Superheated region

Several arbitrary rules of thumb have been suggested to account for the reduction in efficiency when moisture particles are present. One is to multiply the efficiency by the mean vapor content, by weight. For example, if expansion is started in the superheated region and ends with 6% by weight of liquid, the mean vapor content is 97%. If the design efficiency is 78%, the adjusted value of efficiency is 0.78 (0.97) = 0.757 or 75.7%. Some designers prefer to assume a loss equal to twice the value used in the example. 

The calculations show that the liquid pressure monotonically decreases along the heating region. Within the evaporation region a noticeable difference between the vapor and liquid pressures takes place. The latter is connected with the effect of the Laplace force due to the curvature of the interface surface. In the superheated region the vapor pressure decreases downstream. 

COMMENTS. (1) The turbine work produced is very small. It does not pay to install an expansion device to produce a small amount of work. The expansion process can be achieved by a simple throttling valve. (2) The compressor handles the refrigerant as a mixture of saturated liquid and saturated vapor. It is not practical. Therefore, the compression process should be moved out of the mixture region to the superheated region. 

The tables extend to well above the normal melting point to provide data in a metastable region which in this case is a superheated region. Explanations are inserted in the tabulations to indicate the end of the phase stability and any solid-state transitions. 

The correct values at 298.15 K may be obtained from the above relations. A typical liquid table is extrapolated both below the melting point (if the melting point is above 298.15 K) and above the boiling point to facilitate interpolation and to provide data in the metastable regions, both in the supercooled and superheated regions. A glass transition may be included, in which case the tabulation becomes a multiphase table. 

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. 

Draw a p-T chart for water. Label the following clearly vapor-pressure curve, dew-point curve, saturated region, superheated region, subcooled region, and triple point. Show where evaporation, condensation, and sublimation take place by arrows. 

We shall now integrate this expression from the saturated ft-versus-s line into the superheated region, and then consider separately the case of the wet-steam region. 

MPa and 30S.5°C. Most of the expansion takes place in the superheated region and so the value y = 13 was used in equation (A6.2), which assumes implicitly that the vapour remains supersaturated below the saturation line. 

Figures 1 and 2 show the completed T-S diagram for neon for a pressure range of 1-200 atm and a temperature range of 27-320°K. As noted previously, YendalPs equation of state was used to calculate much of the superheated region. It was, however, not used at the higher pressures near the critical point since a plot of the isobars at this point indicated a negative slope. (This deficiency in the original equation of state has now been corrected by Yendall [13].) This explains the blank area immediately above the critical region.

Question by B. F. Dodge, Yale University Do your calculated properties cover only the superheated region ... 

The discharge coefficient of polymer solutions in the subcooled and superheated region is shown in Fig. 16.29. Co differs greatly at 25 °C, hence under subcooled conditions. The shear viscosity of polymer solutions greatly influences the throughput for subcooled flow. The flow regime does not show turbulent but laminar characteristics. The Reynolds numbers (16.12) for a shear viscosity values at 25 °C are compared to those calculated for viscosity values at 80 °C liquid temperature (Fig. 16.28). 

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