Latent heat defined

Figure 28 shows the key features of the humidity chart. The chart consists of the following four parameters plotted as ordinates against temperature on the abscissas (1) Humidity H, as pounds of water per pound of dry air, for air of various relative humidities (2) Specific volume, as cubic feet of dry air per pound of dry air (3) Saturated volume in units of cubic feet of saturated mixture per pound of dry air and (4) latent heat of vaporization (r) in units of Btu per pound of water vaporized. The chart also shows plotted hiunid heat (s) as abscissa versus the humidity (H) as ordinates, and adiabatic humidification curves (i.e., humidity versus temperature). Figure 28 represents mixtures of dry air and water vapor, whereby the total pressure of the mixture is taken as normal barometric. Defining the actual pressure of the water vapor in the mixture as p (in units of mm of mercury), the pressure of the dry air is simply 760 - p. The molal ratio of water vapor to air is p/(760-p), and hence the mass ratio is ... 

The following expressions can be used to estimate the temperature and enthalpy of steam. The expressions are based upon multiple regression analysis. The equation for temperature is accurate to within 1.5 % at 1,000 psia. The expression for latent heat is accurate to within + 3 % at 1,000 psia. Input data required to use these equations is the steam pressure in psia. The parameters in the equations are defined as t for temperature in F, for latent heat in Btu/lb, and P for pressure in psia. 

X Modified latent heat of vaporisation per unit mass defined by (13.68) J/kg 1 T 2... 

The nature of the heat of fusion AHu deserves particular attention, for it represents the heat required to melt one mole of crystalline units it does not refer to the latent heat AJT required to melt such crystallinity as may occur in a given semicrystalline polymer. The depression of the melting point Tm, already defined as the maximum temperature at which crystalline regions may coexist with amorphous poly-... 

The limitation of the storage capacity is, as mentioned before, caused by the limitation of entropy change AS within the storage (see Figure 4). For sensible and latent heat storage (so-called direct thermal energy storage) this is defined by the specific heat... 

The heat transfer across the vapor layer and the temperature distribution in the solid, liquid, and vapor phases are shown in Fig. 13. In the subcooled impact, especially for a droplet of water, which has a larger latent heat, it has been reported that the thickness of the vapor layer can be very small and in some cases, the transient direct contact of the liquid and the solid surface may occur (Chen and Hsu, 1995). When the length scale of the vapor gap is comparable with the free path of the gas molecules, the kinetic slip treatment of the boundary condition needs to be undertaken to modify the continuum system. Consider the Knudsen number defined as the ratio of the average mean free path of the vapor to the thickness of the vapor layer ... 

The thermal quality of the feed is defined as the heat required to raise 1 mole of feed from the feed condition to vapour at the feed plate condition divided by the molar latent heat, and the following values apply q=0 for saturated liquid feed q=l for saturated vapour feed, and q>l for cold feed. The value of q affects the relative liquid and vapour flow rates (L and V) above and (L and V ) below the feed plate, as indicated in Fig. 3.54. 

This implies that the exponents and y defined above are 0 = y = 2( = d) for a first-order transition. Since the symmetry around if = 0 is preserved for finite L, there is no shift of the transition. This feature is different, however, if we consider temperature-driven first-order transitions , since there is no symmetry between the disordered high-temperature phase and the ordered low-temperature phase. In order to understand the rounding of the delta-function singularity of the specific heat, which measures the latent heat for L- oo, it now is useful to consider the energy distribution, for which again a double Gaussian approximation applies ... 

It has already been observed in the case of pure liquids such as substituted hydrocarbons that the surface tension was, if not completely defined by the non-polar portions of the molecule, at least not so markedly affected by substituent groups as we should anticipate if no orientation existed. Again, although vaporisation and condensation at a liquid surface such as water at high temperatures takes place with great speed, yet the life of a molecule on the surface is probably long enough to permit of such adjustment as orientation requires. Even more conclusive in favour of the hjq)othesis of at least partial orientation is the evidence derived from a consideration of the latent heats of evaporation and the... 

The terms AHj, L, AH yUnd i used in Fig. 7.1 are all enthalphy changes defined as follows AHi is the heat of immersion of the solid into the liquid, L is the latent heat of condensation, AH yis the heat of adsorption when the solid is equilibrated with saturated vapor, and i is the heat liberated when solid in equilibrium with saturated vapor is immersed into liquid. Using Hess s law of heat summation... 

If a substance undergoes a transformation from one physical stale to another, such as a polymorphic transition, the fusion or sublimation of a solid, or the vaporization of a liquid, the heat adsorbed hy the substance during the transformation is defined as the latent heat of transformation (transition, fusion, sublimation or vaporization). It is equal in the enthalpy change of the process, which is the difference between the enthalpy of the substance in the two states at (he temperature of the transformation. For the purpose of thcrmochemical calculations, i( is usually reported as a molar quantity with die units of calories (or kilocalories) per mule (or gram formula weight). The symbol L or AH. with a subscript i.f (or in), s. and n is commonly used and the value is usually given at the equilibrium temperature of the transformation under atmospheric pressure, or at 25 C. 

We can rewrite Equation 5.23 in a more usable form by defining average values for the humid heat, Cs, and the latent heat, X, and by neglecting superheat. These are common assumptions applied to cooling towers ... 

If we take AH = La, where La is the latent heat per atom, Jackson s dimensionless parameter a is defined by... 

How does this latent heat relate to other thermodynamic quantities Whether Q is equal to the change in internal energy, or something else, is dependent on the conditions of the experiment. Recalling the first law of thermodynamics and further recalling that work is defined as force through a distance ... 

This number is independent of the heat and mass transfer potentials because it is defined by coefficients , 5, r and Cq (the last two are, respectively, the vaporization latent heat of moisture and the specific heat capacity of the moist body). 

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