TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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TEHNIČKA TERMODINAMIKA__________________________________________ ε q0 q0 h = = w q − q0 Da bismo stupanj rashladnog učina točnije odredili, rastavimo u mislima proces na niz uskih kružnih procesa, slika 113. Jedan od njih prikazan je zasjenjeno. Za ovakav je elementarni kružni proces stupanj rashladnog učina ∂ q0 T ′ h 1 ε ′ h = = = = ∂ w T ′ ′ ′ 0 − T T h 0 −1 ⎛ T ′ ⎜ h ⎝ Za sve takve elementarne procese između tlakova κ −1 ⎛ p ⎞ κ ⎜ ⎟ ⎝ p0 ⎠ p 0 p p 0 1 ⎞ ⎟ ⎠ κ −1 κ −1 . i p vrijednost izraza je ista. Zato je i za sveukupni proces 1 – 2 – 3 – 4 stupanj rashladnog učina ε 0 1 = h κ − 1 κ ⎛ ⎜ ⎝ p p iz čega vidimo da što je veća razlika p0 i p to je manji εh. Najčešće je p 0 ≈1 bar pa treba nastojati da p bude što niži, ali se ipak ne može ići ispod određene granične vrijednosti, kako je ranije prikazano. To uzrokuje previše velike dimenzije cilindra kompresora i jedan je od temeljnih nedostataka zračnog rashladnog stroja. Toplinski kapacitet zraka je malen pa je za određeni rashladni učinak potrebna velika količina zraka. Zato su strojevi glomazni i skupi. Na slici 114. prikazana je usporedba termodinamičkog procesa zračnog rashladnog stroja s Carnotovim. Kao što se vidi, isti efekt hlađenja postigao bi se Carnotovim ciklusom 1 - -2’ – 3 – 4’ – 1, tj. kada bi se ostvarilo izotermno hlađenje radnog tijela (proces 2’ – 3) i izotermno zagrijavanje (proces 4’ – 1). Time bi se izbjeglo beskorisno povišenje temperature radnog tijela do T 2 (proces 2’ – 2), odnosno sniženje njegove temperature do T 4 (proces 4 – 4’). Odvedena toplina u lijevokretnom Carnotovom procesu bila bi veća (za površinu 1 – 4’ – 4 – 1), dok bi utrošeni rad bio manji (površina 1 – 2’ – 3 – 4’ – 1 manja od površine 1 – 2 – 3 – 4 – 1) pa je i koeficijent hlađenja (rashladni učin) 308 ⎞ ⎟ ⎠ −1
_______________________________________________________________Rashladni strojevi zračnog stroja manji od koeficijenta hlađenja lijevokretnog Carnotovog kružnog procesa, tj. ε < ε . To je i ranije bilo naglašeno, uz napomenu o praktičnoj h hc neostvarljivosti Carnotovog kružnog procesa sa zrakom ili nekim drugim idealnim plinom kao radnim tijelom. Zračni rashladni strojevi danas su gotovo potpuno odbačeni i zamijenjeni parnim. Rabe se još sasvim izuzetno za hlađenje rudnika, brodova i u slične svrhe. Slika 113. Utjecaj tlačnog omjera na koeficijent hlađenja zračnog rashladnog stroja p p 309
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