TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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TEHNIČKA TERMODINAMIKA__________________________________________ menuto ohlađivanje nazvano prigušnim efektom ili Joule-Thomsonovim efektom. Joule-Thomsonov efekt ili prigušni efekt Jednu od bitnih oznaka idealnih plinova upoznali smo jednostavnu ovisnost unutarnje energije o toplinskom stanju. Kod idealnog plina unutarnja energija u, a time i entalpija h, ovisne su samo o temperaturi, a ne i o tlaku ili gustoći. Između ostalog, to znači da se temperatura nekog idealnog plina ne mijenja kad se plin prigušuje od tlaka p na niži tlak p 0 . Tako se pri niskim tlakovima vladaju i stvarni plinovi. No, kod prigušivanja stvarnih plinova pojavljuje se pri višim tlakovima veća ili manja promjena temperature koju je mjerenjima ustanovio već Joule. Taj neznatni efekt je za većinu tehničkih procesa posve beznačajan, no za ukapljivanje plinova postao je odlučno važan. Kao prigušni efekt označujemo omjer ⎛ ∆T ⎞ ⎜ p ⎟ . Tu je ∆ T = T − T0 sni- ⎝ ∆ ⎠h ∆p = p − p 0 bar ženje temperature kod prigušenja plina za razliku tlakova ( ) -2 (ili ∆p, N m ), kako je već ranije spomenuto. Indeks h upućuje na proces pri h = konst. koji uvjet označuje prigušivanje bez izmjene topline. Što je slabije prigušivanje ∆p to je manja i promjena temperature ∆T pa kao granični slučaj ⎛ ∂T ⎞ dobivamo diferencijalni prigušni efekt ⎜ p ⎟ . ⎝ ∂ ⎠h Pri pokusima, koji su se kretali u području od 0 °C do 100 °C i od 6 bar naniže, ustanovili su Joule i Thompson da prigušni efekt ovisi samo temperaturi, a ne i o tlaku. Ustanovili su također da se pri temperaturi okoline zraku prigušenom za 1 bar snižava temperatura za otprilike četvrtinu K. Kasniji su pokusi pokazali da je prigušni efekt ovisan i o tlaku. Postoji temperatura na kojoj prigušni efekt prolazi kroz vrijednost nula, tj. mijenja predznak. To je, tzv. temperatura inverzije. 8.2. UKAPLJIVANJE PREMA LINDEU Zamisao C. Lindea, 1895. godine, da primjenom protustrujnog izmjenjivača topline iskoristiti prigušni efekt za ukapljivanje zraka omogućila je ne samo ukapljivanje na industrijskom temelju, već je ono postalo i ekonomski 334
______________________________________________________________Ukapljivanje plinova premoćno. Najjednostavniju napravu za ukapljivanje zraka po Lindeu prikazuje slika 121. Slika 121.Jednostavni uređaj za ukapljivanje plinova po Lindeu Hlađenim kompresorom siše se zrak okolnog stanja 1 ( p 0 , t 0 ) i tlači se po mogućnosti izotermno na visoki tlak p 2 = 200 bar. Važno je da se stlačeni zrak što ječe ohladi rashladnom vodom, stanje 2 ( t 2 = t1 ). S tim stanjem ulazi stlačeni zrak u protustrujni izmjenjivač topline (rekuperator) te stiže do prigušnog ventila. Proces ćemo slijediti i pomoću T, s – dijagrama, slika 122. 335
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TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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