition), Krieger Publishing Company, 1998.

Shah, R.K., Sekulic, D.P., Fundamentals of Heat Exchanger Design, John Wiley & Sons, 2003.

Hesselgreaves, J.E., Compact Heat Exchangers: Selection, Design and Operation, Pergamon Press, 2001.

Sekulic, D.P., Compact Heat Exchanger Design. In: Faghri A., Gupta A.K., Khair K.R., Viskanta R. (eds) Handbook of Thermal Science and Engineering. Springer Reference. Springer International Publishing AG 2018.

Webbusterz Engineering Software. What is a compact heat exchanger and what do we use it for? Retrieved from https://www.webbusterz.org/compact-heat-exchanger/

## Frequently Asked Questions

What is the difference between a conventional heat exchanger and a compact heat exchanger?A conventional heat exchanger is a heat exchanger that has a low ratio of surface area to volume (less than 700 m/m). A compact heat exchanger is a heat exchanger that has a high ratio of surface area to volume (greater than 700 m/m). Compact heat exchangers can transfer more heat per unit volume than conventional heat exchangers.

What are some examples of applications of compact heat exchangers?Some examples of applications of compact heat exchangers are space heating, spacecraft heat exchangers, aircraft heat exchangers, refrigeration systems, power generation systems, chemical processing systems, aerospace systems, etc.

What are some advantages of compact heat exchangers?Some advantages of compact heat exchangers are higher heat transfer rates, lower pressure drops, smaller sizes, lighter weights, lower costs, etc.

What are some challenges of compact heat exchangers?Some challenges of compact heat exchangers are higher complexity, lower reliability, and higher fouling potential.Some challenges of compact heat exchangers are:

Higher complexity: Compact heat exchangers have more complicated geometries and configurations, which means that they are more difficult to design and manufacture. They also require more advanced tools and techniques for analysis and optimization.

Lower reliability: Compact heat exchangers have thinner walls and smaller gaps, which means that they are more prone to failure and damage. They also require more careful inspection and testing for quality assurance.

Higher fouling potential: Compact heat exchangers have smaller passages and higher velocities, which means that they are more susceptible to fouling and corrosion. They a