Section 2 • Theory Of Operation; Compressor; Capacity And Volume Control - Emerson VILTER VSS Installation, Operation And Service Manual

Section 2 • Theory of Operation

Compressor

The Vilter™ Single Screw Compressor is a positive dis-
placement, capacity and volume controlled, oil flooded,
rotary compressor which uses a single main screw inter-
meshed by two opposing gate rotors. Gas compression
occurs when the individual fingers of each gate rotor
sweep through the grooves, or flutes, of the main screw
as the screw rotates. Compression occurs from the time
the screw flute is first closed off by the gate rotor finger,
until the time when the screw flute has rotated to the
point of lining up with the discharge port in the com-
pressor housing. A labyrinth type seal is used to prevent
gas at discharge pressure from leaking past the end of
the screw. Any discharge gas leakage past the labyrinth
seal is vented back to suction via four longitudinal holes
drilled through the body of the screw.
By venting the discharge end of the main screw back to
suction, forces on each end of the screw are equal. This
results in zero net axial forces on the main bearings. With
twin opposing gate rotors, all radial forces are cancelled
out also. Main shaft bearings have no net forces except
the weight of the screw and the shaft assembly.
The compressors are comprised of three rotating as-
semblies: the main screw assembly and the two gate
rotor assemblies. Each of these rotating assemblies use
a common bearing configuration consisting of a single,
cylindrical rolling element bearing at one end, and a
pair of angular contact ball bearings at the other end.
The pair of angular contact ball bearings are used to axi-
ally fix one end of the rotating shafts, and to absorb the
small amount of thrust loads on the shafts. The inner
races of the ball bearings are securely clamped to the ro-
tating shafts, while the outer races are securely held in
the bearing housing, thus fixing the axial position of the
shaft in relation to the bearing housings. The cylindrical
roller bearings at the opposite end of the shafts allow
for axial growth of the shafts while supporting the radial
loads from the shafts.
The suction gas enters the compressor housing through
the top inlet flange, at the driven end of the unit. The
driven end of the compressor housing is flooded with
gas at suction pressure. The gas enters the open end of
the main screw flutes at the driven end, and becomes
trapped in the screw flute as the screw rotates and the
gate rotor tooth enters the end of the flute. At this point,
the compression process begins. Directly after the screw
flute is closed off by the gate rotor tooth, oil is injected
into the groove.
The oil enters the compressor through a connection at
the top of the compressor. The purpose of the injected
oil is to absorb the heat of compression, to seal the gate
rotor tooth in the groove, and to lubricate the moving
parts.
VSS/VSR/VSM/VSH/VSSH Compressor • Installation, Operation and Service Manual • Emerson • 35391S
Additional internal oiling ports are provided at the main
and gate rotor bearings to cool and lubricate the bear-
ings. The mechanical shaft seal housing also contains
oiling ports to lubricate, cool and provide a sealing film
of oil for the mechanical shafts seal. Excess oil flows
through the check valves on the sealing baffle plate.
This oil is directed at the main rotor roller bearing, which
cools and lubricates the front roller bearing.
As the main screw rotates, the gate rotor is also driven,
causing the gate rotor tooth to sweep the groove in the
main screw. This sweeping action reduces the volume
of the groove ahead of the gate rotor tooth and causes
the trapped gas and oil to be compressed in the reduced
volume. As the main screw continues to rotate, the gate
rotor tooth continues to reduce the groove volume to a
minimum, thus compressing the trapped gas to a maxi-
mum pressure. A labyrinth seal arrangement prevents
the compressed gas from leaking past the end of the
screw. As the gate rotor tooth reaches the end of the
groove, the groove rotates to a position that lines up
with the discharge port in the compressor housing and
the gas/oil mixture is discharged from the screw at high
pressure. This completes the compression cycle for a
single flute of the main screw.
Once the gas is swept from the main screw flute through
the discharge port, it passes into the discharge manifold
of the compressor. From the discharge manifold, the
gas/oil exits the compressor housing.

Capacity and Volume Control

The Vilter™ compressors feature the exclusive Parallex™
Slide System, which consists of a pair of slides for each
gate rotor assembly. These two independently oper-
ated slides are referred to as the capacity slide and the
volume ratio slide. On the suction end of the screw, the
capacity slide moves to vary the timing of the beginning
of the compression process. With the slide moved all the
way out to the suction end of the screw (the 100% posi-
tion), the compression process begins immediately after
the gate rotor tooth enters the screw flute and closes off
the end of the groove. In this situation, the maximum
volume of gas is trapped in the screw flute at the start
of the compression process. As the slide is pulled back
away from the suction end of the screw, the start of the
compression process is delayed as some of the suction
gas is allowed to spill back out of the screw flute until the
screw rotates far enough to pass the end of the capac-
ity slide and begin compressing. This causes a reduced
volume of gas to be trapped in the screw flute when the
compression process begins. In this way, the capacity of
the compressor is reduced from 100% down to as low as
10% of the full rated capacity.
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