Appunti Fisica Tecnica Industriale 1

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Università di Roma Tor Vergata – corso Coppa/Petracci

Federico Casucci Federico Casucci ©

2 1 SOMMARIO

1 TERMODINAMICA _______________________________________________________________________________________________________ 7

1.1 Lavoro termodinamico ____________________________________________________________________________________________ 8

1.2 Calore, temperatura ed equilibrio termico _______________________________________________________________________ 8

1.3 Primo principio della termodinamica conservazione dell’energia per i sistemi chiusi _____________________ 9

1.4 Conservazione della massa per sistemi aperti _________________________________________________________________ 10

1.5 Primo principio della termodinamica conservazione dell’energia per i sistemi aperti __________________ 10

1.6 Secondo principio della termodinamica ________________________________________________________________________ 11

1.6.1 Formulazione analitica del secondo principio ____________________________________________________________ 12

1.7 Entropia ___________________________________________________________________________________________________________ 12

1.7.1 Teorema di aumento dell’entropia ________________________________________________________________________ 13

1.7.2 Calore in termini di entropia _______________________________________________________________________________ 13

1.7.3 Energia interna ed entalpia in termini di entropia _______________________________________________________ 13

1.7.4 Lavoro in termini di entropia ______________________________________________________________________________ 13

2 SISTEMI TECNICI ______________________________________________________________________________________________________ 15

2.1 Sistemi tecnici chiusi _____________________________________________________________________________________________ 15

2.1.1 Sistema motore _____________________________________________________________________________________________ 15

2.1.2 Sistema frigorifero __________________________________________________________________________________________ 15

2.1.3 Sistema pompa di calore ___________________________________________________________________________________ 16

2.2 Sistemi tecnici aperti _____________________________________________________________________________________________ 16

2.2.1 Espansore o turbina ________________________________________________________________________________________ 17

2.2.2 Compressore o pompa _____________________________________________________________________________________ 17

2.2.3 Valvola di laminazione _____________________________________________________________________________________ 18

2.2.4 Miscelatore __________________________________________________________________________________________________ 19

2.2.5 Scambiatore _________________________________________________________________________________________________ 19

2.2.6 Condotti, ugelli e diffusori __________________________________________________________________________________ 20

2.3 Sistemi tecnici nei diagrammi ed __________________________________________________________________ 20

3 PROPRIETÀ DELLE SOSTANZE _______________________________________________________________________________________ 21

3.1 Coefficienti di dilatazione e compressione _____________________________________________________________________ 21

3.2 Coefficienti calorimetrici ________________________________________________________________________________________ 21

3.3 Potenziali termodinamici ________________________________________________________________________________________ 22

3.4 Equazioni di Maxwell ____________________________________________________________________________________________ 22

3.5 Energia interna in variabili di stato _____________________________________________________________________________ 23

3.6 Entalpia in variabili di stato _____________________________________________________________________________________ 24

3.7 Calore in variabili di stato _______________________________________________________________________________________ 24

3.8 Entropia in variabili di stato _____________________________________________________________________________________ 24

3.9 Relazione di Mayer _______________________________________________________________________________________________ 24

3.10 Diagramma ________________________________________________________________________________________________ 25

3.11 Diagramma ________________________________________________________________________________________________ 26

4 GAS PERFETTI _________________________________________________________________________________________________________ 28

4.1 Trasformazione isoentropica ___________________________________________________________________________________ 28

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4.2 Trasformazioni politropiche ____________________________________________________________________________________ 29

4.3 Diagramma per i gas perfetti ____________________________________________________________________________ 30

5 PASSAGGI DI STATO PER SOSTANZE PURE _________________________________________________________________________ 32

5.1 Liquido-Vapore nel diagramma __________________________________________________________________________ 32

5.2 Liquido-Vapore nel diagramma _________________________________________________________________________ 33

5.3 Liquido-Vapore nel diagramma _________________________________________________________________________ 33

5.4 Piano delle fasi ___________________________________________________________________________________________________ 33

6 MISCELE DI ARIA UMIDA _____________________________________________________________________________________________ 35

6.1 Psicrometro _______________________________________________________________________________________________________ 36

6.1.1 Diagramma psicrometrico _________________________________________________________________________________ 37

6.2 Trasformazioni di miscele di aria umida _______________________________________________________________________ 37

6.2.1 Riscaldamento isotitolo ____________________________________________________________________________________ 37

6.2.2 Miscelazione ________________________________________________________________________________________________ 38

6.2.3 Raffreddamento isotitolo ___________________________________________________________________________________ 39

6.2.4 Raffreddamento con deumidificazione ___________________________________________________________________ 39

6.2.5 Umidificazione adiabatica con raffreddamento __________________________________________________________ 40

6.2.6 Umidificazione isoterma ___________________________________________________________________________________ 41

7 CICLI MOTORE TERMICI A COMBUSTIONE INTERNA ______________________________________________________________ 43

7.1 Ciclo Otto __________________________________________________________________________________________________________ 43

7.1.1 Ciclo reale e semplificazioni termodinamiche ____________________________________________________________ 44

7.1.2 Parametri di valutazione ___________________________________________________________________________________ 45

7.2 Ciclo Diesel________________________________________________________________________________________________________ 45

7.2.1 Ciclo reale e semplificazioni termodinamiche ____________________________________________________________ 46

7.2.2 Parametri di valutazione ___________________________________________________________________________________ 47

7.2.3 Confronto Otto vs Diesel ___________________________________________________________________________________ 48

7.3 Ciclo misto Sabathé ____________________________________________________________________________________________ 49

7.4 Ciclo Joule _________________________________________________________________________________________________________ 50

7.4.1 Parametri di valutazione ___________________________________________________________________________________ 51

8 CICLI MOTORE A VAPORE ____________________________________________________________________________________________ 54

8.1 Ciclo di Carnot ____________________________________________________________________________________________________ 54

8.1.1 Parametri di valutazione ___________________________________________________________________________________ 55

8.1.2 Limiti realizzativi del ciclo di Carnot ______________________________________________________________________ 55

8.2 Ciclo Hirn o Rankine a vapore surriscaldato _________________________________________________________________ 55

8.2.1 Parametri di valutazione ___________________________________________________________________________________ 57

8.2.2 Confronto Hirn vs Carnot __________________________________________________________________________________ 57

9 IMPIANTI MISTI _______________________________________________________________________________________________________ 59

9.1 Impianto a ciclo combinato ______________________________________________________________________________________ 59

9.2 Impianto di cogenerazione ______________________________________________________________________________________ 59

10 CICLI FRIGORIFERI _________________________________________________________________________________________________ 60

10.1 Ciclo frigorifero a compressione di vapore ____________________________________________________________________ 60

10.1.1 Parametri di valutazione ___________________________________________________________________________________ 61

Federico Casucci ©

4 10.1.2 Fluidi refrigeranti ___________________________________________________________________________________________ 61

10.2 Impianto frigorifero in cascata __________________________________________________________________________________ 62

11 TERMOFLUIDODINAMICA _________________________________________________________________________________________ 63

11.1 Conservazione della massa ______________________________________________________________________________________ 63

11.2 Equazione di Bernoulli generalizzata principio in forma meccanica ___________________________________ 64

11.3 Misurazione della pressione ____________________________________________________________________________________ 65

11.4 Carico potenziale per attrito come caduta di pressione _______________________________________________________ 66

11.5 Viscosità __________________________________________________________________________________________________________ 66

11.6 Esperienza di Reynolds __________________________________________________________________________________________ 67

11.7 Perdite di carico __________________________________________________________________________________________________ 68

11.7.1 Perdite di carico distribuite ________________________________________________________________________________ 68

11.7.2 Perdite di carico concentrate ______________________________________________________________________________ 71

11.7.3 Perdite di carico totali ______________________________________________________________________________________ 71

12 TRASMISSIONE DEL CALORE ______________________________________________________________________________________ 72

12.1 Conduzione _______________________________________________________________________________________________________ 72

12.1.1 Conduzione in regime permanente monodimensionale _________________________________________________ 73

12.1.1.1 Lastra piana ____________________________________________________________________________________________ 74

12.1.1.2 Cilindro cavo ___________________________________________________________________________________________ 74

12.1.1.3 Lastre piane affacciate ________________________________________________________________________________ 74

12.1.1.4 Cilindri cavi affacciati _________________________________________________________________________________ 76

12.1.2 Conduzione a parametri concentrati ______________________________________________________________________ 77

12.1.2.1 Solido a contatto con un fluido _______________________________________________________________________ 78

12.2 Irraggiamento ____________________________________________________________________________________________________ 78

12.2.1 Corpo nero __________________________________________________________________________________________________ 79

12.2.2 Intensità di radiazione di un corpo nero __________________________________________________________________ 80

12.2.3 Corpi reali ___________________________________________________________________________________________________ 81

12.2.4 Legge di Kirchhoff __________________________________________________________________________________________ 81

12.2.5 Scambio termico per irraggiamento – fattori di vista ____________________________________________________ 82

12.2.6 Irraggiamento in cavità nere _______________________________________________________________________________ 84

12.3 Convezione _______________________________________________________________________________________________________ 84

12.3.1 Convezione naturale ________________________________________________________________________________________ 85

12.3.2 Numero di Nusselt per la convezione naturale ___________________________________________________________ 85

12.3.3 Numero di Nusselt per la convezione forzata ____________________________________________________________ 86

12.3.4 Convezione e regime di moto ______________________________________________________________________________ 88

13 COMFORT TERMOIGROMETRICO _________________________________________________________________________________ 90

13.1 Corpo umano _____________________________________________________________________________________________________ 90

13.1.1 Bilancio termico del corpo umano _________________________________________________________________________ 91

13.2 Equazioni del comfort di Fanger ________________________________________________________________________________ 93

13.2.1 Intervallo di comfort ________________________________________________________________________________________ 94

14 IMPIANTI AD ACQUA _______________________________________________________________________________________________ 96

14.1 Tipologie di impianti ad acqua calda ___________________________________________________________________________ 96

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14.2 Componenti degli impianti ad acqua calda _____________________________________________________________________ 97

14.2.1 Caldaia _______________________________________________________________________________________________________ 97

14.2.2 Corpi scaldanti ______________________________________________________________________________________________ 97

14.2.3 Vaso di espansione _________________________________________________________________________________________ 98

14.2.4 Pompa _______________________________________________________________________________________________________ 99

14.2.5 Tubazioni ___________________________________________________________________________________________________ 100

14.2.6 Altri componenti ___________________________________________________________________________________________ 101

14.3 Dimensionamento di un impianto di riscaldamento ad acqua calda ________________________________________ 101

14.3.1 Dimensionamento di un impianto di circolazione a 2 tubi _____________________________________________ 101

14.3.2 Dimensionamento di un impianto a ritorno inverso e ad anello _______________________________________ 103

14.3.3 Dimensionamento di un impianto ad un solo tubo ______________________________________________________ 103

14.3.4 Dimensionamento di un impianto a collettore complanare ____________________________________________ 105

14.4 Regolazione di un impianto di riscaldamento ad acqua calda _______________________________________________ 105

14.5 Installazione delle pompe in un impianto di riscaldamento ad acqua calda _______________________________ 106

15 IMPIANTI DI TELERISCALDAMENTO ____________________________________________________________________________ 108

15.1 Centrale di cogenerazione ______________________________________________________________________________________ 108

15.2 Tipologia di rete _________________________________________________________________________________________________ 109

15.3 Componenti degli impianti di teleriscaldamento _____________________________________________________________ 109

15.3.1 Tubazioni ___________________________________________________________________________________________________ 109

15.3.2 Fluidi termovettori ________________________________________________________________________________________ 109

15.3.3 Vaso di espansione ________________________________________________________________________________________ 110

15.3.4 Serbatoi o accumulatori ___________________________________________________________________________________ 110

15.4 Dimensionamento di un impianto di teleriscaldamento _____________________________________________________ 110

15.5 Posizionamento della pompa___________________________________________________________________________________ 111

16 IMPIANTI AD ARIA ________________________________________________________________________________________________ 113

16.1 Componenti degli impianti ad aria _____________________________________________________________________________ 113

16.1.1 Scambiatori di calore ______________________________________________________________________________________ 113

16.1.2 Umidificatori _______________________________________________________________________________________________ 113

16.1.3 Deumidificatori ____________________________________________________________________________________________ 113

16.1.4 Filtri _________________________________________________________________________________________________________ 114

16.1.5 Bocchette o diffusori _______________________________________________________________________________________ 115

16.1.6 Condotti ____________________________________________________________________________________________________ 115

16.1.7 Ventilatori __________________________________________________________________________________________________ 116

16.2 Tipologie di impianti ad aria ___________________________________________________________________________________ 116

16.2.1 Impianti a tutta aria _______________________________________________________________________________________ 116

16.2.1.1 Im