Vapor steam jet

Steam-Jet Systems. Low pressure water vapor can be compressed by high pressure steam in a steam jet. In this way, a vacuum can be created over water with resultant evaporation and cooling water, therefore, serves as a refrigerant. This method frequently is used where moderate cooling (down to 2°C) is needed. The process is inefficient and usually is economically justified only when waste steam is available for the motive fluid in the steam jet. 

Applications The common Heliflow apphcations are tank-vent condensers, sample coolers, pump-seal coolers, and steam-jet vacuum condensers. Instant water heaters, glycoLwater seivdces, and cryogenic vaporizers use the spiral tube s ability to reduce thermally induced stresses caused in these apphcations. 

There are many means to obtain refrigerating effect, but here three will be discussed mechanical vapor refrigeratiou cycles, absorption and steam jet cycles due to their significance for industiy. 

Steam-Jet (Ejector) Systems These systems substitute an ejector for a mechanical compressor in a vapor compression system. Since refigerant is water, maintaining temperatures lower than the environment requires that the pressure of water in the evaporator must be... 

Condenser water temperature. In comparison with other vapor-compression systems, steam-jet machines require relatively large water quantities for condensation. The higher the inlet-water temperature, the higher are the water requirements (Fig, 11-107),... 

Thermocompression Evaporators Thermocompression-evap-orator calculations [Pridgeon, Chem. Metall. Eng., 28, 1109 (1923) Peter, Chimin Switzerland), 3, II4 (1949) Petzold, Chem. Ing. Tech., 22, 147 (1950) and Weimer, Dolf, and Austin, Chem. Eng. Prog., 76(11), 78 (1980)] are much the same as single-effect calculations with the added comphcation that the heat suppied to the evaporator from compressed vapor and other sources must exactly balance the heat requirements. Some knowledge of compressor efficiency is also required. Large axial-flow machines on the order of 236-mVs (500,000-ftVmin) capacity may have efficiencies of 80 to 85 percent. Efficiency drops to about 75 percent for a I4-mVs (30,000-ftVmin) centrifugal compressor. Steam-jet compressors have thermodynamic efficiencies on the order of only 25 to 30 percent. 

Vacuum is applied to the chamber and vapor is removed through a large pipe which is connected to the chamber in a manner such that, if the vacuum is broken suddenly, the inrushing air will not greatly disturb the bed of material being dried. This line leads to a condenser where moisture or solvent that has been vaporized is condensed. The noncondensable exhaust gas goes to the vacuum source, which may be a wet or diy vacuum pump or a steam-jet ejector. 

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which cannot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. Ideally, it would be possible to operate a vacuum pipe still without ejectors, with the overhead vapors composed only of steam. In practice, however, leakage of air into the system and the minor cracking which occurs make it necessary to provide a means of removing non-condensibles from the system. In addition to the distillation of atmospheric residuum, the lube vacuum pipe still is also used for rerunning of off specification lube distillates. 

Volatile liquid eontaminants ean be removed from PPE or equipment by evaporation followed by a water rinse. Evaporation of volatile liquids ean be enhaneed by using steam jets. With any evaporation or vaporization proeess, eare should be taken to prevent worker inhalation of vaporized ehemieals. And, of eourse, the physieal hazards of steam need to be taken into eonsideration along with proteetion needed to eontrol any splatter of liquid or debris. Regulations pertinent to air emissions must also be taken into eonsideration. 

Figure 6-20A. Air and water vapor mixture data (Dalton s Law)—saturated. Reprinted by permission, Standards for Steam Jet EjectorSj 3rd. Ed., Heat Exchange Institute, 1956 [11].

Figure 11-3. Water-vapor relationship in steam jet refrigeration unit.

The vapor is drawn into a steam jet (discussed in Chap. 16). The steam condensate flows into the boot or hot well. The water in the boot is slightly subcooled. This is accomplished by a pair of baffles that create a small zone of condensate backup. The subcooled condensate, cooled to perhaps 10°F below its boiling or bubbling point, is easier to pump. As the pressure in the hot well is subatmospheric, the hot-well pump typically develops a AP of at least 30 to 50 psi. 

Steam jets are also employed to recompress low-pressure steam to a higher-pressure steam. Jets are sometimes used to compress low-pressure hydrocarbon vapors with higher-pressure hydrocarbon gas (instead of steam). They are really wonderful and versatile machines. 

The second problem was air leaks. Air drawn into the system, would build up in the condenser. This noncondensable vapor was drawn off by using a steam jet. 

The gas that accumulates inside the surface condenser is called the noncondensable load to the steam jets. Some of the noncondensable load consists of C02 accidentally produced when the boiler feedwater is vaporized into steam. Air leaks through piping flanges and valves are other sources of noncondensable vapors. But the largest source of noncondensable vapors is often air drawn into the turbine case, through the shaft s mechanical seals. To minimize this source of leaks, 2 or 3 psig of steam pressure is ordinarily maintained around the seals. However, as the turbine s shaft seals deteriorate, air in-leakage problems can overwhelm the jet capacity. This will cause a loss of vacuum in the surface condenser. 

Uncondensed vapors are removed at the top of the column with a one-stage steam jet ejector equipped with a barometric condenser. 

Figure 3.5. Vacuum control with steam jet ejectors and with mechanical vacuum pumps, (a) Air bleed on PC. The steam and water rates are hand set. The air bleed can be made as small as desired. This can be used only if air is not harmful to the process. Air bleed also can be used with mechanical vacuum pumps, (b) Both the steam and water supplies are on automatic control. This achieves the minimum cost of utilities, but the valves and controls are relatively expensive, (c) Throttling of process gas flow. The valve is larger and more expensive even than the vapor valve of case (a). Butterfly valves are suitable. This method also is suitable with mechanical vacuum pumps, (d) No direct pressure control. Settings of manual control valves for the utilities with guidance from pressure indicator PI. Commonly used where the greatest vacuum attainable with the existing equipment is desired.

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. 

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