Heat overall resistance

It is not practical to determine the contact resistance in power connectors. The resistance of the connection of a specified length of conductor on each side of the connector is measured and is called the overall resistance or the connection resistance. One industry specification (32) defines the included lengths and requites the stabiUty of the connection resistance to be within 5% of its average value throughout the heat-cycle test. 

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

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 ... 

A significant feature of physical adsorption is that the rate of the phenomenon is generally too high and consequently, the overall rate is controlled by mass (or heat transfer) resistance, rather than by the intrinsic sorption kinetics (Ruthven, 1984). Thus, sorption is viewed and termed in this book as a diffusion-controlled process. The same holds for ion exchange. 

The last term on the left-hand side of eq. (3.301) corresponds to the heat transfer to the external fixed-bed wall. The overall heat transfer resistance is the sum of the internal, external, and wall resistances. In an adiabatic operation, the overall heat transfer coefficient is zero so the corresponding term in the energy balance expression drops out, while in an isothermal operation this coefficient is infinite, so that 7 f 7 s 7W. 

This equation indicates that the overall heat transfer resistance, l/fZ, is the sum of the heat transfer resistances of fluid 1, metal wall, and fluid 2. 

The values of k and x are usually known, while the values of and can be estimated, as stated later. It should be noted that, as in the case of electrical resistances in series, the overall resistance for heat transfer is often... 

The first consideration when designing or evaluating heat transfer equipment is as to which side of the heat transfer wall the controlling heat transfer resistance will exist on. for example, when air is heated by condensing saturated steam, the air-side film coefficient may be 30 kcal h m -°(E, while the steam-side film coefficient might be on the order of 10 000 kcalh m" °C . In such a case, we need not consider the steam-side resistance. The overall coefficient would be almost equal to the air-side film coefficient, which can be predicted by... 

In practice, we must consider the heat transfer resistance of the dirt or scale which has been deposited on the metal surface, except when values of U are small, as in the case of gas heater or cooler. Usually, we use the so-called fouling factor h(, which is the reciprocal of the dirt resistance and hence has the same dimension as the film coefficient h. The dirt resistance sometimes becomes controlling, when U without dirt is very large - as in the case of liquid boiler heated by saturated steam. Thus, in case the dirt resistance is not negligible, the overall resistance for heat transfer l/Ll is given by the following equation ... 

A fouling factor of 2000 kcal h m °C is assumed. The overall heat transfer resistance 1/1/ based on the outer tube surface is... 

The sum of these five factors is called the overall resistance to heat transfer. The reciprocal of the overall resistance is termed U, the overall heat-transfer coefficient. 

We again see that the reciprocal of the overall heat transfer coefficient is equivalent to the overall resistance to heat transfer, which is equal to the sum of resistances in the hot film (1 //zG), the wall [r0 n(r0/ri)]/kw and the cold film (r0 r- ), as shown in Equation (22). However in this case, the expressions for the resistances in the wall and the cold film are modified to take the curvature into account. No such modification is required for the outside film resistance, as the overall heat transfer coefficient is defined based on the outside surface area of the cylinder. 

Overall Heat Transfer Coefficients. Overall resistance to heat transfer based upon the outside area for the clean exchanger ... 

We have introduced the concept of thermal resistances and shown how these may be combined to give overall resistances to heat transfer and overall heat transfer coefficients. 

Mechanistic equations describing the apparent radial thermal conductivity (kr>eff) and the wall heat transfer coefficient (hw.eff) of packed beds under non-reactive conditions are presented in Table IV. Given the two separate radial heat transfer resistances -that of the "central core" and of the "wall-region"- the overall radial resistance can be obtained for use in one-dimensional continuum reactor models. The equations are based on the two-phase continuum model of heat transfer (3). 

Although multiplicities of the effectiveness factor have also been detected experimentally, these are of minor importance practically, since for industrial processes and catalysts, Prater numbers above 0.1 are less common. On the contrary, effectiveness factors above unity in real systems are frequently encountered, although the dominating part of the overall heat transfer resistance normally lies in the external boundary layer rather than inside the catalyst pellet. For mass transfer the opposite holds the dominating diffusional resistance is normally located within the pellet, whereas the interphase mass transfer most frequently plays a minor role (high space velocity). 

Related Calculations. The method described for calculating the overall heat-transfer coefficient is also used to calculate the overall resistance to conduction of heat through a composite wall containing materials in series that have different thicknesses and thermal conductivities. For this case, each individual heat-transfer coefficient is equal to the thermal conductivity of a particular material divided by its thickness. The amount of heat transferred by conduction can then be determined from the formula... 

As a first-pass calculation, if it is assumed that the dominant heat transfer resistance is on the reactor side, then the overall heat transfer coefficient (U) from (4) can be used for scale-up. 

The overall coefficient includes the following heat transfer resistances (1) process side film (2) process side fouling (3) wall resistances (4) utility side fouhng, and (5) utility side film. The development of the relationship between U and these individual resistances is given in myriad heat transfer texts. 

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