Chapter 1
HEAT EXCHANGE
EQUIPMENT
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INTRODUCTION
Prior to the 19th century, it was believed that the sense of how hot or cold an object felt was determined by how much "heat" it contained. Heat was envisioned as a liquid that flowed from a hotter to a colder object; this weightless fluid was called "caloric", and until the writings of Joseph Black (1728-1799), no distinction was made between heat and temperature. Black distinguished between the quantity (caloric) and the intensity (temperature) of heat. Benjamin Thomson,
Count Rumford, published a paper in 1798 entitled "An Inquiry Concerning the Source of Heat which is Excited by Friction". Rumford had noticed the large amount of heat generated when a cannon was drilled. He doubted that a material substance was flowing into the cannon and concluded "it appears to me to be extremely difficult if not impossible to form any distinct idea of anything capable of being excited and communicated in the manner the heat was excited and communicated in these experiments except motion"
But it was not until J. P. Joule published a definitive paper in 1847 that the caloric idea was abandoned. Joule conclusively showed that heat was a form of energy. As a result of the experiments of Rumford, Joule, and others, it was demonstrated (explicitly stated by Helmholtz in 1847), that the various forms of energy can be transformed one into another.
When heat is transformed into any other form of energy, or when other forms of energy are transformed into heat, the total amount of energy (heat plus other forms) in the system is constant. This is known as the first law of thermodynamics, i.e., the conservation of energy. To express it another way: it is in no way possible either by mechanical, thermal, chemical, or other means, to obtain a perpetual motion machine; i.e., one that creates its own energy.
A second statement may also be made about how machines operate. A steam engine uses a source of heat to produce work. Is it possible to completely convert the heat energy into work, making it a 100% efficient machine? The answer is to be found in the second law of thermodynamics: No cyclic machine can convert heat energy wholly into other forms of energy. It is not possible to construct a cyclic machine that does nothing, but withdraw heat energy and convert it into mechanical energy. The second law of thermodynamics implies the irreversibility of certain processes - that of converting all heat into mechanical energy, although it is possible to have a cyclic machine that does nothing but convert mechanical energy into heat.
Sadi Carnot (1796-1832) conducted theoretical studies of the efficiencies of heat engines (a machine which converts some of its heat into useful work). He was trying to model the most efficient heat engine possible. His theoretical work provided the basis for practical improvements in the steam engine and also laid the foundations of thermodynamics. He described an ideal engine, called the Carnot engine, that is the most efficient way an engine can be constructed. He showed that the efficiency of such an engine is given by:
efficiency = 1 - T"/T'
where the temperatures, T' and T", are the cold and hot "reservoirs", respectively, between which the machine operates. On this temperature scale, a heat engine whose coldest reservoir is zero degrees would operate with 100% efficiency. This is one definition of absolute zero. The temperature scale is called the absolute, the thermodynamic , or the kelvin scale.
The way, that the gas temperature scale and the thermodynamic temperature scale are shown to be identical, is based on the microscopic interpretation of temperature, which postulates that the macroscopic measurable quantity called temperature, is a result of the random motions of the microscopic particles that make up a system.
About the same time that thermodynamics was evolving, James Clerk Maxwell (1831-1879) and Ludwig Boltzmann (1 844- 1906) developed a theory, describing the way molecules moved - molecular dynamics. The molecules that make up a perfect gas move about, colliding with each other like billiard balls and bouncing off the surface of the container holding the gas. The energy, associated with motion, is called Kinetic Energy and this kinetic approach to the behavior of ideal gases led to an interpretation of the concept of temperature on a microscopic scale.
The amount of kinetic energy each molecule has is a function of its velocity; for the large number of molecules in a gas (even at low pressure), there should be a range of velocities at any instant of time. The magnitude of the velocities of the various particles should vary greatly; no two particles should be expected to have the exact same velocity. Some may be moving very fast; others - quite slowly.
Maxwell found that he could represent the distribution of velocities statistically by a function, known as the Maxwellian distribution. The collisions of the molecules with their container gives rise to the pressure of the gas. By considering the average force exerted by the molecular collisions on the wall, Boltzmann was able to show that the average kinetic energy of the molecules was directly comparable to the measured pressure, and the greater the average kinetic energy, the greater the pressure.
From Boyles' Law, it is known that the pressure is directly proportional to the temperature, therefore, it was shown that the kinetic energy of the molecules related directly to the temperature of the gas. A simple thermodynamic relation holds for this:
average kinetic energy of molecules = 3kT/2
where k is the Boltzmann constant. Temperature is a measure of the energy of thermal motion and, at a temperature of zero, the energy reaches a minimum (quantum mechanically, the zero-point motion remains at 0 'K).
About 1902, J. W. Gibbs (1839-1903) introduced statistical mechanics with which he demonstrated how average values of the properties of a system could be predicted from an analysis of the most probable values of these properties found from a large number of identical systems (called an ensemble). Again, in the statistical mechanical interpretation of thermodynamics, the key parameter is identified with a temperature, which can be directly linked to the thermodynamic temperature, with the temperature of Maxwell's distribution, and with the perfect gas law.
Temperature becomes a quantity definable either in terms of macroscopic thermodynamic quantities, such as heat and work, or, with equal validity and identical results, in terms of a quantity, which characterized the energy distribution among the particles in a system. With this understanding of the concept of temperature, it is possible to explain how heat (thermal energy) flows from one body to another.
Thermal energy is carried by the molecules in the form of their motions and some of it, through molecular collisions, is transferred to molecules of a second object, when put in contact with it. This mechanism for transferring thermal energy is called conduction.
A second mechanism of heat transport is illustrated by a pot of water set to boil on a stove - hotter water closest to the flame will rise to mix with cooler water near the top of the pot. Convection involves the bodily movement of the more energetic molecules in a liquid or gas. The third way, that heat energy can be transferred from one body to another, is by radiation; this is the way that the sun warms the earth. The radiation flows from the sun to the earth, where some of it is absorbed, heating the surface.
These historical and fundamental concepts form the foundation for the design, applications, and operations of a major class of equipment that are used throughout the chemical process industries - heat exchange equipment, or heat exchangers. There are many variations of these equipment and a multitude of applications. However, the design configurations for these equipment are universal, meaning that they generally are not specific to a particular industry sector. In the United States in 1998, the chemical process industries (CPI) invested more than $700 million in capital equipment related to heat transfer.
Much of that investment was driven by a growing body of environmental legislation, such as the U.S. Clean Air Act Amendments. The use of vent condensers, for example, which use heat exchangers to reduce the volume of stack emissions, is increasing. Heat exchanger makers have responded to growing environmental concerns over fugitive emissions, as well by developing a new breed of leak-tight heat exchangers, designed to keep process fluids from leaking and volatile organic compounds from escaping to the atmosphere.
Gasketed exchangers are benefitting from improvements in the quality and diversity of elastomer materials and gasket designs. The use of exchangers with welded connections, rather than gaskets, is also reducing the likelihood of process fluid escape. Throughout the 1990's, the use of heat exchangers has expanded into non-traditional applications. This, coupled with a variety of design innovations, has given chemical engineers a wider variety of heat exchanger options to choose from than ever before. Operating conditions, ease of access for inspection and maintenance, and compatibility with process fluids are just some of the variables CPI engineers must consider when assessing heat exchanger options. Other factors include: maximum design pressure and temperature, heating or cooling applications, maintenance requirements, material compatibility with process fluids, gasket compatibility with process fluids, cleanliness of the streams, and temperature approach. This chapter provides an overview of the most commonly employed equipment. Emphasis is given to practical features of these systems, and typical examples of industrial applications are discussed.
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