Heat transfer coefficients 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. 

There will also be heat loss from tire substrate due to convection cuiTents caused by the teirrperamre differential in the suiTounding gas phase, but this will usually be less than the radiation loss, because of the low value of the heat transfer coefficient, / , of gases. The heat loss by this mechanism, Qc, can be calculated, approximately, by using tire Richardson-Coulson equation... 

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. 

Wall-to-bed heat-transfer coefficients were also measured by Viswanathan et al. (V6). The bed diameter was 2 in. and the media used were air, water, and quartz particles of 0.649- and 0.928-mm mean diameter. All experiments were carried out with constant bed height, whereas the amount of solid particles as well as the gas and liquid flow rates were varied. The results are presented in that paper as plots of heat-transfer coefficient versus the ratio between mass flow rate of gas and mass flow rate of liquid. The heat-transfer coefficient increased sharply to a maximum value, which was reached for relatively low gas-liquid ratios, and further increase of the ratio led to a reduction of the heat-transfer coefficient. It was also observed that the maximum value of the heat-transfer coefficient depends on the amount of solid particles in the column. Thus, for 0.928-mm particles, the maximum value of the heat-transfer coefficient obtained in experiments with 750-gm solids was approximately 40% higher than those obtained in experiments with 250- and 1250-gm solids. 

For the inlet length of a pipe in which the boundary layers are forming, the equations in the previous section will give an approximate value for the heat transfer coefficient. It should be remembered, however, that the flow in the boundary layer at the entrance to the pipe may be streamline and the point of transition to turbulent flow is not easily defined. The results therefore are, at best, approximate. 

Effect of pressure Figure 2.40 shows the heat transfer coefficients for film boiling of potassium on a horizontal type 316 stainless steel surface (Padilla, 1966). Curve A shows the experimental results curve B is curve A minus the radiant heat contribution (approximate because of appreciable uncertainties in the emissivities of the stainless steel and potassium surfaces). Curve C represents Eq. (2-150) with the proportionality constant arbitrarily increased to 0.68 and the use of the equilibrium value of kG as given by Lee et al. (1969). 

Knowing the Reynolds number (to ensure turbulent flow) and the approximate values of the heat-transfer coefficients for the downstream and upstream, we need to relate the length L to hv h2, the specific heats (c1 c2) and mass flow (mly m2) of the two streams (see Fig. 5.11). 

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... 

A precise theoretical solution is neither necessary nor possible, since during the operation of the evaporator, variations of the liquor levels, for example, will alter the heat transfer coefficients and hence the temperature distribution. It is necessary to assume values of heat transfer coefficients, although, as noted previously, these will only be approximate and will be based on practical experience with similar liquors in similar types of evaporators. 

The results show clearly that a much better design can be obtained with a variable heat transfer coefficient. The length of the original reactor is now at its upper bound, which translates into increased conversion of nitrogen to ammonia. This is reflected in the exit ammonia concentration of 23.93 mol %, a 15.2% increase over the initial value. Further, this also represents a 15.4% increase over the corresponding result with a constant U case. The final objective function value of 5.6359 denotes a 12.25% rise over the initial value. In addition, the accuracy of the approximation profiles and adequacy of the final element partitions are also evident. 

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

In the case where fluid flows in the shell side space of a shell-and-tube-type heat exchanger, with transverse baffles, in directions that are transverse, diagonal, and partly parallel to the tubes, very approximate values of the heat transfer coefficients at the tube outside surfaces can be estimated using Equation 5.12a, if is calculated as the transverse velocity across the plane, including the shell axis [1]. 

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... 

In some forced convective flows it has been found that the Nusselt number is approximately proportional to the square root of the Reynolds number. If, in such a flow, it is found that h has a value of 15 W/m2K when the forced velocity has a magnitude of 5 m/s, find the heat transfer coefficient if the forced velocity is increased to 40 m/s. 

Table 1-2 Approximate Values of Convection Heat-Transfer Coefficients...

Boiling and condensation phenomena are very complicated, as we have shown in the preceding sections. The equations presented in these sections may be used to calculate heat-transfer coefficients for various geometries and fluid-surface combinations. For many preliminary design applications only approximate values of heat flux or heat-transfer coefficient are required, and Tables 9-4 to 9-6 give summaries of such information. Of course, more accurate values should be obtained for the final design of heat-transfer equipment. 

Table 9-4 Approximate Values of Condensation Heat-transfer Coefficients tor Vapors at 1 atm. According to Refs. 3 and 45.

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

In the common types of baffled shell-and-tube exchangers, the shell-side fluid flows across the tubes. The equations for predicting heat-transfer coefficients under these conditions are not the same as those for flow of fluids inside pipes and tubes. An approximate value for shell-side coefficients in a cross-flow exchanger with segmental baffles and reasonable clearance between baffles, between tubes, and between baffles and shell can be obtained by using the following correlation ... 

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

It can be seen from Fig. 7.17 that the condensing heat-transfer coefficient for a fluid with a Prandtl number of approximately 2 (for instance, steam) is not strongly dependent on flow rate or Reynolds number. For this reason, heat-transfer coefficients for steam condensing on vertical tubes are frequently not calculated, but are assigned a value of 1500 to 2000Btu/(h)(ft2)(°F) [8500 to 11,340 W/(m2)(K)]. 

When the heat-transfer coefficients for laminar flow and for vapor shear are nearly equal, the effective heat-transfer coefficient is increased above the higher of the two values. The following table permits the increase to be approximated ... 

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

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