A heat exchanger is a device that is designed to transfer thermal energy from one fluid to another. Heat exchangers are everywhere in our modern society. Nearly all thermal systems employ at least one and usually several heat exchangers. The background material related to conduction and convection, presented in Chapters 2 through 11, is required to analyze and design heat exchangers. Section 12.1 reviews the applications and types of heat exchangers that are commonly encountered. The subsequent sections provide the theory and tools required to predict and understand the performance of these devices.

Heat transfer is the term used to describe the movement of thermal energy (heat) from one place to another. Heat transfer drives the world that we live in. Look around. Heat transfer is at work no matter where you currently are.

Chapters 2 through 6 consider heat transfer in a stationary medium where energy transport occurs entirely by conduction and is governed by Fourier’s Law. Thus far, convection has been considered primarily as a boundary condition for these conduction problems. Convection refers to the transfer of energy that occurs between a surface and a moving medium, most often a liquid or gas flowing through a duct or over an object. Convection processes include fluid motion and the related energy transfer. The additional terms in an energy balance related to the fluid flow often dominate the now familiar energy transport by conduction. The presence of fluid motion complicates heat transfer problems substantially and links the heat transfer problem with an underlying fluid dynamics problem. The complete solution to most convection problems therefore requires sophisticated computational fluid dynamic (CFD) tools that are beyond the scope of this book.

Chapter 8 provides correlations that can be used to solve external flow forced convection problems where an external flow is defined as one where the boundary layer can grow without bound. For flow over a flat plate located sufficiently far from any other surface, the boundary layer is never confined by the presence of another object and therefore continues to grow from the leading edge to the trailing edge. An internal flow is defined as a flow situation where the growth of the boundary layer is confined; that is, the boundary layers can only grow to a certain thickness before being constrained. Internal flows are often encountered in engineering applications (e.g., the flow through tubes or ducts).

From a thermodynamic perspective, thermal energy can be transferred across a boundary (i.e., heat transfer can occur) by only two mechanisms: conduction and radiation. Conduction is the process in which energy exchange occurs due to the interactions of molecular (or smaller) scale energy carriers within a material. The conduction process is intuitive; it is easy to imagine energy carriers having a higher level of energy (represented by their temperature) colliding with neighboring particles and thereby transferring some of their energy to them. Radiation is a very different heat transfer process because energy is transferred without the involvement of any molecular interactions. Radiation energy exchange is related to electromagnetic waves and therefore can occur over long distances through a complete vacuum. For example, the energy that our planet receives from the Sun is a result of radiation exchange. This chapter presents an introduction to radiation heat transfer with a focus on providing methods for solving radiation problems.

Chapter 5 discussed transient problems in which the spatial temperature gradients within a solid object can be neglected and therefore the problem is approximately zero-dimensional (0-D). In these lumped capacitance transient problems the solution is a function only of time. Lumped capacitance problems essentially ignore the process of conduction as being unimportant. This chapter discusses transient problems where internal, spatial temperature gradients related to energy transfer by conduction are nonnegligible (i.e., the Biot number is not much less than unity). The first section provides some conceptual tools that are not exact, but can be used to develop an understanding of transient conduction problems. More sophisticated analytical and numerical solutions that provide more exact solutions are presented in the remaining sections.

Mass transfer occurs whenever fluid flows; that is, some mass is transferred from one place to another. However, the focus in this chapter is on the transport of one chemical species (or component) within a mixture that occurs as a direct result of a concentration gradient, independent of a pressure gradient. This type of mass transfer is called diffusion. A familiar example of diffusion mass transfer is the humidification process that occurs when an open container of water is allowed to sit out in a room. The gas in the room is a mixture of air (which is itself a mixture of oxygen, nitrogen, and other gases) and water vapor. The air–water mixture in contact with the surface of the liquid water is nearly saturated with water vapor and so it has a relatively high concentration of water vapor. The air–water mixture further from the liquid has a lower concentration of water vapor. Therefore, there is a concentration gradient that drives a mass transfer process causing water to be transported from the liquid surface to the air in the room, thereby humidifying it.