TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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__________________________________Tehnička postrojenja za provedbu kružnih procesa 6.1.3. Unutarnji stupanj djelovanja Unutarnji stupanj djelovanja uzima u obzir sekundarne nepovrativosti pa prema tome ovisi o kvaliteti izvedenih strojeva. Za ekspanziju je definiran kao omjer između stvarno izvršenog mehaničkog rada i maksimalno mogućeg pa je w h − h 1 2 ′ η u.eks. = = . wmax. h1 − h2 Unutarnji stupanj djelovanja kompresije definiran je kao omjer između minimalno potrebne mehaničke energije i stvarno potrebne mehaničke energije w − h min . 2 1 η u.komp. = = . w h2 ′ − h1 U navedenim jednadžbama uporabljene su oznake iz prethodnih dijagrama. Unutarnji stupanj djelovanja iznosi od 0.80 do 0.90 što ne ovisi samo o kvaliteti izvedbe stroja nego i o njegovu tipu. h 6.1.4. Dovod i odvod topline Najčešće se toplina dovodi i odvodi uz konstantan tlak, npr. Jouleov kružni proces. Budući da se dovod i odvod topline provodi pomoću izmjenjivača topline kroz koji struji radna tvar, a za njezin tok potrebna je razlika tlakova, stvarni dovod i odvod topline se ne ostvaruje po izobari. Da bi se osiguralo strujanje, tlak mora opadati u smjeru strujanja. Budući da toplina pri tome ostaje nepromijenjena i neovisna o gubitku tlaka, entalpija je na kraju procesa dovoda i odvoda topline nepromijenjena bez obzira koliko se je smanjio tlak u izmjenjivaču topline. Na slici 90. prikazane su promjene stanja tijekom dovoda topline i to u p, v – dijagramu i h, s – dijagramu. Kada ne bi bilo gubitka tlaka, promjena stanja tekla bi po izobari 1 – 2. Zbog gubitka tlaka proces završava u točki 2’ na krivulji h 2 = konst. na kojoj je i točka 2. Analogno tome vrijedi i za odvod topline. Promjene stanja vide se na dijagramima na slici 91. 275
TEHNIČKA TERMODINAMIKA__________________________________________ Slika 90. Dovod topline s gubicima tlaka u p, v – dijagramu i h, s – dijagramu Slika 91. Odvod topline s gubicima tlaka u p, v – dijagramu i h, s – dijagramu 276
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