Resistance, to heat transfer

To do this one must sum all the resistances to heat transfer. The reciprocal of this sum is the heat transfer coefficient. For a heat exchanger the resistances are... 

Transparent or translucent insulating materials (TIMs) can provide light or solar gains without view. TIMs typically have thermal properties similar to conventional opaque insulation and are thicker than conventional insulating glass units, providing significant resistance to heat transfer. 

Because the condensate builds up along the bottom portion of horizontal tubes, the layer builds up thicker and offers more resistance to heat transfer. Kem proposes good agreement with practical experience using the following... 

B = overall resistance to heat transfer less shell-side resistance (for finned tube calculations), 1/Btu (hr) (ft ) (°F). 

It is defined as the sum of the individual resistances to heat transfer as heat passes from one fluid to another, and can be written as ... 

The Biot number is essentially the ratio of the resistance to heat transfer within the particle to that within the external fluid. At first sight, it appears to be similar in form to the Nusselt Number Nu where ... 

Bi very large. The resistance to heat transfer in the fluid is then low compared with that in the solid with the temperature of the surface of the particle being approximately equal to the bulk temperature of the fluid, and the heat transfer rate is independent of the Biot number. Equation 9.44 then simplifies to ... 

Bi very small, (say, <0.1). Here the main resistance to heat transfer lies within the fluid this occurs when the thermal conductivity of the particle in very high and/or when the particle is very small. Under these conditions, the temperature within the particle is uniform and a lumped capacity analysis may be performed. Thus, if a solid body of volume V and initial temperature Oo is suddenly immersed in a volume of fluid large enough for its temperature 0 to remain effectively constant, the rate of heat transfer from the body may be expressed as ... 

Intermediate values ofBi. In this case the resistances to heat transfer within the solid body and the fluid are of comparable magnitude. Neither will the temperature within the solid be uniform (case 1), nor will the surface temperature be equal to that in the bulk of the fluid (case 2). 

In such a vessel, the thermal resistances to heat transfer arise from the water film on the inside of the coil, the wall of the tube, the film on the outside of the coil, and any scale that may be present on either surface. The overall transfer coefficient may be expressed by ... 

As the thickness of the lagging is increased, resistance to heat transfer by thermal conduction increases. Although the outside area from which heat is lost to the surroundings also increases, giving rise to the possibility of increased heat loss. It is perhaps easiest to think of the lagging as acting as a fin of very low thermal conductivity. For a cylindrical... 

The heat-transfer coefficient for the liquid is often large compared with that for the gas phase. As a first approximation, therefore, it can be assumed that the whole of the resistance to heat transfer lies within the gas phase and that the temperature at the water-air interface is equal to the temperature of the bulk of the liquid. Thus, everywhere in the tower, 0/ = [.. This simplifies the calculations, since the lines AC, HJ, and so on, have a slope of -co, that is, they become parallel to the enthalpy axis. 

The specific heat and density of the oil may be assumed constant at 1.9 kJ/kg K and 900 kg/m respectively, and any resistance to heat transfer on the steam side neglected. 

Neglect any resistance to heat transfer in the pipe walls. 

Catalyst pellets often operate with internal temperatures that are substantially different from the bulk gas temperature. Large heats of reaction and the low thermal conductivities typical of catalyst supports make temperature gradients likely in all but the hnely ground powders used for intrinsic kinetic studies. There may also be a him resistance to heat transfer at the external surface of the catalyst. 

The overall coefficient is the reciprocal of the overall resistance to heat transfer, which is the sum of several individual resistances. For heat exchange across a typical heat-exchanger tube the relationship between the overall coefficient and the individual coefficients, which are the reciprocals of the individual resistances, is given by ... 

Empirical methods approximate methods, in which the resistance to heat transfer is considered to control the rate of condensation, and the mass transfer resistance is neglected. Design methods have been published by Silver (1947), Bell and Ghaly... 

Consider first the resistance to heat transfer across the wall of the tubes. 

Figure 15.3 illustrates the resistance to heat transfer across the wall of the tube. There are five resistances to heat transfer. Each can be characterized by a heat transfer coefficient. 

Equation 15.78 gives the criterion for heat transfer enhancement to cater to the new duty without increasing the heat transfer area. If it is assumed that the resistance to heat transfer across the tube wall is negligible (i.e. hw goes to infinity) and the difference between the inside and outside diameters is negligible (i.e. d0 = df), then Equation 15.78 simplifies to9 ... 

Resistance to heat transfer across the tube wall for shell-and-tube heat exchangers is made up of five individual resistances to heat transfer ... 

The temperature of the reactor could theoretically be controlled by changing the flow rate or the temperature of the water in the jacket. It will now be shown that the former is impractical. The over-all heat transfer coefficient is given in the major equipment section as around 50 BTU/hr ft2°F or greater. This means that the major resistance to heat transfer is the film on the inside of the reaction vessel. 

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