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- Reciprocating – A reciprocating compressor or piston compressor is a positive displacement compressor that uses pistons driven by a crankshaft to reduce the volume of the aspirated gas and to deliver gases at high pressure. Reciprocating compressors are available in either single stage or multi-stage compression depending upon the design for the required output pressure, both single or double acting.
The intake gas enters the suction manifold or intake filter and the suction valves, then flows into the compression cylinder where it gets compressed by a piston driven in a reciprocating motion via a crankshaft and is then discharged.
Positive displacement compressors provide increased air pressure by limiting the volume of air. They’re typically capable of outputs ranging from 1 to 11 kW or then 45-355 kW. Working pressure depends on number of compression stages: 1 stage is usually up to 3.5 bar; 2 stage – up to 15 bar, 3 stage up to 45 bar. These are made in lubricated or non-lubricated / oil-free types.
The number and size of the working chambers is defined by the thermodynamic function of the compressor. Several working chambers can be fitted within one cylinder.
The arrangement of the cylinder axes influences the design of the crank shaft and the mechanical stresses of the compressor. In a compressor with in-line cylinders, the axes of the cylinders are parallel and are at right angles to the crankshaft.
In general, the axes are in a vertical plane, therefore also called a vertical piston arrangement. If the cylinders are arranged in two opposite groups, the compressor is of the boxer type (balanced-opposed piston). The axis of the cylinders is then mostly in a horizontal plane. If the cylinders are arranged in two planes which are at an angle to each other it is called a V design. A V-angle of 90 degrees is favourable for the balance of the mass forces. A special case is the arrangement of both vertical and horizontal cylinder axis (vertical / horizontal design).
The capacity of these compressors is regulated in the basic version by closing and opening the suction valves – usually it is a 3-stage regulation (100% and 50% capacity, idle run and stop). These types of compressors usually operate at low rotational speeds, so the use of inverters only applies to machines with speeds higher than 500 rpm, but these regulation ranges are usually narrow.
They are famous for their high durability and low idle power consumption (thanks to the use of a flywheel) and low total cost of ownership during their operation. It should also be added that they emit a high level of low-frequency noise, and those intended for continuous operation (24/7) must be water-cooled. The water-cooled version is extremely resistant to high ambient temperatures.
(See also, ‘Oil-free Reciprocating compressors’ and ‘Oil lubricated Reciprocating compressors’)
(See also, “Compressors Capacity Control”)
- Rotary screw – Positive displacement compressors that are considered simple to operate and maintain. Favoured for their ability to provide continuous duty, their design provides cooling within the compressor’s interior, saving the individual parts from extreme operating temperatures and enabling them to deliver outputs that range from 4 to 900 kW. Apart from single stage they are also available in two stages (with inter-stage cooling) for higher pressure or to increase efficiency of compression.
These are either lubricated or non-lubricated (sometimes called oil injected or oil flooded), oil-free (Dry Screw) as per the application. Available at working pressures: 2.5 bar (1stage oil-free); 3.5-15 bar (1 stage oil injected); 5-13 bar (2 stage oil injected); 5-13 bar (2 stage oil free). They are famous for their compact construction, taking up little space in the compressor room and low noise level.
Oil-injected compressors and oil-free compressors with water cooling make it very easy to install energy-sensitive waste heat recovery systems from these compressors. Thanks to heat recovery systems, it is possible to obtain thermal power equivalent to more than 60-70% of the electrical power consumed by the compressor operating on load in the form of hot water at a temperature above 70-75oC.
The capacity of these compressors is regulated in the basic version by closing and opening the suction valve (open/closed – with an idle phase) or by means of a modulated suction valve (with stepless opening) or by changing the length of the active compression chamber.
(See also, “Compressors Capacity Control”)
Screw compressors have great possibilities of using variable speed motors. Whether 2-speed motors or motors with frequency converters. 2-speed motors were popular in the 1980s and early 1990s, but the “inverter” technology at the end of 1990s (sometimes called VSD or VFD) opened up new possibilities. “Inverter” screw compressors offer an incredibly wide adjustment range – for oil-injected machines, it is 20% to 100%, and for oil-free machines, it is usually 40% to 100%.
Mono (single) Screw – The single-screw compressor is a rotating, oil-injected, positive-displacement machine. It comprises a central helical grooved screw, with a pair of gate rotors with flat star-shaped teeth on the sides (see figures la and lb). The gate rotors are engaged with the screw and form a wall or seal of the compression chamber. The single screw can be thought of as two compressors in one, since each side of the compressor functions as a separate compressor. By a revolution of the screw, the compression and discharge cycle occurs on both sides of the machine. The result of this two-sided compressor is that the radial forces on the screw, due to the pressure of the gas, are balanced. In addition, the discharge side of the screw incorporates a labyrinth seal that allows both the suction and discharge ends of the screw to be maintained at the suction pressure. In addition, the forces of the gas inside the screw propellers do not produce unbalanced axial loads. As a result, axial loads are kept to a minimum. In this way, the bearings for the screw have a light load, which translates into a long life cycle. The bearings on the gate rotors carry an eccentric axial load due to the pressure along the blades of the gate rotors that are engaged with the screw. However, these loads are also relatively light, providing a long life cycle to the bearings of the gate rotors. The lubrication of the bearings in the single-screw compressor is simplified by the fact that all bearings are located in low-pressure regions. This means that they can be lubricated without the need for an oil pump. The pressure difference along the compressor from suction to discharge is adequate to provide the required oil flow to the bearings. Gate rotors are produced in two pieces comprising a floating part of lightweight compound and a ductile iron support to transport the loads. (See Figures 1a and 1b.) These two parts are not rigidly fastened but can rotate relative to each other within the limits of a torsional damper, hence the term “floating gate rotor” for the lightweight composite part. The reasoning behind this design is to reduce the driving forces between the screw and the gate rotors.
- Water injected Twin-screw – consists of two interlocking rotors that compress the air, a water jacket surrounding the rotors to cool the compressed air, and a water injection system that injects water into the compression chamber.
The water injection system works by spraying a fine mist of water into the compression chamber during the compression process. The water absorbs the heat generated by the compression process, cooling the compressed air and reducing the energy required to compress the air.
To avoid corrosion from water, the material of construction of the rotor elements is of composite resin or polymer ceramic type.
Hook and Claw – Hook and claw compressors utilise a positive-displacement operating principle, similar to rotary vane pumps. Claw pumps are 100% free of oil and contact during operation. To achieve this, two claw-like rotors rotate within a compression chamber. They rotate in opposite directions without touching one another or the chamber. These are 2-stage type.
- Centrifugal – A dynamic compressor in which air or gas is compressed by the mechanical action of rotating impellers impacting velocity and pressure to the air or gas. Air enters the compressor through its modulating inlet guide vane (IGV) and flows radially in to the first stage where impeller imparts velocity (kinetic) energy to the air. The air then proceeds through diffuser section which converts the velocity (kinetic) energy of the air stream into its pressure energy. These compressors are most effective when running at their full capacity, making them ideal for operating at base loads where demand is continuous and output needs start around 75 kW. The most attractive efficiencies start from 500 kW and above. These are available in multi stages as per the pressure requirements, but are always non-lubricated (oil-free) type. They are famous for their great durability. Most are delivered as water-cooled machines. They require a supply of high-purity dust-free air. Compressors with a power above 500 kW are usually supplied with a voltage higher than the standard industrial voltage (several thousand Volts – depending on the local specifications of the electrical network), due to the amount of current consumed, especially starting current. Their capacity is regulated through the above-mentioned IGV valve, which can usually reduce the intake air flow to a minimum of 65-70% of the nominal value, and below this value (to avoid abnormal vibrations that threaten the machine), the blow-off valve (Blow Off Valve – BOV) is opened, which causes large energy losses of the already compressed air. The amount of energy consumed when closing the IGV valve is not directly proportional to the percentage of its opening. The latest generations of centrifugal compressors use a motor or motors that enable the use of frequency converters, which expands their capacity control capabilities.
(See also “Compressors Capacity Control”)
- Rotary Vane – Rotary / Sliding Vane are positive displacement type that use rotating vanes in a cylindrical housing to compress gas or air. The vanes are mounted on a rotor that rotates inside the housing. The rotor is eccentric in the housing, thereby the vanes can slide within their slots outwards & inwards. As the rotor spins, the vanes are forced outward by centrifugal force and make contact with the inside of the housing. This creates a seal between the vanes and the housing, which traps a volume of gas between them. As the rotor continues to rotate, the volume of gas is compressed and discharged through an outlet port.
Thanks to their low operating speed, these compressors are famous for their long durability. They have the ability to recover heat, similar to screw compressors. The capacity is regulated in the basic version using the suction valve (open/closed) and with the help of inverters, usually in the range of 50-100% of the capacity.
Scroll- A scroll compressor is a positive displacement machine, where gas is compressed between two spiral-shaped metallic scrolls, leading to an increase in both pressure and temperature of the gas.
Axial – are a crucial component of the prime mover / driver known as a Gas Turbine. They have rotor blades (instead of impeller) that rotate with the Shaft. Each rotor blade is paired with a stator blade. A stator blade is a stationary blade (non-moving) that is attached to the interior of the Compressor casing. Each rotor blade-stator blade combo makes a “stage” in the Axial Flow Compressor. The compressed gas is buffeted between the stages of rotor blade-stator blades which creates a zig-zag flow pattern down the axis of the Shaft.
