The 4 Main Components of a Reciprocating Compressor
Compressors that are used in the gas, chemical, and petrochemical industrial processes are made of several components, each of which strives to offer some level of support to the machine.
These components have a role to play either in managing heat, aiding in rotation, enhancing the machine’s durability, narrowing gas flow, and even enabling other parts to carry out their task efficiently.
While these components may be similar in centrifugal processors, their setup uniquely differs from those in the reciprocating compressors.
But then, what are the major components of reciprocating compressors?
Read on to find out!
What is a Reciprocating Compressor?
A reciprocating compressor is also known as a piston compressor because it uses pistons that are aided by a crankshaft to increase the level of pressure in gases.
The gas, in this case, will first have to move through the suction manifold before flowing to the compression cylinder where it is compressed.
The compression is brought about by the reciprocating motion of the piston before the gas is discharged.
Major Components of a Reciprocating Compressor
The major components of a reciprocating compressor and their functions give the machine its level of functionality. This positive displacement machine’s parts include:
- Distance Piece
- Piston rings
- Cross Head
- Connecting Rod
The first thing you’ll notice in a compressor is its heavy and rugged frame which encloses all the rotating components such as the cylinder and crosshead guide.
The frame is also called a crankcase and it has a square or rectangle shape. Its role is to offer support to the crankshaft.
Notwithstanding, you can find Separable compressors or Integral compressors and in the case of the latter, the compressor and engine-power cylinders are fitted to the same frame and driven by the same crankshaft.
Cylinders are pressure vessels that contain the gas that is to be compressed, and as such, they are one of the most important components of a reciprocating compressor.
In large low-pressure cylinders, these parts are made up of cast iron and are removable from the major frame.
They are also connected to the frame through an intermediary known as a distance piece.
Alternatively, small high-pressure cylinder compressors made of steel are directly fixed to the main body of the compressor.
Cylinders support the discharge valve plates and suction and sometimes feature replaceable liners or sleeves which give the cylinder’s wearing portion a renewable surface.
The liners do not slip from the surface. Therefore, they ensure that if the cylinder is worn or damaged, it can easily be replaced instead of buying a new system entirely which is more expensive.
The function of cylinders in reciprocating compressors is to cool the temperature of the machine during its compression cycle which tends to generate heat.
This is made possible with the use of a water jacket or fins in the cylinder which provides cooling air.
3. Distance Piece
A distance piece separates the compressor cylinder from the frame. It can either have a single, double or even an extra long compartment arrangement.
- The single design uses a structure where the distance between the diaphragm and cylinder packing is extended in order to prevent any part of the rod entering the crankcase and cylinder stuffing box.
- A double-design also uses the same structure where there is no part of the rod entering the compartment and crankcase placed adjacent to the gas cylinder.
The long distance piece, on the other hand, helps to separate the part of the piston rod that enters the crankcase. The other part of the rod which enters the cylinder sends its lubrication to the long distance piece.
As such, lubricant cannot enter the cylinder to contaminate the compressed gas.
There are four major classifications of distance piece and these are: type A, type B, type C, type D.
The mode of operation of the compressor greatly relies on the piston. This is because the piston is the component that actually compresses air.
The piston must have weight, strength, and be compatible with the gas to be compressed.
It also moves the energy from the crankcase to the gas contained in the cylinder in a bid to prevent the refrigerant from leaking through the gap. In between the piston and cylinder walls, this component is usually covered with piston rings.
Furthermore, a piston in a reciprocating compressor can either be made of aluminum or cast iron and it moves upwards and downwards in the cylinder.
Its movement brings about the suction and compression of the refrigerant.
5. Piston Rings
The piston rings are wound on the piston and as the piston moves up and down the cylinder, the piston rings come in contact with the walls of the cylinder.
Due to the amount of friction created during this reciprocating motion, rings have to be replaced frequently in order to maintain the seamless operation of the compressor.
In some cases, a wear band or rider ring is used as an additional piston ring.
The function of rider ring in the reciprocating compressor is to reduce the probability of wear and tear occurring between the cylinder and piston.
While rings have to be softer than the liner wall and cylinder, different types of rings can be used for the piston and these are metallic rings and non-metallic rings.
- Non-metallic rings made up of fluorocarbon compounds, for instance, are mostly employed in non-lubricated service. They also weigh less than metallic rings and as such, they have gained wider adoption than the latter.
- Metallic rings like bronze or cast iron, as another option, are used for lubricating service.
6. Cross Head
A crosshead enables the piston to be inserted in the cylinder bore.
The use of a crosshead enables a compressor to use a narrow piston and it enables longer strokes and higher efficiency.
The crankshaft is the major shaft in a compressor, the other being a motor shaft.
The shaft revolves around the frame axis and it drives the piston, piston rod, and connecting rod.
- This component is built as a forged steel crankshaft. It is in large compressors that operate above 150 kW (200 hp).
- For machines that work with less than or equal 150kW machines, a ductile iron crankshaft is used.
Over and above that, a crankshaft in a reciprocating compressor is either connected to the electric motor directly or indirectly with the use of a belt and pulley system.
As the motor shaft rotates, the crankshaft also performs a rotary motion which gives the piston the ability to carry out its reciprocating motion inside the cylinder.
But first, a rod has to be connected to the crankshaft and rod to enable the piston to perform an upward and downward movement.
8. Connecting Rod
When it comes to the crankshaft and piston, a connecting rod is linked between either.
Depending on the power which the machine operates, a forged steel connecting rod or ductile iron material can be used.
The former is in compressors that operate above 150 kW(200 hp) while the latter finds use in compressors that work around 150kW or less.
The physical connection has one side of the rod linked to the piston using a piston pin.
Alternately, the other side of the rod is connected to the crankshaft using a connecting cap rod.
While the connection may sound simple, it is what is required to give the crankshaft its rotary motion which also helps the piston to translate within the cylinder.
Therefore, the connecting rod changes the rotary motion to reciprocating motion.
A compressor valve’s function is to allow gas to flow in the right direction while blocking those that may tend to flow in an undesired direction.
Accordingly, these valves are placed in each operating end of a cylinder. One end will have a set of inlet valves to allow gas into the cylinder while another end features two discharge valves.
The valves in a reciprocating compressor are also of various configurations. These include:
- Ring valve
- Poppet valve
- Plate valve
– Ring Type Valve
This type of valves uses one or more circular rings that have a narrow channel embedded in them. They can then be placed on the center line of the valve seat through the stud.
Their advantage is that they have a low-stress level since there are no stress concentration points.
Alternatively, the difficulty to maintain a uniform flow control in each of the rings gives it its downside.
Ring type valves also have a recommended differential of 30 Mpa or 60 Mpa discharge pressure and a 600rpm
– Poppet Type Valve
This type of valve consists of several sealing elements and ports of the same size and it is employed in reciprocating compressors that operate at a medium pressure range.
It has the advantage of high flow efficiency. This isattributed to the streamlined shape of the sealing element and the high lift that is employed.
Its disadvantage, in contrast, is its inability to withstand uneven flow distribution.
They are, therefore, recommended up to a differential pressure of 15 Mpa or 30 Mpa discharge pressure and 600rpm.
– Plate Type Valve
Plate valves have the same concentric design of the ring valve even though this time around, the rings are joined together.
The recommended differential is 20 Mpa and the discharge pressure of 40 Mpa.
Bearings are not the least component given that they can be found throughout the compressor’s frame.
They ensure other components in the compressor are properly positioned radially and axially.
An instance is that of the main bearings which are fixed in the frame to ensure that the crankshaft is well fitted.
There is also a crank pin bearing that is positioned between the connecting rod and crankshaft.
Other bearings that can be found are the crosshead bearings and wrist pin bearings.
These are the major components of a reciprocating compressor and each works hand-in-hand to give it its high level of performance.
It is also worthy to note that while these machines may be made by different manufacturers, their components are similar.
Nonetheless, the quality of the material this time around will determine the level of functionality it can provide.
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