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TEHNIČKA TERMODINAMIKA__________________________________________ Slika 69. Proces s mokrom parom u T, s – dijagramu 6 Slika 70. Proces s mokrom parom u p, v – dijagramu Stupanj djelovanja Carnotova procesa ovisi samo o temperaturama T i T 0 , a određen je relacijom T − T0 η = . T 228
______________________________________________________Stvarni ili realni plinovi i pare Prijelazom na proces prikazan na slikama 69. i 70., koji se naziva Rankineov proces ili Clausius-Rankineov proces, dobiva se više mehaničke energije iz 1 kg pare, budući da je mehanička energija nastala prema Rankineovom procesu proporcionalna površini 5 – 2 – 3 – 4 , a iz Carnotovog procesa proporcionalna je površini 1 – 2 –3 – 4. To je posljedica veće količine dovedene topline. Međutim, termički stupanj djelovanja Rankineovog procesa slabiji je od Carnotovog. Dovedena toplina q u Rankineovom procesu proporcionalna je površini c – 5 – 2 – 3 – b, a odvedena toplina q 0 površini c – 5 – 4 – b te je stupanj djelovanja q 0 η =1 − . q Ako se površina c – 5 – 2 – 3 – b zamijeni pravokutnikom jednake površine, ali tako da osnovica bude jednaka razlici entropija s4 − s5 , dobiva se iz visine pravokutnika c – 5’ – 3’ – b srednja temperatura T sr. dovoda topline te će termički stupanj djelovanja Rankineovog procesa biti T η = 1 − 0 . T sr. Budući da je T sr. < T , termički stupanj Rankinovog procesa manji je od stupnja djelovanja Carnotova procesa. Kako je već spomenuto, uporaba h, s – dijagrama ima znatne prednosti jer su u njemu entalpije prikazane kao dužine. Na slici 71. predočen je proces mokrom parom u h, s – dijagramu. Stanja su označena isto kao u T, s – dijagramu i p, v – dijagramu prethodnih slika. Kao u T, s – dijagramu tako i u h, s – dijagramu točke 5 i 6 padaju praktički zajedno jer su izobare vrlo guste u području tekućeg agregatnog stanja (lijevo od donje granične krivulje). Ako se uvrste razlike entalpija, termički stupanj djelovanja procesa je h4 − h5 h3 − h4 η = 1 − = h3 − h5 h3 − h5 jer je razlika entalpija h3 − h 4 upravo jednaka mehaničkoj energiji koja se dobiva iz procesa ako ekspanzija teče po adijabati. Termički stupanj djelovanja procesa s mokrom parom ovisi o tlaku svježe pare p1 i tlaku p2 na kraju procesa, što je prikazano na dijagramu na slici 72. U ovim slučajevima računalo se da temperatura napojne vode, koja se dovodi crpki, odgovara temperaturi isparavanja pri tlaku p 2 . Proces s vlažnom parom radi s niskim stupnjem djelovanja, 229
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