Chapter 1
Introduction to Process Heat Transfer

Introduction

The science of thermodynamics deals with the quantitative transitions and rearrangements of energy as heat in bodies of matter. Heat transfer is the science which deals with the rates of exchange of heat between hot and cold bodies called the source and receiver, respectively. The equipment employed to bring about this heat exchange is referred to as a heat exchanger.

Perhaps the simplest example of the various types of heat exchangers is the double pipe heat exchanger, a unit that will receive extensive treatment in Part II, Chapter 6. A simple diagram representing this exchanger is provided in Figure 1.1. This unit consists of two concentric pipes. Each of the two fluids - hot and cold - flow either through the inside of the inner pipe or through the annulus formed between the outside of the inner pipe and the inside of the outer pipe. Generally, it is more economical (from heat efficiency and design perspectives) for the hot fluid (the source) to flow-through the inner pipe and the cold fluid (the receiver) through the annulus, thereby reducing heat losses from the hot fluid to the surroundings.

Figure 1.1 Line diagram of a cocurrent flow heat exchanger.

Fundamentally, a temperature difference between the two bodies in close proximity (or between two parts of the same body) results in heat flow from higher to lower temperatures. There are three different (and classic) mechanisms by which this heat transfer can occur: conduction, convection, and radiation. When the heat transfer is the result of molecular motion (e.g., the vibrational energy of molecules in a solid being passed along from molecule to molecule), the mechanism of transfer is conduction. When heat transfer results from macroscopic motion, such as currents in a fluid, the mechanism is convection. When heat is transferred by electromagnetic waves, radiation is the mechanism. In most industrial applications, multiple mechanisms are usually involved in the transmission of heat. However, since each mechanism is governed by its own set of physical laws, it is beneficial to discuss them independently of each other (see Chapters 2-4).

This introductory chapter consists of the following eight sections:

1. 1.1 Units and Dimensional Analysis
2. 1.2 Key Physical Properties
3. 1.3 Key Process Variables and Concepts
4. 1.4 Laws of Thermodynamics
5. 1.5 Heat-related Theories and Transfer Mechanisms
6. 1.6 Fluid Flow and Pressure Drop Considerations
7. 1.7 Environmental Considerations
8. 1.8 Process Heat Transfer

1.1 Units and Dimensional Analysis

This section is primarily concerned with units. The units used in the text are consistent with those adopted by the engineering profession in the United States [1-3]. One usually refers to them as English or engineering units. Since engineers are often concerned with units and conversion of units, both the English and SI (International System of Units) units are used throughout the book. All quantities of the physical and chemical properties to be discussed are expressed using either or both of these two systems. Although the English (engineering) system of units is primarily employed in this text (Kern originally used engineering units), a discussion on the metric and SI system of units is warranted. Background histories are provided in the next two subsections.

The Metric System [1, 2]. The need for a single worldwide coordinated measurement system was recognized nearly 350 years ago. In 1670, Gabriel Mouton, Viscar of St. Paul's church in Lyon, proposed a comprehensive decimal measurement system based on the length of one minute of arc of a great circle of the Earth. In 1671, Jean Picard, a French astronomer, proposed that the period of an instrument called a "seconds pendulum" be precisely two seconds - thereby denoting that one unit of time, a second, be one swing of the seconds pendulum. (Such a pendulum would have been fairly easy to reproduce, thus facilitating the widespread distribution of uniform standards.) Other proposals were made, but over a century elapsed before any action was taken.

In 1790, in the midst of the French Revolution, the National Assembly of France requested the French Academy of Sciences to "deduce an invariable standard for all the measures and weights." The Commission appointed by the Academy created a system that was, at once, simple and scientific. The unit of length was to be a portion of the Earth's circumference. Measures for capacity (volume) and mass (weight) were to be derived from the unit of length, thus relating the basic units of the system to each other and to nature. Furthermore, the larger and smaller versions of each unit were to be created by multiplying or dividing the basic units by 10 and its multiples. This feature provided a great convenience to users of the system by eliminating the need for such calculations as dividing by 16 (to convert ounces to pounds) or by 12 (to convert inches to feet). Similar calculations in the metric system could be performed simply by shifting the decimal point. Thus, the metric system is a base-10 or decimal system.

The Commission assigned the name metre (which is now spelled meter) to the unit of length. This name was derived from the Greek word metron meaning "a measure." The physical standard representing the meter was to be constructed so that it would equal one ten-millionth of the distance from the north pole to the equator along the meridian of the Earth running near Dunkirk in France and Barcelona in Spain.

The metric unit of mass, called the gram, was defined as the mass of one cubic centimeter (a cube that is 1/100 of a meter on each side) of water at its temperature of maximum density. The cubic decimeter (a cube 1/10 of a meter on each side) was chosen as the unit of fluid capacity. This measure was given the name liter.

Although the metric system was not accepted with enthusiasm at first, adoption by other nations occurred steadily after France made its use compulsory in 1840. The standardized character and decimal features of the metric system made it well suited to scientific and engineering work. Consequently, it is not surprising that the rapid spread of the system coincided with an age of rapid technological development. In the United States, by Act of Congress in 1866, it was made "lawful throughout the United States of America to employ the weights and measures of the metric system in all contracts, dealings, or court proceedings."

By the late 1860s, even better metric standards were needed to keep pace with scientific advances. In 1875, an international treaty, the "Treaty of the Meter," set up well-defined metric standards for length and mass. The treaty also established permanent machinery to recommend and adopt further refinements in the metric system. This treaty, known as the Metric Convention, was signed by 17 countries, including the United States. As a result of the treaty, metric standards were constructed and distributed to each nation that ratified the Convention. Since 1893, the internationally agreed to metric standards have served as the fundamental weights and measures standards of the United States.

By 1900, a total of 35 nations - including the major nations of continental Europe and most of South America - had officially accepted the metric system. Today, with the exception of the United States and a few small countries, the entire world is predominantly using the metric system or is committed to such use. In 1971, the Secretary of Commerce, in transmitting to Congress the results of a 3-year study authorized by the Metric Study Act of 1968 recommended that the U.S. change to the use of the metric system through a coordinated national program.

The International Bureau of Weights and Measures located at Sevres, France, serves as a permanent secretariat for the Metric Convention, coordinating the exchange of information about the use and refinement of the metric system. As measurement science develops more precise and easily reproducible ways of defining the measurement units, the General Conference of Weights and Measures - the diplomatic organization made up of adherents to the Convention - meets periodically to ratify improvements in the system and the standards.

The SI System [1, 2]. In 1960, the General Conference adopted an extensive revision and simplification of the system. The name Le Systeme International d'Unites (International System of Units), with the international abbreviation SI, was adopted for this modernized metric system. Further improvements in and additions to SI were made by the General Conferences in 1964, 1968, and 1971.

The basic units in the SI system are the kilogram (mass), meter (length), second (time), Kelvin (temperature), ampere (electric current), candela (the unit of luminous intensity), and radian (angular measure). All are commonly used by the practicing engineer and scientist. The Celsius scale of temperature (0 °C = 273.15 K) is commonly used with the absolute Kelvin scale. The important derived units are the newton (SI unit of force), the joule (SI unit of energy), the watt (SI unit of power), the pascal (SI unit of pressure),...