Evacuating apparatus

An evacuating apparatus comprising:

• a housing having a fluid suction opening and at least one fluid exhaust opening;
• a first pump section mounted in said housing and having a suction side and an exhaust side;
• a second pump section, having a suction side and an exhaust side and being mounted in said housing on the exhaust side of said first pump section, for exhausting a smaller amount of fluid than said first pump section;
• wherein said first and second pump sections comprise a plurality of rotors accommodated in said housing;
• wherein bearings are mounted in said housing and rotatably support said rotors;
• wherein said first pump section comprises a positive displacement pump which positively displaces fluid by utilizing a volume change of a space defined by said rotors and said housing;
• wherein said first pump section comprises a pair of rotors having respectively interengaging thread grooves;
• wherein said second pump section comprises a pair of rotors having respectively interengaging thread grooves;
• and
• wherein the following equations are satisfied:
• B.sub.2 >B.sub.1 ;
• and
• h.sub.1 >h.sub.2 ;
• wherein B.sub.1 is an average width of the thread grooves formed on said rotors of said first pump section, B.sub.2 is an average width of the thread grooves formed on said rotors of said second pump section, h.sub.1 is an average depth of the thread grooves formed on said rotors of said first pump section, and h.sub.2 is an average depth of the thread grooves formed on said rotors of said second pump section.