Reciprocating compressors are the best known and most widely used compressors of the positive displacement type. They operate on the principal of reducing the volume (increase in pressure), of specified quantity of trapped gas in an enclosure and then compressed gas is pushed out of the enclosure.
Reciprocating compressors have inherent advantages over other compressors in the ability to adopt a wide range of loads and pressure ratios. The load may vary from 0 to 100% of the capacity.
Reciprocating compressors have inherent advantages over other compressors in the ability to adopt a wide range of loads and pressure ratios. The load may vary from 0 to 100% of the capacity.
RECIPROCATING COMPRESSOR ADVANTAGES:
1.High Compression ratios
Due to the positive displacement characteristics, reciprocating compressors are capable of extremely high compression ratios and pressure levels. They are limited by the resultant discharge temperature (best approximated by assuming adiabatic compression) and mechanical strength of the drive train and cylinder. Polythylene machines are today compressing this material to 50,000 psig. at which point the
fluid behaves more like a liquid than a gas.
fluid behaves more like a liquid than a gas.
2.Constant Volume, Variable Pressure
As has been illustrated in the figure showing characteristic performance curves, a reciprocating compressor can be advantageous in its ability to deliver a fairly constant volume over a wide pressure range. However, due to increased slippage, inlet gas preheating, valve efficiency, and volumetric efficiency effects at higher compression ratios, less volume will be moved.
3.Molecular Weight Flexibility
Whether a low molecular weight gas such as hydrogen or an extremely high molecular weight gas is to be compressed, this machine can achieve high compression ratios within the discharge temperature and mechanical limitations. This has proven to be beneficial when handling a service which has a fairly wide molecular weight range due to diversity of feed stocks, plant operational flexibility,
catalyst activity changes etc.
4.Basic Technology
It is one of the oldest, tested and proven technology over the years. With good manufacturing and design techniques, once built and installed properly, reciprocating compressors are more easily understood and maintained than the dynamic machines.
5.High efficiency
Up until a few years ago, most users had an unflinching desire to use dynamic machines wherever possible. In today’s world of high kilowatt-hours, reciprocating compressors are winning a few more battles when the utilities are evaluated. The efficiency advantage is greater at lower flows and for services requiring a wide molecular weight range.
RECIPROCATING COMPRESSOR DISADVANTAGES:
1.Separate Heavy Foundation:
Depending on the resultant unbalanced forces for a given application, the mass of concrete required for the foundation can be considerable.
2.Pulsating Flow
Due to pulsation on the suction and discharge of the machines, pulsation suppression devices are required to dampen the pressure wave amplitudes, which are damaging to the compressor valves and connecting piping. Piping must be sufficiently supported to avoid fatigue failures. At higher pressures, higher horsepower, more stages of compression, and multiple machines, the analysis grows more complex and an analog study is used to insure a satisfactory piping design. In addition to the comp lexity added to the piping design, the pulsating flow aggravates flow measurement. Also, the pulsation can be transmitted to the other parts of the process. Exchangers in reciprocating compressor circuits must be designed in consideration of the potentially damaging pulse waves.
3.Vulnerable to dirt and liquid
Packing, rings, and valves are extremely sensitive to dirt and liquid. Prior to start-up, the gas circuit is acidified to remove mild scale and other dirt. Suction vessels and inlet lines are designed to minimize liquid entertainment and condensation. Liquid carry-over can cause catastrophic damage, unsafe load reversals.
4.Low reliability:
Due to reciprocating motion the parts are subjected to more wear and tear, this requires opening of machine periodically.
5.Maintenance
This is an area which the user usually has a full appreciation for and requires multiple machines to improve plant on-stream efficiency. A reasonable good rule of thumb for maintenance costs is $35 / BHP / Year.
6.Plot area
These compressors usually take up a considerable area as are located at lower level, whereas centrifugal compressors can be installed at elevation. There are more machines, and each one can be fairly large, requiring substantial clearances for maintenance.
7.Large volume capability
For very large volumes, the size, number, and cost of cylinders make these compressors unattractive. Many machines could be required to meet the capacity.
8.Torsional Implications
Due to the characteristic pulsating torque for a reciprocating machine, the drive train and electrical circuit have to be protected. A flywheel is one way of smoothing out the torque demand. Coupling, gears, motors, and steam turbine have to be selected with careful consideration given to the particular compressor and its torsional characteristics.
CHARACTERISTIC DIMENSIONS :
Suction pressure – is the pressure measured at the compressor cylinder inlet flange.
Discharges pressure – is the total pressure measured at the discharge flange of the compressor
Compression ratio – is the ratio of the absolute discharge pressure to absolute suction pressure.
Top dead centre (TDC) - is the outer (at cylinder end) dead centre of the piston stroke
Bottom dead centre (BDC) – is the inner (crank side) dead centre of the piston stroke
Stroke – It is the distance traversed by the piston in moving between two dead centres (top and Bottom)
Suction volume – is the volume of gas that enters the compressor in each unit of time.
Piston displacement – is the volume of a gas displaced in a stroke
Volumetric efficiency – is the ratio of actual capacity of compressor to displacement (theoretical capacity)
Compression capacity – is ratio of the theoretical horsepower to indicated horsepower.
Mechanical efficiency – is the ratio of the indicated horsepower to the break horsepower delivered at the shaft.
Overall efficiency – is the product of compression efficiency and mechanical efficiency.
Compressor capacity - is expressed in ICFM (intake cubic feet per minute) or ACFM (actual cubic feet per minute) at intake conditions.
See Also:
RECIP RECIPROCATING COMPRESSOR
CLASSIFICATION OF RECIP RECIPROCATING COMPRESSOR
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