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Rotary Screw Compressors
Rotary Screw Compressors are the most widely applied industrial compressors in the 40 (30kW) to 500 hp (373 kW) range. They are available in both lubricated and oil-free configurations. The popularity of rotary compressors is due to the relatively simple design, ease of installation, low routine maintenance requirements, ease of maintenance, long operating life and affordable cost.
The simplicity and compactness of an oil injected rotary screw compressor allows the compressor package to include all of the components necessary to produce high quality compressed air.
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| Figure RS-1.1 Oil Lubricated Rotary Screw Airend |
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| Figure RS-1.2 Base Mounted 15 hp Rotary Screw Compressor Package with Silencing Enclosure. |
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| Figure RS-1.3 Tank Mounted Rotary Screw Compressor |
The heart of the oil lubricated rotary screw compressor is the compression module or airend (fig. RS-1.1). The airend consists of two (2) precision ground helical rotors fitted inside the outer housing or stator. Bearing sets are fitted to each rotor to absorb axial and radial loads that develop during normal operation. Lubricating oil is injected directly into the compression chamber to seal the rotors and to cool the compressed air. This cooling of the compressed air allows air to be compressed to as high as 200 psig (13.8 bar) in a single stage configuration. The remainder of the compressor package consists of an intake filter, drive motor, direct or v-belt drive system, oil separation system, controls, oil and aftercoolers. All components of the compressor are packaged on a common base frame that is either floor mounted (fig. RS-1.2) or tank mounted (fig. RS-1.3). A noise silencing enclosure is frequently specified when the compressor will be installed near the factory workforce. Compressors can also be provided with refrigerated air dryers and filters built into the package. Compressor flow, pressure and horsepower ratings are shown in table RS-1.3.
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Single-Stage |
Two-Stage |
| Discharge pressure, psig (bar) |
70 to 175 (4.8 to 12) |
70 to 150 (4.8 to 10) |
| Flow rating, cfm (m3/min) |
20 to 1500 (0.57 to 42.5) |
500 to 2600 (14.2 to 73.6) |
| Horsepower, hp (kW) |
5 to 350 (3.7 to 261) |
100 to 500 (75 to 373) |
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| Table RS-1.3 Oil Lubricated Rotary Screw Compressor Ratings |
During normal operation of an oil lubricated rotary screw compressor (fig. RS-1.4), atmospheric air is drawn into the intake air filter due to the rotation of the rotors in the airend. Air enters the airend where it is mixed with lubricating oil. The air/oil mixture is compressed, exits the airend and flows into the oil separator tank. The oil separator tank acts as a reservoir for the oil, allows for the primary separation of the air from the oil, and houses the air/oil separator element. The air/oil separator reduces the oil content of the air to approximately 5 ppm by weight. Compressed air then flows into the aftercooler where the temperature is reduced from approximately 200ºF (93ºC) to 15ºF (-9ºC) to 25ºF (-4ºC) above the cooling medium temperature. The cooled air then enters the moisture separator where the condensed moisture is separated from the air. Compressed air then exits the compressor and the drain valve removes condensed moisture.
(Note: Since the condensate will contain a mixture of water and lubricating oil, be sure to drain all condensate in a manner approved by all federal, state and local regulatory agencies. Oil/water separators are available to assist in this function.)
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| Figure RS-1.4 Oil Lubricated, Single Stage Rotary Screw Air Compressor Process Flow Schematic |
Hot oil from the oil separator tank flows into the oil-cooler through the temperature control valve. The temperature control valve is a mixing valve that maintains lubricating oil temperature (compressor operating temperature) high enough to maintain the required oil viscosity and to prevent formation of condensation in the oil. Circulation of the oil is maintained by the pressure differential between the oil separator tank and bearing and airend injection points.
Control of oil lubricated rotary screw compressors is either load/no load, modulation or a combination of load/no load and modulation.
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 located on the oil separator tank. 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. Although the inlet valve is closed, a small quantity of air flows through a port in the inlet valve. Since oil is circulated by differential pressure across the compressor, some air pressure is required. 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 25% to 30% of its full load power. Auto Start/Stop control is typically used to shut the compressor down when unloaded run time is excessive.
Modulation Control - this control system consists of a control valve mounted on the airend inlet, a control regulator to throttle the control valve and sense compressor discharge pressure, and vent valve located on the oil separator tank. 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. Although the inlet valve is closed, a small quantity of air flows through a port in the inlet valve. Since oil is circulated by differential pressure across the compressor, some air pressure is required. 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 control regulator senses that the air demand is decreasing. This decrease in demand causes the inlet valve to modulate or throttle towards the closed position. Modulation control is very effective in maintaining a stable discharge pressure. Power requirements in the modulated mode are quite high.
Load/No Load and Modulation - a combination of the two control methods allows the compressor to modulate as compressor demand falls. However, when the demand is reduced to below 70% of full load capacity, the control system will unload the compressor thereby reducing power consumption. Auto Start/Stop control can also be incorporated to shut the compressor off should the unloaded run time become excessive.
A comparison of the energy consumption of each control type is shown in figure RS-1.5.
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| Figure RS-1.5 Control Energy Consumption Curve |
Lubricating fluids are the life-blood of oil-lubricated rotary screw compressors. The lubricating fluids circulating within the compressor serve a number of functions including:
- Removal of the heat of compression generated during the compression process. The normal discharge temperature of a single stage, 125 psig (8.6 bar), oil -lubricated rotary compressor is approximately 200ºF (93ºC). Discharge temperatures would be in excess of 700ºF (371ºC) if the lubricant were removed.
- Sealing of the rotors and compression chamber to minimize the effect of "slip" and increase volumetric efficiency. Slip is the leakage of air from clearances between the two rotors and between the rotors and stator.
- Lubrication of bearings that support the rotors.
Rotary screw compressor lubricants are available in two (2) basic categories, mineral and synthetic. Synthetic lubricants have been developed specifically for the unique demands of rotary compressors and varying application requirements. Available synthetic lubricants are described in figure CL-1.1:
| Synthetic Lubricant Type |
General Description |
| Diester | General purpose, wide operating temperature range. |
| Food Grade (Polyalphaolefin) | >Used in food applications when incidental contact with food may occur. |
| Polyglycol | >Oils and coolants used for high temperature, long life applications |
| Polyolester | Severe duty, biodegradable, long life applications. |
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| Figure CL-1.1 Synthetic Lubricant Types |
Use of synthetic lubricants offers many advantages over mineral base oils. Typically synthetic lubricants offer:
- Superior lubricating properties reduce frictional losses and increase bearing life.
- High viscosity index for a wide range of operating temperatures.
- Outstanding mechanical separation reduces carryover and downstream contamination.
- Long operating life reduces maintenance costs.
Typical properties for synthetic lubricants are listed in table CL-1.2.
| Base Fluid |
ISO Grade |
Viscosity 40ºC cSt |
Viscosity 100ºC cSt |
SAE Grade |
Specific Gravity (15.6ºC) |
Flash Point ºC |
Pour Point ºC |
Diester
2,000 to 4,000 hrs. |
32 |
29 |
5.5 |
10 |
0.92 |
245 |
-55 |
| 46 |
44 |
6.4 |
10W-20 |
0.96 |
245 |
-50 |
| 68 |
64 |
7.4 |
20 |
0.95 |
245 |
-33 |
Polyol Ester
10,000 hrs. |
32 |
31 |
6.1 |
10 |
0.92 |
285 |
-65 |
| 46 |
44 |
7.9 |
10W-20 |
0.91 |
270 |
-65 |
| 68 |
32 |
10.2 |
20 |
0.91 |
270 |
-60 |
| 46 |
43.8 |
7.6 |
10W20 |
0.96 |
250 |
-55 |
Food Grade (PAO)
4,000 hrs. |
32 |
31 |
5.9 |
5 |
0.83 |
245 |
-50 |
| 46 |
47 |
7.8 |
10 |
0.83 |
245 |
-50 |
| 68 |
65 |
10.2 |
20 |
0.84 |
250 |
-50 |
Polyglycol (PAG)
8,000 hrs. |
32 |
31 |
6.2 |
10 |
1 |
240 |
-70 |
| 46 |
47.2 |
8.9 |
10W-20 |
1 |
240 |
-60 |
| 68 |
67 |
11.9 |
20 |
1 |
230 |
-70 |
Table CL-1.2 Synthetic Lubricant Properties
Note: Certain materials and paint finishes utilized in compressed air equipment and piping systems may not be compatible with all synthetic lubricants. Table CL-1.3 contains a general guide of various synthetic lubricants and their compatibility with certain materials. Be sure to contact eCompressedair application engineers for assistance on materials not listed.
When converting the type of lubricant from either mineral base oil or other synthetic fluid, be sure to follow the lubricant and compressor manufacturer's recommendations.
| Synthetic Lubricant Type |
Compatible Materials |
Non-Compatible Materials |
| Diester |
Epoxy paint, Celcon, Viton®, High Nitrile Rubber (Buna N, NBR greater than 36% Acrylonitrile), Medium Nitrile Rubber (Buna N, NBR 30% - 36% Acrylonitrile), Teflon, Oil-Resistant Alkyd and Nylon. |
Neoprene, Low Nitrile Rubber (Buna N, NBR less than 30% Acrylonitrile), SBR Rubber, PVC, ABS, Acrylic Paint and Lacquer. |
| Food Grade (Polyalphaolefin) |
Neoprene, Viton®, Buna-N, Buna-N24, Buna-N19, Epoxy Paint, Compounded Urethane, Nylon, Teflon, Plastisol, PVC, Acrylic Paint, Lacquer. |
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| Polyglycol |
High and Medium nitrile Buna-N, Viton®, Teflon, Silicone, Butyl Rubber, Ethylene Propylene Rubber, Polyurethane Foams, Modified Alkyds, Epoxies and Phenolic Paints.. |
Low nitrile Buna N, Neoprene, Acrylic Paints, Polyurethane Varnishes, Polystyrene Plastics, PVC Plastics, Polycarbonate Plastics and ABS Plastics. |
| Polyolester |
Viton®, High Nitrile Buna N, Teflon, Epoxy Paint, Oil-Resistant Alkyd, Nylon, Delrin, Celcon and PBT. |
Neoprene, SBR Rubber, Low Nitrile Buna N, Acrylic Paint, Lacquer, Polystyrene Plastics, PVC Plastics and ABS Plastics. |
Table CL-1.3 Material Compatibility Table for Synthetic Lubricant
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