TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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_________________________________________________Drugi glavni zakon termodinamike radne sposobnosti davatelja rada njegovo stanje, dakle, treba izjednačiti (uravnotežiti) sa stanjem okoline tako da postane i p 2 = p 0 T 2 = T 0 . Slijedi da je maksimalni mogući rad nekog davatelja rada pri zadanom stanju okoline ovisan samo o početnom stanju davatelja rada time i o fizikalno-kemijskim svojstvima davatelja rada, npr. toplinskom kapacitetu i sl. Slijedi da za dobivanje maksimalnog rada mora davatelj rada povrativim načinom prijeći u stanje koje je u ravnoteži s okolinom. Prikazana jednadžba za maksimalan rad predstavlja izraz u najopćenitijem obliku. To znači da ovaj izraz vrijedi za sve povrative procese, bez obzira na vrstu radnog tijela. Za stvarne, tj. nepovrative procese rad je uvijek manji od maksimalnog. 4.5.1. Maksimalan rad nekih tipičnih povrativih procesa Pretpostavimo da je davatelj rada idealan plin, odnosno neki stvarni plin (zrak, vrući produkti sagorijevanja itd.), koji je po svojim svojstvima vrlo blizak idealnom plinu pa se na njega mogu primijeniti zakoni idealnog plina i izrazi koje smo za povrative promjene stanja idealnih plinova dobili na temelju I. i II. zakona termodinamike. Razmotrit ćemo nekoliko primjera. 4.5.1.1. Maksimalan rad zraka stlačenog u rezervoaru Razmotrit ćemo koliki maksimalni rad može izvršiti zrak, stlačen u nekom rezervoaru volumena V1 , a pod tlakom p1 (viši tlak od atmosferskog), ako mu je temperatura jednaka temperaturi okolnog zraka, ( T 1 = T 0 ). Zadatak, kako se vidi, sastoji se u tome da se pomoću nekog uređaja (ili ako bi dno rezervoara bilo pokretno) omogući širenje stlačenog zraka pri T 1 = T 0 = konst. do tlaka okoline p 0 (v. dijagram). Ovakav proces je tipičan povrativ proces jer se odigrava pri razlici temperatura stlačenog zraka (davatelja rada) i okoline jednakoj nuli ( T 1 = T0 ) pa će i rad koji se tim procesom ostvari biti maksimalan. 143
TEHNIČKA TERMODINAMIKA _________________________________________ Dakle, u stanju 2 postignut je okolni tlak p o . Budući da je temperatura zraka u rezervoaru (spremniku) stalno bila jednaka okolnoj temperaturi, u stanju 2 zrak spremnika prilagodio se okolnom stanju pa novo dobivanje rada više nije 144
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TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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