Ejector and Vacuum Systems

Application ranges of the various kinds of devices for maintenance of subatmospheric pressures in process equipment are shown in Table 7.3. The use of mechanical pumps—compressors in reverse— for such purposes is mentioned earlier in this chapter. Pressures also can be reduced by the action of flowing fluids. For instance, water jets at 40psig will sustain pressures of 0.5-2.0psia. For intermediate pressure ranges, down to O.lTorr or so, steam jet ejectors are widely favored. They have no moving parts, are quiet, easily installed, simple, and moderately economical to operate, and readily adaptable to handling corrosive vapor mixtures. A specification form is in Appendix B. 

Several ejectors are used in parallel when the load is variable or because the process system gradually loses tightness between maintenance shutdowns—then some of the units in parallel are cut in or out as needed. 

Finding Work of Compression with a Thermodynamic Chart 

Hydrogen sulfide is to be compressed from 100°F and atmospheric pressure to 50 psig. The isentropic efficiency is 0.70. A pressure-enthalpy chart is taken from Starling (Fluid Thermodynamic Properties for Light Petroleum Systems, Gulf, Houston, TX, 1973). The work and the complete thermodynamic conditions for the process will be found. 

The path followed by the calculation is 1-2-3 on the sketch. The initial enthalpy is —86Btu/lb. Proceed along the isentrop S = 1.453 to the final pressure, 64.7 psia, and enthalpy H2 = —27. The isentropic enthalpy change is 

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Ejector and Vacuum Systems 162 Ejector Arrangements 162 Air Leakage 164 Steam Consumption 165 Ejector Theory 166 Glossary for Chapter 7 166 References 167... 

Volume lA hird Edition, which covers process planning, scheduling, and flowsheet design, fluid, flow, pumping of liquids, mechanical separations, mixing of liquids, ejector and vacuum systems, and pressure-relieving devices. 

Vacuum Distillation - Heavier fractions from the atmospheric distillation unit that cannot be distilled without cracking under its pressure and temperature conditions are vacuum distilled. Vacuum distillation is simply the distillation of petroleum fractions at a very low pressure (0.2 to 0.7 psia) to increase volatilization and separation. In most systems, the vacuum inside the fractionator is maintained with steam ejectors and vacuum pumps, barometric condensers, or surface condensers. 

Other factors that favor the choice of the steam ejector are the presence of process materials that can form soflds or require high alloy materials of constmction. Factors that favor the vacuum pump are credits for pollution abatement and high cost steam. The mechanical systems require more maintenance and some form of backup vacuum system, but these can be designed with adequate reflabiUty. 

Air is usually the basic load component to an ejector, and the quantities of water vapor and/or condensable vapor are usually directly proportional to the air load. Unfortunately, no reliable method exists for determining precisely the optimum basic air capacity of ejectors. It is desirable to select a capacity which minimizes the total costs of removing the noncondensable gases which accumulate in a process vacuum system. An oversized ejector costs more and uses unnecessarily large quantities of steam and cooling water. If an ejector is undersized, constant monitoring of air leaks is required to avoid costly upsets. 

Figure 2-44. Friction factor for streamlined flow of air at absolute pressures from 50 microns Hg. to 1mm Hg. By permission, Standards for Steam Jet Ejectors, 3rd. Ed., Heat Exchange Institute, 1956 [54] and Standards for Steam Jet Vacuum Systems, 4th Ed., 1988. Note f on same basis as Figure 2-3 [58].

Standards for Steam Jet Ejectors, 3rd ed.. Heat Exchange Institute, 1956, and Standards Jor Steam Jet Vacuum Systems, 4th ed., 1988, Cleveland, Ohio. 

It is necessary to consult manufacturers for final and specific selecdons. However, the followang guide data is reliable and should serve to check recommendations or to specify a system. It is advisable to try to accomplish the specific operation wth as few ejectors as possible, because this leads to the most economical operation and lowest first cost in the majority of cases. Figures 6-9A, B, and C are a basic comparison guide for vacuum systems. 

Few vacuum systems are completely airtight, although some may have extremely low leakage rates. For the ideal system the only load for the ejector is the non-condens-ables of the process (absorbed gases, air, etc.) plus the saturated vapor pressure equivalent of the process fluid. Practice has proven that allowance must be made for air leakage. Considering the air and non-condensables. For base ejector capacity determine inert gases only by ... 

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