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Kao jedan od glavnih procesnih uređaja, izmjenjivači topline predmet su intenzivnih istraživanja usmjerenih na različita područja interesa koji uključuju optimalan dizajn, poboljšanja strukture, poboljšanja prijenosa topline, dinamičko ponašanje, automatsko upravljanje, sintezu mreže, numeričku simulaciju, eksperimentalne metode, proizvodnju i razvoj novih vrsta površina za prijenos topline. U ovom radu se za zadanu ukupnu površinu izmjenjivača topline traži optimalno mjesto priključka dodatne struje, određenog protoka i temperature, iz dostupne mreže izmjenjivača topline kako bi se ispunila dva cilja: ostvarenje maksimalnog toplinskog toka kao prvi cilj te iznalaženje mjesta priključka dodatne struje za postizanje maksimalne entropije kao drugi cilj. Definirani su kriteriji koji određuju pri kojim uvjetima se lokalni maksimum toplinskog toka postiže za sve vrijednosti 3 značajke, pri kojim uvjetima se pojavljuje za neke vrijednosti te iste značajke te kada se maksimum ne pojavljuje. Razvijen je i algoritam za izračun generirane entropije te je istraženo pri kojim uvjetima se pojavljuje njezin maksimum. Rezultati istraživanja prikazani su u odgovarajućim dijagramima te su interpretirani, s posebnim osvrtom na slučajeve koji udovoljavaju kriteriju maksimalnog toplinskog toka za sve radne točke protusmjernog rekuperatora, odnosno za pojedine radne točke rekuperatora. Također su prikazani i interpretirani slučajevi kod kojih niti jedna radna točka rekuperatora neće udovoljiti postavljenoj hipotezi za realizaciju maksimalnog toplinskog toka. Dijagramski su prikazani te interpretirani rezultati bezdimenzijske generirane entropije te omjer bezdimenzijskog toplinskog toka i bezdimenzijske generirane entropije. U ovom radu provedeno je i eksperimentalno istraživanje s ciljem potvrde izvedenog matematičkog modela za ostvarenje maksimalnog toplinskog toka. Rezultati matematičkog modela uspoređeni su s eksperimentalnim rezultatima kojima su pridružene izračunate sastavljene standardne mjerne nesigurnosti. As one of the main process equipment, heat exchangers are the subject of intensive research focused on various areas of interest including optimal design, structure improvements, enhanced heat transfer techniques, dynamic response behavior, automatic control, network synthesis, numerical simulation, experimental methods, manufacturing, and development of new types of heat transfer surfaces. For the purpose of this thesis, a counterflow heat exchanger of a given total heat transfer area was selected, in which the required amount of heat load needs to be exchanged, having various mass flow rates and supply temperatures of the streams available from the heat exchanger network. Those inlet parameters are such that by connecting either only stream A or only stream B to the heat exchanger with a given surface it would not be possible to achieve the targeted heat flow rate. Thus, the question arises as to whether the targeted amount of exchanged heat flow rate can be achieved by the simultaneous action of both stream A and stream B on a given overall heat exchange area. This means that such simultaneous operation of both streams does not a priori allow greater exchanged heat flow rate than in the case where these streams act individually. This thesis aims to obtain the general criterion that needs to be met in order not only to achieve an increase in heat flow rate in such connected (networked) heat exchangers, but to maximize the exchanged heat flow rate. Fulfillment of this criterion means finding the optimal position on a heat exchanger for connection of the stream A, so that the maximum heat flow rate can be, along with the stream B, achieved in a single networked heat exchanger. In other words, within this mathematical model, the criterion of the existence of the maximum heat flow rate, as a local extreme, should be found, which is greater than the heat flow rate achieved only with stream A or stream B. Also, an algorithm for entropy generation in a heat exchanger network is developed. Entropy generation due to heat transfer between streams at finite temperature differences is considered in the model. Due to the complexity of the expression for the calculation of entropy generation, it was not possible to extract the analytical criteria for achieving the local maximum entropy, so it is necessary to determine the maximum of the function numerically, for each case separately. The research is based on the following hypotheses: maximum heat flow rate and maximum entropy generation is possible to achieve, as local extrema, if the criteria that can be written in explicit dimensionless form are fulfilled. The goal of this research is to obtain these criteria, which will contain the given overall surface of the heat flow rate, inlet temperatures of the weaker and stronger stream as well as the temperature of the weaker stream which is taken from the heat exchanger network. The first criterion must relate to maximum heat flow rate goal and the second to maximum entropy generation goal. A very important parameter M is formulated, representing the ratio between temperature difference of the weaker inlet connecting stream from heat exchanger network and inlet stronger stream and the temperature difference of the inlet weaker stream and inlet stronger stream of the observed (separated) counterflow heat exchanger. The results of the research are presented in related diagrams and interpreted, with special emphasis on cases fulfilling the maximum heat flow rate criterion for all operating points of the counterflow heat exchanger as well as for certain operating points of the counterflow heat exchanger. Cases where none of the operating points of heat exchanger will meet the hypothesis for achieving the maximum heat flow rate are also presented and interpreted. Finally, the results of dimensionless entropy generation and ratio between dimensionless heat flow rate and dimensionless entropy generation are presented in diagrams and interpreted. An experimental study was also conducted with the aim of confirming the derived mathematical model for developing the criterion for the maximum heat flow rate. The results of the mathematical model are compared with the experimental results with associated calculated composed standard uncertainties. The dissertation confirms the thesis regarding the existence or non-existence of operating parameters of a networked heat exchanger with the aim of achieving the desired maximum heat flow rate, maximum entropy or the desired maximum ratio between dimensionless heat flow rate and dimensionless entropy. For given available (specific) data on flows and stream temperatures as well as the overall exchange surface of the networked heat exchanger, using developed computer program it is possible to give a very quick and easy answer to given demands or criteria.

For many years now, heat exchanger optimization has been a field of research for a lot of scientists. Aims of optimization are different, having in mind heat exchanger networks with different temperatures of certain streams. In this paper mathematical model in dimensionless form is developed, describing operation of one heat exchanger in a heat exchanger network, with given overall area, based on the maximum heat-flow rate criterion. Under the presumption of heat exchanger being a part of the heat exchanger network, solution for the given task is resting in a possibility of connecting an additional fluid stream with certain temperature on a certain point of observed heat exchanger area. The connection point of additional fluid stream determines the exchanging areas of both heat exchangers and it needs to allow the maximum exchanged heat-flow rate. This needed heat-flow rate achieves higher value than the heat-flow rate acquired by either of streams. In other words, a criterion for the existence of the maximum heat-flow rate, as a local extremum, is obtained within this mathematical model. Results of the research are presented by the adequate diagrams and are interpreted, with emphasis on the cases which fulfill and those which do not fulfill the given condition for achieving the maximum heat-flow rate.