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Oil-Free Rotary Screw Air Compressor

Introduction to Air Compressors
Air Compressor Principals
Air Compressor Types (1.5 to 100 horsepower)
Positive Displacement Compressors
Reciprocating Compressors
Single Acting
Double Acting
Reciprocating Compressor Capacity Controls
Rotary Screw Compressors
Oil Lubricated Rotary Screw Compressor
Oil Lubricated Rotary Screw Capacity Controls
Rotary Compressor Lubricants
Oil-Free Rotary Screw Compressor
Oil-Free Rotary Screw Controls
Motors and Motor Control for Air Compressors
Air Compressor Installation and Maintenance
Compressed Air Purification

Technological advances in the manufacturing of textile products, foods and beverages, electronics, chemicals, medical and plastics have created a significant demand for clean and dry compressed air. Oil-free compressors are required by these high technology industries since any contact with compressor lubricants can result in reduced quality of the end product.

Oil-Free Rotary Screw Air Compressors have become the standard in the 50 (37 kw) to 100 (75 kW) horsepower range. Oil-free rotary screw compressors offer many of the same performance, reliability and cost savings of lubricated rotary designs but without compressor lubricant in the compression chamber.
Oil-Free rotary screw compressors are typically two (2) stage designs. This is done to gain the improved efficiency of an intercooler and to minimize the final discharge temperature. The first stage generally compresses ambient air to 30 psig (2 bar). The 30 psig (2 bar) air then flows through intercooler and into the second stage. The second stage increases the pressure usually to either 100 psig (7 bar) or 125 psig (8.6 bar). The high-pressure compressed air then flows into the aftercooler. Cooling of the intercooler can either be air or water-cooled. Water-cooling is normally the preferred method since the lower operating temperatures will reduce the moisture load on downstream purification equipment. Both the low and high-pressure airends are flanged onto the main drive gear housing. The main drive gear

Figure OFS-1.1 Oil-Free Rotary Screw Air Compressor Process Flow Schematic

housing also serves as the lube oil sump. The main drive gear also drives the positive pressure lube oil pump. The lube oil pump provides lubricant to the bearings in both airends. Bearing housings on both airends have a redundant shaft seal system to prevent oil migration into the compression chamber.

Oil-free rotary screw compressors are available fully packaged (fig. OFS-1.2) with all components, including the airends, drive system, motor, coolers and controls mounted on a common base plate. The entire assembly also includes a full enclosure to reduce noise to acceptable levels.

Figure OFS-1.2 Oil-Free Rotary Screw Compressor with water cooled coolers and with the enclosure removed.

Flow, pressure and horsepower ratings are listed below (table OFS-1.3)

Single-Stage

Discharge pressure, psig (bar) 100 & 125 (7 & 8.6)

Flow rating, cfm (m3/min) 200 to 450 (5.7 to 12.7)

Horsepower, hp (kW) 50 to 100 (37 to 75)

Table OFS-1.3 Oil - Free Rotary Screw Compressor Ratings

Oil-Free Rotary Screw Compressor Control System

Load/No Load is the preferred control method for oil-free rotary screw compressors.

Load / No Load Control - this control system consists of a control valve mounted on the airend inlet, a pressure switch that senses the compressor discharge pressure and vent valve. During compressor start-up, the inlet valve is closed and vent valve is open. This permits the compressor to start unloaded to avoid overloading or overheating the main drive motor. When the pressure switch senses a need for compressor capacity, the inlet valve opens fully and the vent valve closes. The compressor operates fully loaded until the pressure switch senses that the air demand has been satisfied. The inlet valve closes and the vent valve opens. While running unloaded, the compressor will consume approximately 10% of its full load power. Auto Start/Stop control is typically used to shut the compressor down when unloaded run time is excessive.

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Oil-Free Rotary Screw Air Compressor

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Oil-Free Rotary Screw Compressor Control System