Proficiency in technical writing requires practice. But the effort is worth- while because good communications skills are a prerequisite for success in engineering and in nearly every other field. Further discussion of technical report writing relating specifically to thermal system design is provided in the literature [ 10,
Process Flow Sheets (Flow Diagrams). As indicated in Figure 1.2, the third stage of the design process culminates in the final flow sheets (also known as process flow diagrams). Process flow sheets, analogous to the circuit diagrams of electrical engineering, are conventional and convenient ways to represent process concepts. In a rudimentary form the flow sheet may be no more than a block diagram showing inputs and outputs. More detailed flow sheets include the general types of major equipment required: pressure ves- sels, compressors, heat exchangers, and so on. Complete flow sheets provide details on the devices to be used, operating temperatures and pressures at key locations, compositions of flow streams, utilities, feed and product stream details, and other important data. As every language, including the language of mathematics, has rules and conventions to foster communication, so does flow sheet preparation. There are standard symbols for various types of equip- ment and conventions for numbering equipment and designating utility and process streams. Special symbols or conventions are commonly employed to label the temperature, pressure, and chemical composition at various loca- tions. Further details concerning flow sheet preparation are provided else- where
Other The life-cycle design of a thermal system normally involves documentation in addition to what already has been discussed. This documentation includes, for instance, construction drawings, installation instructions, operating instructions (including how to operate the system in its normal service range and various operating modes: startup, standby, shutdown, and emergency), information about how to cope with equipment failure, diagnostic and maintenance procedures, quality control and qual- ity assurance procedures, and retirement instructions (procedures for decommissioning and disposing of the system). in all, the docurnentation required is considerable and underscores the value to industry of engineers with good communication skills.
1.4 THERMAL SYSTEM DESIGN ASPECTS
An integrated, well-structured design process engaging a design team with a broad range of experience and featuring good communications is an approach that can be recommended generally and is not limited to the thermal systems serving as the present focus. Accordingly, much of the discussion of Section
16 INTRODUCTION TO THERMAL SYSTEM DESIGN
1.3 applies to engineering design of all kinds. In the present section we take a closer look at some key design aspects within the context of thermal sys- tems.
1.4.1 Environmental Aspects
Compliance with governmental environmental regulations has customarily featured an end-of-the-pipe approach that addresses mainly the pollutants emitted from stacks, ash from incinerators, thermal pollution, and so on. In- creasing attention is being given today to what goes into the pipe, however.
This is embodied in the concept of design for the environment (DFE), in which the environmentally preferred aspects of a system are treated as design objectives rather than as constraints.
In DFE, designers are called on to anticipate negative environmental im- pacts throughout the life cycle and engineer them out. In particular, efforts are directed to reducing the creation of waste and to managing materials better, using methods such as changing the process technology and/or plant operation, replacing input materials known to be sources of toxic waste with more benign materials, and doing more in-plant recycling. Concurrent design with its multifaceted approach is well suited for considering environmental objectives at every decision level. Moreover, the quality function deployment design strategy naturally allows for environmental quality to be one of many quality factors taken into account.
Compliance with environmental regulations should be considered through- out the design process and not deferred to the end when options might be foreclosed owing to earlier decisions. Addressing such regulations early may result in fundamentally better process choices that reduce the size of the required cleanup. Costs to control pollution are generally much higher if left for resolution after the facility has begun operation. Still, under the best of circumstances some end-of-the-pipe cleanup might be required to meet fed- eral, state or local environmental regulations. The cost of this may be con- siderable.
Design engineers should keep current on what is legally required by the federal EPA (Environmental Protection Agency) and OSHA (Occupational Safety and Health Administration) and corresponding state and local regula- tory groups. An important reporting requirement is the formulation of an environmental impact providing a full disclosure of project features likely to have an adverse environmental effect. The report includes the iden- tification of the specific environmental standards that require compliance by the project, a summary of all anticipated significant effluents and emissions, and the specification of possible alternative means to meet standards.
Specification of appropriate pollution control measures necessitates con- sideration of the type of pollutant being controlled and the features of the available control equipment. The size of the equipment needed is generally related to the quantity of pollutant being handled, and so equipment costs can be reduced by decreasing the volume of effluents. Depending on the nature
1.4 THERMAL SYSTEM DESIGN ASPECTS
of the processes taking place in the system, several types of pollution control may be needed: air, water, thermal, solid waste, and noise pollution.
Air pollution control equipment falls into two general types: particulate removal by mechanical means, such as cyclones, filters, scrubbers, and pre- cipitators, and gas component removal by chemical and physical means, in- cluding absorption, adsorption, condensation, and incineration. For liquid waste effluents, physical, chemical, and biological waste treatment measures can be used. To avoid costly waste treatment facilities or reduce their cost, it is advisable to consider the recovery of valuable liquid-borne proclucts prior to waste treatment. Thermal pollution resulting from the direct discharge of warm water into lakes, rivers, and streams is commonly ameliorated by cool- ing towers, cooling ponds, and spray ponds.
Solid wastes can be handled by incineration, pyrolysis, and removal to a sanitary landfill adhering to state-of-the-art waste management practices. As for liquid wastes, it is advisable to recover valuable substances from the solid waste before treating it. Coupling waste incineration with steam or hot water generation may provide an economic benefit. For effective and practical noise control it is necessary to understand the individual equipment and process noise sources, their acoustic properties and characteristics, and how they in- teract to cause the overall noise problem.
1.4.2 Safety and Reliability
Safety should be designed in from the beginning of the life-cycle design process. As for environmental considerations, the concurrent design approach is well suited for considering safety at every decision level, and quality func- tion deployment naturally allows safety to be one of the quality factors taken into account.
The service life of a system will not be trouble free and the occasional failure of some piece of equipment is likely. The design team is responsible for anticipating such events and designing the system so that a local failure cannot mushroom into an overall system failure or even disaster. A tolerance to failure is an important feature of every system. One approach for instilling such a tolerance involves testing the response of each component via com- puter simulation at extreme conditions that are not part of the normal oper- ating plan.
Personnel safety is an area where there can be no compromise. Safety studies should be undertaken throughout the design process. Deferring safety issues to the end is unwise because decisions might have been made earlier that foreclose effective alternatives. Hazards have to be anticipated and dealt with; exposure to toxic materials should be prevented or minimized; machin- ery must be guarded with protective devices and placarded against unsafe uses; and first-aid and medical services must be planned and available when needed. The design team should use safety checklists for identifying hazards that are provided in the literature [ 1, The design team must be aware of
18 INTRODUCTION TO THERMAL SYSTEM DESIGN
the requirements of the federal Occupational Safety and Health Act and ap- plicable state and local requirements.
Published codes and standards must also be considered. For some types of systems both the required design calculations and performance levels to be achieved are specified in design codes and standards promulgated by govern- ment, professional societies, and manufacturing associations. The
(American Society of Mechanical Engineers) Test Codes are well-known examples. The NRC (Nuclear Regulatory Commission) regula- tions apply to nuclear power generation technologies. The U.S. armed forces standard MIL-STD 882B (System Safety Program Requirements) aims at en- suring safety in military equipment. For shell-and-tube heat exchangers there are the TEMA (Tubular Exchanger Manufacturers Association) and APA (American Petroleum Association) standards. The ANSI (American National Standards Institute) standards may also be applicable. At the outset of the design process it is advisable that relevant codes be identified and accessed for subsequent use.
Reliability is a crucially important feature of systems and products of all kinds. As for other qualities discussed, reliability must be designed in from the beginning and considered at each decision level. Reliability is closely related to maintainability and availability. Reliability is the probability that a system will successfully perform specified functions for specified environ- mental conditions over a prescribed operating life. Since no system will ex- hibit absolute reliability, it is important that the system can be repaired and maintained easily and economically and within a specified time period. Main- tainability is the probability that these features will be exhibited by the sys- tem. Availability is a measure of how often a system will be, or was, available (operational) when needed. Reliability, availability, and maintainability (RAM) contribute importantly to determining the overall system cost. Further discussion of these important design qualities is found elsewhere Of particular interest to thermal systems design is UNIRAM, a personal computer software package that has been developed to perform RAM analyses of ther- mal systems
Ideally, a system is designed so that its performance is insensitive to factors out of the control of designers: external factors such as ambient temperature and air quality, internal factors such as wear, and imperfections related to manufacturing and assembly. The relative influence of different internal and external factors on reliability can sometimes be investigated via computer simulations, but in many cases an experimental approach is required. As tra- ditional experimental approaches are often time and costly, more effective testing strategies have been developed for determining statistically how systems may be designed to allow them to be robust (reliable) in the face of disturbances, variations, and uncertainties. Such methods, including the popular methods aim to achieve virtually flawless perform- ance economically.
Each piece of equipment of a thermal system should be specified to carry out its intended function. Still, to perform reliably, protect from uncertainty
1.4 THERMAL SYSTEM DESIGN ASPECTS
in the design data, and allow for increases in capacity, reasonable
tors are usually applied. It is important to engineer on the safe side; but an indiscriminate application of safety factors is not good practice because it can result in so much overdesign that the system becomes uneconomical.
1.4.3 Background and Data Sources
Design engineers should make special efforts to keep current on advances in their fields and allied fields. This includes reading the technical literature, attending industrial expositions and professional society meetings, and de- veloping a network of professional contacts with individuals having kindred interests. Considerable background information and data normally exist for just about every type of design situation. The difficulty is to locate such material and have it available when needed. Both private and public sources of information and data may be available to support a design project.
Private sources of information consist of the usually closely guarded pro- prietary information accumulated by individual companies. Little of this is ever made available to outsiders. The project file containing correspondence leading up to the formulation of the primitive design problem is an example of a private source of information. Companies also may have a collection of files, reports, and data compilations related to the project under consideration.
Special questionnaires soliciting input about the project may be available from respondents both inside and outside the company. Internal design standards may fix limits on the choices that can be made throughout the design process.
Personal contact with experts within the company is especially recommended, for a wealth of useful information often can be obtained simply by asking.
Public sources of information include the open technical literature. This potentially rich source should be explored, but owing to the rapid growth of the literature it is increasingly difficult to search effectively for specific types of information. This has given rise to commercial online databases as a way to facilitate searches. A database is an organized collection of information on particular topics. An online database is one accessible by computer, normally for a fee. A survey of online databases relevant to thermal system design is provided in Reference 15. Included in the survey are thermal property data- bases and databases on environmental protection, safety and health, and pat- ents.
Handbooks are another valuable source of information 16- Textbooks can also provide background information and data. Review articles and arti- cles describing current technology are published in technical periodicals such as Energy-The International Journal, Chemical Engineering Progress, RAE Transactions, Chemical Engineering, Hydrocarbon Processing, and Power. Considerable useful information often can be obtained by contacting the authors of pertinent recent publications.
The Thomas Register [ provides an exhaustive listing of manufactured items. An excellent way to obtain information on components and materials is to locate sources from the Thomas Register and telephone company
INTRODUCTION TO THERMAL SYSTEM DESIGN
resentatives. Design engineers often spend considerable time on the telephone with vendor sales representatives or engineering departments to obtain current performance and cost information.
The patent literature is a potentially high payoff public source of infor- mation. Though frequently underutilized by industry, the return on time spent studying patents is probably as great as for any other engineering activity.
The patent literature not only can provide ideas that can assist in achieving a design solution but also can help in avoiding approaches that will not work.
Patents are classified alphabetically by class and subclass in the to A weekly publication of the U.S. patent office, the
Gazette, lists in numerical order an abstract of each patent issued in an earlier week. The Index and Gazette are available in many libraries. To find the numbers of the specific patents that have been filed in a particular class, a computer index can be used: CASSIS (Classification and Search Support In- formation System).
Codes and standards have been mentioned in Section 1.4.2. The wisdom of adhering to codes and standards cannot be overemphasized. Using them can shorten design time, reduce uncertainty in performance, and improve product quality and reliability. Federal, state, and local codes and standards provide information in the form of allowable limits on the performance of various systems. Useful background information is also available in standards published by professional associations and manufacturers’ groups. Courts of law normally consider it a sign of good engineering practice if designs adhere to applicable standards, even if there is no legal compulsion to do so.
1.4.4 Performance and Cost Data
Equipment performance and cost data are required at various stages of the design process. These data might be obtained from vendors located via the Thomas Register or other sources mentioned in Section 1.4.3. Detailed cost estimates are conducted by specialists, usually in a cost-estimating depart- ment. Working from the details of a completely designed system, this group is normally able to estimate costs within 5% for a plant without major new components. Design engineers frequently make approximate cost estimates at various stages. Quick, back-of-the-envelope calculations may have an inac- curacy of estimate of or more. For more detailed calculations, esti- mates may be in the to range.
To facilitate equipment cost estimating, relatively easy-to-use tabular and graphical compilations are provided in References 9-1 and the accompa- nying reference lists. The design engineer must also estimate the final product cost. The total product cost is the sum of costs related to investment, resources and utilities, and labor. The subject of cost calculation in design is detailed in Chapter 7.