Ma{instvo 2(3), 63 - 78, (1999) N.Sto{i},...: SAVREMENI RAZVOJ KONSTRUKCIJE... Slika 1. Upore|enje nekih rotora vij~anih <strong>kompresora</strong> Figure 1. A comparison of several pairs of screw compresor rotors - 72 -
Ma{instvo 2(3), 63 - 78, (1999) N.Sto{i},..: SAVREMENI RAZVOJ KONSTRUKCIJE... razlika izme|u njih se uo~ava na vode}em rotoru koji po~inje sa ekscentri~nom kru`nicom a nastavlja se linijom, elipsom ili hiperbolom respektivno. Pokazuje se da je hiperbola najbolje mogu}e primjenjivo geometrijsko rje{enje. Na kraju grupe se nalaze dva 'rack generisana' profila [37] i [50]. Mo`e se vidjeti da posljednji profil ima najve}u dobavu. Tako|er, mogu se uo~iti jo{ dvije osobine koje karakteri{u te rotore i favorizuju ih u odnosu na ostale. Postignuto je zaptivanje du` cijele linije dodira i time manji nezaptiveni prostor, {to nije slu~aj kod profila [37]. Dodatno, kontaktne povr{ine sa jedne i druge strane profila u blizini podionog kruga su ravne linije na zup~astom {tapu tako da formiraju evolventu na rotorima. Zbog toga {to je relativno kretanje jednog rotora u odnosu na drugi gotovo ~isto kotrljanje, to je najbolji dodir {to ga je mogu}e posti}i izme|u rotora. Rack generisani rotori konfiguracije 5/6 su prikazani na slici 2. Posljednja grupa rotora sa 4 i 5 rotora u vode}em i vo|enom vijku respektivno, postaje sve vi{e popularna za uljnopodmazivane zra~ne kompresore. Prvi par rotora su "Cyclon" rotori [19]. Iza njih su SRM "D" rotori [4] i na kraju grupe rotori generisani pomo}u zup~astog {tapa [50]. 3.2 Konstrukcija <strong>kompresora</strong> Iako je moderan profil rotora neophodan uslov da bi vij~ani kompresor bio efikasan, sve ostale komponente tako|er moraju biti konstruisane tako da se na najbolji na~in iskoristi potencijal rotora i ostvare najbolje mogu}e performanse <strong>kompresora</strong>. Zbog toga je vrlo va`no pravilno odabrati zazor izme|u ku}i{ta i rotora, posebno na strani visokog pritiska. To, povratno, zahtijeva ili skuplje le`ajeve sa manjim zazorima ili jeftinije le`ajeve kod kojih su prednaprezanjem zazori svedeni na prihvatljivu veli~inu. Vij~ani kompresor je op}enito, a slu~aju uljnopodmazivanog <strong>kompresora</strong>, kada radi sa velikom razlikom pritisaka, posebno optere}en aksijalnim i radijalnim silama koje se na ku}i{te prenose preko le`ajeva. Za vij~ane kompresore malog i srednjeg kapaciteta obi~no se koriste kotrljaju}i le`ajevi, koji u tom slu~aju moraju biti tako dobro odabrani da zadovolje sve zadane uslove rada. Obi~no se koriste dva le`aja na tla~noj strani vratila rotora kako bi se aksijalne i radijalne sile na ku}i{te prenijele odvojeno. Tako|er je bitno je da je me|uosno rastojanje djelimi~no odre|eno i veli~inom le`ajeva i njihovim unutra{njim zazorima. Kontaktne sile, koje su definisane obrtnim momentom koji se prenosi izme|u rotora, igraju klju~nu ulogu kod <strong>kompresora</strong> sa direktnim dodirom rotora. Ove su sile relativno male u slu~aju pogona preko vode}eg imity of the pitch circles are straight lines on the rack which form involutes on the rotors. Hence the relative motion between the rotors is close to pure rolling which is the best possible. 3.2 Compressor Design Screw compressor design has gradually evolved and the trend is to get as small as possible a machine to meet the required performance. This means that rotor tip speeds are as high as possible within the limits imposed by efficiency requirements. Normal practice is to use rolling element bearings wherever possible because these permit smaller clearances than journal bearings. Similarly, the ports are made as large as possible to minimize suction and discharge gas speeds. These features lead to great similarity in all designs for any specified application. Although advanced rotor profiles are a necessary condition for a screw compressor to be efficient, all other components must be designed to minimise losses if maximum improvements are to be achieved. Thus the rotor to housing clearances must be properly selected, especially at the high pressure end. This in turn requires either expensive bearings with small clearances or cheaper bearings with their clearances reduced to an acceptable value by preloading. A screw compressor, especially of the oil flooded type, operates with high pressure differences. These create large axial and radial bearing forces. Rolling element bearings are normally chosen for small and medium screw compressors and these must be carefully selected to obtain a satisfactory design since, inter alia, the distance between the rotor centre lines is in part determined by them. It must be added that recent advances in the development of advanced low friction rolling element bearings greatly contributes to this choice [31]. Usually two bearings are fitted at the high pressure end of the rotor shafts in order to absorb the radial and axial loads separately. The contact force between the rotors is determined by the torque transferred between them and is very significant when they make direct contact. It is relatively small when the compressor drive is through the main rotor. However, when the drive is through the gate rotor, the contact force is substantially larger and, as far as possible, this arrangement should be avoided. - 73 -
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