Suction volume

Due to the constancy of the circulating liquid quantity, the deceleration leads to an extension of the liquid ring to the impeller hub  

One can infer from both of these equations that with increasing power consumption, the liquid ring decelerates more strongly (Eq. (3.4)) and thus the suction chamber is reduced (Eq. (3.5)). It is to be recognised, for example that highly viscous liquids not only raise the power consumption of the vacuum pump but also impair the suction capacity. 

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Because of the low efficiency of steam-ejector vacuum systems, there is a range of vacuum above 13 kPa (100 mm Hg) where mechanical vacuum pumps are usually more economical. The capital cost of the vacuum pump goes up roughly as (suction volume) or (l/P). This means that as pressure falls, the capital cost of the vacuum pump rises more swiftly than the energy cost of the steam ejector, which iacreases as (1 /P). Usually below 1.3 kPa (10 mm Hg), the steam ejector is more cost-effective. 

Cylinders can be selected in the same manner as previously presented, remembering that it is the volume at the suction to the cylinder that is important. Therefore, the effect of the compressibility and moisture content must be reflected in the suction volume being considered. Be careful not to apply these factors twice, when calculating actual [(PD) (E, )] from the horsepower equation. 

The inlet or suction volume to a compressor can be determined in several ways, depending on the data available and the processing conditions. A common relationship is... 

Figure 12-69. Correction factor for compressor bhp/million ft per day at 14.7 psia and suction temperature versus suction volume. (Used by permission Dresser-Rand Company.)...

Usually a constant speed, motor-driven compressor will be equipped with inlet guide vanes to the first wheel to allow suction volume control. If the guide vanes are not used, some other throtding device should be available. Figure 12-62 presents the constant speed performance of a centrifugal machine with inlet guide vanes. 

After operating at essentially design conditions for one year, the process has been changed to have the inlet temperature drop to 50°F. What will be the new operating conditions (1) discharge pressure or (2) bhp The suction volume remains the same at 12,000 cfm. 

Figure 12-95. Axial compressor Type AV100-16, during erection. Note stationary and rotating blades. Two identical steam turbine-driven machines supply air to blast furnace at steel works. Suction volume = 560,000 NmVh discharge pressure = 6.2 bar power input = 52,000 kW each. (Used by permission Bui. 26.13.10.40-Bhj. Sulzer Turbo Ltd.)...

The range of suction volumes is approximately 175-35,300 scfm per minute, and the units are capable of discharging up to 580 psi. In vacuum application, the units can draw down to 1.3 psi abs. ... 

This calculation assumed the gas to be ideal. For comparison, the calculation can be based on the Peng-Robinson Equation of State (see Chapter 4). A number of commercial physical property software packages allow the prediction of gas density and y for a mixture of hydrogen and methane using the Peng-Robinson Equation of State. Using this, the gas density at normal conditions is 0.1651 kg-rn 3. At 40°C and 81 bar, the density is 11.2101 kg-rn 3. Thus, suction volume of gas... 

If the compressor is suction-volume-limited, we say that it is speed-limited. If the compressor is driven by an AC motor, its speed is determined by the motor speed and any connecting gears. If the compressor is driven by a turbine, then the rated speed of the turbine limits the compressor speed. 

Figure 7.27. Efficiencies of centrifugal and reciprocating compressors, (a) Polytropic efficiencies of centrifugal compressors as a function of suction volume and compression ratio (Clark Brothers Co.), (b) Relation between isentropic and polytropic efficiencies, Eqs. (7.22) (7.23). (c) Isentropic efficiencies of reciprocating compressors (De Laval Handbook, McGraw-Hill, New York, 1970). Multiply by 0.95 for motor drive. Gas engines require 7000-8000 Btu/HP.

Economics The advantages of this process are low equipment costs (viz. the deethanizer system and ethylene/ethane separation) and reliability of the acetylene hydrogenation due to low excess hydrogen at the reactor inlet. The refrigeration compressor benefits from low specific power and suction volume, while the cracked-gas compressor processes above-ambient-temperature gas. 

Reference to the stored characteristic curve then gives the volumetric flow rate, Qo, at the design speed and this allows the current suction volume flow to be calculated from ... 

The suction volume of the compressor consists of the flow from the loading terminal and the recycled permeate flow. The reduced stage cut affects the relation regarding the flows share of both and causes an increase of VRU capacity, which is in direct relation to the specific energy consumption. The second influence on the stage cut is the required vent gas purity. Any increase of the allowed hydrocarbon concentration in the vent gas leads to a decrease of the stage cut... 

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