Vaporization latent heat of, 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 latent heat of vaporization per unit mass of a pure substance at a given temperature, X, is defined as the difference in enthalpy between the saturated vapor and saturated liquid at the given temperature, T. Since enthalpy is a thermodynamic function of state, show that X can be evaluated from a known value of X0 at a reference temperature T0 from the equation... 

In addition to the fact that water is relatively inexpensive and is usually available in large quantities, there are two specific properties of water that make it invaluable. Those properties are its latent heat of vaporization and its specific heat. The latent heat of vaporization of a substance is defined as the amount of heat a material must absorb when it changes from a liquid to a vapor or gas. The specific heat of a substance is defined as the ratio between the amount of heat necessary to raise the temperamre of a substance and the amount of heat necessary to raise the same weight of water by the same number of degrees. 

The above is often approximated in different ways. For example, when dealing with liquid-gas equilibria one frequently introduces the approximations Vi critical point, but at temperatures sufficiently high for the perfect gas approximation to hold. On defining L = Hg — Hi as the molar latent heat of vaporization of the liquid Eq. (2.3.3) assumes the approximate form... 

Debye length (3.1.10b), mean free path of a gas molecule (3.1.114), filter coefficient (7.2.187), parameter for a dialyzer (8.1.399), parameter for a distillation plate/stage (8.3.38), latent heat of vaporization/condensation molecular conformation coordinate (3.3.89c) electrode spacings (7.3.18) retention parameter for species i (7.3.213), ionic equivalent conductance of ion i (3.1.108r) value of Xi for a cation value of A, for an anion value of A, at infinite dilution (Table 3.A.8) defined by (5.4.100) equivalent conductance of a salt (an electrolyte) (3.1.108s)... 

This is an intuitively satisfying idea. Liquids that have a high latent heat of vaporization require a large energy input to overcome the strong intermolecular attractions to change to a vapor. A more dense liquid (one with a lower value of Vzj must have a higher intermolecular attractive force per unit mass than one with a lower density. They then defined... 

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. 

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). 

The term on the left describes the heat of the gas to the particle. Radiative heat transfer is excluded. The first term on the right describes the required heat for increasing temperature of the particle. The second term shows the heat needed to cover the latent heat of evaporating water. The effect of bound water is excluded, because it is small compared to heat of vaporization [8]. Drying the fiiel particle is described with an experimentally defined function z(u). The last term describes the heat needed to rise the temperature of water vapour from evaporation to leaving air temperature. 

The regions of the atmosphere are defined by the vertical temperature profile. At the bottom is the troposphere where temperature decreases with height from the surface (which is warmed by the sun). The rate of change of temperature (the lapse rate) depends on the amount of moisture in the air since the latent heats of condensation and evaporation affect the heat of a rising or descending air parcel. For dry air the dry adiabatic lapse rate is — 9.8 K km but a more typical value of the environmental lapse rate (for air containing some water vapor) is — 6.5 K km The troposphere extends up to about 10 km, though this varies with... 

Within the bubble boiling regime, thermal induced disintegration occurs when the vapor pressure unbalances the equilibrium between surface tension, viscous forces and inertial forces. The nature of this mechanism is different from those observed onto cold surfaces, as it is triggered by combined effects induced by the liquid surface tensirm and the latent heat of evaporation, /ifg, and the analysis requires the use of dimensionless groups complementary to those in Table 8.1. The most important is the Jakob number, defined as/a = Cp(Tw — 7 sat)//tfg where Cp is the specific heat of the liquid. 

The mixing ratio of water vapor q is defined by q = Pw/P, where /0 is the density of water vapor and p is the density of dry air. When the mixing ratio exceeds the saturation mixing ratio Qs, the excess water vapor over Qs ordinarily gives rise to condensation in the form of rain or to subhmation in the form of snow. Latent heat of condensation or sublimation is liberated in the phase transition and becomes available to heat the air. The opposite of condensation is the evaporation that acts to cool the air. The rate of heating or cooling from this process is designated by Qc inEq. (17). 

Latent Heat. The heat of vaporization, commonly referred to as the latent heat, is usually defined in terms of Btu required to vaporize 1 lb of a liquid at its atmospheric boiling point. If vaporization takes place at another pressure (or temperature), the latent heat should be specified as such. The latent heat varies with the temperature (or pressure) at which vaporization occurs and with the type of hydrocarbon. 

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 available heat is defined as the quantity of heat released within a combustion chamber minus (1) the sensible heat carried away by the dry flue gases, and (2) the latent heat and sensible heat carried away in water vapor contained in the flue gases. Thus, the available heat represents the net quantity of heat remaining for useful heating. 

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