Pressure change, latent heat

The change of /, with temperature is important in evaporation under reduced pressure. The latent heat 4 includes the energy spent by the vapour overcoming its own pressurep when tWs is subtracted we obtain the internal latent heat hi=le p(vg—vi)r= l—pVgp l —RTlM,. .. (1)... 

The change of 4 with temperature is important in evaporation under reduced pressure. The latent heat 4 includes the energy spent by the vapour overcoming its own pressurep when this is subtracted we obtain the internal latent heat hi=h-p(vg-vi) le-pvg Ie—RTIM,. .. (1) where /, vg are the specific volumes of liquid and vapour, R is the gas constant (in the same energy units as 4. e.g. 1-9875 g.cal./l°C.), and M is the. molar weight of the vapour. If E and H are the energy and heat content, then ... 

Latent heats vary as a function of temperature and to a smaller extent pressure. The latent heat for a phase change at 1 atm (101.29 kPa) is often called the standard heat of phase change and is available for many materials in the literature (Perry and Green 1997). 

Ideal gas enthalpy, Hfv> is obtained from (4-60). The derivative of pure component liquid fugacity coefBcient with respect to temperature leads to the following relation for combined effects of pressure and latent heat of phase change from vapor to liquid. 

More usually, however, they are measured at constant pressure and are then equal to the change in enthalpy (provided that pdF is the only form of work). Fmr example, in the vaporization of a pure liquid at constant pressure, the latent heat, as it used to be called, is... 

The term to describe the change of enthalpy during a phase transition at constant pressure is latent heat. Latent means hidden, and it is called latent because we cannot sense the heat input by detecting the temperature change, as is the case with sensible heat, described previously. 

A = effective surface area for heat and mass transfer in m L = latent heat of vaporization at in kj/kg k = mass-transfer coefficient in kg/ (sm kPa) t = mean source temperature for all components of heat transfer in K t = Hquid surface temperature in K p = Hquid vapor pressure at in kPa p = partial pressure of vapor in the gas environment in kPa. It is often useful to express this relationship in terms of dry basis moisture change. For vaporization from a layer of material ... 

Cluusius-Clupeyron Eijliation. Derived from equation 1, the Clapeyron equation is a fundamental relationship between the latent heat accompanying a phase change and pressure—volume—temperature (PVT data for the system (1) ... 

The entropy change AS/ - and the volume change AV/ - are the changes which occur when a unit amount of a pure chemical species is transferred from phase I to phase v at constant temperature and pressure. Integration of Eq. (4-18) for this change yields the latent heat of phase transition ... 

The situation changes when the system pressure becomes high enough for refrigerant to condense in the condenser and reject the resulting latent heat to the environment. Further addition of heat to the adsorbate desorbs more refrigerant which condenses in the condenser and trickles down into the receiver. The system pressure stays approximately constant as desorption and condensation proceed. 

The term latent heat is also pertinent to our discussions. The process of changing from solid to gas is referred to as sublimation from solid to liquid, as melting and from liquid to vapor, as vaporization. The amount of heat required to produce such a change of phase is called latent heat. If water is boiled in an open container at a pressure of 1 atmosphere, its temperature does not rise above 100° C (212° F), no matter how much heat is added. The heat that is absorbed without changing the temperature is latent heat it is not lost, but is expended in changing the water to steam. 

Latent Heat of Vaporization The heat required to change a liquid into a vapor without a change in the temperature or pressure. 

The heat necessary to change the state of a substance from solid to liquid or from liquid to gas, or the heat given up during the reverse process. There is no change in temperature during these processes. For example, continuing to boil a kettle of water previously raised to I00°C to steam requires the addition of latent heat, but there is no change in temperature if the pressure remains constant. 

A liquid boils and condenses - the change between the liquid and gaseous states - at a temperature which depends on its pressure, within the limits of its freezing point and critical temperature. In boiling it must obtain the latent heat of evaporation and in condensing the latent heat must be given up again. 

Heat is put into the fluid at the lower temperature and pressure and provides the latent heat to make it boil and change to a vapour. This vapour is then mechanically compressed to a higher pressure and a corresponding saturation temperature at which its latent heat can be rejected so that it changes back to a liquid. 

If the percentage saturation of an air sample is less than 100, i.e. it is less than saturated, and it comes into contact with water at the same temperature, there will he a difference in vapour pressures. As a result, some of the water will evaporate. The latent heat required for this change of state will he drawn from the sensible heat of the water, which will he slighdy cooled. This drop in the water temperature provides a temperature difference, and a thermal balance will be reached where the flow of sensible heat from the air to the water (Figure 23.2) provides the latent heat to evaporate a part of it. 

Definition.—The heat absorbed in producing a change of physical state or chemical composition of a system, at constant temperature and pressure, is called the latent heat of the given transition, and is measured by the number of calories absorbed during the transition of unit mass of the substance from the initial to the final state. 

Le — latent heat of evaporation. v2 — Vi = Ar = volume change accompanying unit mass of phase transition at the pressure p. 

The equation of Planck ( 93) for the dependence of the latent heat of a change of state on the temperature and pressure applies, of course, to fusion as well as evaporation ... 

The specific volume curves intersect at a point where Ar = 0 the latent heat, on the contrary, changes only very slightly with the temperature, and its curve is either horizontal, or exhibits a maximum, falling off slightly at higher pressures, and probably approaching the p axis. Thus, when Ar = 0, L f has a considerable positive value, and when L/ = 0, Ar has (probably) a considerable negative value. 

Thus, if we find how the electromotive force changes when the temperature of the cell is altered on open circuit, i.e., when no current is passing, we can at once calculate A, the latent heat, just as we can calculate the latent heat of evaporation of a liquid when we know the variation of its vapour pressure with temperature. Since E changes only slightly with T, we can evaluate dE... 

The variation of enthalpy for binary mixtures is conveniently represented on a diagram. An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of ammonia and water versus concentration with pressure and temperature as parameters. It covers the phase changes from solid to liquid to vapour, and the enthalpy values given include the latent heats for the phase transitions. 

Second-order phase transitions are those for which the second derivatives of the chemical potential and of Gibbs free energy exhibit discontinuous changes at the transition temperature. During second-order transitions (at constant pressure), there is no latent heat of the phase change, but there is a discontinuity in heat capacity (i.e., heat capacity is different in the two... 

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