TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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__________________________________Tehnička postrojenja za provedbu kružnih procesa sori se u praksi grade do oko 6 do 9 bar tlaka kao jednostupanjski, a za više tlakove kao višestupanjski. Slika 108. Proces s trostupanjskom kompresijom 301
_______________________________________________________________Rashladni strojevi 7. RASHLADNI STROJEVI Termodinamički procesi za postizavanje niskih temperatura rabe se kada niske temperature ne možemo postići hlađenjem vodom ili zrakom. Najstariji način umjetnog hlađenja je uporaba rashladnih smjesa nekih soli s ledom koje daju niske temperature. Npr. 22 %-tna smjesa NaCl i leda daje temperaturu od – 21 °C, a 30 %-tna smjesa CaCl 2 i leda daje temperaturu od –55 °C. Danas se za postizavanje niskih temperatura rabe rashladni strojevi. U njima se hladnoća postiže na sljedećim principima: a) Isparavanjem niskohlapivih tekućina, npr. tekućeg amonijaka; što je niži tlak isparavanja tekućine i postignute su temperature niže. b) Ekspanzijom komprimiranih plinova u ekspanzijskom stroju; kada plin vrši vanjski rad na račun smanjenja svoje unutarnje energije, temperatura se snizuje. c) Prigušivanjem komprimiranih plinova i para; ovdje se ne vrši vanjski rad, a proces je praćen sniženjem temperature. Zadatak rashladnih strojeva je postizavanje i održavanje nižih temperatura od temperatura okoline. Jedan od najvažnijih zadataka rashladnih strojeva je hlađenje nesavršeno izoliranih prostorija u kojima treba održavati temperaturu nižu od okoline. U takve će prostorije prodirati toplina iz toplije okoline. Drugi vrlo česti zadatak je proizvodnja leda od vode. U oba slučaja treba odvoditi toplinu kod pretežno konstantnih temperatura. Od takvog hlađenja razlikuje se zadatak ohlađivanja zadane tvari s okolne temperature na nižu jer se ovdje toplina mora odvoditi kod promjenjivih temperatura. Na to ćemo naići u proizvodnji krutog ugljikovog dioksida poznatog kao “suhi led”, pri ohlađivanju većih količina kapljevina od više temperature na nižu i kod ukapljivanja zraka. Toplina koja se na taj način odvodi rashladnoj prostoriji ili vodi koja se smrzava mora se negdje predati kod više temperature. Konačno se ova toplina mora uvijek predati okolini, odnosno rashladnoj vodi kao predstavniku okoline. 301
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TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet