Rotary Screw Compressors

In a rotary screw compressor, air enters the compressor and is trapped between mating male and female rotors and compressed to the required discharge pressure. In the basic single-stage design, the compressor consists of a pair of rotors meshing in a one-piece, dual-bore cylinder. Oil lubricates, seals, and cools the compressor. Oil-flooded rotary screw compressors are typically available in capacities ranging from 25 to 3,000 cubic feet per minute (cfm) or 0.7 to 85 cubic meters per minute (m3/m) at pressures up to 600 psig (42 bar). Most units are ported for a pressure of 100 psig (7.9 bar), but are suitable for operation between 50 and 200 psig (4.5 to 14.8 bar) without large losses of efficiency. Single-, two-, and three-stage designs are available.

Figure 30-8 is an illustration of a rotary helical-screw compressor. In a single-stage unit, the air inlet is usually located at the top of the cylinder. The cylinder provides air inlet passages, oil injection points, compression zone, and discharge port. The male rotor typically has four helical lobes that are spaced 90 degrees apart. The female rotor has corresponding helical grooves, usually six, spaced 60 degrees apart. Typically, the male rotor is driven directly or indirectly by an electric motor or prime mover and the female rotor is driven by the male rotor. The thrust bearings (at the discharge end) take the rotor axial thrust and carry radial loads. The floating bearings on the opposite end allow for unequal thermal expansion of the rotor and cylinder.

Rotary Compressor Roller
Fig. 30-8 Rotary Helical Screw Compressor Showing Thrust-Carrying Roller Bearings at One End and Floating Bearings at the Other End. Source: Compressed Air and Gas Institute

Figure 30-9 illustrates the compression cycle for a single-stage helical screw-type compressor. With oil injected rotary screw compressors, air is drawn into the cavity between the main rotor lobes and secondary rotor grooves. As they continue to rotate, the rotor lobes pass the edges of the inlet ports, trapping the air in a cell that forms between the rotor cavities and the cylinder wall. Continued rotation causes the main rotor lobe to roll into the secondary rotor groove, reducing volume and thereby raising pressure. After the cell is closed to the inlet, oil is injected to seal the clearances and remove heat. Compression ceases when the rotor lobes pass the edge of the discharge port and release the compressed air/oil mixture.

As the mixture passes to the oil reservoir, velocity change and impingement cause much of the oil to fall from the air. The air then passes through a separation device which removes most of the rest of the oil. Oil carryover to discharged air typically ranges from 0.002 to 0.005 ounces/cf (2.09 to 5.22 ml/m3). An effective oil filtering system is also required to protect bearings and rotating elements.

Oil-free or dry screw compressors are also available, but are much more expensive and less efficient. In an oil-free screw compressor, the rotors have to be geared so that they do not touch. There is also no oil to seal the rotor tips or absorb the heat of compression. Oil-free screw compressors are typically two-stage units.

Capacity modulation can be affected in several ways:

• Air inlet throttling utilizes an inlet air valve that modulates in response to pressure sensing controls. A typical inlet throttled screw compressor operating at 70% capacity will require nearly 90% full-load power (22% efficiency loss).

• On-line/off-line control cycles the inlet air valve between the fully-open and fully-closed position. This method is more efficient than air inlet throttling, but results in pressure swings and usually requires a discharge air receiver. It can also produce increased axial stress on bearings.

• Geometry control uses a turn- or slide-valve to change the geometry within the compression chamber, changing the effective length of the rotors.

• Speed control can be used to efficiently reduce airflow capacity. Like all positive displacement machines, screw compressors continue to operate near full-load volumetric efficiency when speed is reduced. Input power requirement is reduced roughly in proportion to airflow. Since the ability to reduce speed is limited by the driver's capability, it is usually necessary to rely on an additional control method for operation under low-load conditions.

Rotary screw compressors now dominate the midsized compressed air market. They are characterized by low vibration and simple foundation requirements, broad pressure and capacity ranges, low maintenance, and long service life. A critical advantage of the rotary screw compressor over the reciprocating compressor is reduced maintenance requirements. While reciprocating units will typically require minor overhaul (e.g., valves) every 8,000 hours, screw compressor intervals may be 20,000 to 40,000 hours. Screw compressors are also smaller, quieter, and less expensive than reciprocating units in most mid-range and larger capacities and use less oil.

Screw compressors are usually less efficient than reciprocating units of comparable capacity, particularly under part-load conditions. In many applications, compressors operate at less than full load all or most of the time. As noted above, operating cost can exceed capital investment cost by several times during the life of the compressor, and the inability of constant-speed screw compressors to unload as efficiently as reciprocating units can thus be a significant disadvantage in many applications.

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