Overall heat-transfer coefficient approximate values

The following values of overall heat-transfer coefficients are based primarily on results obtained in ordinary engineering practice. The values are approximate because variation in fluid velocities, amount of noncondensable gases, viscosities, cleanliness of heat-transfer surfaces, type of baffles, operating pressure, and similar factors can have a significant effect on the overall heat-transfer coefficients. The values are useful for preliminary design estimates or for rough checks on heat-transfer calculations. 

The value of f should be greater than 0.8 because low values of F means that substantial additional area must be supplied in the heat exchanger to overcome the inefficient thermal profile. The approximate heat transfer area A can be calculated using reasonable guess for the overall heat transfer coefficient selected values are available in Table 4.1 [2]. The next step is to determine the approximate number of tubes Nj, needed to do the job. 

The calculation of overall heat transfer coefficient U using the equations jireviously presented can be rather tedious. Fleat transfer specialists have computer programs to calculate this value. There are some quick approximation techniques. Table 2-8 comes from the Gas Processors Suppliers Association s Engineering Data Book and gives an approximate value of U for shell and tube heat exchangers. 

Overall heat transfer coefficients for any form of evaporator depend on the value of the film coefficients on the heating side and for the liquor, together with allowances for scale deposits and the tube wall. For condensing steam, which is a common heating medium, film coefficients are approximately 6 kW/m2 K. There is no entirely satisfactory... 

Approximate ranges of values of the overall heat transfer coefficient, h... 

We can assume as a first approach that the values of ho, and h, will have an approximate value of the overall heat transfer coefficient... 

Tabla 10-1 Approximate Values of Overall Heat-transfer Coefficients. 

Approximate design values of overall heat-transfer coefficients... 

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,... 

Overall heat transfer coefficient is either a function of (1) local position only (laminar gas flow) U, (2) temperature only (turbulent liquid flow) U, or (3) both local position and teirj perature (a general case) U. U(T) in Urepresents a position average overall heat transfer coefficient evaluated at a local temperature. Integration should be performed numerically and/ or can be approximated with an evaluation at three points. The values of the correction factor K are presented in Fig. 17.29. 

A change in viscosity by a factor of 100 or, in other words, by two orders of magnitude, reduces the overall heat transfer coefficient to approximately 1/5 of its original value. This dramatic effect is shown graphically in Figure 4-65 for the relevant range of possible viscosity changes. 

Some typical values of overall heat transfer coefficients are given in Table 7.2. These values are very approximate and should only be used for order-of-magnitude estimates. 

Determine the optimal recovery of IPA and acetone for this system using an objective function of the EAOC based on an 8000h operating year. Any simulations should be carried out using a suitable process simulator using the UNIFAC K-value option and the latent heat enthalpy option. The overall heat transfer coefficient for E-403 maybe taken as 200 W/m K. Utility costs should be obtained from Chapter 8, and the cost of E-403 is approximated by the following equation ... 

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