TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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TEHNIČKA TERMODINAMIKA__________________________________________ dira adijabatski u parnom stroju od stanja 3 do 4. Za vrijeme ekspanzije dio pare se kondenzira jer se adijabate sve više udaljuju od granične krivulje te u kondenzator dolazi smjesa pare i tekućine. Na kraju ekspanzije mokra para ima temperaturu T 0 koja je i najniža temperatura u procesu. Slika 67. Carnotov proces s mokrom parom u p, v – dijagramu Slika 68. Shema postrojenja za provedbu kružnog procesa s parom kao radnom tvari 226
______________________________________________________Stvarni ili realni plinovi i pare U kondenzatoru se preostala para postupno kondenzira zbog toga što se toplina odvodi rashladnom vodom koja struji kroz kondenzator. Odvedena toplina proporcionalna je površini b – 4 – a – 1 (T, s – dijagram). Da bi se adijabatskom kompresijom mogla dobiti vrela voda stanja 2, morala bi se iz kondenzatora isisati smjesa tekućine i pare koja odgovara stanju 1. Prilikom kompresije ugrijava se samo para, a tekućina ostaje pretežno hladna zbog loših uvjeta pri izmjeni topline. Zato se na kraju kompresije neće dobiti vrela voda stanja 2, već neka smjesa pregrijane pare i hladne vode; dakle, neće nastati stanje koje odgovara pretpostavljenom kružnom procesu. To znači da je praktički nemoguće ostvariti Carnotov proces mokrom parom. Stoga se u praksi kompresor zamjenjuje napojnom crpkom, a to znači da se sva para koja izlazi iz parnog stroja mora kondenzirati u kondenzatoru i da na izlazu mora biti postignuto stanje na izobari p1 = konst. Izobara je ujedno i izoterma T 0 = konst., a postignuto stanje je na donjoj graničnoj krivulji (x = 0) (stanje 5 na slikama 69. i 70.). Napojna crpka siše vodu temperature T 0 i tlaka p1 pa je adijabatski komprimira na tlak p2 koji je jednak tlaku u kotlu. Pri tome temperatura tekućine naraste za vrlo mali iznos (vidi povećani dio T, s – dijagrama) budući da stanja 5 i 6 padaju gotovo zajedno. Proces kompresije napojnom crpkom prikazan je krivuljom 5 – 6 na p, v – dijagramu u kojem su prilike lijevo od granične krivulje nacrtane uvećano kako bi se jasnije prikazao proces kompresije. Zapravo, na p, v – dijagramu donja ili lijeva granična krivulja, adijabata i izoterma padaju skoro zajedno i to u jednu okomitu crtu koja je gotovo paralelna s ordinatnom osi. Potrebna energija za crpenje proporcionalna je površini a – 5 – 6 – b na prikazanom p, v – dijagramu. Uzme li se da je volumen v5 prije kompresije praktički jednak volumenu v6 nakon kompresije, potrebna mehanička energija za crpenje je w p ( p2 − p1 ) ⋅ v5 = , koja je vrlo malena u usporedbi s mehaničkom energijom koja se dobiva ekspanzijom pare, te se može i zanemariti. Temperatura vode koja je pod tlakom p 2 dovedena u kotao (stanje 6) niža je od temperature isparavanja pri tlaku p 2 te se mora prije isparavanja ugrijati od T6 do T2 (vidi T, s – dijagram). To se postiže ili predgrijavajući vodu prije nego se uvede u kotao ili ucrpavajući je u hladnom stanju u kotao gdje se miješa s vrućom vodom parnog kotla. 227
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