Heat transfer coefficients individual

The problem with Eq. (7.5) is that the overall heat transfer coefficient is not constant throughout the process. Is there some way to extend this model to deal with the individual heat transfer coefficients ... 

Note that individual heat transfer coefficients are not additive, but their reciprocals, or resistances, are... 

F = MTD correction factor, dimensionless FA = face area, fF FV = standard air face velocity, sfm G = mass velocity, lb/ (sec) (fF) h = individual heat transfer coefficient,... 

Procedures for estimating the individual heat-transfer coefficients and the exchanger pressure drops are given in this chapter. 

For a preliminary design, except for very large or expensive heat exchangers, it is usually adequate to use approximate heat transfer coefficients. These can be found in references 22,23 and 24. When calculating individual heat transfer coefficients, it may simplify calculations to note that for streams that have a viscosity greater than 5 cp the tube-size coefficient is two or three times what the shell-side coefficient would be for the same material.2 This is often the deciding factor in determining which fluid should flow within the tubes. 

Fig. 27. Individual heat transfer coefficients for boiling metals. Data from Fig. 26 are included (L3).

The effects of the many variables that bear on the magnitudes of individual heat transfer coefficients are represented most logically and compactly in terms of dimensionless groups. The ones most pertinent to heat transfer are listed in Table 8.8. Some groups have ready physical interpretations that may assist in selecting the ones appropriate to particular heat transfer processes. Such interpretations are discussed for example by GrOber et al. (1961, pp. 193-198). A few are given here. 

TABLE 8.10. Recommended Individual Heat Transfer Coefficient Correlations ... 

Cp = heat capacity at constant pressure C = heat capacity at constant volume gc = gravitational constant (numerical values in Table A1) h = individual heat transfer coefficient H = enthalpy k=thermal conductivity k=C iCv... 

The overall heat transfer coefficient is a composite number. It depends on the individual heat transfer coefficients on each side of the tube and the thermal conductivity of the tube material. The individual heat transfer coefficient in turn depends on the fluid flow rate, physical properties of the fluid, and dirt factor. The temperature along the tube is not uniform. The hot and the cold fluids may flow in the same (cocurrent) or in opposite (countercurrent) directions. Generally the hot and cold fluids come in contact only once, and such an exchanger is called single pass. In a multipass exchanger, the design of the... 

One objective of this work is to calculate fluid temperatures inaccessible to direct measurement from the model equations by synchronizing the model with the real time process via the measured wall temperatures. For this purpose the individual heat transfer coefficients between fluid and wall k and between wall and sand bath ks have to be determined. First... 

Individual heat-transfer coefficients to account for fouling... 

Related Calculations. The method described for calculating the overall heat-transfer coefficient is also used to calculate the overall resistance to conduction of heat through a composite wall containing materials in series that have different thicknesses and thermal conductivities. For this case, each individual heat-transfer coefficient is equal to the thermal conductivity of a particular material divided by its thickness. The amount of heat transferred by conduction can then be determined from the formula... 

Individual Heat-Transfer Coefficient The local rate of convective heat transfer between a surface and a fluid is given by Newton s law of cooling... 

Individual heat-transfer coefficients and the fouling resistance or fouling factor, are listed... 

Select approximate values of the individual heat-transfer coefficients and fouling resistance from Equations 4.5.10 to 4.5.13. 

Use Table 4.3 to obtain approximate values of the individual heat-transfer coefficients and fouling resistances. Then, calculate the overall heat-transfer coefficient from Equation 4.5.9 after selecting a conservative heat-transfer coefficient of 5000 W/m -K for water on both the shell and tube sides. Also, select a high... 

Selecting an approximate overall heat-transfer coefficient is a problem because of insufficient data. Although there are correlations available for calculating the individual heat-transfer coefficients and hence the overall heat-transfer coefficients, at the preliminary stage of the process design, we try to avoid detailed calculations. The best we can do is to select a coefficient that best matches the conditions in the CSTR. Because the jacket liquid is water, and the reactor liquid is a dilute aqueous solution, we find that from Table 7.6, Uj varies from 60 to 110 Btu/h-ft -°F (341 to 625 wW-°F) The average value is 85 Btu/h fP "F (483 W/m -K). From Equation 7.4.9, we find that the standard 8000 gal (30.3 m ) reactor has a jacket area of 466 tf (43.3 m ). From Equation 7.4.7, the heat that can be transferred to the jacket,... 

Note that the fouling resistances have been subtracted from the fouled individual heat transfer coefficients to obtain the overall clean coefficient, compared with a normal value of... 

The effects of the many variables that bear on the magnitudes of individual heat transfer coefficients are represented most logically... 

Individual heat transfer coefficient correlations are summarized in Table 10.2. Penney (1983) has given the recommended geometry for anchors and helical ribbons ... 

Use Eq. (14-18) to obtain the individual heat transfer coefficient. Assume that (/////, ) essentially unity. 

In order to calculate the overall heat transfer coefficient in the equation above, the heat transfer properties of the heating medium and the liquor are described in terms of individual heat transfer coefficients, and the heat transfer properties of the separating solid in terms of its thermal conductivity. Deposits at the interface, which one might expect to be described by a thermal conductivity, are usually described instead in terms of either a fouling (heat transfer) coefficient or a fouling factor (which is usually defined to have a value of 1000/[fouling coefficient]). 

Individual heat transfer coefficients for convection are defined by the equation ... 

The individual heat transfer coefficients for the medium and the liquor can be calculated from general correlations using the properties of the fluid and the velocity fields in the system.Additional information can be found in the continuing series of the American Society of Mechanical Engineers. Detailed studies and bibliographies on prediction of individual heat transfer coefficients for evaporating films on/in horizontal/vertical tubes, can be found in other publications. " ... 

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