Over the years machines of all kinds have been improved and made more reliable. However, machines typically operate as components of larger systems, such as transportation systems, communication systems, manufacturing systems, defense systems, health care systems, and so on. While many aspects of such systems can be and have been automated, the human operator is retained in many cases. This may be because of economics, tradition, cost, or (most likely) capabilities of the human to perceive patterns of information and weigh subtle factors in making control decisions which the machine cannot match.

Although the public as well as those responsible for system operation usually demand that there be a human operator, “human error” is a major reason for system failure. And aside from prevention of error, getting the best performance out of the system means that human and machine must be working together effectively — be properly “impedance matched.” Therefore, the performance capabilities of the human relative to those of the machine must be taken into account in system design.

Efforts to “optimize” the human-machine interaction are meaningless in the mathematical sense of optimization, since most important interactions between human and machine cannot be reduced to a mathematical form, and the objective function (defining what is good) is not easily obtained in any given context. For this reason, engineering the human-machine interaction, much as in management or medicine, remains an art more than a science, based on laboratory experiments and practical experience.

In the broadest sense, engineering the human-machine interface includes all of ergonomics or human factors engineering, and goes well beyond design of displays and control devices. Ergonomics includes not only questions of sensory physiology, whether or not the operator can see the displays or hear the auditory warnings, but also questions of biomechanics

, how the body moves, and whether or not the operator can reach and apply proper force to the controls. It further includes the fields of operator selection and training, human performance under stress, human factors in maintenance, and many other aspects of the relation of the human to technology. This section focuses primarily on human-machine interaction in control of systems.

The human-machine interactions in control are considered in terms of Figure 6.1.1. In Figure 6.1.1a the human directly controls the machine; i.e., the control loop to the machine is closed through the physical sensors, displays, human senses (visual, auditory, tactile), brain, human muscles, control devices, and machine actuators. Figure 6.1.1b illustrates what has come to be called a supervisory control system , wherein the human intermittently instructs a computer as to goals, constraints, and procedures, then turns a task over to the computer to perform automatic control for some period of time.

Displays and control devices can be analogic (movement signal directions and extent of control action, isomorphic with the world, such as an automobile steering wheel or computer mouse controls, or a moving needle or pictorial display element). Or they can be symbolic (dedicated buttons or generalpurpose keyboard controls, icons, or alarm light displays). In normal human discourse we use both speech (symbolic) and gestures (analogic) and on paper we write alphanumeric text (symbolic) and draw pictures (analogic). The system designer must decide which type of displays or controls best suits a particular application, and/or what mix to use. The designer must be aware of important criteria such as whether or not, for a proposed design, changes in the displays and controls caused by the human operator correspond in a natural and common-sense way to “more” or “less” of some variable as expected by that operator and correspond to cultural norms (such as reading from left to right in western countries), and whether or not the movement of the display elements correspond geometrically to movements of the controls.

Thermal design frequently aims at the most effective system from the cost viewpoint. Still, in the cost optimization process, particularly of complex energy systems, it is often expedient to begin by identifying a design that is nearly optimal thermodynamically; such a design can then be used as a point of departure for cost optimization. Presented in this section are guidelines for improving the use of fuels (natural gas, oil, and coal) by reducing sources of thermodynamic inefficiency in thermal systems. Further discussion is provided by Bejan et al. (1996).

To improve thermodynamic effectiveness it is necessary to deal directly with inefficiencies related to exergy destruction and exergy loss. The primary contributors to exergy destruction are chemical reaction, heat transfer, mixing, and friction, including unrestrained expansions of gases and liquids. To deal with them effectively, the principal sources of inefficiency not only should be understood qualitatively, but also determined quantitatively, at least approximately. Design changes to improve effectiveness must be done judiciously, however, for the cost associated with different sources of inefficiency can be different.

For example, the unit cost of the electrical or mechanical power required to provide for the exergy destroyed owing to a pressure drop is generally higher than the unit cost of the fuel required for the

exergy destruction caused by combustion or heat transfer.

Since chemical reaction is a significant source of thermodynamic inefficiency, it is generally good practice to minimize the use of combustion. In many applications the use of combustion equipment such as boilers is unavoidable, however. In these cases a significant reduction in the combustion irreversibility by conventional means simply cannot be expected, for the major part of the exergy destruction introduced by combustion is an inevitable consequence of incorporating such equipment. Still, the exergy destruction in practical combustion systems can be reduced by minimizing the use of excess air and by preheating the reactants. In most cases only a small part of the exergy destruction in a combustion chamber can be avoided by these means. Consequently, after considering such options for reducing the exergy destruction related to combustion, efforts to improve thermodynamic performance should focus on components of the overall system that are more amenable to betterment by cost-effective conventional measures. In other words, some exergy destructions and energy losses can be avoided, others cannot. Efforts should be centered on those that can be avoided.

Nonidealities associated with heat transfer also typically contribute heavily to inefficiency. Accordingly, unnecessary or cost-ineffective heat transfer must be avoided. Additional guidelines follow:

• The higher the temperature T at which a heat transfer occurs in cases where T > T0 , where T0 denotes the temperature of the environment (Section 2.5), the more valuable the heat transfer and, consequently, the greater the need to avoid heat transfer to the ambient, to cooling water, or to a refrigerated stream. Heat transfer across

T0 should be avoided.

• The lower the temperature T at which a heat transfer occurs in cases where T < T0 , the more valuable the heat transfer and, consequently, the greater the need to avoid direct heat transfer with the ambient or a heated stream.

• Since exergy destruction associated with heat transfer between streams varies inversely with the temperature level, the lower the temperature level, the greater the need to minimize the streamto-stream temperature difference.

• Avoid the use of intermediate heat transfer fluids when exchanging energy by heat transfer between two streams

Although irreversibilities related to friction, unrestrained expansion, and mixing are often secondary in importance to those of combustion and heat transfer, they should not be overlooked, and the following guidelines apply:

• Relatively more attention should be paid to the design of the lower temperature stages of turbines and compressors (the last stages of turbines and the first stages of compressors) than to the remaining stages of these devices.

• For turbines, compressors, and motors, consider the most thermodynamically efficient options.

• Minimize the use of throttling; check whether power recovery expanders are a cost-effective alternative for pressure reduction.

• Avoid processes using excessively large thermodynamic driving forces (differences in temperature, pressure, and chemical composition). In particular, minimize the mixing of streams differing significantly in temperature, pressure, or chemical composition.

• The greater the mass rate of flow, the greater the need to use the exergy of the stream effectively.

• The lower the temperature level, the greater the need to minimize friction. Flowsheeting or process simulation software can assist efforts aimed at improving thermodynamic effectiveness by allowing engineers to readily model the behavior of an overall system, or system components, under specified conditions and do the required thermal analysis, sizing, costing, and optimization. Many of the more widely used flowsheeting programs: ASPEN PLUS, PROCESS, and

CHEMCAD are of the sequential-modular type. SPEEDUP is a popular program of the equation-solver type. Since process simulation is a rapidly evolving field, vendors should be contacted for up-to-date information concerning the features of flowsheeting software, including optimization capabilities (if any). As background for further investigation of suitable software, see Biegler (1989) for a survey of the capabilities of 15 software products.

In combustion reactions, rapid oxidation of combustible elements of the fuel results in energy release as combustion products are formed. The three major combustible chemical elements in most common fuels are carbon, hydrogen, and sulfur. Although sulfur is usually a relatively unimportant contributor to the energy released, it can be a significant cause of pollution and corrosion.

The emphasis in this section is on hydrocarbon fuels, which contain hydrogen, carbon, sulfur, and possibly other chemical substances. Hydrocarbon fuels may be liquids, gases, or solids such as coal.

Liquid hydrocarbon fuels are commonly derived from crude oil through distillation and cracking processes.

Examples are gasoline, diesel fuel, kerosene, and other types of fuel oils. The compositions of liquid fuels are commonly given in terms of mass fractions. For simplicity in combustion calculations, gasoline is often considered to be octane, C8H18, and diesel fuel is considered to be dodecane, C12H26.

Gaseous hydrocarbon fuels are obtained from natural gas wells or are produced in certain chemical processes. Natural gas normally consists of several different hydrocarbons, with the major constituent being methane, CH4. The compositions of gaseous fuels are commonly given in terms of mole fractions.

Both gaseous and liquid hydrocarbon fuels can be synthesized from coal, oil shale, and tar sands. The composition of coal varies considerably with the location from which it is mined. For combustion calculations, the makeup of coal is usually expressed as an ultimate analysis giving the composition on a mass basis in terms of the relative amounts of chemical elements (carbon, sulfur, hydrogen, nitrogen, oxygen) and ash. Coal combustion is considered further in Chapter 8, Energy Conversion.

A fuel is said to have burned completely if all of the carbon present in the fuel is burned to carbon dioxide, all of the hydrogen is burned to water, and all of the sulfur is burned to sulfur dioxide. In practice, these conditions are usually not fulfilled and combustion is incomplete. The presence of carbon monoxide (CO) in the products indicates incomplete combustion. The products of combustion of actual combustion reactions and the relative amounts of the products can be determined with certainty only by experimental means. Among several devices for the experimental determination of the composition of products of combustion are the Orsat analyzer, gas chromatograph, infrared analyzer, and flame ionization detector. Data from these devices can be used to determine the makeup of the gaseous products of combustion. Analyses are frequently reported on a “dry” basis: mole fractions are determined for all gaseous products as if no water vapor were present. Some experimental procedures give an analysis including the water vapor, however.

Since water is formed when hydrocarbon fuels are burned, the mole fraction of water vapor in the gaseous products of combustion can be significant. If the gaseous products of combustion are cooled at constant mixture pressure, the dew point temperature (Section 2.3, Ideal Gas Model) is reached when water vapor begins to condense. Corrosion of duct work, mufflers, and other metal parts can occur when water vapor in the combustion products condenses.

Oxygen is required in every combustion reaction. Pure oxygen is used only in special applications such as cutting and welding. In most combustion applications, air provides the needed oxygen. Idealizations are often used in combustion calculations involving air: (1) all components of air other than oxygen (O2) are lumped with nitrogen (N2). On a molar basis air is then considered to be 21% oxygen and 79% nitrogen. With this idealization the molar ratio of the nitrogen to the oxygen in combustion air is 3.76; (2) the water vapor present in air may be considered in writing the combustion equation or ignored. In the latter case the combustion air is regarded as dry; (3) additional simplicity results by regarding the nitrogen present in the combustion air as inert. However, if high-enough temperatures are attained, nitrogen can form compounds, often termed NOX, such as nitric oxide and nitrogen dioxide.

Even trace amounts of oxides of nitrogen appearing in the exhaust of internal combustion engines can be a source of air pollution.

The minimum amount of air that supplies sufficient oxygen for the complete combustion of all the combustible chemical elements is the theoretical, or stoichiometic, amount of air. In practice, the amount of air actually supplied may be greater than or less than the theoretical amount, depending on the application. The amount of air is commonly expressed as the percent of theoretical air or the percent excess (or percent deficiency) of air. The air-fuel ratio and its reciprocal the fuel-air ratio, each of which can be expressed on a mass or molar basis, are other ways that fuel-air mixtures are described. Another is the equivalence ratio: the ratio of the actual fuel-air ratio to the fuel-air ratio for complete combustion with the theoretical amount of air. The reactants form a lean mixture when the equivalence ratio is less than unity and a rich mixture when the ratio is greater than unity.