Emerson VSG & VSSG Specifications download pdf (Page 13)

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Description

COMPRESSOR

The Vilter Single Screw Compressor is a positive

displacement, capacity and volume controlled,

oil flooded, rotary compressor which uses a

single main screw intermeshed by two oppos-

ing gate rotors. Gas compression occurs when

the individual teeth 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 compressor 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 VSG compressors are comprised of three

rotating assemblies: 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 axially 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 rotating

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 hous-

ing 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 connec-

tion at the top of the compressor. The purpose of

the injected oil is to absorb the heat of compres-

sion, to seal the gate rotor tooth in the groove,

and to lubricate the moving parts.

Additional internal oiling ports are provided at the

main and gate rotor bearings to cool and lubricate

the bearings. 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 di-

rected 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

maximum 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

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Emerson VSG & VSSG Specifications Page 13

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