Latent enthalpy

No heat is being added or removed in this process, so the enthalpy must remain constant, and the process is shown as a movement along the line of constant enthalpy. Latent heat will be taken in by the water, from the sensible heat of the air, until the mixture reaches saturation, when no more water can be evaporated. 

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

Tempera- ture, °F Absolute pressure, psi Volume of vapor, ft3/lb Enthalpy Latent heat of evaporation, Btu/lb... 

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

E = enthalpy, latent heat of evaporation, BTU/lb f = friction factor Flf = laminar flow factor G = rate of flow or quantity of liquid to be heated or cooled, ft /hr... 

First-Order Transitions. The principal transitions in macromolecules are those concerned with enthalpy (latent heat) and entropy changes. They are called first-order transitions according to a classification requiring the first derivative of the change in free enthalpy with respect to temperature not to be zero (92). For a single-component system (pure compound), thermodynamic equilibrium states... 

Stream properties required for solving material and energy balance equations and other process calculations are predicted from component properties. The properties of petroleum pseudocomponents can be estimated from their boiling points and specific gravities. The component properties include the molecular weight, critical constants, acentric factor, heat of formation, ideal gas enthalpy, latent heat, vapor pressure, and transport properties. These are predicted mainly by empirical correlations based on experimental data. Many of these correlations are documented in the American Petroleum Institute Technical Data Book (API, 1983). 

A CHEMCAD simulation is the basis for the design. The thermodynamics models used were K-val = UNIT AC and Enthalpy = Latent Heat. 

Similarly, the vapor enthalpy can be expressed in terms of humid heat and latent heat in relation to a base condition ... 

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 ... 

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. 

Cp = Speeific heat of the desuperheating water, Btu/lb-°F h, = Enthalpy of superheated system, Btu/lb hj = Enthalpy of saturated steam, Btu/lb Lh = latent heat of vaporization of water at t,j Btu/lb m = Mass of superheated steam into the system, Ib/hr mj = Mass of saturated steam out, Ib/hr n = Mass of desuperheating water used, Ib/hr t[ = Temperature of superheated steam, °F = Temperature of saturated steam, °F t,j, = Temperature of desuperheating water, °F... 

It is beneficial to develop the enthalpy expressions for the gaseous VOC-laden stream as its temperature is cooled from T to some arbitrary temperature, T, which is below T . Assuming that the latent heat of the VOC remains constant over the condensation range, the enthalpy change (e.g., kJ/kmole of VOC-ftee gaseous stream) can be approximated through... 

If a phase change occurs in the process stream for which heat duties are being calculated, it is best to perform a flash calculation and determine the heat loss or gain by the change in enthalpy. For a quick hand approximation it is possible to calculate sensible heat for both the gas and liquid phases of each component. The sum of all the latent and sen -i-ble heats is the approximate total heat duty. 

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. 

Assume using enthalpy and latent heat tables/charts. 

It carries very large amounts of heat as enthalpy of evaporation or latent heat in relatively small weights of steam ... 

High-pressure condensate forms at the same temperature as the high-pressure steam from which it condenses, as the enthalpy of evaporation (latent heat) is transferred from it. When this condensate is discharged through a steam trap to a lower pressure the energy it contains is greater than it can hold while remaining as liquid water. The excess... 

The total furnace heat absorption may be estimated by using the calculated furnace exit gas temperature and analysis to determine the enthalpy (excluding the latent heat of water vapor) and thus deducting the heat rejection rate from the net heat input rate. 

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.)... 

The boiling point of water is 100 °C (212 °F) at atmospheric pressure, and at this same point the enthalpy of evaporation (latent heat of vaporization) is 970 Btu/lb. 

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. 

Thus, the isothermal is a straight line of slope [Cw(9 — On) + A] with respect to the humidity axis. At the reference temperature 90, the slope is X at higher temperatures, the slope is greater than X, and at lower temperatures it is less than X. Because the latent heat is normally large compared with the sensible heat, the slope of the isothermals remains positive down to very low temperatures. Since the humidity is plotted as the ordinate, the slope of the isothermal relative to the X-axis decreases with increase in temperature. When 9 > Bq and Jf > Jf0, the saturation humidity, the vapour phase consists of a saturated gas with liquid droplets in suspension. The relation between enthalpy and humidity at constant temperature 9 is ... 

It can be seen from Figure 13.5 that for the air-water system a straight line, of slope equal to the enthalpy of dry saturated steam (2675 kJ/kg), is almost parallel to the isothermals. so that the addition of live steam has only a small effect on the temperature of the gas. The addition of water spray, even if the water is considerably above the temperature of the gas, results in a lowering of the temperature after the water has evaporated. This arises because the. latent heat of vaporisation of the liquid constitutes the major part of the enthalpy of the vapour. Thus, when steam is added, it gives up a small amount of sensible heat to the gas, whereas when hot liquid is added a small amount of sensible heat is given up and a very much larger amount of latent heat is absorbed from the gas. 

The vapour enthalpies are calculated from the molar heat capacity functions for the vapour components and the latent heats of vaporisation at standard temperature. The vapour overflow, V, is then obtained from the energy balance as... 

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