What is a compressor?
As we know liquids are incompressible (theoretically) whereas gases or air can be compressed to a desired pressure (above atmospheric pressure). Practically, Gases can not be used for engineering purposes (such as in industrial pneumatic systems, blowers, household gas supply for heating, hospital oxygen supply systems, air-conditioning, etc.) at atmospheric pressure. Moreover, to store gas (of course large amounts) at atmospheric pressure would require vessels of enormous storage capacity which is practically not possible. So, They have to be compressed, which means reducing the volume and increasing the pressure, for storing in the specially designed and manufactured vessel and later on utilization for the desired purpose. So, the machine used for compressing the air or gas is called Compressor. Depending on the amount (volume or flow) and pressure of the gas there can be various types or designs of compressors and their design is based on applicable ISO, EN, or other equivalent standards.
Compressors: Categories
There are 2 main categories of compressors
a) Displacement Compression
b) Dynamic compression
Displacement compressors are machines where a static increase in the pressure is built by the displacement of a “moving part of gas volume” which is sucked into closed space, compressed, and then ejected to store in a pressure vessel.
On the other hand, Dynamic compression is based on accelerating the gas medium at high speed through one or more impellers that convert the kinetic energy of gas (or air) molecules into static pressure.
The application areas for various compressors are defined by volume (flow) capacity and pressure. For example, Piston compressors produce high pressure for small volumes. Whereas Tubro compressors are for the largest volume capacity (although dependent on the pressure ratio). Rotary piston machines are positioned between the Piston and turbo compressors.
Piston Compressors
The distinguishing features of this type of compressor are primarily the number of stages And whether these are single-action or dual-action. Single-acting Work stages are common in small machines and high-pressure compressors. If the compression exit exceeds 5 to 6 bar then the compressor is generally designed for multi-stage compression. It is desirable that all the stages have the same pressure ratio as this gives the lowest power requirements. Also, depending on design and application requirements, an intercooler may be used when compressed air goes from one stage to the next. For example, a two-stage unit with a final pressure of 8 bar (0,8 Mpa), first stage 2.83 bar, second to 8 bar.
The cylinders of the 2-cylinder compressor are usually arranged in a V formation (crank angle 90 degrees) and where there are 3 cylinders, a fan-shaped arrangement is usually (Crank angle 45 degrees. In Principle, however, all known cylinder arrangements can be used.
Diaphragm compressors
Diaphragm compressors are used to produce a vacuum or dry compressed air. The Flexible diaphragm (flexible disk) oscillates by a rotating eccentric shaft (or crankshaft) which generates compressed air or can also be used to create a vacuum. The crankshaft is driven by an electric motor via a coupling or by a vehicle engine via Belt drive. The drive is sealed from the process fluid by the flexible disc, and thus there is no possibility of lubricant coming into contact with any gas. So these compressors produce Oil Free compressed air. They are generally low-capacity small units (low air flow and pressure) intended for a specific application with very clean air requirements such as for medical equipment, laboratories, the dental industry, special paint spraying equipment, or vacuum pumps for passenger car brake effect reinforcement.
The RPM of the crankshaft is from 1500 to 6000 depending on the application requirements. The working pressure is between 2 to 6 bar and down to approximately 150 Torr Vacuum.
Rotary Piston Machines
Whenever high flow rates are required then the reciprocating piston compressors have limitations due to the limited surface of the piston. So to counter this situation, a design with a higher surface area of the pistons is required such as Rotary piston machines. Rotary piston compressors (also used as vacuum pumps) work with large quantities of air at speeds of up to approximately 3000 RPM at low pressures.
Rotary piston compressors have two identical rotary pistons (also called Lobe) which rotate in opposite directions and operate in a casing with flat ends and a cylindrical outside surface. Compression occurs when the space limited by the cylinder and wing of a piston comes into contact with the pressure side. This is why there is no continuous compression and the machine operated against full pressure.
The driven main piston transmits the rotary movement via toothed wheels (spur gear or helical gears) to the auxiliary piston. The two rotors have the same speed but are not in direct contact with each other or the casing hence they do not require lubrication. They do require a bearing arrangement that has radial and axial clearance as close to zero as possible.
Rotary Compressors
Rotary compressors also known as Lamellar compressors, are machines with pulsating compression as are the piston compressors. An enclosed quantity of gas is brought to the desired pressure by reducing its volume. The capacity is proportional to the speed and only slightly influenced by the opposing pressure.
Rotary compressors are single-stage for pressure up to 3 Bar and 2-stage for pressure up to 8 bar. The economically viable suction performance is up to 5000 m3/h.
The major advantage is these compressors are suitable for direct coupling to economic asynchronous electric motors. They operate without producing vibrations with constant torque and therefore require a small lightweight foundation.
The primary applications of these compressors are for production of the compressed air for pneumatically driven machines and tools of all kinds in foundries, queries, mines, and the engineering industry. In addition to the stationary equipment, many mobile units are used which are then provided with appropriate water cooling systems. As vacuum pumps they find application in the chemical industry for evacuation purposes and are also used to remove dust.
A rotary drum, supported at both sides in cylindrical roller bearings, is eccentrically mounted in a water-cooled housing and forms a sickle-shaped chamber with the housing. The drum is provided with a number of longitudinal slots into which lamellae, with complete freedom of movement, are introduced. As the drum rotates, the centrifugal force ejects these lamellae. The volume between each pair of lamellae is reduced during the movement in the direction of the pressure chamber into which compressed air (or gas) is pressed. The heat produced during the compression is removed by cooling the housing. The cooling water enters via a valve at the lowest point of each stage and is then distributed evenly by means of an internal system of pipes. The working chamber, entrance, and exit supports, as well as the housing cover, are all water-cooled so that cooling is good all around.
Screw Compressor
These compressors have gained A lot of popularity Overtime because of their quiet operation And the continuous compressed air supply they deliver. The screw compressor has no acceleration or retardation forces to overcome operation in contrast to piston compressors where the oscillating parts have to be accelerated and retarded twice per shaft revolution. Screw compressors can therefore operate at high speeds and can be made compact there is no rotor contact to the casing in case the main rotor and auxiliary rotor are connected by timing gears they do not contact each other either, otherwise they do as the main rotor drives the auxiliary rotor. Tolerances including the bearing clearance values must be very narrow.
Compression takes place between the screw threads and the router housing. It begins when the rotors “separate” at the intake slot allowing the medium to enter the space Between the threads. When the rotors reach the end of this intake slot, the air is enclosed between the threads of the two rotors and is transported axially by the rotors. Its volume is continuously reduced and the pressure increases when the exit vent is reached and the compressed medium is injected. The figure below shows a diagrammatic representation of the compression cycle of a screw compressor.
The two rotors are differently designed in practice the following profile combinations have been found particularly suitable (Refer to the figure below). The Main rotor has four helical threads (passages) at 90 degrees to each other. The auxiliary rotor has 6 helical grooves at 60 degrees to each other which enable meshing with the main rotor. Originally the profile was symmetrical (semicircular) but this design has been superseded by an asymmetric profile that has lower gap losses and which by virtue of its efficiency and economy brought about the industrial breakthrough for the screw compressor.
There are 2 types of screw compressors which differ in their mode of operation
a) Dry running screw compressor
b) Oil Lubricated screw compressor
In dry-running screw compressors, the movement of the driven main rotor is Transmitted to the auxiliary rotor via a synchronization or timing gear set with the appropriate ratio.
The Torsional flank clearance of the synchronizing gear wheels must be smaller than that of the screw rotors. Compression is polytropic for a peripheral speed of the main rotor Which is near the optimum as possible (approximately 90 meters per second). This type of screw compressor supplies completely dry compressed air. They are used in the chemical and process industries, in food processing, and also as mobile units for starting the engines of jet aircraft.
The more common type is however the design where oil is sprayed into the pressure chamber in order to remove the heat generated during compression from the medium being compressed (isothermal compression). The oil also serves to lubricate the few positions at which inter-metallic contact occurs. The drive main roto in this type of compressor drives the auxiliary rotor “automatically” without any constant velocity gearing. Only some 10% of the torque is transmitted to the auxiliary rotor.
The considerable quantities of oil sprayed into the compressor not only serves to lubricate, but also seal the very narrow gaps between the rotor pair and the housing. In this way, moderate peripheral speeds (between 20 and 50 m/s) and low rotational speeds can be used. A spin-off effect is the lower nose level resulting from the damping of the oil. This type of compressor is used where traces of oil in the compressed air can be accepted like – a pneumatic cylinder for pressing and tensioning., measuring and control apparatus, pneumatic tools, and refrigeration plant.
Machines with Dynamic Compression
Compression in these types of machines depends upto the transportation of energy from one set of rotating blades to a gas. The rotor produced this energy transfer by altering the pulse and pressure of the gas. The pulse- as a measure of kinetic energy – is transformed into compression energy in the associated impeller machine or diffuser.
Dynamic compressors, either single-stage or two-stage, are characterized by a pulsation-free delivery. Within the chamber, no lubrication is required, so that oil-free compressed gas can be delivered. For this reason, these compressors are used in the chemical and process industries.
Depending on the direction of the flow of gas, the compressors are classified as radial or axial compressors. The diagonal compressor is a combination of both, but it is not particularly common.
Radial Turbo Compressors
The below figure shows a radial unit and air streams out radially from the impeller. This type of compressor is suitable for small to medium air flow rates (34,000 to 10,00,000 m3/h) at power ratings of a maximum of up to 26 MW. The operating speeds reach up to 22,000 to 25,000 RPM and there is a tendency to design even higher RPM compressors. The operating pressure of multistage units is up to 40 bar. There are certain special designs where a pressure of up to 350 bar has been achieved.
Axial Turbo Compressors
These compressors have capacities of 43,000 to 17,00,000 m3/h, requiring upto 75 MW. The operating speed is lower than for the radial machines and normally does not exceed 10,000 to 15,000 RPM.
The pressure increase per compression stage is smaller for the axial turbo-compressors but the efficiency is generally better than that of the redial turbo-compressors. The operating pressure does not normally exceed 7 bar, although peak values upto 35 bat have been known.
Because of the relatively high speeds, turbo compressors, particularly radial compressors are especially suitable for direct drive by a steam turbine or gas turbine. For smaller units, the requisite speed level is attained by using a gearbox.
Rolling element bearings are only used (generally) on the impellers of small radial turbo-compressors. Large units and multistage axial turbo-compressors are fitted with plain bearings.
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