Superheated vapor Superheaters

In the context of a system containing a single condensable species and noncondensable gases, explain in your own words the terms saturated vapor, superheated vapor, dew point, degrees o f superheat, and relative saturation. Explain the following statement from a weather report in terms a first-year engineering student could understand The temperature is 75 F, barometric pressure is 29.87 inches of mercury and falling, the relative humidity is 50%. and the dew point is 54°E... 

If a gas containing a single superheated vapor A is cooled at constant pressure, the temperature at which the vapor becomes saturated is the dew point of the gas. The dew point may be determined from Raoult s law, paP = pX(T[Pg.278]

The region to the right of the vapor-pressure curve in Fig. 3.9 is called the superheated region and the one to the left of the vapor-pressure curve is called the sub-cooled region. The temperatures in the superheated region, if measured as the difference (0-N) between the actual temperature of the superheated vapor and the saturation temperature for the same pressure, are called degrees of superheat. For example, steam at 500 F and 100 psia (the saturation temperature for 100 psia is 327.8°F) has (500 — 327.8) = 172.2 F of superheat. Another new term you will find used frequently is the word quality. A wet vapor consists of saturated vapor and saturated liquid in equilibrium. The mass fraction of vapor is known as the quality. 

A superheated vapor may be condensed directly from the vapor state by contact with a surface that is below the saturation temperature or the dew point of the vapor. The heat-transfer coefficient is predicted by the equations in this section if the saturation temperature (not the superheat temperature) is used as the effective temperature for heat transfer, i.e., a desuperheater/condenser may be... 

CONDENSATION OF SUPERHEATED VAPORS. If the vapor entering a condenser is superheated, both the sensible heat of superheat and the latent heat of condensation must be transferred through the cooling surface. For steam, because of the low specific heat of the superheated vapor and the large latent heat of condensation, the heat of superheat is usually small in comparison with the latent heat. For example, 50°C of superheat represents only 100 J/g, as compared with approximately 2300 J/g for latent heat. In the condensation of organic vapors, such as petroleum fractions, the superheat may be appreciable in comparison with the latent heat, When the heat of superheat is important, either it can be calculated from the degrees of superheat and the specific heat of the vapor and added to the latent heat, or if tables of thermal properties are available, the total heat transferred per pound of vapor can be calculated by subtracting the enthalpy of the condensate from that of the superheated vapor. 

Because of the low individual coefficient on the gas side, the overall coefficient in the desuperheater section is small, and the area of the heating surface in that section is large in comparison with the amount of heat removed. This situation should be avoided in practice. Superheat can be eliminated more economically by injection of a spray of liquid directly into the superheated vapor, since small drops evaporate very rapidly, cooling the vapor to the saturation temperature. The desuperheating section is thereby eliminated, and condensation occurs with high coefficients. 

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. 

Although sensible heat transfer coeflicients are considerably lower than condensing coeflicients, heat transfer rates are quite high in the desuperheating zones of distillation condensers. The low heat transfer coefficient in the desuperheating zone is compensated for by the higher temperature difference between the superheated vapor and the coolants (compared to the temperature difference between the saturated vapor and the coolant). In most cases, moderate variation in superheat has little effect on condenser performance and is seldom troublesome in distillation operation. 

Solvent is discontinuously removed from the contents of a distillation still (1) by blowing water vapor into the still. The impure vapor is condensed in a steam converter (a tube heat exchanger (2), slightly inclined). The condensate is fed to a regeneration unit for further treatment. Pure steam generated at the jacket during vapor condensation is compressed to the injection pressure in a steam compressor (4) and is simultaneously superheated. The superheat is removed in a saturator (5) by the injection of condensate or water and the saturated steam is injected into the distillation unit (1). 

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.)... 

Finally, the high pressure liquid is brought back to a superheated vapor state in the boiler. It is in this step that energy released from the combustion of fuel is transferred to the working fiuid. The fuel provides the high-temperature reservoir for the boiler. The boiler isobarically heats the liquid to saturation, vaporizes it, and then superheats the vapor. The rate of heat transfer in the boiler is given by ... 

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. 

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.

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... 

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. 

Thermodynamic and mechanical equilibrium on a curved vapor-liquid interface requires a certain degree of superheat in order to maintain a given curvature. Characteristics of homogeneous and heterogeneous nucleation can be estimated in the frame of classical theory of kinetics of nucleation (Volmer and Weber 1926 Earkas 1927 Becker and Doring 1935 Zel dovich 1943). The vapor temperature in the bubble Ts.b can be computed from equations (Bankoff and Flaute 1957 Cole 1974 Blander and Katz 1975 Li and Cheng 2004) for homogeneous nucleation in superheated liquids... 

For a bulk liquid at pressure pL, the vapor pressure pG of the superheated liquid near the wall can be related to the amount of superheat, (TG — Tsat), by the Clausius-Clapeyron equation,... 

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

Let us consider a superheated liquid which has attained the limit of superheat and a vapor embryo forms in equilibrium with the liquid. The bubble radius is ro, the pressure in the bulk liquid is Pq, and the temperature is Tq. Assume the liquid is pure. 

In Fig. 12, we show the computed values of bubble radius for superheated liquid propane at two pressure levels 1 and 5 atm. Consider the inertial rate first. At 1 atm, liquid superheated propane attains the limit of superheat at about 328 K, where the vapor pressure is —18.9 atm. With Eq. (12), Tinertiai 52f m, where t is in seconds. At 5 atm, the driving force [Eyp(To) - Eq] is less than that at 1 atm, but the difference is slight. Thus, the 1 and 5 atm radii are shown as a single line in Fig. 12. 

This brief commentary on superheated liquids has indicated that they are readily formed if one prevents heterogeneous nucleation of vapor embryos. Also, there is a limit to the degree of superheat for any given liquid, pure or a mixture. This limit may be estimated either from thermodynamic stability theory or from an analysis of the dynamics of the formation of critical-sized vapor embryos. Both approaches yield very similar predictions although the physical interpretation of the results from both differ considerably. 

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