Rotary-screw compressor

The compression action of a twin-screw compressor with a six-lobe female screw and a five-lobe male screw - the compression of one pair of cavities is highlighted: air is sucked in through the intake port, compressed (shown by the colour change from cyan to red) and released through the outlet port.
Rotary-screw air compressor internal view

A rotary-screw compressor is a type of gas compressor that uses a rotary-type positive-displacement mechanism. They are commonly used to replace piston compressors where large volumes of high-pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers.

The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors.

Operation

Cross-section through the rotors of a typical screw compressor with a male rotor having 5 lobes and a female rotor having 6. For five rotations of the female rotor, the male rotor makes six rotations. Click here for the animated diagram

Rotary-screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry-running rotary-screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary-screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws.[1]

The effectiveness of this mechanism is dependent on precisely fitting clearances between the helical rotors and between the rotors and the chamber for sealing of the compression cavities. However, some leakage is inevitable, and high rotational speeds must be used to minimize the ratio of leakage flow rate over effective flow rate.

In contrast to Roots blowers, screw compressors are made with different profiles on the two rotors: the male rotor has convex lobes which mesh with the convex cavities of the female rotor. Usually the male rotor has fewer lobes than the female rotor, so that it rotates faster. Originally, screw compressors were made with symmetrical rotor cavity profiles, but modern versions use asymmetrical rotors, with the exact rotor designs being the subject of patents.[2]

Size

Rotary-screw compressors tend to be compact and smoothly running with limited vibration, thus not requiring spring suspension. Many rotary-screw compressors are, however, mounted using rubber vibration-isolating mounts to absorb high-frequency vibrations, especially in rotary-screw compressors that operate at high rotational speeds. Rotary-screw compressors are produced in sizes that range in pumping speeds from 10 cubic feet per minute to several thousand CFM. Rotary-screw compressors are typically used in applications requiring more airflow than is produced by small reciprocating compressors but less than is produced by centrifugal compressors.

Applications

Typically, they are used to supply compressed air for general industrial applications. Trailer-mounted diesel-powered units are often seen at construction sites and are used to power air-operated construction machinery.

Oil-free

In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximal discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psi (10 atm) and output volume of over 2,000 cubic feet per minute (57 m3/min).

Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for filtration, as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point of use. Subsequently, air treatment identical to that used for an oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air.

Oil-injected

Diagram of a rotary-screw compressor

In an oil-injected rotary-screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed-air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed-gas stream downstream of the compressor. In many applications, this is rectified by coalescer/filter vessels.[3] In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be removed from the compressed-air system by condensate-management equipment.

Control schemes

Among rotary-screw compressors, there are multiple control schemes, each with differing advantages and disadvantages.

Start/stop

In a start/stop control scheme, compressor controls actuate relays to apply and remove power to the motor according to compressed air needs.

Load/unload

In a load/unload control scheme, the compressor remains continuously powered. However, when the demand for compressed air is satisfied or reduced, instead of disconnecting power to the compressor, a device known as a slide valve is activated. This device uncovers part of the rotor and proportionately reduces capacity of the machine down to typically 25% of the compressors capability, thereby unloading the compressor. This reduces the number of start/stop cycles for electric motors over a start/stop control scheme in electrically-driven compressors, improving equipment service life with a minimal change in operating cost. This scheme is utilised by nearly all industrial air-compressor manufacturers. When a load/unload control scheme is combined with a timer to stop the compressor after a predetermined period of continuously unloaded operation, it is known as a dual-control or auto-dual scheme.[4]

Modulation

Rotary-screw air compressor in a housing for sound attenuation

Instead of starting and stopping the compressor, a slide valve as described above modulates capacity to the demand. While this yields a consistent discharge pressure over a wide range of demand, overall power consumption may be higher than with a load/unload scheme, resulting in approximately 70% of full-load power consumption when the compressor is at a zero-load condition.

Due to the limited adjustment in compressor power consumption relative to compressed-air output capacity, modulation is a generally inefficient method of control when compared to variable-speed drives. However, for applications where it is not readily possible to frequently cease and resume operation of the compressor (such as when a compressor is driven by an internal-combustion engine and operated without the presence of a compressed-air receiver), modulation is suitable.

Variable displacement

Utilized by compressor companies Quincy Compressor, Kobelco, Gardner Denver, and Sullair, variable displacement alters the percentage of the screw compressor rotors working to compress air by allowing air flow to bypass portions of the screws. While this does reduce power consumption when compared to a modulation control scheme, a load/unload system can be more effective with large amounts of storage (10 gallons per CFM). If a large amount of storage is not practical, a variable-displacement system can be very effective, especially at greater than 70% of full load.[5]

One way that variable displacement may be accomplished is by using multiple lifting valves on the suction side of the compressor, each plumbed to a corresponding location on the discharge. In automotive superchargers, this is analogous to the operation of a bypass valve.

Variable speed

While an air compressor powered by a variable-speed drive can offer the lowest operating-energy cost without any appreciable reduction in service life over a properly maintained load/unload compressor, the variable-frequency power inverter of a variable-speed drive typically adds significant cost to the design of such a compressor, negating its economic benefits if there are limited variations in demand. However, a variable-speed drive provides for a linear relationship between compressor power consumption and free air delivery. In harsh environments (hot, humid or dusty), variable-speed drives may not be suitable due to the sensitivity of the equipment.[6]

Superchargers

Lysholm screws. Note the complex shape of each screw. The screws run at high speed and with closely engineered tolerances.

The twin-screw type supercharger is a positive displacement type device that operates by pushing air through a pair of meshing close-tolerance screws similar to a set of worm gears. Twin-screw superchargers are also known as Lysholm superchargers (or compressors) after their inventor, Alf Lysholm.[7] Each rotor is radially symmetrical, but laterally asymmetric. By comparison, conventional "Roots" type blowers have either identical rotors (with straight rotors) or mirror-image rotors (with helixed rotors). The Whipple-manufactured male rotor has three lobes, the female five lobes. The Kenne-Bell male rotor has four lobes, the female six lobes. Females in some earlier designs had four. By comparison, Roots blowers always have the same number of lobes on both rotors, typically 2, 3 or 4. The working area is the inter-lobe volume between the male and female rotors. It’s larger at the intake end, and decreases along the length of the rotors until the exhaust port. This change in volume is the compression. The intake charge is drawn in at the end of the rotors in the large clearance between the male and female lobes. At the intake end the male lobe is much smaller than its female counterpart, but the relative sizes reverse proportions along the lengths of both rotors (the male becomes larger and the female smaller) until (tangential to the discharge port) the clearance space between each pair of lobes is much smaller. This reduction in volume causes compression of the charge before being presented to the output manifold.

Comparative advantages

The rotary screw compressor has low leakage levels and low parasitic losses vs. Roots type. The supercharger is typically driven directly from the engine's crankshaft via a belt or gear drive. Unlike the Roots type supercharger, the twin-screw exhibits internal compression which is the ability of the device to compress air within the housing as it is moved through the device instead of relying upon resistance to flow downstream of the discharge to establish an increase of pressure.[8]

The requirement of high-precision computer-controlled manufacturing techniques makes the screw type supercharger a more expensive alternative to other forms of available forced induction. With later technology, manufacturing cost has been lowered while performance increased.

All supercharger types benefit from the use of an intercooler to reduce heat produced during pumping and compression.

A clear example of the technology applied by the twin-screw in companies like Ford, Mazda, Mercedes and Mercury Marine can also demonstrate the effectiveness of the twin screw. While some centrifugal superchargers are consistent and reliable, they typically do not produce full boost until near peak engine rpm, while positive displacement superchargers such as Roots type superchargers and twin-screw types offer more immediate boost.

The term "blower" is commonly used to define a device placed on engines with a functional need for additional airflow, such as a 2-stroke Diesel engine, where positive intake pressure is needed to "scavenge", or clear spent exhaust gasses from the cylinder and force a fresh intake charge into the cylinder before the compression stroke. The term blower is applied to rotary screw, roots-type, and centrifugal compressors when utilized as part of an automotive forced induction system.

See also

References

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