Friday, December 26, 2008

Positive Displacement Compressors-Screw

Rotary Twin Screw Air Compressors

Description:
Rotary, helical screw, oil-injected, positive-displacement compressors are constant volume, variable pressure machines. They are available in a range from 25 cfm to 3000 cfm at pressure up to 600 psig in single, 2-stage & 3-stage designs.


USES
The most common uses of the rotary screw compressors are engine-driven portable compressors for mining, construction, and energy exploration.

And as electric-motor driven stationary compressors for industrial and mining application.

Rotary helical screw compressors of this types are characterized by compact, low vibration, simple foundation, broad pressure and capacity ranges, and long life with minimum maintenance.

They are used for pressure or vacuum services.

Design:
The single-stage design consists of a pair of rotor meshing in a one-piece, dual-bore cylinder. The male rotor, usually 4 helical lobes, spaced 90 degrees apart. The female rotor, usually has six corresponding helical grooves, spaced 60 degrees apart.

The rotor speed ratio is inversely proportional to the lobe-groove ratio. When the male rotor rotates at 3000 rpm, the female rotor would rotate ...... ....... rpm?

The male rotor can be directly driven, belt driven or gear driven by engine or electric motor. The female rotor is driven by the male rotor without metal-to-metal contact through the thin oil film developed between the rotors. Female rotor drive is possible in some design.

The rotors inter-mesh in the dual bores of a one piece cylinder (housing, or case). The cylinder provides air passages, oil injection holes, compression chamber, and discharge ports.

Each rotor is supported by rolling element bearings located at the ends of the rotor body. The bearings at one end, usually the discharge end, take the rotor axial thrust, carry radial loads, and provide for small axial running clearances necessary.

Bearings at the opposite end are floating bearings, which allow for uneven thermal expansion of the rotors and cylinder.

For 2 and 3 stages, the individual stages are designed to be similar to the single stage as far as the rotor configuration is concerned.

2 Styles

Two styles of multi-stage compressors are in common use;

Those axially mounted, in which the second stage is driven directly from the rear of the first stage.
And side or parallel mounted, in which the stages are each mounted directly on a gear housing and are independent of each other.

Some designs remove the oil and cool the air between stages in an intercooler, while others discharge the air and oil directly into the next stage.

Multi-stages
Multi-stages are used in compressors for either to improved efficiency or for higher pressure.


Performance
The overall efficiency of the unit depends almost entirely on the achievement of proper clearance in the rotors during manufacture. The normal manufacturer's warranty allows for plus +- minus of 3% variation in performance.

Discharge temperatures are much lower than in other types of compressors. Most manufacturers supply a thermostatic oil control valve to control the minimum discharge air temperature to avoid condensation in the reservoir or oil sump.

A 2-stage, 100n psig compressor improves performance by lowering the pressure differential across the leakage paths within the rotors [Blowhole] and end plates. The altitude [sea level vs mountain top] on which the compressor is located also has an effect upon performance.

The oi-injected screw machine is capable of operating on vacuum service and produces maximum vacuum equal to that of the 2-stage, double acting reciprocating machine. The oil flow are normally reduced because of lower mass flow.

The power consumption of the rotary-screw compressor during unloaded operation is normally higher than of reciprocating types. Recent developments by some manufacturers have led to systems in with the unloaded horsepower is 15% to 25% of loaded power.

This systems are normally used with electric-motor, constant-speed drives rather than engine-drives in which the engine speed can be reduced during unload, for reducing fuel consumption.

Application of the unit as a based load compressor is also recommended to avoid any disadvantage due to unloaded power coat.

source from CAGI handbook
edited version

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