Overall transfer coefficient

When the equilibrium curve is not straight, there is no strictly logical basis for the use of an overall transfer coefficient, since the value of m will be a function of position in the apparatus, as can be seen from Fig. 5-27. In such cases the rate of transfer must be calculated by solving for the interfacial compositions as described above. 

Experimentally observed rates of mass transfer often are expressed in terms of overall transfer coefficients even when the eqmlibrium lines are curved. This procedure is empirical, since the theory indicates that in such cases the rates of transfer may not vary in direct proportion to the overall bulk concentration differences y — y°) and (x° — x) at all concentration levels even though the rates may be proportional to the concentration difference in each phase taken separately, i.e., Xi — x) and y — y ). 

The overall transfer coefficient for the tracer annulus space is... 

Overall transfer coefficient for absorbers = 100 Btu/hr (fF) ° Distillation Column... 

Because of the difficulties in determining x, the thickness of the film between the two vapor pressures, an overall transfer coefficient is introduced. Based on the two film theory, the overall transfer coefficient is used. In the case of water evaporation, the gas film is the controlling mechanism and the resulting equation is... 

Similar results have been obtained by Bonilla and Perry 79>, Insinger and Bliss 801, and others for a number of organic liquids such as benzene, alcohols, acetone, and carbon tetrachloride. The data in Table 9.9 for liquids boiling at atmospheric pressure show that tile maximum heat flux is much smaller with organic liquids than with water and the temperature difference at this condition is rather higher. In practice the critical value of AT may be exceeded. Sauer et al.m] found that the overall transfer coefficient U for boiling ethyl acetate with steam at 377 kN/m2 was only 14 per cent of that when the steam pressure was reduced to 115 kN/m2. 

In such a vessel, the thermal resistances to heat transfer arise from the water film on the inside of the coil, the wall of the tube, the film on the outside of the coil, and any scale that may be present on either surface. The overall transfer coefficient may be expressed by ... 

In the case of a storage tank with liquor of mass m and specific heat C heated by steam condensing in a helical coil, it may be assumed that the overall transfer coefficient U is constant. If 7 is the temperature of the condensing steam, Tt and To the initial and final temperatures of the liquor, and A the area of heat transfer surface, and T k the temperature of the liquor at any time t, then the rate of transfer of heat is given by ... 

A spiral plate exchanger is illustrated in Figure 9.90 in which two fluids flow through the channels formed between the spiral plates. With this form of construction the velocity may be as high as 2.1 m/s and overall transfer coefficients of 2.8 kW/m2 K are frequently obtained. The size can therefore be kept relatively small and the cost becomes comparable or even less than that of shell and tube units, particularly when they are fabricated from alloy steels. 

Usually, the values of the concentration at the interface are not known and the mass transfer coefficient is considered for the overall process. Overall transfer coefficients are then defined by ... 

What is the overall transfer coefficient K(j based on the gas phase ... 

Equation 10.156 can be written in terms of the film coefficient for the second phase (hoi) or either of the overall transfer coefficients (Jf 1, AY). Transfer units based on either film coefficient or overall coefficient can therefore be defined, and the following equations are analogous to equation 10.159 ... 

Determine the overall transfer coefficient and the height of the transfer unit. 

By analogy with the derivation for film coefficients, a series of overall transfer coefficients and overall transfer units based on these overall driving forces may be defined. Thus, the number of overall gas transfer units is given by ... 

The validity of using equations 12.17 and 12.18 in order to obtain an overall transfer coefficient has been examined in detail by King117 . He has pointed out that the equilibrium constant /7 must be constant, there must be no significant interfacial resistance, and there must be no interdependence of the values of the two film-coefficients. 

The relations for mass transfer may also be written in terms of overall transfer coefficients Kr and Ke defined by ... 

Transfer relationships of a more mWoscopic overall form are also used for example, film coefilcients and overall co cients in heat transfer. Here the difference in the bulk properties between two locations is the driving force. The proportionality constant is an overall transfer Coefficient. Table 2.1 summarizes some to the various relationships used in developing models. 

To model sulfur dioxide absorption by the blood through the walls of the upper airways, as demonstrated by Frank et one must include the transport rates of sulfur dioxide across a mucus-tissue interface, a tissue layer, and a tissue-blood interface (Figure 7-2). For the case of release of dissolved gas back into the exhaled air, which is depleted of gas in the lower lung, the mucus layer would still represent the greatest resistance to transfer. Consequently, the overall transfer coefficient, kg, would still be given by ki/H. 

When the Schmidt (or Prandtl) number is sufficiently large, the transfer coefficient near the wall, as calculated in the preceding section, can approximate the overall transfer coefficient. For smaller values of the Schmidt... 

Ux = overall transfer coefficient, finned tube extended area basis, Btu/h °F ft2... 

REM AIRCLR8.BAS PROGRAM FOR AIR COOLER RATING VERSION 50 REM INPUT LINES 50-300 FOR PROGRAM VARIABLES 60 UX=4.5 FIRST ASSUMPTION OF OVERALL TRANSFER COEFFICIENT... 

To obtain the parameters presented in table 1 we considered that the particle diameter is the weight mean diameter calculated by sieving, the extract was considered as terpineol, the diffusion coefficient was obtained by the equation proposed by Tan et al (1988) and the overall transfer coefficient for model II is related to the diffusivity and to the external mass transfer coefficient by the relationship proposed by Catchpole etal( 1990). 

Overall transfer coefficient (cm /h) = (mass on total dermal surface)/[(exposure duration) x(dislodgeable concentration in total environmental surface)] ... 

Table 4.6.3 Overall transfer coefficients for oxygenation using different aeration systems...

Apparent overall transfer coefficients were calculated and used to compare the extraction kinetics of the various systems (Table IV). Although some scattering was observed, it appears that all extractions are rapid and that no significant differences can be noted between the solvents. 

---