Theory and Calculations of Gas Compression

The sliding vane compressor can deliver pressures of 50 psig or pull a vacuum of 28 in. of mercury. A two-stage unit can deliver 250 psig. A generous supply of lubricant is needed for the sliding vanes. Table 7.9 shows that power requirements are favorable in comparison with other rotaries. 

The main concern of this section is how to determine the work requirement and the effluent conditions of a compressor for which the inlet conditions and the outlet pressure are specified. Theoretical methods allow making such calculations for ideal and real gases and gas mixtures under isothermal and frictionless adiabatic (isen-tropic) conditions. In order that results for actual operation can be found it is neecessary to know the efficiency of the equipment. That depends on the construction of the machine, the mode of operation, and the nature of the gas being processed. In the last analysis such information comes from test work and its correlation by manufacturers and other authorities. Some data are cited in this section. 

Under isentropic conditions and with constant heat capacities, the pressure-volume relation is 

Since k ordinarily is a fairly strong function of the temperature, a suitable average value must be used in Eq. (7.20) and related ones. Under adiabatic conditions the flow work may be written as 

Upon substitution of Eq. (7.20) into Eq. (7.24) and integration, the isentropic work becomes 

The main concern of this section is how to determine the work requirement and the effluent conditions of a compressor for which the inlet conditions and the outlet pressure are specified. 

TABLE 7.7. Some Sizes of One- and Two-Stage Reciprocating Compressors (a) Horizontal, One-Stage, Belt-Driven 

Diameter Cylinder (in.) Stroke (in.) Displacement (cuft/min.) Air Pressure (Ib/sq in.) Brake HP at Rated Pressure Openings (in.)  

---

A specification form is included in Appendix B and as Table 4.4. Efficiency data are discussed in Section 7.6, Theory and Calculations of Gas Compression Efficiency. 

A two-stage double-acting compressor with water cooled cylinder jackets and intercooler is shown in Figure 7.18(c). Selected dimensional and performance data are in Table 7.7. Drives may be with steam cylinders, turbines, gas engines or electrical motors. A specification form is included in Appendix B. Efficiency data are discussed in Section 7.6, Theory and Calculations of Gas Compression Temperature Rise, Compression Ratio, Volumetric Efficiency. 

THEORY AND CALCULATIONS OF GAS COMPRESSION 155 TABLE 7.9. Five Rotary Compressors for a Common Service... 

---