Thermal Systems

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THERMAL SYSTEMSThermal components, processes and systems .1Thermal systems .1Thermal processes .3Thermal components .4Thermal (system) engineering (projects) .4Thermal engineering tasks .4Thermal design.5Thermal instrumentation .7Thermal data .8Thermal sciences: Thermodynamics, Fluid flow and Heat and mass transfer.9Thermal applications .11Thermal conditioning. HVAC&R .11Ventilation.12Space heating .13Heat dissipation .15Coolers .16Heat generation .17Heat sources (electrical, chemical.) .17Heating systems (heaters, furnaces, boilers.) .20Power generation. Heat engines .22Steam power plants .23Reciprocating power plants.23Gas turbine power plants .24Cold generation. Refrigerators and freezers .24Cold-producing processes .25Materials thermal processing .26Biological processing .26Chemical processing .27Physical processing .27Type of problems .27THERMAL COMPONENTS, PROCESSES AND SYSTEMSGreek prefix therm means heat (causes and effects, generation and usage), and Latin prefix temper meansmixed (originally used for 'temperatura caeli', the sky combination).THERMAL SYSTEMSIn Thermodynamics, a (thermodynamic) system is the part of the physical world that the observer selectsto analyse separately from the rest; in between, a frontier is defined through which mass and energyexchanges are precisely identified: an isolated system has no interaction with the rest, a closed systemmay only exchange heat and work, and an open system may also exchange mass.A thermal system, on the contrary, is a complex assembly of coupled components (some of themthermal), showing a common structured behaviour; e.g. a refrigerator is a combination of pipes,compressor, electric motor, heat exchangers, valves, insulation, casing, doors, lamp, etc., interacting to aThermal systems1

common goal of cold production within. Thus, a refrigerator is a thermal system, whereas the refrigerantfluid or the interior space are thermodynamic systems. Small thermal systems like a refrigerator, usuallycome fully assembled from the manufacturer (but a simple split-air-conditioning system needsinstallation), whereas large thermal systems like a refrigerated store, are built on the field. Thermalsystems usually demand some services as electrical supply (for power or at least for control), water intakeand exit, air intake and exit, fuel supply and flue stack, etc.Thermal system engineering is not usually thought of as a first rank engineering discipline as Mechanical,Civil, Electrical and Chemical Engineering, and it is usually ascribed to the leading one (like Aerospace,Naval, and Automotive Engineering) because the paradigmatic thermal systems has always been the heatengine, but its importance pervades all other branches (e.g. thermal control systems are needed fromcomputer-chip design to communication satellites).A basic grouping of thermal systems may be established according to whether heat exergy, Q, workenergy, W, or mass, mi, is transferred through the frontier of the system of interest (' ' if going out, ' ifgoing in): (Q ) Heat dissipation systems: coolers. Heat flows out of the system naturally or accelerated. (Q ) Heat generation systems: heaters, furnaces, boilers, heat pumps. Heat flows into the system. (W ) Power generation systems: heat engines, chemical engines, propulsion. Work flows out ofthe system. (W ) Cold generation systems: refrigerators and air conditioners. Work flows into the system toproduce cold (considered above if for heating). (mi ) Mixture separation and chemical synthesis by thermal processes: dryers, distillers andreactors. An specific substance must go out of the system. (mi ) Mixing and dilution: preparation (compounding) or dispersion (waste disposal). An specificsubstance must go into of the system.However, other possible grouping may be established according to the type of industry implied, as forthermal systems in (materials) manufacturing (furnaces, casting, extrusion, welding, oxy-cut, refining), orin food processing (cooking, sterilisation, drying, dehydration, the cold chain).In this Chapter 14 we aim at understanding what thermal systems are, and how to analyse them (to bepostponed to subsequent chapters), but it is important to broaden for a while our target to see how actualthermal systems are designed, at least to a conceptual stage. Even a sales engineer (and a purchaser) needsto know how thinks work, what are their functionalities, and what performances have been accomplished.In summary, learning is an iterative process like exploring: you first have a rough plan and start walking,dealing with close problems, but from time to time you have to make a stop to integrate what you havelearnt and plan further challenges (that you may view as 'conquests' or 'requirements' according to your'pusher' or 'dragged' mood).Thermal systems2

THERMAL PROCESSESThermal processes are those giving way to thermal effects. Natural thermal effects include, besides thetendency for temperature gradients to die out in an isolated system (strict thermal effects), the tendencyfor velocity gradients to die out in a suitable reference frame (for an isolated system), and the tendencyfor concentration gradients (really chemical potential gradients, in absence of external force fields) to dieout for an isolated system. Artificial thermal effects aim at forcing the gradients not to die out (at expenseof some external exergy consumption). All physico-chemical processes taking place at high or lowtemperature involve thermal processes, at least to maintain the desired environment and to measure it(high-T thermometry and low-T thermometry are special subjects).A process is a sequence of steps taking place (by natural or artificial forces) between two states of thesystem, called initial and final states, that may coincide and then the process is said to be cyclic (or steadyif no time variation is apparent between the end states).Steady state processes are the most common in thermal systems; cyclic processes are also analysed as asequence of steady states. Other thermal processes of great interest are iterative process, which are nearcyclic processes where the sequence of steps is repeated always the same, but the working substance donot recover the same initial conditions (e.g. multi-stage refining).The basic thermal processes have already been studied in previous chapters: Heat transfer: by pure diffusion, by combined diffusion and advection, or by radiation Chemical reaction (Chapter 9) Mixing (Chapter 7, including air drying and humidification in Chapter 8) Phase change (Chapter 6) Compression and expansion (Chapter 5),as well as the general evaluation of thermal energy variations by heat or work exchange (Chapter 1), ofentropy variations and entropy sources (Chapter 2), of exergy sources and exergy requirements (Chapter3), and of data requirements and data sources (Chapter 4).Besides those basic thermal processes, some fluid flow processes must be considered at the same timebecause most thermal systems are fluid systems. In order of complexity one may quote: One-dimensional incompressible flow in pipes connecting thermal subsystems (piping,pumps, restrictions, valves). Pressure losses. Empirical thermo-fluiddynamic relations for convective heat transfer. One-dimensional compressible flow in nozzles. Real fluid flow processes (boundary layer, wake, turbulence, CFD).More complex thermal processes, as combustion, materials thermal treatments, and combined processesfor power, heat or cold generation, are treated here or in following chapters.Thermal systems3

THERMAL COMPONENTSThe components of a thermal system (e.g. a power plant, a refrigeration plant, a liquefaction plant) maybe other subsystems (e.g. heat exchangers, compressors, valves, mixers, distillers) or basically rawmaterials (working fluids, solid insulators), and the ancillary elements in thermal installations (e.g.sensors, actuators, controllers, piping and wiring).During operation, the whole thermal system, its components, and particularly the working substancesinside, are subjected to natural or artificial thermal processes.Every component, including the working substances and the whole system, has some temperature rangesoutside which they do not behave as wanted. Thermal conditioning must then be properly cared about(and there are many components very sensitive to temperature, as electronics components are).THERMAL (SYSTEM) ENGINEERING (PROJECTS)THERMAL ENGINEERING TASKSEngineering is the art and science of solving practical problems; thermal engineering addresses thethermal problems, i.e. problems with relevant thermal effects. But problems in real-life are seldomexclusively thermal: even a heat exchanger is a mechanical entity that, besides is thermal goal, possesstructural problems (thicknesses, joints, dilatations), chemical problems (corrosions, materialscompatibility), handling problems (assembly, inspection, cleaning), logistic problems (availability ofmaterials, machinery, man-power, workshops, warehouses), economic problems (cost of raw materials,manufacturing, marketing, selling), etc. There are many "e" aspects to consider even in genuine energysystems: Energy, Exergy, Economy, Ecology, Ergonomics, Esthetics. (e.g., many air-conditioner buyerstake their purchasing decision based on lower noise and not in higher efficiency equipment).Real-life problems must then be solved from all the engineering points-of-view. The approach is toproceed like in sewing, i.e. a combination of parallel interaction and serial iterations, depending on theratio of man-power to time-allocation, in an interdisciplinary mode called system engineering, toconceptualise, design, built, operate, maintain, or dispose of, the system under consideration.Thermal engineers get from system engineers some general goals and constraints (to which the formersmay have contributed a lot, if the problem is focussed on thermal aspects), and start to work to solve thethermal problems, assuming that the other engineering aspects (structural, chemical, electrical, etc.) willbe solved "as usual" (thermal engineers need a broad engineering background to know what is "as usual"in the other disciplines, having a common understanding is a prerequisite to collaborative work).The iterative problem-solving process of system engineering may be applied to understand the basicfunctioning of a thermal system, but where it is really needed is in the design of thermal systems, the mostcomplex of thermal engineering projects.Thermal systems4

A project is a case (an instance) in system engineering. The purpose of a thermal project is to deliver to acustomer (and maybe to support or operate it, if required) a system comprising one or more elementsrelated to production, consumption or processing of thermal energy.The project activities carried out by the thermal system engineer may be: Project specification (goal adviser), responsible for creation and maintenance of an envelope ofconditions that satisfy the customers (according to some written requirements and some assumedstandards) without too much impact on the rest of the world (other customers and the environment). Project management (time and money manager), responsible for achievement of the totality of theproject objectives, and specifically for organisation of the project, and its timely and cost-effectiveexecution. Project engineering (designer), responsible for definition of the system, verification that thecustomer's technical requirements are achieved, and compliance with the applicable projectconstraints. Many models of the system are developed in support of design: mathematical models,mock-ups, engineering models, prototypes and pilot plants. Production (builder), responsible for materials procurement, manufacture, assembly and integration ofthe system, in accordance with the design. Marketing (seller), responsible for tailoring the product to satisfy the customers. Except for the case ofintricate installations demanding tailored work, this is not properly engineering. Operations (operator), responsible for exercising and supporting the system in order to achieve thecustomer's objectives during the operational phase. Product assurance (reviewer), responsible for the implementation of the quality assurance element ofthe project and also for certain other specialist activities as reliability, availability, maintainability andsafety.THERMAL DESIGNDesign is making choices to achieve a goal (i.e. to work out a non-trivial solution). The design task mayrange from a more or less elaborated sketch to represent an idea (not yet existing or not available at thetime and place), with the intention just to be communicated (like in technical descriptive drawing orartistic emotive painting), or may include a prototype realisation of the idea conceived (as in industrialand urban design). Thermal design is just a part of industrial design.To design a physical system (or a component) is to devise (to plan) means to accomplish a stated purpose(user requirements), under explicit and implicit constraints (time, budget, user and social acceptance),from anew or retrofitting. A project is the actual development of the design. Sometimes project anddesign are used indistinctly to mean the organisation of resources (information, materials, equipment,man-power, financial-power) to accomplish a common objective.The following 'rules of thumb' for system engineering may help to focus the attention also when dealingwith thermal system design: Everything has a cost (even respiration demands an effort). A trade-off is always implied.Requirements should be weighed against foreseeable cost; rigidity is just a first approximation inThermal systems5

design. Efficiency, the ratio of benefit to cost, must be pursued (the challenge is to accomplishmuch with little). Most times cost (including environmental costs) can be reduced to economicterms. A project not worth doing, is not worth doing well.Everything interacts with everything else. You may (usually you must) decompose the system intosubsystems down to basic components, but a shared interface between any two blocks alwaysremains. Best system engineering approach is to decompose the system along the most trivialinterfaces.Everything goes somewhere. It is a special case of the previous rule when applied to interactionswith the surroundings (environment).Everything needs to be done, and under the same global budget. The performances of the wholesystem will normally be lowered to the poorer link in the chain. Everything may look simple to theperson who doesn't have to do it. Don't polish work done before finishing the whole. Anypostponed task may soar.Everything must be verified. One test is worth a thousand expert opinions. A test will be believedby everyone (except perhaps the tester); an analysis may be believed by no-one but the analyst.Everything may fail. Minimise risk, plan for contingencies, provide redundancies. Expect theworst and you'll not be disappointed.A design, even a conceptual design, cannot reduce to a wording description of a solution, not even to agraphical layout of its components and interfaces; a proper design is always based on mathematicalquantitative modelling, that establish in more or less detail a set of governing equations (i.e. idealisedphysical constraints), pinpointing and relating amongst them the most important of the problem variables.And the designer has to verify that the small set of variables selected are really the leading ones, and allthe rest have only second order effects.Design is always driven by an extremum goal, that can be: Minimisation of effort, cost, risk, input resources, environmental impact, or an appropriate mix. Maximisation of achievement, profit, benefit, output, appeal, or an appropriate mix.Experience helps a lot, but sooner or later some experimental trials, with breadboards or prototypes, areneeded to elucidate the validity of the model or to find new empirical data to feed the model.Experimental breadboards are set up when difficulties arise in the modelling of the behaviour orinteractions of subsystems, with instrumentation for testing, trying to set up a minimum rig, decoupling itfrom the rest of the system by the most simple and clear interfaces to ensure that partial simulation isrepresentative.The designer has to deal with instrumentation not only for testing during project development, but also fornormal operation of the thermal system and for maintenance.Thermal systems6


Thermal system engineering is not usually thought of as a first rank engineering discipline as Mechanical, Civil, Electrical and Chemical Engineering, and it is usually ascribed to the leading one (like Aerospace, Naval, and Automotive Engineering) because the paradigmatic thermal systems has always been the heat engine, but its importance pervades all other branches (e.g. thermal control .

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