Superheated vapor properties

Condensation of pure vapors under laminar conditions in the presence of noncondensable gases, interfacial resistance, superheating, variable properties, and diffusion has been analyzed by Minkowycz and Sparrow [Int. ]. Heat Ma.s.s Tran.sfer, 9, 1125 (1966)]. 

If the steam leaves the turbine part liquid and vapor, the properties of the exit stream are determined by a mass fraction average of the properties of pure liquid and vapor as given by Equation 5.11.11 to 5.11.13. According to the phase mle, these properties are a function of one thermodynamic variable. Because the inlet steam is superheated, the properties depend on two variables as given by Equation 5.11.14 and 5.11.17. Problem 5.3 illustrates the calculation procediue given in Table 5.12. 

Table B.7—which is referred to as the superheated steam table—lists V, U, and H of water (the latter two properties relative to liquid water at the triple point) at any temperature and pressure, not just at points on the VLE curve. If you are given a temperature and a pressure, you can locate the properties of water at the intersection of the column corresponding to the given temperature and the row corresponding to the given pressure. If the intersection falls within the closed region on the table bounded by the vertical line to the left of the 50 C column, the horizontal line below the 221.2 bar row, and the zigzag hypotenuse, the water is a liquid outside this region, it is a superheated vapor.

In areas surrounding the phase envelope, the system exists as a single phase. Below the dew point curve and at higher temperatures, it is a superheated vapor, while areas above the bubble point curve and to the left of it represent a sub-cooled liquid. In areas above the phase envelope between the sub-cooled liquid and the superheated vapor, the mixture is a dense or supercritical fluid, with properties changing gradually from those typical of a liquid to those typical of a vapor. 

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. 

Solution We will demonstrate the calculation by obtaining all properties along an isotherm, from the compressed liquid region to the superheated vapor. The calculation can then repeated with other temperatures to obtain the complete phase diagram. We pick T = 260 K. 

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. 

At a sufficiently high temperature, the evaporation rate from a wetted surface into its pure superheated vapor is higher than into perfectly dry air. The temperature at which this transition rate occurs is called the inversion value. Experimental values for this temperature range between 160 [103] and 230°C [104] in comparing steam with air. The inversion occurs because of the difference in properties between steam and air (which have different temperature coefficients) and the absence of a gas side mass transfer resistance when... 

The first tables of thermodynamic properties of Freon-22 were compiled by Graham and McHamess in 1945 based on the analysis of data by Benning and coworkers [2.23-2.26]. These tables were published practically unaltered in a number of handbooks [0.7, 0.45, 2.22, and others]. They include the values of V, hy and s on the saturation line at T = 170-325 K and in the region of superheated vapor at r 430 K and 1.8 MPa. 

In the work of I. I. Perelshtein [0.28] the tables of thermodynamic properties of saturated and superheated vapor of Freon-22 were calculated from the following equation ... 

Given two properties, identify the phases present on a PT or a Pv phase diagram, including solid, subcooled liquid, saturated liquid, saturated vapor, and superheated vapor and two-phase regions. Identify the critical point and... 

Your company has just developed a new refrigeration process. This process uses a secret gas, called Gas A. You are told that you need to come up with thermodynamic property data for this gas. The following data have already been obtained for the superheated vapor ... 

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. 

G. Wobsa, Zeit. Kdlteind., 14. 61, 1907 C. H. Lees, Phil. Trans., 191. 399, 1898 G. A. Good-enough and W. E. Mosher, The Properties of Saturated and Superheated Ammonia Vapor, Urbana, 1913 Bull. Vniv. Illinois, 66, 1913,... 

Hua et al. (1995) proposed a supercritical water region in addition to two reaction regions such as the gas phase in the center of a collapsing cavitation bubble and a thin shell of superheated liquid surrounding the vapor phase. Chemical transformations are initiated predominantly by pyrolysis at the bubble interface or in the gas phase and attack by hydroxyl radicals generated from the decomposition of water. Depending on its physical properties, a molecule can simultaneously or sequentially react in both the gas and interfacial liquid regions. 

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