Screw compressor
Encyclopedia
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 jackhammer
s.
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.
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.
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.
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. 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).
.
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.
, 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 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 loads greater than 70% of full load.
One way that variable displacement may be accomplished 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.
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.
, 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.
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 supercharger
s and twin-screw types offer more immediate boost.
system.
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
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 jackhammer
Jackhammer
A jackhammer is a pneumatic tool that combines a hammer directly with a chisel that was invented by Charles Brady King. Hand-held jackhammers are typically powered by compressed air, but some use electric motors. Larger jackhammers, such as rig mounted hammers used on construction machinery, are...
s.
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
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.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.
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.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 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.
Oil-flooded
Coalescer
A Coalescer is a technological device performing coalescence. They are primarily used to separate emulsions into their components via various processes; operating in reverse to an emulsifier....
/filter vessels. 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).
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, relays actuate to apply and remove power to the compressor controls according to compressed air needs.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.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 DenverGardner Denver
Gardner Denver, Inc. designs, manufactures, and markets engineered industrial machinery and related parts and services primarily in North America, Europe, Asia, South America, Africa, and Australia...
.
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, Gardner DenverGardner Denver
Gardner Denver, Inc. designs, manufactures, and markets engineered industrial machinery and related parts and services primarily in North America, Europe, Asia, South America, Africa, and Australia...
, 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 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 loads greater than 70% of full load.
One way that variable displacement may be accomplished 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.
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.Superchargers
Positive displacement
Positive displacement may refer to:* Positive displacement pump* Positive displacement meter...
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
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
) after their inventor, Alf Lysholm.
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 superchargerRoots type supercharger
The Roots type supercharger or Roots blower is a positive displacement lobe pump which operates by pumping fluids with a pair of meshing lobes not unlike a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust...
, 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.
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
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...
to reduce heat produced during pumping and compression.
A clear example of the technology applied by the twin-screw in companies like Ford, Mazda
Mazda
is a Japanese automotive manufacturer based in Fuchū, Aki District, Hiroshima Prefecture, Japan.In 2007, Mazda produced almost 1.3 million vehicles for global sales...
, Mercedes
Mercedes-Benz
Mercedes-Benz is a German manufacturer of automobiles, buses, coaches, and trucks. Mercedes-Benz is a division of its parent company, Daimler AG...
and Mercury Marine
Mercury Marine
Mercury Marine, founded in 1939, is a division of Brunswick of Lake Forest, Illinois, in the United States. Mercury provides engines for private, commercial and government sales. Mercury also has its own line of very successful racing engines tailored for power and speed. The company's primary...
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 supercharger
Roots type supercharger
The Roots type supercharger or Roots blower is a positive displacement lobe pump which operates by pumping fluids with a pair of meshing lobes not unlike a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust...
s and twin-screw types offer more immediate boost.
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 inductionForced induction
Forced induction is the process of compressing air on the intake of an internal combustion engine . A forced induction engine uses a gas compressor to increase the pressure, temperature and density of the air...
system.
See also
- Gas compressorGas compressorA gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
- Guided rotor compressorGuided rotor compressorThe guided rotor compressor is a positive displacement rotary gas compressor. The compression volume is defined by the trochoidally rotating rotor mounted on an eccentric drive shaft with a typical 80 to 85% adiabatic efficiency.-History:...
- Variable speed air compressorVariable speed air compressorA variable-speed drive air compressor is an air compressor that takes advantage of variable-speed drive technology. This type of compressor uses a special drive to control the speed of the unit, which in turn saves energy compared to a fixed speed equivalent.The most common form of VSD...
- Vapor-compression refrigerationVapor-compression refrigerationVapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...
- Reciprocating CompressorReciprocating compressorA reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure....