TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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TEHNIČKA TERMODINAMIKA______________________________________________ Rješenje Prema I. glavnom zakonu bit će 3 U 2 − U1 = Q − W = −110 ⋅10 + 70 ⋅ 3600 = 142000 J = 142 kJ 102
___________________________________________________________________Kružni procesi 3. KRUŽNI PROCESI Kod promjena koje smo do sada razmatrali, osim kod promjene pri konstantnom volumenu, dobiva se neki rad. Veličina dobivenog rada ovisila je o načinu kojim je promjena izvršena. Prilikom tih promjena plin ekspandira od nekog početnog stanja određenog početnim tlakom, temperaturom i volumenom na neko konačno stanje koje je određeno konačnim uvjetima. Obzirom na to da ne raspolažemo neiscrpnim rezervoarima plina kod početnih uvjeta, to je za ponovno dobivanje rada iz nekog sustava nužno radnu tvar, odnosno plin vratiti u isto početno stanje. Ako pokušamo ponovo dovesti plin u početno stanje, vidjet ćemo da ćemo rad, koji smo ekspanzijom dobili, morati utrošiti za kompresiju. Kao rezultat sveukupne promjene dobit ćemo, dakle, početnu količinu topline i bez rada. Zato, ako želimo dobiti rad koji ćemo moći iskoristiti, moramo radnu tvar, tj. plin vratiti u početno stanje nekim drugim putem koji se mora razlikovati od puta ekspanzije i pri kojem nećemo utrošiti sav rad kojeg smo dobili ekspanzijom. Ponavljanje ovakvih procesa za tehniku je od najveće važnosti jer gotovo svi strojevi rade ritmički, tj. oni trajno ponavljaju radni kret koji se može sastojati i od više elemenata. Uzmimo da smo pošli od stanja 1 do stanja 2 putem a, slika 21. Promjenom stanja po putu a dobili smo neki rad, W a . Radnu tvar ne smijemo istim putem vratiti u početno stanje jer bi sav dobiveni rad, W a utrošili za izvođenje tog suprotnog procesa. Zato za vraćanje u početno stanje odabiremo put b. I u ovom slučaju trošimo neki rad, W b za kompresiju, ali ovaj rad je ipak manji od rada dobivenog ekspanzijom na putu a. U p, V – dijagramu, rad se predočuje kao površina ispod krivulje promjene stanja, a ova površina je pozitivna kada se integrira u smjeru pozitivne osi V, a negativna kada se ide u smjeru negativne osi V. 103
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Prof. dr. sc. Nedjeljka Petric Prof
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