Compressor sizing

While theoretical compressor power requirements are reduced slightly by going to lower evaporating temperatures, the volume of vapor to be compressed and hence compressor size and cost increase so rapidly that low-temperature operation is more expensive than high-temperature operation. The requirement of low temperature for fruit-juice concentration has led to the development of an evaporator... 

Flooded compressors use the asymmetric profile rotor extensively because the rotor s efficiency is most apparent in this size range. Flooded compressor size has, over the more recent times, been increased. The upper range is in the 7000 cfm range. While most applications are in air and refrigeration, certain modifications can make it applicable for process ga.s service. One of the considerations is the liquid used for the flooding. 

Figure 12-73. Centrifugal compressor size versus capacity. (Used by permission Dresser-Rand Company.)...

Speed. From Figure 12-75, the rpm is approximately 6,800 rpm for a (first compressor) size No. 3 case. This is not unusual for different manufacturers to operate at significantly different speeds, as this is a function of wheel design. 

Well spacing/flow requirements Compressor size Blower size/well placement Vapor treatment... 

Compressor (size) Pressure (psi) Capacity (cuft/min) Motor (HP) Speed (rpm)... 

In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is given by... 

Once the number of compressors is determined, the work requirement (kW) for these compressors is found using Equation (10). This work requirement, or necessary compressor size, is used to determine the total energy requirement for the compressor, based on the capacity factor. It also is used to determine the capital cost of the compressor using the same method as the compressors in hydrogen storage. 

Improvements in membrane selectivity have now reached a point of diminishing returns. A major improvement in selectivity, from 8 to 12, would reduce compressor size by 20%. Unfortunately, based on Robesoifs trade-off curve, such an improvement in selectivity is likely to be accompanied by a tenfold reduction in membrane permeability, and a tenfold reduction in permeabihty means a tenfold increase in membrane area. These effects are shown graphically in Fig. 7.9. 

Significant efforts have been made for increasing O2/N2 selectivity of the polymeric membranes these two molecules have a very close kinetic diameter (nitrogen, 3.64 A oxygen, 3.46 A), thus the use of a membrane whose selective properties are related only on the molecular size selection was very difficult. As reported by Baker [7], the first membranes used for this separation showed an O2/N2 selectivity of ca. 4. Significant improvement in membrane selectivity allowed values ranging from 8 to 12 to be reached, implying relevant reduction in compressor size and equipment costs. 

Careful analysis of life cycle costs and equipment size is important. Little can be done to correct an error in compressor sizing without incurring significant expense. 

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