Rotary screw air compressor basics
There is no metal-to-metal contact, which minimizes wear.
Variable speed drives match the performance of the compressor to demand and reduce power consumption.
Installation is simplified.
At one time, compressed air was considered free. It came out of a hose along with some water and oil spray, and the compressor in the back room went chugga-chugga. Today we know compressed air is an expensive fourth utility not to be wasted. We want it dry and clean, and the efficient compressor should come in a box that is quiet.
Times have changed and the most popular air compressor design today that meets the industrial plant's demands is the rotary screw. It has evolved from a simple circular profile to an asymmetric profile (Fig. 1), which has improved efficiency due to lower leakage or bypass in the discharge area. Because size is a leakage factor, efficiency is greater in smaller compressors.
A rotary screw uses two rotors to push air through the compressor, which creates pressure. Compression is accomplished by the main and secondary rotors synchronously meshing in a one-piece, dual-bore housing. The main rotor has helical lobes and the secondary rotor has helical grooves (Fig. 2). The air inlet port is located on top of the housing near the drive shaft end. The discharge port is near the bottom at the opposite end of the housing. Figure 2 (below) is inverted to show both ports.
In oil or water injected designs, fluid is introduced in step B to remove the heat of compression, seal internal clearances, and prevent rotor-to-rotor contact. In oil-free designs, timing gears are attached to the rotor shafts to prevent rotor-to-rotor contact. Precision tolerances effect a seal. Water could still be injected into the compression chamber for cooling and sealing for higher pressures.
Limiting factors in rotary screw air compressors are the discharge temperature and pressure, and temperature and pressure differentials across the machine. These factors have an effect on rotor and housing expansion and deflections, housing strength, and bearing loads.
Multiple stages are used for improved efficiency and higher pressures. 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.
There are three basic types of drives for air compressors: V-belt, direct, and gear. The choice depends on several factors, including the speed requirement of the compressor airend and the manufacturer's preference. A fourth type is a combination of any of the above with a variable speed motor.
V-belt drives allow a smaller footprint for the enclosure. Alignment is not critical and maintenance is simplified.
Direct drives have alignment built-in. Belt and gear maintenance are eliminated. The footprint may be larger but the machine can run quieter.
Gear-driven airends eliminate alignment problems and are usually used for higher horsepowers. As with V-belts, compressor speed can be different from motor speed.
Variable speed drives (VSDs) usually alter the frequency of the incoming motor power. This approach is a simple way to vary compressor output and can be efficient if the airend has a wide efficiency range or flat efficiency curve over a wide speed range.
VSDs can boost the motor power factor to eliminate penalties from utility companies. The soft start feature reduces high input starting amps, another cost-saving aspect. Some drawbacks are additional initial cost and a power loss of 2­3% during full load operation. However, payback time is often realized within 3 yr (Fig. 3).
These controls have been used on constant speed compressors to regulate air output when the unit is operating below full load capacity. For example, if plant air requirements decline during a shift change, the control system allows the compressor to produce only the required amount of air, which lowers the power needed and maintains efficiency. The key to selecting the proper system is to determine the plant load cycles and evaluate the control system performance based on these cycles.
Until VSDs became popular, there were two basic types of capacity control systems: inlet throttling and rotor length adjustment. They both automatically regulate to match system demand with no over or under pressuring.
A version of inlet throttling is a simple load/no-load system. The compressor is producing at full capacity or doesn't deliver any air. In this system there is over and under pressuring.
To take advantage of the load/no-load system the compressor must run unloaded and an air storage tank must be used. This setup allows the compressor, under load, to make and store more air than is being used. The larger the storage tank, the longer the no-load cycle time and greater the energy savings.
Inlet throttling restricts the compressor inlet opening to admit only the amount of air demanded by the system. At less than full capacity, a partially closed valve creates a vacuum at the inlet, which lowers inlet pressure. More work is now required to raise the inlet pressure to the discharge pressure. Overall, less power is required to compress the smaller volume of air. However, with system inefficiencies, more energy is expanded per cu ft of air compressed. This system should be used for base load applications only, where system needs rarely drop below 70%.
Rotor length adjustment controls air output by varying the seal-off point in the rotor housing as the demand for air declines. Unneeded air is allowed to return to the inlet before being compressed. There are two common types of rotor length adjustment: turn/spiral valves and poppet valves. These methods are efficient control systems for any part-load application above a 40% load cycle.
Turn/spiral valves are actuated to expose a greater or lesser number of ports in the compressor housing bore (Fig. 4). The valve rotates continuously, providing instantaneous and infinite positioning among these ports. These ports are as deep as the housing is thick. Air in the compression pocket can leak around the rotor tips, which reduces efficiency.
Poppet valves open sequentially, as needed, and allow air to return to the inlet. Control is similar to turn valves, but in finite steps of about 15­16 psig. Compressed air can leak by flat poppet valves, but some designs have a face that conforms to the housing bore and reduces leakage (Fig. 5).
Many compressors now have electronic control panels with clear graphics, are user friendly, and are computer-compatible. Compressor operating conditions, set points, and configuration parameters are displayed by easy-to-read alphanumeric readouts. System adjustments to meet varying operating requirements are easily made from the control panel.
Maintenance costs are lowered because there are automatic indications when service is required for filters, separators, and the oil system. Status of the compressor can be remotely monitored with modem connections.
Since there are no unbalanced dynamic forces with a rotary compressor, only a concrete foundation of sufficient size to support its mass and maintain any alignment between compressor and driver is required. Foundation bolts may be required to hold the unit in place, depending on the design. Where units must be bolted on a base plate or sole plates, these items should be leveled and grouted to the foundation.
Piping should be supported and aligned so its dead weight and any thermal or mechanical stresses are not applied to the inlet and discharge flanges of the compressor housing.
Compressor maintenance can be performed according to a standard timetable or by monitoring running hours. The latter approach is more practical, but requires accurate and reliable maintenance indicators. Fortunately, the latest compressor controls contain troubleshooting diagnostics and fault indicators.
There are generally five areas of the compressor that require maintenance: airend, motor, drive train, lubricants, and filters.
Airends and their bearings are damaged by contamination in the air and lubricants and by excessive heat. Inspect for unusual noises and vibrations. Monitors should be able to detect impending problems and sound alarms. Most manufacturers recommend rebuilding airends between 50,000­60,000 hr to avoid expensive, forced shutdowns.
Motor bearings should always be lubricated with the right amount and type of grease. Replace bearings on a conservative schedule. Periodically check ampere draw to ensure the motor is not overloaded. Providing proper ventilation and ambient temperature in the compressor room increases motor life.
Drive train systems must be kept aligned. If the frame or mounting block settles, misalignment will damage the coupling. Gear drives must always be properly lubricated. Inspect and adjust the tension on V-belt drives at least every 500 operating hours. Worn or frayed belts should be replaced.
Lubricants cool, seal, protect, and remove contaminants in a compressor. Always drain all existing lubricant before refilling to ensure maximum service life. Change oil on a schedule recommended by the manufacturer for the application. Regular oil sampling helps maximize lubricant life. Avoid lubricants not specifically designed for a particular compressor; they may impact service life.
Filters protect the compressor from wear and damage. Proper filtration is more cost effective than repairing damaged equipment. Inspect and replace air inlet filters on a regular basis (about every 2000 hr). This timing reduces component contamination and airend wear, which improves operating efficiency. An inlet filter with an increase of pressure drop of 1% reduces compressor capacity by 1%. Oil filters should be changed and strainers cleaned about every 1000 hr.
Keep the compressor clean. Fouled compressor and intercooler surfaces increase the compressed air temperature. This increase has the effect of reducing output, but not total energy input. This added energy per unit output shows up as higher temperature of the discharge air. The aftercooler and dryer must reduce this temperature. The result is less compressed air output and increased energy input.