A rotary-screw compressor is a type of gas compressor that uses a rotary type positive displacement mechanism. They are generally used to replace piston compressors where large volumes of high pressure air are required, either for large industrial applications or for operating high-powered air tools such as jackhammers.
The gas compression process of the rotary screw is a continuous sweeping motion, so there is very little pulsation or flow surge, as happens with a piston compressor.
Video Rotary-screw compressor
Operation
The rotary-screw compressor uses two helical meshing screws, known as the rotor, to suppress the gas. In a dry rotary compressor, the pinch gear ensures that the male and female rotor maintain the correct alignment. In an oil-drenched rotary-screw compressor, the lubricating oil bridges the space between the rotor, both providing a hydraulic seal and transferring mechanical energy between the drive rotor and the drive. The gas enters the suction side and moves through the screw when the screw spins. The rotor meshing forces the gas through the compressor, and the gas exits from the tip of the screw.
The effectiveness of this mechanism depends on the right fit loosen between the helical rotor and between the rotor and the space for sealing the compression cavity. However, some leaks are unavoidable, and high rotational speeds should be used to minimize leak flow rate ratio above effective flow rate.
Unlike the Roots blower, screw compressors are made with different profiles on two rotor: the male rotor has a convex lobe connected to the concave cavity of the female rotor. Usually the male rotor has fewer lobes than the female rotor, thus rotating faster. Initially, screw compressors were made with symmetrical rotor cavity profiles, but the modern version used asymmetric rotor, with the exact rotor design being the subject of the patent.
Maps Rotary-screw compressor
Size
The capacity of rotary-screw compressors is usually assessed in horsepower (HP), Standard Cubic Feet per Minute (SCFM) and pound per square inch (PSI.) For units in the 5 to 30 HP range, the physical size of these units is proportional to the compressor two distinct phases. As horsepower increases, there is a large economical scale that supports rotary compressors. For example, a 250 HP compound compressor is a large piece of equipment that typically requires a special foundation, building accommodation and riggers trained to place equipment. On the other hand, a 250 HP rotary-screw compressor can be placed on the shop floor using a standard forklift. In industry, 250 HP rotary-screw compressors are generally considered to be compact equipment.
Rotary-screw compressors are generally available in the 5 to 500 HP range and can generate over 2500 SCFM airflows. While there is a high-pressure rotary-screw compressor, in a compressed air community, the upper pressure limit is generally about 125 PSI.
The rotary-screw compressor tends to run smoothly with limited vibration, so it does not require a special foundation or mounting system. Typically, rotary-screw compressors are installed using a standard rubber insulation holder designed to absorb high-frequency vibrations. This is especially true of rotary-screw compressors operating at high rotation speeds.
* For lower levels, some compressors are rated in Actual Cubic Feet per Minute (ACFM). Others are rated in Cubic Feet per Minute (CFM). Using CFM to assess an incorrect compressor as it represents a flow rate independent of the pressure reference. ie 20 CFM at 60 PSI.
Apps
Rotary-screw compressors are commonly used to supply compressed air for larger industrial applications. They are best applied in applications that have continuous air demand such as food packaging plants and automated manufacturing systems. In larger facilities, which may only have intermittent applications, the average use among many work stations will place a continuous demand on the compressor. In addition to fixed units, rotary-screw compressors are usually mounted on the back of the trailer and powered by small diesel engines. This portable compression system is commonly referred to as a construction compressor. Construction compressors are used to provide pressurized air hammers for jack hammers, riveting tools, pneumatic pumps, sand blasting operations and industrial paint systems. They are usually seen at construction sites and are tasked with road repair crews around the world.
Oil free
In an oil-free compressor, the air is compressed entirely through the action of the screw, without the aid of oil seals. They usually have a lower maximum discharge release capability as a result. However, an oil-free multi-stage compressor, in which air is compressed by multiple sets of screws, can reach a pressure of more than 150 psi (10 atm) and an output volume of more than 2,000 cubic feet per minute (57 m 3 /mnt).
Oil-free compressors are used in applications where carry-over of entrained oil is unacceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for screening, since hydrocarbons and other contaminants ingested from ambient air should also be discarded before the point of use. Furthermore, identical air treatment with those used for muddy screw compressors is often necessary to ensure the quality of compressed air provided.
Injected oil
In an oil-injected rotary-screw compressor, the oil is injected into the compression cavity to help seal and provide a cooling sink for the gas load. The oil is separated from the effluent stream, then cooled, filtered and recycled. Oil captures non-polar particles from incoming air, effectively reducing particulate filtration particle loading of compressed air. It is common for some entrained compressor oil to carry into the compressed gas stream of downstream compressor. In many applications, this is fixed by coalescer/filter vessels. In other applications, this is improved by the use of a receiving tank which reduces the local speed of compressed air, allowing oil to condense and exit airflow to be removed from the air compression system by condensate management equipment.
Control scheme
Among the rotary-screw compressors, there are several control schemes, each with different advantages and disadvantages.
Start/stop
In the start/stop control scheme, the compressor control runs a relay to apply and transfer power to the motor according to compressed air requirements.
Loading/unload
Compressed Air In a load/unload control scheme, the compressor remains powered continuously. However, when compressed air demand is met or reduced, instead of releasing power to the compressor, a device known as a shift valve is activated. This device discloses part of the rotor and proportionally reduces engine capacity by up to 25% of the compressor's capability, thus disassembling the compressor. This reduces the number of start/stop cycles for the electric motor through the start/stop control scheme on an electrically driven compressor, increasing equipment life with minimal changes in operating costs. This scheme is used by almost all manufacturers of industrial air compressors. When the load/unload control scheme is combined with a timer to stop the compressor after a predetermined period of continuous downhill operation, this is known as a dual-control or auto-dual scheme.
Modulation
Instead of starting and stopping the compressor, the sliding valve as described above modulates capacity to demand. While this results in consistent discharge pressure over demand, overall power consumption may be higher than with a load/demolition scheme, which accounts for about 70% of full load power consumption when the compressor is in zero-load state.
Because of the limited adjustment in compressor power consumption relative to the compressed-air output capacity, modulation is a control method that is generally inefficient when compared to variable speed drives. However, for applications that do not allow to frequently stop and resume compressor operations (such as when the compressor is driven by an internal combustion engine and operated in the absence of an air-compressed receiver), the modulation fits.
Variable shift
Utilized by the compressor company Quincy Compressor, Kobelco, Gardner Denver, and Sullair, variable displacements change the percentage of screw rotor compressors that work to compress air by allowing airflow to cut off the screw parts. While this reduces power consumption when compared to modulation control schemes, the load/unload system can be more effective with large amounts of storage (10 gallons per CFM). If large amounts of storage are impractical, variable displacement systems can be very effective, especially in more than 70% of full load.
One way that variable displacement can be achieved is to use a double lifter valve on the suction side of the compressor, each thrown to the appropriate location at the discharge. In automotive superchargers, this is analogous to the operation of bypass valves.
Variable speed
While air compressors powered by variable speed drives can offer the lowest operating cost of energy without significant reductions in service life through a well-maintained load/disassembly compressor, variable frequency drive inverters from variable speed drivers typically add significant costs to such compressor design , negating its economic benefits if there is limited demand variation. However, variable speed drivers provide a linear relationship between compressor power consumption and free air delivery. In harsh environments (hot, humid or dusty), variable speed drivers may not be suitable due to the sensitivity of the equipment.
Supercharger
The twin-screw type supercharger is a positive displacement type device that operates by pushing the air through a pair of near-tolerance screws similar to a set of worm gears. Twin-screw superchargers are also known as Lysholm superchargers (or compressors) after its discoverer, Alf Lysholm. Each rotor is symmetrically radial, but laterally asymmetric. By comparison, a conventional type "Roots" blower has an identical rotor (with a straight rotor) or a mirror-image rotor (with a ribbed rotor). Whipple-produced male rotor has three lobes, five female lobes. The male rotor of the Kenne-Bell has four lobes, six lobes females. Women in some previous designs have four. By comparison, the Roots blower always has the same number of lobes on both rotor, usually 2, 3 or 4. The work area is the inter-lobe volume between the male and female rotor. It is larger at the end of the intake, and decreases along the rotor to the exhaust port. This volume change is compression. The cost of the intake is pulled at the end of the rotor in the large gap between the male and female lobes. In the latter intake the male lobe is much smaller than the female partner, but the relative size is reversed proportion along the length of the two rotor (the male becomes larger and the female smaller) to (tangential to the exhaust port) the empty space between each pair of lobes much smaller. This volume reduction causes the charge compression before it is presented to the output manifold.
Comparative advantage
The rotary screw compressor has a low leakage rate and low parasitic losses vs the Roots type. Superchargers are usually driven directly from the engine crankshaft via a belt or a gear drive. In contrast to the Roots-type supercharger, twin-screw shows internal compression which is the ability of the device to compress the air inside the housing as it moves through the device instead of relying on resistance to downstream discharge to build up pressure.
The requirements of high precision computer controlled manufacturing techniques make this type of alternative supercharger screw more expensive for other forms of forced induction available. With technology in the future, production costs have been lowered while performance is improving.
All types of superchargers benefit from the use of intercoolers to reduce the heat generated during pumping and compression.
Real examples of technologies employed by twin screw at companies like Ford, Mazda, Mercedes, and Mercury Marine can also show the effectiveness of twin screw. While some centrifugal superchargers are consistent and reliable, they typically do not generate full impulse until the engine revolves close to the peak, while positive charge superchargers such as Roots and type twin-screw superchargers offer a faster boost.
Related terms
The term "blower" is generally used to define devices placed on machines with functional requirements for additional airflow, such as a 2-step Diesel engine, where positive intake pressure is required to "scavenge", or clean the exhaust gases removed from the cylinder and forcibly charge the new intake into the cylinder before the compression step. The term "blower" is applied to the rotary screw, root type, and centrifugal compressor when used as part of the automated forced induction system.
See also
- Gas compressor
- Guided reccdors
- Reciprocating Compressors
- Vapor compression cooling
- Variable speed air compressor
- White-Westinghouse
References
Source of the article : Wikipedia
0 Comments