savremeni razvoj konstrukcije i proizvodnje vij^anih kompresora ...
savremeni razvoj konstrukcije i proizvodnje vij^anih kompresora ...
savremeni razvoj konstrukcije i proizvodnje vij^anih kompresora ...
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Ma{instvo 2(3), 63 - 78, (1999)<br />
N.Sto{i},...: SAVREMENI RAZVOJ KONSTRUKCIJE...<br />
Drugi na~in na koji je mogu}e definisati profil rotora<br />
je da se osnovne krive izaberu na nekom<br />
zami{ljenom, "nefizikalnom" rotoru. Po{to su sve<br />
jedna~ine zup~anika neovisne od koordinatnog sistema<br />
u kojem su izra`ene, primarni luk je mogu}e<br />
definisati i u nekom koordinatnom sistemu koji je ne<br />
ovisi ni o jednom rotoru. U ve~ini slu~ajeva se na<br />
taj na~in jedna~ine mogu pojednostaviti. Upotreba<br />
jednog koordinatnog sistema za definisanje svih krivih<br />
na rotoru tako|er mo`e pojednostaviti proces<br />
profilisanja. Tako se polazni profil u cijelosti mo`e<br />
definisati u koordinatnom sistemu koji ne zavisi od<br />
rotora, {to vrijedi i za rotor sa beskona~nim<br />
pre~nikom koji se naziva zup~asti {tap. Prvi ikada<br />
publikovani patent koji prikazuje ovakvu generaciji<br />
profila rotora [30] objavljen 1977, iako nije prakti~no<br />
primjenljiv, vje{to koristi pomenutu teoriju. [37] i kasnije<br />
[50] prvi daju dobru osnovu za generaciju profila<br />
vij~anog <strong>kompresora</strong>.<br />
Efikasan vij~ani kompresor mora imati rotore ~iji profil<br />
osigurava veliki proto~ni presjek, kratku liniju zaptivanja<br />
i mali nezaptiveni prostor na vrhu rotora. To<br />
zna~i, da je za ve}i proto~ni presjek i broj obrtaja<br />
<strong>kompresora</strong>, i dobava <strong>kompresora</strong> ve}a. Kra}e linije<br />
dodira i manji nezaptiveni prostor smanji}e unutra{nje<br />
curenje. Ve}a dobava i manje curenje }e pove}ati<br />
volumetrijski stepen iskori{tenja <strong>kompresora</strong>, koji je<br />
defini{an kao dobava podijeljena zbirom ukupnog<br />
protoka i curenja. Time se dalje pove}ava adijabatski<br />
stepen iskori{tenja, jer se za kompresiju gasa koji<br />
recirkuli{e unutar <strong>kompresora</strong> tro{i manje snage.<br />
Izbor odnosa veli~ine nezaptivenog prostora i broja<br />
radnih zapremina zavisi}e od namjene <strong>kompresora</strong>.<br />
Kako je za manje razlike pritisaka curenje proporcionalno<br />
manje, dobici postignuti velikim proto~nim<br />
presjekom mogu poni{titi ili smanjiti gubitke nastale<br />
usljed velikog nezaptivenog prostora. Na sli~an na~in<br />
se mo`e dobiti optimalni broj zuba vij~anog <strong>kompresora</strong>,<br />
po{to manji broj zuba daje ve}i proto~ni<br />
presjek ali i rezultira u ve}oj razlici pritiska izme|u<br />
radnih prostora.<br />
Sa smanjenjem zazora izmedju rotora, bez obzira na<br />
podmazivanje <strong>kompresora</strong> uljem, vjerovatno}a direktnog<br />
dodira me|u rotorima sa pove}ava. Dodir<br />
me|u rotorima dovodi do pove}anja kontaktnih sila i<br />
optere}enja rotora {to dalje uzrokuje deformaciju<br />
vo|enog rotora. Zbog toga, njegov profil mora biti<br />
konstruisan tako da se rizik od pucanja ili plasti~ne<br />
deformacije rotora smanji na najmanju mogu}u mjeru.<br />
Brzi <strong>razvoj</strong> metoda matematskog modeliranja i<br />
ra~unarske simulacije je, u posljednje vrijeme, podstakao<br />
i olak{ao istra`ivanje novih profila rotora.<br />
use of one coordinate system to define all the<br />
curves, simplifies the design process. Typically, the<br />
template is specified in a rotor independent coordinate<br />
system. This is valid for a rotor of infinite<br />
radius, which is a rack. From this, a secondary arc<br />
on some of the rotors is obtained by a procedure,<br />
which is called 'rack generation'. The first patent<br />
on rack generation published, [30], is based on<br />
this theory but lacks practicality. [37] and, more<br />
recently, [50] give a good basis for rotor profile<br />
generation.<br />
For a screw compressor to be efficient, the rotor<br />
profile must form a large flow cross section area, a<br />
short sealing line and a small blow-hole area. The<br />
larger the cross section area, the higher the flow<br />
rate for the same rotor sizes and speeds. Shorter<br />
sealing lines and a smaller blow-hole reduce leakages.<br />
Higher flow and smaller leakage rates both<br />
increase the compressor volumetric efficiency, which<br />
is the rate of flow delivered as a fraction of the<br />
sum of the flow plus leakages. This in turn increases<br />
the adiabatic efficiency because less power is<br />
wasted in the compression of gas which is recirculated<br />
internally.<br />
The optimum choice between blow hole and flow<br />
areas depends on the compressor duty since, for<br />
low pressure differences, the leakage rate will be<br />
relatively small and hence the gains achieved by<br />
a large cross section area may outweigh the losses<br />
associated with a larger blow-hole. Similar considerations<br />
determine the best choice for the number<br />
of lobes since fewer lobes imply greater flow<br />
area but increased pressure difference between<br />
them.<br />
As precise manufacture permits rotor clearances to<br />
be reduced, despite oil flooding, the likeliehood of<br />
direct rotor contact is increased. Hard rotor contact<br />
leads to deformation of the gate rotor, increased<br />
contact forces and ultimately rotor seizure. Hence<br />
the profile should be designed so that the risk of<br />
seizure is minimised.<br />
The search for new profiles has been both stimulated<br />
and facilitated by recent advances in mathematical<br />
modelling and computer simulation. These<br />
analytical methods may be combined to form a<br />
powerful tool for process analysis and optimisation<br />
and thereby eliminate the earlier approach of intuitive<br />
changes, verified by tedious trial and error testing.<br />
As a result, this approach to the optimum<br />
design of screw rotor lobe profiles has substantial-<br />
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