Enthalpy transfer coefficients

K = overall enthalpy transfer coefficient, Ib/hr (ft transfer area) (lb water/lb dry air)... 

Overall enthalpy transfer coefficient, lb/ (hr) (ft- transfer area) (lb water/lb dry air or, mass transfer coefiRcient, lb water/(hr) (ft ) Enthalpy coefficient of total heat transfer,... 

Method for the Calculation of Over-All Volumetric Enthalpy Transfer Coefficients in Cross-Flow Cooling Towers... 

Now consider the heat transfer area required by enthalpy interval k, in which the overall heat transfer coefficient is allowed to vary... 

Thus, the enthalpy and temperature of the vapor—Hquid interface are related to the Hquid temperature and gas enthalpy at any point in the column through a ratio of heat- and mass-transfer coefficients. 

The thermal design of cooling towers follows the same general procedures already presented. Integration of equation 35 is usually done numerically using the appropriate software, mass-transfer coefficients, saturation enthalpies, etc. In mechanical-draft towers the air and water dows are both suppHed by machines, and hence dow rates are fixed. Under these conditions the design procedure is straightforward. 

Overall enthalpy mass-transfer coefficient kmoP[(s-m )(mole fraction)] ... 

The heat-transfer coefficient for the liquid is often large compared with that for the gas phase. As a first approximation, therefore, it can be assumed that the whole of the resistance to heat transfer lies within the gas phase and that the temperature at the water-air interface is equal to the temperature of the bulk of the liquid. Thus, everywhere in the tower, 0/ = [.. This simplifies the calculations, since the lines AC, HJ, and so on, have a slope of -co, that is, they become parallel to the enthalpy axis. 

Fig. 3.2 shows the case of a jacketed, stirred-tank reactor, in which either heating by steam or cooling medium can be applied to the jacket. Here V is volume, Cp is specific heat capacity, p is density, Q is the rate of heat transfer, U is the overall heat transfer coefficient, A is the area for heat transfer, T is temperature, H is enthalpy of vapour, h is liquid enthalpy, F is volumetric flow... 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

If the variation in the physical properties is too large for these simple methods to be used it will be necessary to divide the temperature-enthalpy profile into sections and evaluate the heat-transfer coefficients and area required for each section. 

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

Cy.jt = overall heat transfer coefficient between hot stream i and cold stream j in enthalpy interval k... 

The Gibbs energy of activation in Eq. (5.4) can be split into an enthalpy and an entropy term AGjx = AH x - T AS]x. Define two transfer coefficients... 

This result is quite in contrast to the common expectation that the electrode potential changes the activation barrier at the interface which would result in a temperature independent transfer coefficient a. Following Agar s discussion (30), such a behavior indicates a potential dependence of the entropy of activation rather than the enthalpy of activation. Such "anomalous" behavior in which the transfer coefficient depends on the temperature seems to be rather common as recently reviewed by Conway (31). 

If a more complex mathematical model is employed to represent the evaporation process, you must shift from analytic to numerical methods. The material and enthalpy balances become complicated functions of temperature (and pressure). Usually all of the system parameters are specified except for the heat transfer areas in each effect (n unknown variables) and the vapor temperatures in each effect excluding the last one (n — 1 unknown variables). The model introduces n independent equations that serve as constraints, many of which are nonlinear, plus nonlinear relations among the temperatures, concentrations, and physical properties such as the enthalpy and the heat transfer coefficient. 

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