TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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TEHNIČKA TERMODINAMIKA__________________________________________ Kod ljevokretnog Carnotovog kružnog procesa rad se troši pri čemu se oduzima toplina rashladnom spremniku, a dovodi toplina ogrjevnom spremniku. Na taj način u neku ruku dižemo toplinu od niže temperature rashladnog spremnika na višu temperaturu ogrjevnog spremnika (dizalica topline). Da bi se to postiglo moraju se temperature lijevokretnog procesa odabrati tako da je t > t g , a t 0 < t h jer samo u tom slučaju toplina će prelaziti u žaljenom smjeru. U principu je dovoljno da se ove razlike temperatura odaberu po volji malene, tako da uz određene toplinske spremnike mogu u graničnom slučaju temperature plina u desnom procesu i u lijevom procesu biti iste i jednake temperaturama dotičnih (zadanih) toplinskih spremnika. Iako bi pri tome prijelaz topline tekao beskonačno polagano ipak taj krajnji slučaj smatramo neobično važnim. Kako vidimo toplinski spremnici sa svojim temperaturama stavljaju granice kružnom procesu te se i time očituje njihova presudna važnost za dobivanje rada. Pokušaji da se ostvari toplinski stroj prema Carnotovu kružnom procesu nisu uspjeli pa on danas ima teorijsko značenje i to za usporedbu s tehnički izvedivim procesima. 3.2. JOULEOV KRUŽNI PROCES Za tehničku praksu bolje odgovara kružni proces koji se sastoji od dvije izobare i dvije adijabate. To je Jouleov kružni proces, slika 25., a može se ostvariti postrojenjem prikazanim na shemi a, slika 26. Plinu se u zagrijaču Z a dovodi toplina Q pri konstantnom tlaku i uz povećanje volumena od V1 do V2 . Pri tome se temperatura povećava od T1 na T2 . Toplinu zagrijaču predaju, npr. plinovi izgaranja ili se ona odvodi, npr. iz nuklearnog reaktora. Tako zagrijani plin struji u ekspanzijski stroj, npr. turbinu i tu adijabatski ekspandira do tlaka p 3 = p 0 pa prilikom ekspanzije predaje mehaničku energiju (mehanički rad) rotoru turbine. Nakon ekspanzije u turbini, plin dolazi u izmjenjivač topline Z b u kojem se hladi pri konstantnom tlaku pomoću vode ili zraka kojima predaje toplinu Q0 . Kada je postigao temperaturu T4 , plin se komprimira adijabatski u kompresijskom stroju, kompresoru, do tlaka p 1 = p . Temperatura T 2 je najviša, a T 4 najniža u procesu. Da bi se osigurao prijelaz topline između ogrjevnog spremnika i plina, odnosno između plina i rashladnog spremnika, mora temperatura Tg biti nešto veća od temperature T2 , odnosno Th nešto manja od T4 . Dakle, toplina se odvodi i dovodi u Jouleovom procesu uz konstantni tlak. 110
___________________________________________________________________Kružni procesi Slika 25. Jouleov kružni proces Slika 26. Shema postrojenja za primjenu Jouleovog kružnog procesa 111
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