Compression Reciprocating Compressors

Gas turbine driven centrifugal compressors are very efficient under the right operating conditions but require careful selection and demand higher levels of maintenance than reciprocating compressors. Compression facilities are generally the most expensive item in an upstream gas process facility. 

Figure 12-12. Reciprocating compressor compression diagrams. Actual losses and effect of k = cjc on performance.

Reciprocating compressors compress gases by a piston moving backwards and forwards in a cylinder. Valves control the flow of low-pressure gas into the cylinder and high-pressure gas out of the cylinder. The mechanical work to compress a gas is the product of the external force acting on the gas and the distance through which the force moves. Consider a cylinder with cross-sectional area A containing a gas to be compressed by a piston. The force exerted on the gas is the product of the pressure (force per unit area) and the area A of the piston. The distance the piston travels is the volume V of the cylinder divided by the area A. Thus ... 

Reciprocating Compressors. Prior to 1895, when Linde developed his air Hquefaction apparatus, none of the chemical processes used industrially required pressures much in excess of I MPa (145 psi) and the need for a continuous supply of air at 20 MPa provided the impetus for the development of reciprocating compressors. The introduction of ammonia, methanol, and urea processes in the early part of the twentieth century, and the need to take advantage of the economy of scale in ammonia plants, led to a threefold increase in the power required for compression from 1920 to 1940. The development of reciprocating compressors was not easy Htfle was known about the effects of cycles of fluctuating pressure on the behavior of the... 

Other variations in compression using the reciprocating engine driver are the reciprocating compressor frame and the centrifugal compressor. 

A reciprocating compressor is a positive-displacement machine in which the compressing and displacing element is a piston moving linearly within a cylinder. Figure 10-1 shows the action of a reciprocating compressor. 

Non-lubricated screw compressors have very close clearances and thus they are designed for limited ranges of discharge temperature, temperature rise, compression ratio, etc., all of which can cause changes in thesc clearances. Lubricated compressors have a somewhat broader tolerance to changes in operating conditions, but they are still more limited than reciprocating compressors. 

There is a disadvantage in centrifugal machines in that they are low efficiency. This means it requires more brake horsepower (bhp) to compress the same flow rate than would be required for a reciprocating compressor. If the compressor is driven with a turbine, there is even a greater disadvantage because the turbines are low in fuel efficiency. The net result is that turbine-driven centrifuea machines do not use fuel very... 

Oil is miscible with all the refrigerants except ammonia. This may create foaming in the crankcase and an unsatisfactory compression condition for reciprocating compressors. 

Piston displacement is the actual volume of the cylinder displaced as the piston travels its stroke from the start of the compression (condition (1)) to the end of the stroke (condition (e)) of Figure 12-12 expressed as fF of volume displaced per minute. Displacement values for specific cylinder designs are available from the manufacturers, Table 12-6. Neerken is a useful reference. Reciprocating compressors are usually rated in terms of piston displacement, which is the net volume in ft per minute displaced by the moving piston. Note that the piston does not move through the clearance volume of Figure 12-12 therefore this volume is not displaced during the stroke. 

Used and compiled by permission Plain Talks on Air and Gas Compression, Fourth of Series, Worthington. Dresser-Rand Corporation. Also compiled by permission from Reciprocating Compressor Calculation Data, 1956. Dresser-Rand Corporation. 

Figure 12-25. Combined compression and mechanical efficiency of reciprocating compressors. (Used by permission Campbell, J. M. Oil and Gas Journal and Ridgway, R. S. California Natural Gasoline Association Meeting, 1945. All rights reserved.)...

The fundamental characteristics of compression are the same for centrifugal and reciprocating compressors. The manner in which these fundamentals are interpreted must be adapted to the particular machine type and operating characteristics, and this accounts for the difference in design procedures. 

The only compressor that is ideally suited for loadfollowing applications is the reciprocating type. These units have an absolute ability to absorb the variations in pressure and demand without any impact on either reliability or life cycle cost. The major negative of the reciprocating compressor is the pulsing or constant variation in pressure that is produced by the reciprocating compression cycle. Properly sized accumulators and receiver tanks will resolve most of the pulsing. 

Few machines involve linear reciprocating motion exclusively. Most incorporate a combination of rotating and reciprocating linear motions to produce work. One example of such a machine is a reciprocating compressor. This unit contains a rotating crankshaft that transmits power to one or more reciprocating pistons, which move linearly in performing the work required to compress the media. 

Estimate the power required to compress 1000 m3/h air from ambient conditions to 700 kN/m2 gauge, using a two-stage reciprocating compressor with an intercooler. 

Estimate the work required to compress ethylene to 25 MPa in a two-stage compressor. A reciprocating compressor will be used. The gas is at an initial temperature of 15°C and is cooled to 25°C after the first-stage compression. 

Estimate the work required to compress ethylene from 32 MPa to 250 MPa in a two-stage reciprocating compressor where the gas is initially at 30 °C and leaves the intercooler at 30 °C. See Example 3.13. 

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