Specific latent enthalpy

Usually any material undergoes a reversible transformation of state with an increase of its temperature in the order depicted in Figure 1.8. Each reversible transformation requires a certain amount of energy to occur. At constant pressure, this energy per unit amount of matter of the material is called the standard latent molar enthalpy of the transformation (formerly the latent heat), denoted and expressed in J.moF For practical reasons, engineers prefer to use the latent enthalpy per unit mass of the substance called the specific latent enthalpy of the transformation, denoted and expressed in J.kg The simple relationship between the two quantities is as follows ... 

Specific Energy (heat/mass) (e.g., calorific value, mass basis, specific latent heat, enthalpy)... 

Specific free internal energy gravitational acceleration Specific free enthalpy Specific enthalpy Latent heat... 

Chemical name (lUPAC) Surface tension Y(T)=A-BT Surface tension (293.15K) Thermal conductivity (293.15K) Specific heat capacity Latent enthalpy vaporization Refractive index (589nm) UV cut- off Relative electrical permittivity (293.15K) Dipole moment Henry s law constant Vapor pressure (293.15K)... 

The physical meanings of many thermodynamic parameters such as specific heat, enthalpy, entropy, and the Gibbs function were discussed, and several methods for determining their values for nonreacting and reacting liquid- and gas-phase mixtures were discussed. The heat of formation, sensible enthalpy, and latent heat of phase change make up the total enthalpy of a substance at a given temperature ... 

Cg = humid heat for humidity H in units of kj / (kg-K) and = latent heat of vaporization at / in kj /kg. The slope of the constant-enthalpy adiabatic cooling line is —C j which is the relationship between temperature and humidity of gas passing through a totally adiabatic direct-heat dryer. The humid heat of a gas—vapor mixture per unit weight of dry gas includes the specific heat of the vapor... 

Single-Effect Evaporators The heat requirements of a singleeffect continuous evaporator can be calculated by the usual methods of stoichiometry. If enthalpy data or specific heat and heat-of-solution data are not available, the heat requirement can be estimated as the sum of the heat needed to raise the feed from feed to product temperature and the heat required to evaporate the water. The latent heat of water is taken at the vapor-head pressure instead of at the product temperature in order to compensate partiaUv for any heat of solution. If sufficient vapor-pressure data are available for the solution, methods are available to calculate the true latent heat from the slope of the Diihriugliue [Othmer, Ind. Eng. Chem., 32, 841 (1940)]. 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by ... 

To accommodate the step-by-step, recycling and checking for convergences requires input of vapor pressure relationships (such as Wilson s, Renon s, etc.) through the previously determined constants, latent heat of vaporization data (equations) for each component (or enthalpy of liquid and vapor), specific heat data per component, and possibly special solubility or Henry s Law deviations when the system indicates. 

The specific enthalpy will increase with dry hulh (sensible heat of the air) and moisture content (sensible and latent heat of the water). The adiabatic (isoenthalpic) lines for an air-water vapour mixture are almost parallel with the wet bulb lines so, to avoid any confusion, the enthalpy scale is placed outside the body of the chart, and readings must be taken using a straight-edge. (See Figure 23.7.)... 

If H is the enthalpy of the humid gas per unit mass of dry gas, Ha the enthalpy of the dry gas per unit mass, Hw the enthalpy of the vapour per unit mass, Ca the specific heat of the gas at constant pressure, Cw the specific heat of the vapour at constant pressure. 0 the temperature of the humid gas, 0a the reference temperature, X the latent heat of vaporisation of ihe liquid at 6 0 and the humidity of the gas. 

Enthalpy can be calculated from specific and latent heat data see Section 3.5. 

The fluid physical properties required for heat-exchanger design are density, viscosity, thermal conductivity and temperature-enthalpy correlations (specific and latent heats). Sources of physical property data are given in Chapter 8. The thermal conductivities of commonly used tube materials are given in Table 12.6. 

Heat transfer coefficient Specific enthalpy (latent heat, etc.) Specific heat capacity Thermal conductivity... 

One would expect the enthalpy of sublimation (d) to be the largest of the four quantities cited. Molar heat capacities are quite small, on the order of fractions of a kilojoule per mole-degree. (Remember that specific heats have values of joules per gram-degree.) All of the heats of transition (or latent heats) are positive numbers and on the order of kilojoules per mole. Since the heat of sublimation is the sum of the heat of fusion and the heat of vaporization, AHsubl must be the largest of the three. 

FIG. 8 Schematic illustration of the steps in the phase diagram and the energy required for ice starting at — 20 °C to become superheated gas (steam) at 120°C at atmospheric pressure (1 atm). The type and amount of heat (sensible or latent) required to change the temperature or phase are given, where Cp is the specific heat and AH is the change in enthalpy. 

The term specific heat refers to the sensible-heat content of either vapor or liquid. The specific heat is the amount of heat needed to raise the temperature on one pound of the vapor or liquid by 1°F. The term latent heat refers to the heat of vaporization, or the heat of condensation, needed to vaporize or condense one pound of liquid or vapor at constant temperature. Note that the heat of condensation is equal to the heat of vaporization. Each is referred to as the latent heat. The sum of the sensible heat, plus the latent heat, is called the total heat content, or enthalpy. 

Baxter (B3) uses an enthalpy-flow temperature method, due originally to Dusinberre (D5, D6) and Eyres et al. (E4), whereby the movingboundary effect is reduced to a property variation. To begin with, the melting of a slab of finite thickness initially at the fusion temperature is considered. At the surface of the melt, which is of the same density as the solid, a heat transfer boundary condition is applied. The technique takes into account latent heat effects by allowing the specific heat to become infinite at the fusion temperature in such a way that... 

Latent heat of vaporization of the liquid phase at inlet conditions. It is the difference between the vapour- and the liquid-specific enthalpies kJ/kg... 

The last database of the eight key data items promised is enthalpy. I have broadly used the term enthalpy to signify all thermal properties that include specific heat, latent heat, and an absolute enthalpy value, expressed as Btu/lb. This section presents a table which, by interpolation, may be applied to any single component or component mixtures, or to any petroleum characterized component groupings. This enthalpy source table (Table 1.10) may be used conveniently and quickly to derive energy or heat values of both liquids and gases. It is compiled from data in Maxwell (pp. 98 to 127) [5]. 

The following properties belong to the calorimetric category (1) specific and molar heat capacities, (2) latent heats of crystallization or fusion. It will be shown that both groups of properties can be calculated as additive molar quantities. Furthermore, starting from these properties the molar entropy and enthalpy of polymers can be estimated. 

The specific enthalpy change associated with the transition of a substance from one phase to another at constant temperature and pressure is known as the latent heat of the phase change (as distinguished from sensible heat, which is associated with temperature changes for a singlephase system). For example, the specific enthalpy change AH for the transition of liquid water to steam at I00°C and 1 atm, which equals 40.6 kJ/mol, is by definition the latent heat of vaporization (or simply the heat of vaporization) of water at this temperature and pressure. 

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