Efficiency, compression volumetric

Similar to volumetric efficiency, isentropic (adiabatic) efficiency T is the ratio of the work required for isentropic compression of the gas to work input to the compressor shaft. The adiabatic efficiency is less than one mainly due to pressure drop through the valve ports and other restricted passages and the heating of the gas during compression. 

Earlier in the chapter, when the compression cycle was described, a portion of the indicator. Path 3-4, was referred to as the expansion portion of the cycle. The gas trapped in the clearance area expands and partly refills the cylinder taking away some of the capacity. The following equation reflects the expansion effect on capacity and is referred to as the theoretical volumetric efficiency Ev,. 

Step L Calculate volumetric efficiency using Equations 3.5 and 3.6. To complete the calculation for volumetric efficiency, the compressibilities are needed to evaluate the f term of Equation 3.6. Using Equations 2.11 and 2.12 for the inlet conditions,... 

Step 1. Find the suction and discharge volumetric efficiencies using Equations 3.5 and 3.21 with rp = 831/514 = 1.617. The natural gas compressibility values can be obtained by using the gravity/compressibility charts (see Appendix B-29 through B-35) for a specific gravity of. 60. Both Zj and Z2 values are. 93. Applying Equation 3.6, the value of f may be obtained as follows ... 

The capacity of the cylinder is a function of piston displacement and volumetric efficiency. This is in turn a function of cylinder clearance, compression ratio, and gas properties. 

Compute discharge temperature, volumetric efficiency, required clearance, rod load, and required horsepower for the given conditions. Also calculate the lowest suction pressure at which this unit can compress lOOMMscfd. 

In some cases, it is desirable to use two-stage compression without intercooling. If the composition of the gas must remain constant throughout the compression and the temperature does not limit, intercoolers cannot be used if condensables are present. Sometimes two stages are used on low k or n value gases to improve the volumetric efficiency. When this is the case and high compression temperatures or economy of operation do not control, it may be advantageous to omit the intercooler. 

Vg = PD (Ey) cfm cylinder will compress at suction pressure and temperature E = volumetric efficiency, is based... 

E, or sometimes Ev, is the volumetric efficiency of a cylinder and is the ratio of the amount of gas that is actually compressed to the amount of gas that could be compressed if no clearance existed in the cylinder, see Figure 12-12. E, can be obtained from Figures 12-18A-F. 

Compression efficiency is the ratio of the work required to adiabatically compress a gas to the work actually done within the compressor cylinder as shown by indicator cards. Figures 12-12 and 12-16. The heat generated during compression adds to the work that must be done in the cylinder. Valves may vary from 50-95% efficient depending on cylinder design and the ratio of compression. Compression efficiency (or sometimes termed volumetric efficiency) is affected by several details of the systems ... 

The general steps in cylinder selection wiU be outlined. However, actual selection can be accomplished only by referring to a specific manufacturer s piston displacement and the volumetric efficiency of a cylinder. The volumetric efficiency is a function of the compression ratio and k value of gas (both independent of cylinder) and the % clearance, a function of cylinder design. 

The variable available for control is the volumetric efficiency, E, which is a function of the compression ratio of the process requirement and the % clearance of the cylinder. The % clearance can be varied in the cylinder for capacity control by... 

For suction drums, AV is evaluated using the suction volumetric efficiency, E . E = cfm at suction conditions/PD at the selected rpm, or E can he obtained from curves of volumetric efficiency versus the compression ratio for varying cylinder clearances. 

There is no clearance in a rotary compressor. However, there is leakage of air within the internal seal system and around the vanes. Thus, the typical volumetric efficiency for the sliding vane compression is of the order of 0.82 to 0.90. The heavier the gas, the greater the volumetric efficiency. The higher the pressure ratio through the stage, the lower the volumetric efficiency. 

Volumetric efficiency is determined by the leakage at tips of the lobes. The leakage is referred to as slip. Slippage is a function of rotor diameter, differential pressure, and the gas being compressed. 

Where the ratio of suction to discharge pressure is high enough to cause a serious drop in volumetric efficiency or an unacceptably high discharge temperature, vapour compression must be carried out in two or more stages. Two basic systems are in use. 

Screw compressors have no clearance volume, and may work at high compression ratios without loss of volumetric efficiency . In all screw compressors, the gas volume will have been reduced to a pre-set proportion of the inlet volume by the time the outlet port is uncovered, and this is termed the built-in pressure ratio. At this point, the gas within the screws is opened to condenser pressure and gas will flow inwards or outwards through the discharge port if the pressures are not equal. 

In addition, the compression ratio has increased considerably and there must be a correction for loss of volumetric efficiency. 

The factor [ I + c — < PilP )l-Y] is called the theoretical volumetric efficiency and is a measure of the effect of the clearance on an isentropic compression. The actual volumetric efficiency will be affected, in addition, by the inertia of the valves and leakage past the... 

Assuming that the same compression ratio is used in each cylinder, then the theoretical volumetric efficiency of the first stage is ... 

Equation (2.16) implies that the compression k attainable with a Roots pump must always be less than the grading k, between Roots pump and backing pump since volumetric efficiency is always < 1. When combining equations (2.13) and (2.16) one obtains for the efficiency the well known expression... 

Volumetric efficiency applies only to reciprocating compressors. A reduction in volumetric efficiency reduces the gas flow through the compressor. A reduction in volumetric efficiency need not reduce the adiabatic 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. 

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