Volumetric compressor efficiency

Figure 12-18A. Compressor volumetric efficiency curves for gas with k or n of 1.15. (Used by permission Natural Gasoline Supply Men s Association Data Book, 1957. Origin Ingersoll-Rand Co. All rights reserved.)...

Equation (8.5) does not account for the volumetric efficiency (VE) loss of reciprocating-type compressors. Volumetric efficiency is simply the clearance allowed in the compressor cylinder head in which this compressed gas volume is allowed to mix with the inlet gas to be compressed in the next compression stroke. Thus this compressed gas in the cylinder clearance is recycling, which makes the output of the compressor less efficient, the larger this clearance volume. Volumetric efficiencies usually range fron 6 to 20%. An average reciprocating compressor VE should be 10%. For conservative cost calculating, use 15% of overall GHP for the VE efficiency. 

Ratio of the ac tual flow rate (entering compressor) to the displacement rate is the volumetric efficiency T va- The volumetric efficiency is... 

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. 

Because there is no clearance volume expansion, as in the reciprocating compressor, the volumetric efficiency is a function of the rotor slip. This is the internal leakage from the higher pressure to the lower pressure side, reducing potential volume capacity of the compressor. 

Figure 4-6. Pressure ratio vs. volumetric efficiency for an SRM compressor.

The example demonstrates that operating the compressor off the built-in pressure ratio means operating at a lower efficiency. This could be anticipated from Figure 4-3. The optimum port configuration for the various types of screw compressors was determined from a series of prototype tests. The change in volumetric efficiency is not a result of the built-in volume ratio, but is due to the increased slip (internal leak " n... 

Volumetric efficiency is determined by the tip leakage past the rotors, not unlike the rotary screw compressor. The leakage is referred to as slip. 

Output volume is calculated from displaced volume multiplied by the difference between a desired speed and the slip speed, rather than using the concept of volumetric efficiency as in the other positive displacement compressors. Output volume is expressed by... 

Thus, in all compressors of two or more stages, the volumetric efficiency of the low-pressure cylinder determines the volumetric efficiency of the entire compressor (not recognizing packing leaks). ... 

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 ... 

In many instances, the design of suction and discharge pulsation dampening drums (or bottles) for reciprocating compressors is based on piston displacement and volumetric efficiency, and this design normally will suffice to reduce peak pulsation to approximately 5% of the line pressure. In special or other cases, experience has shown that operational difficulties (vibrations, meter pulsations, etc.) may indicate that the peak pulse pressure of 5% line pressure is inadequate. Thus, the pressure in pulsation-reduction design selection is... 

The first three factors are present in compressors, but they are small and on the whole can be neglected. The clearance volume problem, however, is unique to reciprocating piston compressors. The volumetric efficiency e estimates the effect of clearance. The volumetric efficiency can be approximated as... 

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

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. 

Where water is used as a cooling medium for compressor and ancillary equipment it should be within the temperature and pressure levels prescribed by the compressor supplier and should be free from harmful impurities. The cooling water should have a low inlet temperature in order to assist in achieving a high volumetric efficiency in the compressor and to cool the air passing through the aftercooler to a temperature adequate for effective condensation of water vapor. 

This loss of useful working stroke will increase with the ratio of the suction and discharge absolute pressures, and the compressor efficiency will fall off. This effect is termed the volumetric efficiency [11]. Typical figures are shown in Figure 2.8. 

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. 

For instance, suppose we are working with a reciprocating compressor. This particular machine has a very small volumetric clearance and a very high volumetric efficiency (these mysterious terms are fully explained in Chap. 29, Reciprocating Compressors ). With this compressor, a significant decrease in discharge pressure will result in only a tiny increase in vapor flow. Therefore, the load on the driver will decrease. 

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 reduction in volumetric efficiency reduces the work required from the driver. For reciprocating compressors, we intentionally reduce the volumetric efficiency, to reduce the load on the driver. Sometimes this is done to save energy sometimes this is done to prevent a motor driver from tripping off on high amps. 

In the plant, we use the term unloading to indicate various ways of reducing the volumetric efficiency of a reciprocating compressor. There are two sorts of unloaders ... 

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. 

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. 

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