Sunday, August 15, 2010

GEARED CENTRIFUGAL COMPRESSORS(Design,Operation,Basics,Capacity Range)

GEARED CENTRIFUGAL COMPRESSORS



A multistage, multishaft integral gear compressor consists of individual scroll casings flanged to the gearbox and connected by piping. This compressor type needs fewer stages for a given pressure ratio than a multistage, single shaft turbo compressors. The reduced number of stages, i.e. the increased stage pressure ratio, results from the open-type impeller design which can be used here as well as from the higher speeds which are achieved with this design. The resulting compact manner of construction, which involves considerable savings in terms of weight and space required, is an interesting aspect for investment planning.

Not only are the investment costs reduced in comparison to single shaft compressor, the operating costs are lower as well, since all stages can be operated in their optimum efficiency range by selecting the appropriate pinion shaft speed.

Further plus points of gear compressors:

- The medium can be cooled following each stage
- Intermediate sidestreams and extraction are possible following each stage (multi service operation)
- Can be easily adapted to modified operating conditions by means of a timesaving exchange of impellers.
- Each stage can easily be equipped with inlet guide vane control and adjustable diffuser vane control, thus enabling the control elements to become effective for the entire compressor.

COMPRESSOR DESIGN

The multistage, multi shaft integral gear compressor consists of individual scroll casings flange-connected to the gear casing. The im1pellers in the scroll casings are mounted on the extended pinion shafts (“overhung design”) . Each compressor stage has an axial suction nozzle, thus making it possible to arrange an inlet guide vane in front of each stage. In each compressor stage sealing thus takes place only at the rear wall.

Compressor Scroll Casing:

Each scroll casing consists of two castings, there in adjustable diffuser vane are installed. The entire frame consists of the scroll casing with the rear wall and the inlet diaphragm as well as the diffuser front wall. These castings are centered radially and bolted together. Rear wall and diffuser vane control, a vaned diffuser in which a part of the kinetic energy (velocity energy) of the gas is transformed into potential energy (pressure increase). The transformation of the residual kinetic energy takes place in the conical discharge nozzle of the scroll.

Gear – Design:

The gear is a compact, multi shaft spur-type single helical design. The central gear wheel and two pinion shafts, which run at different speeds, are located in the plane of the horizontal joint. After the top half of gearbox is removed, the central gear wheel and the pinion shafts with their bearings and seals are freely accessible for maintenance and inspection.

The central gear wheel is driven via a coupling. Plain journal bearings are used for the low-speed shaft. One is a radial bearing, the other a combined journal and thrust bearing. The bearings of the pinion shafts are tilting pad bearings with floating-ring seal and five tilting pads per bearing. The design feature increases operating safety and minimizes undesired mechanical losses. The tilting pad bearings give the turbo compressor, which runs in supercritical ranges, a very high degree of operating stability and minimize rotor vibrations.

The axial thrust generated by the stages is transferred to the central gear shaft via thrust collars of the pinion shafts and the residual thrust is absorbed by the thrust bearing. A mechanically driven pump is integrated into the gear; it guarantees the lube oil supply in normal operation and in case of power failures supplies enough oil for coast-down.






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