Vaporizers superheater

Unal, C., K. Tuza, O. Bach, S. Neti, and J. C. Chen, 1991b, Convective Boiling in a Rod Bundle Transverse Variation of Vapor Superheat Temperature under Stabilized Post-CHF Conditions, Int. J. Heat Mass Transfer 34/1695-1706. (4)... 

Do film subcooling and vapor superheat cause the overall heat transfer and condensation rates to increase or decrease Explain the answers you give. 

Prepare a heat balance diagram of the preheater-vaporizer-superheater-reactor system. 

Drop-wall interaction. Hie earlier models ignored the contribution of drop-wall interaction. The direct evaporation of droplets at the hot channel wall can play an important role when the wall temperatures are relatively low just upstream of the dryout point. Evans et al. [344] made measurements of vapor superheat just downstream of the dryout point and found that for approximately one-third of a meter downstream, the vapor remained at its saturation temperature, indicating that, in this region, the heat flux was being absorbed by... 

It must be noted that predictions of heat transfer coefficients in all mentioned situations may be treated, as a rule, as conservative as long as the correlation is based on the Nusselt theory. Two important additional phenomena, though, are not included vapor superheat and vapor shear effects. The influence of superheating can be included (although the effect is usually small) by the sixth correlation from the top in Table 17.23. 

Holding the parts basket (and parts) in the vapor, superheat, or freeboard zones for the appropriate time periods. 

The entering vapor quickly is cooled to the adiabatic saturation temperature with respect to water. Thus, the vapor superheat, plus the heat of vaporization of the higher-boiling hydrocarbons, is converted to heat of vaporization of water. The vapor then progresses up the column at the water dew point as well as the hydrocarbon dew point. The vapor leaving the quench tower normally is at a temperature about 8° to 14°F above the water feed temperature to the column top when trays are used. 

The nodalization shown in Figure 2 was used in conjunction with the reactor vessel nodalization from TMLB initiation to core heatup. In this assessment, it was assumed that the onset of core heatup coincided with a core exit vapor superheat of 2.78 K. During this portion of the transient, full loop natural circulation of subcooled and saturated liquid can develop. As the core heats the primary coolant toward saturation, however, voids begin to form and collect at the top of the steam generator U-tubes. Once that occurs, full loop natural circulation of liquid is interrupted. 

Tubular low-temperature vaporizers/superheaters Combination large flow rate liquid heatup and subsequent boiling or superheating of mixed fluids with diverse boiling points. Needs special stress analysis and mechanical design. Can preheat, boil, and superheat in same vessel. (See Fig. V-9.)... 

FIG. V-9 Tubular low-temperature vaporizers/superheaters. (Source Armstrong Engineering Associates.)... 

The output from the turbine might be superheated or partially condensed, as is the case in Fig. 6.32. If the exhaust steam is to be used for process heating, ideally it should be close to saturated conditions. If the exhaust steam is significantly superheated, it can be desuperheated by direct injection of boiler feedwater, which vaporizes and cools the steam. However, if saturated steam is fed to a steam main, with significant potential for heat losses from the main, then it is desirable to retain some superheat rather than desuperheat the steam to saturated conditions. If saturated steam is fed to the main, then heat losses will cause excessive condensation in the main, which is not desirable. On the other hand, if the exhaust steam from the turbine is partially condensed, the condensate is separated and the steam used for heating. 

Solution The fraction of liquid vaporized on release is calculated from a heat balance. The sensible heat above saturated conditions at atmospheric pressure provides the heat of vaporization. The sensible heat of the superheat is given by... 

Propylene is a colorless gas under normal conditions, has anesthetic properties at high concentrations, and can cause asphyxiation. It does not irritate the eyes and its odor is characteristic of olefins. Propjiene is a flammable gas under normal atmospheric conditions. Vapor-cloud formation from Hquid or vapor leaks is the main ha2ard that can lead to explosion. The autoignition temperature is 731 K in air and 696 K in oxygen (80). Evaporation of Hquid propylene can cause skin bums. Propylene also reacts vigorously with oxidising materials. Under unusual conditions, eg, 96.8 MPa (995 atm) and 600 K, it explodes. It reacts violentiy with NO2, N2O4, and N2O (81). Explosions have been reported when Hquid propylene contacts water at 315—348 K (82). Table 8 shows the ratio TJTp where is the initial water temperature, and T is the superheat limit temperature of the hydrocarbon. 

Fig. 28. Rankine cycle for superheat, where ( ) represents adiabatic (isentropic) compression ( ), isobaric heating ( ), vaporization (x x x x), superheating turbine expansion and (-----), heat rejection. To convert kPa to psi, multiply by 0.145. To convert kj to kcal, divide by 4.184.

Superheater Heats a vapor above the saturation temperature. 

Material stored at a reduced temperature has little or no superheat and there will be little flash in case of a leak. Vaporization will be mainly determined by liquid evaporation from the surface of the spilled liqrtid, which depends on weather conditions. 

A common cause of a BLE T] in plants of the hydrocarbon-chemical industry is exposure to fire. With an external fire below the liquid level in a vessel, the heat of vaporization provides a heat sink, as with a teakettle evolved vapors exit tnrough the relief valve. But if the flame impinges on the vessel above the liquid level, the metal will weaken and may cause the vessel to rupture suddenly, even with the relief valve open. The explosive energy for a BLE T] comes from superheat. This energy is at a maximum at the superheat hmit temperature. (SLT is the maximum temperature to which a hquid can be heated before homogeneous nucleation occurs with explosive vaporization of the hquid and accompanying overpressure.) The SLT... 

Superheaters and Reheaters A superheater raises the temperature of the steam generated above the saturation level. An important function is to minimize moisture in the last stages of a turbine to avoid blade erosion. With continued increase of evaporation temperatures and pressures, however, a point is reached at which the available superheat temperature is insufficient to prevent excessive moisture from forming in the low-pressure turbine stages. This condition is resolved by removing the vapor for reheat at constant pressure in the... 

G = Gas specific gravity = mol. wt./29 Pi = Valve inlet pressure, psia AP = Pressure drop across valve, psi Q = Gas flow rate, SCFH Qs = Steam or vapor flow rate, Ib/hr T = Absolute temperature of gas at inlet, °R T5I1 = Degrees of superheat, °F... 

The tendency is greatest, however, where pressures are close to atmospheric and "superheat" relative to atmosphere is least. Pipestill atmospheric towers and cat unit fractionators tend to fall in this category. Some operators consider that the likelihood is great that calculated condensation (dew) will coalesce to droplets which will gravitate (rain) when the partial pressure of condensibles at the dew point exceeds 1/3 atmosphere. With this factor and environmental protection in mind, some plants have diverted such releases into closed systems. Generally, however, this has not been of sufficient concern, and such releases have been treated as though they were all vapor. 

Material stored at or below its atmospheric pressure boiling point has no superheat. Therefore there will be no initial flash of liquid to vapor in case of a leak. Vaporization will be controlled by the evaporation rate from the pool formed by the leak. This rate can be minimized by the design of the containment dike, for example, by minimizing the surface area of the liquid spilled into the dike area, or by using insulating concrete dike sides and floors. Because the spilled material is cold, vaporization from the pool will be further reduced. 

The superheat-temperature-limit locus for propane is plotted by Reid (1979) in a 77-diagram together with the vapor pressure (Figure 6.2). When the liquid is heated, for example, from A to B, a sudden drop in pressure to 1 atmosphere (C)... 

Figure 6.2. Pressure-vapor curve and superheat limit locus for propane (Reid 1979).

Another theory of liquid-liquid explosion comes from Board et al. (1975). They noticed that when an initial disturbance, for example, at the vapor-liquid interface, causes a shock wave, some of the liquid is atomized, thus enhancing rapid heat transfer to the droplets. This action produces further expansion and atomization. When the droplets are heated to a temperature equal to the superheat temperature limit, rapid evaporation (flashing liquid) may cause an explosion. In fact, this theory resembles the theory of Reid (1979), except that only droplets, and not bulk liquid, have to be at the superheat temperature limit of atmospheric pressure (McDevitt et al. 1987). 

The formation of droplets and their rapid, efficient vaporization is the reason that there is more vapor in the cloud than the amount which flashed off originally. Schmidli et al. (1990) determined that 5 to 50% of the mass of the original fuel can be found in droplets. This value depends upon initial mass and degree of superheat, that is, amount by which the fuel s temperature exceeds its boiling point. 

As described in Section 6.2.1., British Gas performed full-scale tests with LPG BLEVEs similar to those conducted by BASF. The experimenters measured very low overpressures firom the evaporating liquid, followed by a shock that was probably the so-called second shock, and by the pressure wave from the vapor cloud explosion (see Figure 6.6). The pressure wave firom the vapor cloud explosion probably resulted from experimental procedures involving ignition of the release. The liquid was below the superheat limit temperature at time of burst. 

Thus, the BLEVE theory predicts that, when the temperature of a superheated liquid is below T, liquid flashing cannot give rise to a blast wave. This theory is based on the solid foundations of kinetic gas theory and experimental observations of homogeneous nucleation boiling. It is also supported by the experiments of BASF and British Gas. However, because no systematic study has been conducted, there is no proof that the process described actually governs the type of flashing that causes strong blast waves. Furthermore, rapid vaporization of a superheated liquid below its superheat limit temperature can also produce a blast wave, albeit a weak... 

Figures 6.30 and 6.31 present the same information for saturated hydrocarbons. In Figure 6.30, the saturated liquid state is on the lower part of the curve and in Figure 6.31 it is on the upper part of the curve. Below T y, the line width changes, indicating that the liquid probably does not flash below that level. Note that a line has been drawn only to show the relationship between the points a curve reflecting an actual event would be smooth. Note that a liquid has much more energy per unit of volume than a vapor, especially carbon dioxide. Note It is likely that carbon dioxide can flash explosively at a temperature below the superheat limit temperature. This may result from the fact that carbon dioxide crystallizes at ambient pressure and thus provides the required number of nucleation sites to permit explosive vaporization.

The above methods assume that all superheated liquids can flash explosively, yet this may perhaps be the case only for liquids above their superheat-limit temperatures or for pre-nucleated fluids. Furthermore, the energies of evaporating liquid and expanding vapor ate taken together, while in practice, they may produce separate blasts. Finally, in practice, there are usually structures in the vicinity of an explosion which will reflect blast or provide wind shelter, thereby influencing the blast parameters. 

The explosion of a vessel full of liquid above the superheat limit temperature has much more energy, and therefore, causes a much more severe blast than a gas- or vapor-filled vessel. 

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