Section 1, General Information; Figure 1-1 - Compression Cycle - Gardner Denver ELECTRA SAVER II G2 STG2-40 Operating & Service Manual

SECTION 1
GENERAL INFORMATION
COMPRESSOR - The Gardner Denver Rotary Screw compressor is a single stage, positive displacement
rotary machine using meshing helical rotors to effect compression. Both rotors are supported between high
capacity roller bearings located outside the compression chamber. Single width cylindrical roller bearings
are used at the inlet end of the rotors to carry part of the radial loads. Tapered roller bearings at the
discharge end locate each rotor axially and carry all thrust loads and the remainder of the radial loads.
COMPRESSION PRINCIPLE (Figure 1-1) Compression is accomplished by the main and secondary rotors
synchronously meshing in a one-piece cylinder. The main rotor has five (5) helical lobes 72 apart. The
secondary rotor has six (6) matching helical grooves 60 apart to allow meshing with main rotor lobes.
The air inlet port is located on top of the compressor cylinder near the drive shaft end. The discharge port
is near the bottom at the opposite end of the compressor cylinder. It is then routed to the top of the
integrated cast housing. Figure 1-1 is an inverted view to show inlet and discharge ports. The compression
cycle begins as rotors unmesh at the inlet port and air is drawn into the cavity between the main rotor lobes
and secondary rotor grooves (A). When the rotors pass the inlet port cutoff, air is trapped in the interlobe
cavity and flows axially with the meshing rotors (B). As meshing continues, more of the main rotor lobe
enters the secondary rotor groove, normal volume is reduced and pressure increases.
Oil is injected into the cylinder to remove the heat of compression and seal internal clearances. Volume
reduction and pressure increase continues until the air/oil mixture trapped in the interlobe cavity by the
rotors passes the discharge port and is released to the oil reservoir (C). Each rotor cavity follows the same
"fill-compress-discharge" cycle in rapid succession to produce a discharge air flow that is continuous,
smooth and shock free.
AIR FLOW IN THE COMPRESSOR SYSTEM (Figure 5-3, page 54) Air enters the air filter and passes
through the inlet unloader valve to the compressor. After compression, the air/oil mixture passes into the
oil reservoir where most of the entrained oil is removed by velocity change and impingement and drops
back into the reservoir. The air and remaining oil passes into the separator and separator housing where
the oil is separated and passes through tubing connecting the separator housing and compressor. The air
passes through the minimum pressure valve, discharge check valve and cooler, then to the plant air lines.
Figure 1-1 COMPRESSION CYCLE
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(Ref. Drawing)