Heat transfer coefficient typical values

An alternative method of estimating dryer size very roughly is to estimate a volumetric heat-transfer coefficient [typical values are around 2000 I/(m s K)j and thus calculate dryer volume. 

Typical overall heat-transfer coefficients are given in Tables 11-3 through 11-8. Values from these tables may be used for preliminaiy estimating purposes. They should not be used in place of the design methods described elsewhere in this section, although they may serve as a useful check on the results obtained by those design methods. 

For low values of the Reynolds number, such as 10, where sn eamline flow should certainly apply, the Nusselt number has a value of about 2, and a typical value of the average heat transfer coefficient is 10 ". For a Reynolds number of 104, where the gas is certainly in turbulent flow, the value of the Nusselt number is typically 20. Hence there is only a difference of a factor of ten in the heat transfer coefficient between tlrese two extreme cases. 

We see that the heat transfer coefficient is inversely proportional to the square root of the wire diameter, which is the reason for the development of fine wire heat exchangers after all. With an air velocity v of 0.5 m/s and a wire of 100 m, we have a=226 W/m K, which is around ten times the typical value of flat plate heat exchangers to air. 

This approach is much easier to use than the first, but may give less accurate results since it assumes an average or typical value for the heat transfer coefficient for all heated source geometries. 

Typical values of thermal resistances and individual and overall heat transfer coefficients are given in Tables 9.15-9.18. 

Typical values of the overall heat-transfer coefficient for various types of heat exchanger are given in Table 12.1. More extensive data can be found in the books by Perry et al. (1997), TEMA (1999), and Ludwig (2001). 

For a heating system that heats from a warm surface, the heating power q = A a A T, where A is the surface area, a the heat transfer coefficient and A T the temperature difference between the surface and the ambient air. Typical values are ... 

Calculation of the overall heat transfer coefficient from Equation 15.13 requires knowledge of the film transfer coefficients. Although Table 15.1 presents typical values,... 

Other typical values of heat transfer coefficients achieved in drying operations are ... 

In the following analysis, both p-z-n and junction barrier Schottky diodes will be evaluated for use in a 3-kV, 30A SiC bridge rectifier module. Four of these modules will replace the 10 Si diode bridge rectifiers and will reduce system volume and increase efficiency. To optimize the design of the module, we will evaluate the power density at the die level as a function of the number of paralleled diodes in each rectifier leg. A typical value of the heat-transfer coefficient of conventional, power components is 100 W/cm In the present analysis, we have a design limit of 200 W/cm and will determine the number of JBS and p-z -n diode needed to meet this goal. 

Table 3.4-2 summarizes typical values of the overall heat-transfer coefficient for heat exchangers and high-pressure gases. 

Note Heat-transfer coefficients for heat-exchanger design are calculated using the nomographs and typical values suggested in Refs. TD6 and TD7. This enables the determination of a preliminary heat-transfer area using the formula ... 

A provisional heat-transfer coefficient is obtained from Ref. E2 (p.513, Table 12.1), assumed to be typical of a transfer coefficient for two gaseous streams. This reference suggests a value of U0 = 25 W/m2 K), however this value is multiplied by a factor of four because of the very large temperature difference and good gas velocities encountered in this application, Therefore, the value of U0 = 100 W/m2 K) is used in the design calculations. 

Some typical values of heat transfer coefficients are given in Table 9.5. The values provided for hr without stirrer and hc without flow, show the influence of failure of the stirrer or of the cooling system on the heat transfer. 

The convective heat transfer coefficients hi and h0 must be calculated from equations that involve the geometry of the system, the physical properties of the fluid, and the velocity with which it is flowing. These equations are obtained variously by more or less fundamental analysis of the heat transfer and fluid flow mechanisms, or by correlation of experimental data, or by combinations of these methods. A few typical values of the film coefficients are... 

Typical values for the mean heat transfer coefficient... 

Typical values of the mean convective heat transfer coefficient for various flow situations are listed in Table 1.1. 

A Typical values of the mean heat transfer coefficient for a variety of situations were listed in Table 1.1. Discuss some of the physical reasons why these values vary so ereatlv from one situation to another. 

Calculations of the convection heat-transfer coefficients for use in the overall heat-transfer coefficient are made in accordance with the methods described in later chapters. Some typical values of the overall heat-transfer coefficient are given in Table 10-1. 

Typical values for the heat transfer coefficient lie around 104 W mj"2 K-1. Heat transfer effects rarely affect rates measured in slurry reactors. 

Correlation 7.181 should be used with care at low Reynolds numbers. Typical values for gas-solid transfer are 1 m mi"2 s 1 for the mass transfer coefficient and 102 W m-2 K-1 for the heat transfer coefficient. 

Calculate the heat-transfer coefficient using both mechanisms, and select the higher value calculated as the effective heat-transfer coefficient hL. The vapor-shear effects vary for each typical baffle section. The condenser should be calculated in increments, with the average vapor velocity for each increment used to calculate vapor-shear heat-transfer coefficients. 

Here we present examples of how the reactor type and heat transfer fluid affect the heat transfer coefficient. When the reactor fluid has a low viscosity, the dominant heat transfer resistance tends to be on the jacket side. When the reactor fluid has a high viscosity, however, the dominant resistance is typically on the reactor side. Parameter values for the studies are presented in Figures 5-7 and are given in the literature [18],... 

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