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A rotary screw compressor is a type of gas compressor which 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.
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[edit]Operation
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][2]
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.
[edit]Size
Rotary screw compressors tend to be compact and smooth running with limited vibration and thus do not require spring suspension. Many rotary screw compressors are, however, mounted using elastomer 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 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.
[edit]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.
[edit]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 maximum 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 psig, and output volume of over 2000 cubic feet (56.634 cubic meters) per minute (measured at 60 °C and atmospheric pressure).
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.
[edit]Oil-flooded
In an oil-flooded 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 via condensate management equipment.
Standard oil-flooded compressors are capable of achieving output pressures over 200 psig, and output volumes of over 1500 cubic feet per minute (measured as Free Air Delivery at 60 °C and atmospheric pressure).
[edit]Control Schemes
Among rotary screw compressors, there are multiple control schemes, each with differing advantages and disadvantages.
[edit]Start/Stop
In a start/stop control scheme, relays actuate to apply and remove power to the compressor controls according to compressed air needs.
[edit]Load/Unload
In a load/unload control scheme, the compressor airend remains continuously powered. However, when the demand for compressed air is satisfied, instead of disconnecting power to the compressor airend, the inlet valve is closed, 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].
[edit]Modulation
Instead of starting and stopping the compressor or actuating the inlet valve between two distinct positions, a modulation control scheme proportionally adjusts the inlet valve open and closed, altering the compressor discharge according to demand. While this yields a consistent discharge pressure over a wide range of demand, power consumption is significantly 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. This control scheme was popularized in industrial air compressor applications by Kaeser and Gardner Denver[5].
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.
[edit]Variable Displacement
Utilized by compressor companies Quincy Compressor, 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/no load system can be more effective when 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. [6]
One way that variable displacement may be accomplished is via the use of multiple lifting valves on the suction side of the compressor airend, each plumbed to a corresponding location on the discharge. In automotive superchargers, this is analogous to the operation of a bypass valve.
[edit]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. [7]
[edit]Superchargers
The twin-screw type supercharger or twin-screw blower 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.[8]
[edit]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.[9]
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.
[edit]Related terms
The term "blower" is commonly used to define a device placed on engines with a functional need for additional airflow. The term blower is applied to rotary screw, roots-type, and centrifugal compressors when utilized as part of an automotive forced induction system.
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