Vapor superheated, defined

Heat of vaporization, J/g or Btu/lb A, average difference in enthalpy between boiling liquid and superheated vapor, defined by Eq. (13.23) Wavelength of smallest wave that can grow on flat horizontal surface, m or ft [Eq. (13.24)]... 

What happens at the feed stage depends on the condition of the feed, whether it is subcooled, saturated liquid, partially vaporized, saturated vapor or superheated vapor. To define the condition of the feed, the variable q is introduced, defined as ... 

The thermal condition of die feed stream introduces the need for nn energy balance at die feed stage. The feed may be a subcooled liquid, a saturated liquid, a mixture of liquid and vapor, a saturated vapor, or a superheated vapor. A special term may be defined diet can acoonnt for the thermal condition of the... 

Stable states defined by positive curvature on the A-V diagram at constant temperature and composition, where (dp/dV)T,aa Ni < 0. Stable equilibrium states exist outside the binodal region where single-phase behavior prevails. Subcooled liquids and superheated vapors represent examples of stable states. 

Gases and superheated vapors exist at temperatures above the saturation or boiling temperature for a given pressure. Pressure and temperature are independent properties and knowledge of both, according to the state postulate, fully defines the state and all other properties. 

The feed can be in different aggregation states as compressed liquid, saturated liquid, a vapor-liquid mixture, saturated vapor, and superheated vapor. We define the liquid and vapor fraction as l and respectively ... 

Va.por Pressure. Vapor pressure is one of the most fundamental properties of steam. Eigure 1 shows the vapor pressure as a function of temperature for temperatures between the melting point of water and the critical point. This line is called the saturation line. Liquid at the saturation line is called saturated Hquid Hquid below the saturation line is called subcooled. Similarly, steam at the saturation line is saturated steam steam at higher temperature is superheated. Properties of the Hquid and vapor converge at the critical point, such that at temperatures above the critical point, there is only one fluid. Along the saturation line, the fraction of the fluid that is vapor is defined by its quaHty, which ranges from 0 to 100% steam. 

It is not obvious how AF varies with the size of a cluster, because vv depends on the size, but an indirect scheme is available for determining the desired information. For one particular size, R0, there is assumed to be a value of AF for a condition of stability. This means that for a superheated liquid at a stated temperature and pressure, one and only one cluster size is capable of existence for long. This cluster is called a nucleus. A stable cluster is really in a metastable state, as discussed later. However, for any degree of equilibrium, AF must be unaffected by infinitesimal changes in the cluster size. So d(AF)/dR = 0. If vv is defined as the volume occupied by one vapor molecule, then ny = 47r.fi o8/(3ty)- These two manipulations produce a solution for the quantity r v — vl in Eq. (40)... 

Let us now continue with our discussion of how to relate the chemical potential to measurable quantities. We have already seen that the chemical potential of a gaseous compound can be related to pressure. Since substances in both the liquid and solid phases also exert vapor pressures, Lewis reasoned that these pressures likewise reflected the escaping tendencies of these materials from their condensed phases (Fig. 3.9). He thereby extended this logic by defining the fugacities of pure liquids (including subcooled and superheated liquids, hence the subscript L ) and solids (subscript s ) as a function of their vapor pressures, pil ... 

When does a liquid boil Clearly, boiling at constant pressure—say, atmospheric pressure—begins when we increase the temperature of a liquid or solution and the vapor pressure reaches a pressure of one atmosphere. Alternatively, the pressure over a liquid or solution at constant temperature must be reduced until it reaches the vapor pressure at that temperature (e.g., vacuum distillation). Yet it is well known that liquids can be superheated (and vapors supersaturated) without the occurrence of phase transfer. In fact, liquids must always be superheated to some degree for nucleation to begin and for boiling to start. That is, the temperature must be raised above the value at which the equilibrium vapor pressure equals the surrounding pressure over the liquid, or the pressure must be reduced below the vapor pressure value. As defined earlier, these differences are called the degree of superheat. When the liquid is superheated, it is metastable and will reach equilibrium only when it breaks up into two phases. 

Some valve manufacturers use the valve-flow coefficient C for valve sizing. This coefficient is defined as the flow rate, in lb/h, through a valve of given size when the pressure loss across the valve is 1 lb/in2. Tabulations such as Tables 6.16 and 6.17 incorporate this flow coefficient and are somewhat, easier to use. These tables make the necessary allowances for downstream pressures less than the critical pressure (= 0.55 x absolute upstream pressure, in lb/in2, for superheated steam and hydrocarbon vapors, and 0.58 x absolute upstream pressure, in lb/in2, for saturated steam). The accuracy of these tabulations equals that of valve size determined by using the flow coefficient. 

Physical changes of state are observable under suitable conditions as well-defined phenomena. However the very frequent occurrence of superheating and supercooling in liquids, supersaturation of vapors (e.g., in closed chambers), and the persistence of metastable solids (e.g., monoclinic sulfur at 0°C) show that these phase changes can be at times exceedingly... 

Define vapor pressure, triple point, equilibrium, dew point, bubble point, saturated, superheated, subcooled, and quality, and be able to locate the region or point in a p-T chart in which each term applies. 

It is obvious from the conditions defined above that the rate-based model equations and variables are more numerous and complex than those in the equilibrium stage model described in Chapter 13. Other features of the rate-based model are that the exiting liquid and vapor from a stage can be at different temperatures since separate balance equations are written for each phase. Each phase on a stage can have a different externally transferred heat duty. The exiting phases in general are not at equilibrium the liquid may be subcooled and the vapor may be superheated. In a rate-based model the phase interface must be defined. The variables defining the interface include the liquid and vapor compositions and the temperature at the interface, and the molar flux across the interface. 

A comparison of the liquid temperatures and pressures with the equilibrium values indicated that, immediately after venting, the liquids were superheated the temperatures were higher than indicated by equilibrium conditions. As the pressure increased, the bulk-liquid temperature also increased, but at a rate considerably lower than indicated by equilibrium (saturated) conditions. Stratification of the liquid temperatures provided a surface condition which within a short time followed the equilibrium vapor-pressure curve, while the bulk of the liquid became subcooled. The liquid-surface locations were not well defined but the level measurements (capacitance) indicated that the surface was around the 20-in. height at the beginning. The vapor temperature increased from about 43 to 46.5 R during the pressure rise without any major fluctuations. 

---