Piston Compressor for

Multiple-stage turbo compressors are by far the most frequently used machines in cryogenic air separation. Positive displacement machines are foimd in niche applications, such as piston compressors for oxygen compression and screw compressors for small plants processing less than 4000 m h of air. 

Figure 13.17 Piston compressor for 300 MPa for carbon dioxide (according to Hofer) (source Institute of Process Machinery and Systems Engineering, University of Erlangen-Nuremberg, Germany).

After all the previous statements, it would seem very difficult to select a piston. speed. For someone without direct experience, the following guidelines can be used as a starting point. Actual gas compressing experience should be solicited when a new compressor for the same gas is being eonsidered. These values will apply to the industrial process type of compressor with a double-acting cylinder construction. For horizontal compressors with lubricated cylinders, use 700 feet per minute (fpm) and for nonlubricated cylinders use 600 fpin. For vertical compressors with lubricated cylinders, use 800 fpm and for nonlubricated cylinders use 700 fpm. 

Calculate the suction capacity, horsepower, discharge temperature, and piston speed for the following single-stage double acting compressor. 

In much the same manner as pumps, compressors are classified as one of two general classes positive displacement or dynamic (see Figure 3-68) [23]. These two general classes of compressors are the same as that for pumps. The positive displacement class of compressors is an intermittent flow device, which is usually a reciprocating piston compressor or a rotary compressor (e.g., sliding vane, screws, etc.). The dynamic class of compressor is a continuous flow device, which is usually an axial-flow or centrifugal compressor (or mix of the two). 

Figure 3-73. Volumetric efficiency for reciprocating piston compressors (with ciearance) [4].

It is possible that the maintenance problems with multi-stage screw compressors are substantially smaller than with piston compressors, but so far only one publication has been found [2.30] comparing the technical data and operation cost of piston- and screw compressors for two brands. The maintenance cost of both systems are not discussed. 

The air required for combustion is compressed by rotating piston compressors or turbo blowers. The air is preheated in heat exchangers by hot gases containing carbon black leaving the reactor. This conserves energy and thus improves the carbon black yield. Preheated air temperatures of 500-700 °C are common. 

Fig. 4.1-29. Power range of various compressor types for high-pressure [24], D, Diaphragm compressor PI, Piston compressor, dry running (piston ring, labyrinth) P2, Piston compressor, lubricated (piston ring) P3, Ultra-high-pressure compressor, lubricated (plunger piston) TR, Radial turbo-compressor TA, Axial turbo-compressor S, Screw compressor.

The so-called hypercompressors for the production of LDPE represent a special case. The ethylene is compressed in a primary piston compressor, with several stages up to around 200 to 300 bar the hypercompressor (or secondary compressor) brings the gas up from there to 3000 bar. The hypercompressors show pairwise-opposite-cylinders, and are built with up to fourteen cylinders in a multiplex arrangement. The components loaded by an internally pulsating pressure are either shrunk and/or autofrettage-treated in order to implement protective compressive residual stresses (Fig. 4.1-34). 

Fig. 4.1-33. Reciprocating drive units for high-pressure piston compressors...

At evaporating temperatures between -30 and -45 °C, the screw compressor is more economical. At temperatures of -55 °C and lower, the piston and screw compressors are equally efficient. The reason for this lies in the lower power input of the piston compressor at low evaporating temperatures. Another advantage of the screw compressor is the low noise level. The disadvantage might be the price, but this must be evaluated in each individual case. Another alternative should be mentioned to complete the picture this alternative to screw compressors would be the use of cooling cascades. 

Figures 8.10 and 8.11 are based on the driver FOB fabrication shop cost only. For the completed driver package, additional items, such as a surface condenser for the condensing steam turbine or a starting motor or air piston starter for the gas engine, are required. Tables 8.29 to 8.32 are each dedicated to providing factoring for these specific costs. Such costs will again be based on the compressor horsepower, BHP.

Operating speeds of larger units are as high as 277 rpm with piston speeds for air service up to 4.3 m/sec. The larger compressors with provision for multiple services reduce the number of motors or drivers and minimize the accessory equipment, resulting in lower maintenance cost. 

Compressors for oxygen service are characteristically operated at lower piston speeds, of the order of 3.3 m/sec. Maintenance of these machines requires rigid control of cleaning procedures and inspection of parts to ensure the absence of oil in the working cylinder and valve assemblies. 

A sample cell was designed to accomodate supercritical fluids at high pressures up to 20 MPa and at moderate temperatures using stainless steel and fused silica glass, as schematically shown in Fig. 1. The fluid was prepared by compressing the low pressure sample gas from the tube using a piston compressor outside the cell. The cell and the attached part for high pressure lines were immersed in an air thermostat. The pressure and temperature were... 

Piston Compressors and Blowers.—The large quantities of air required for blast-furnaces and Bessemer converters are usually supplied by piston compressors of large capacity, driven either by steam or gas engines. Turbo-blowers directly driven by steam turbines, however, have been recently developed for this work. 

Actual Values of Efficiencies.—Tests of piston compressors show extreme variations of mechanical efficiency from 76 to 97 per cent, with approximate averages for the more common sizes of 85 per cent. The true volumetric efficiency of piston air compressors will vary from 80 to 97 per cent, and the cylinder efficiency for water-jacketed compressors from 80 to 85 per cent. This will result in efficiencies of compression varying from 64 to 82 per cent. 

---