Heat-transfer coefficients magnitude

These small positive and negative errors partially cancel each other. The result is that capital cost targets predicted by the methods described in this chapter are usually within 5 percent of the final design, providing heat transfer coefficients vary by less than one order of magnitude. If heat transfer coefficients vary by more than one order of magnitude, then a more sophisticated approach can sometimes be justified. ... 

The concept of a transfer unit is useful in the design of heat exchangers and in assessing their performance, since its magnitude is less dependent on the flowrate of the fluids than the heat transfer coefficient which has been used so far. The number of transfer units N is defined by ... 

For normalization of the value of the heat transfer enhancement, we used its magnitude at the maximum for each curve. The result of such normalization is shown in Fig. 2.59. In this figure, C is the solution concentration, Cq is the characteristic concentration, h is the heat transfer coefficient at given values of the solution concentration and the heat flux q, /zmax is the maximum value of the heat transfer coefficient at the same heat flux, and /zw is the heat transfer coefficient for pure water at the same heat flux q. Data from all the sources discussed reach the same value of 1.0 at the magnitude of relative surfactant concentration equal to 1.0. 

For a thermometer to react rapidly to changes in the surrounding temperature, the magnitude of the time constant should be small. This involves a high surface area to liquid mass ratio, a high heat transfer coefficient and a low specific heat capacity for the bulb liquid. With a large time constant, the instrument will respond slowly and may result in a dynamic measurement error. 

The gas-phase wall heat-transfer coefficient can be evaluated by using the gas-phase Reynolds number and Prandtl number in Eq. (33). The thermal conductivities of liquids are usually two orders of magnitude larger than the thermal conductivities of gases therefore, the liquid-phase wall heat-transfer coefficient should be much larger than the gas-phase wall heat-transfer coefficient, and Eq. (34) simplifies to... 

The absolute magnitude of the heat transfer coefficient is several folds greater than single-phase gas convection at the same superficial velocity. 

Similar behavior has been reported by other researchers for both vertical and horizontal tubes, though the magnitude of the heat transfer coefficients change with operating conditions, (Mickley and Trilling, 1949 Vreedenberg, 1960 Botterill and Desai, 1972 Ozkaynak and Chen, 1980). 

Magnitude of the heat transfer coefficient generally decreases with elevation along the bed. 

Magnitude of the heat transfer coefficient is higher than that for equivalent air convection, but lower than that found in dense bubbling beds. 

The high rates of heat transfer obtainable are due to a number of reasons. Firsf, fhe presence of particles in a fluidized bed increases the heat transfer coefficient by up to two orders of magnitude, compared with the value obtained with gas alone at the same velocity. This is because the particles tend to reduce the thickness of the boundary layer at the heat transfer surface (Jowitt, 1977). The bed particles are responsible for fhe fransfer of heat and, because of the high rate of particle movement (and very short residence times close to the heat transfer... 

Assume magnitudes of the tube-diameter, D, mass velocity, G, and the fluid properties. Divide the length into equal increments of quality change, Ax, and calculate the heat-transfer coefficient, hz at the mid-quality magnitude of each Ax and assume that h2 is uniform in this quality range. Then the length required to change the quality Ax is... 

The effects of the many variables that bear on the magnitudes of individual heat transfer coefficients are represented most logically and compactly in terms of dimensionless groups. The ones most pertinent to heat transfer are listed in Table 8.8. Some groups have ready physical interpretations that may assist in selecting the ones appropriate to particular heat transfer processes. Such interpretations are discussed for example by GrOber et al. (1961, pp. 193-198). A few are given here. 

Maintenance of proper temperature is a major aspect of reactor operation. The illustrations of several reactors in this chapter depict a number of provisions for heat transfer. The magnitude of required heat transfer is determined by heat and material balances as described in Section 17.3. The data needed are thermal conductivities and coefficients of heat transfer. Some of the factors influencing these quantities are associated in the usual groups for heat transfer namely, the Nusselt, Stanton, Prandtl, and Reynolds dimensionless groups. Other characteristics of particular kinds of reactors also are brought into correlations. A selection of practical results from the abundant literature will be assembled here. Some modes of heat transfer to stirred and fixed bed reactors are represented in Figures 17.33 and 17.18, and temperature profiles in... 

In general, and to give an order of magnitude, the film heat transfer coefficient with natural convection is approximately 10% of the heat transfer coefficient with agitation [4]. 

The scale-up criterion of constant heat transfer coefficient is suitable when the predominant problem of the reactor involves the removal of heat. The magnitude of the heat transfer coefficient is governed by the intensity of stirring within the reactor, and is represented by ... 

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