Heat transfer factor

Example 2 Calculation of j Factors in an Annulus Calculate the heat-transfer / factors for both walls of an annulus for the following condi-... 

A heat transfer factor (a) between the fluid and surface is defined as... 

If steam condenses on a surface, there is no boundary layer the resistance to heat flow is due to scale, metal thickness, and the condensed liquid layer, resulting in a high heat transfer factor. A thin layer of air or other noncondensing gas forms at the surface through which the steam diffuses. The heat transfer factor diminishes rapidly but is considerably higher than in dry convection. 

The net heat transfer between two surfaces according to Eq. (4.159) is proportional to the first or second power of the temperature difference hence the radiation heat transfer dominates at a high temperature or for large temperature differences. When the temperature difference is small, a heat transfer factor is used similar to that used for convective heat transfer ... 

For conduction the heat resistance is the distance divided by the heat conductivity, R = 8/X.A, and the heat conductance is heat conductivity divided by distance, U = X.A/8. For convection and radiation the heat resistance is 1 divided by the heat transfer factor, 1/aA, and the heat conductance is the same as the heat transfer factor, U aA. A coefficient of heat flow is also used, the K value, which is the total conductance ... 

FIGURE 4.33 Heat transfer factor representing blackbody radiation for various mean temperatures and temperature differences. 

We can apply this result to determine the analogy between mass and heat transfer factors. Mass flow density /a (mol/m s) can be given as... 

The dimensionless quantity Sh is called the Sherwood number. The heat transfer factor a is defined bv... 

Heat Transfer Factors. These involve the removal, or addition of the heat of reaction. 

Surfaces will absorb radiant heat and this factor is expressed also as the ratio to the absorptivity of a perfectly black body. Within the range of temperatures in refrigeration systems, i.e. - 70°C to + 50°C (203-323 K), the effect of radiation is small compared with the conductive and convective heat transfer, and the overall heat transfer factors in use include the radiation component. Within this temperature range, the emissivity and absorptivity factors are about equal. 

Pressure losses due to pipe friction can be calculated from the basic formulas established by Reynolds and others. However, as with the calculation of heat transfer factors, this would be a time-consuming process and some of the parameters are not known accurately. Recourse is usually made to simplified estimates or tables published in works of reference [32, 33]. 

As discussed in Section 9.4.3, heat transfer data are conveniently correlated in terms of a heat transfer factor j/ again modified by the viscosity correction factor ... 

It may be noted that whilst Figure 9.77 is similar to Figure 9.24, the values of ji, differ due to the fact that Kern(28) and other workers define the heat transfer factor as ... 

Figure 9.77. Heat transfer factor for flow inside tubes...

Figure 9.81. Shell-side heat-transfer factors with segmental baffles1 ...

It is often convenient to correlate heat-transfer data in terms of a heat transfer factor, which is similar to the friction factor used for pressure drop (see Volume 1, Chapters 3 and 9). The heat-transfer factor is defined by ... 

Note. Kern (1950), and other workers, define the heat transfer factor as ... 

Figure 12.23. Tube-side heat-transfer factor...

Figure 12.31. Heat-transfer factor for cross-flow tube banks...

The heat transfer factor jh can be correlated for turbulent flow (Re > 4000) as1 ... 

Correlations for Mass and Heat Transfer Factors in Fluidized Beds... 

For the conditions cited, it was shown that jD was 0.235 and, by the Chilton-Colburn analogy Jh = 0.235. From the definition of the heat transfer factor jH, it follows that... 

When handling gases, the heat transfer factor for a fluid flowing inside a tube [8,9] is best expressed as follows ... 

The basis for estimating the purchase cost of the heat exchangers was the area available for heat transfer. Factors were then applied to allow for different construction materials, operating pressures and tube dimensions. 

This correlation is widely accepted. All the variables relate to the fluid properties inside the tubes. As a check a second correlation recommended by Kern is used. It employs the heat-transfer factor (jh) which is a function of Reynolds number. This equation is shown below. 

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