Estimation of Film Coefficients

Forced Flow of Fluids through Tubes (Conduits) 

For the individual (film) coefficient h for heating or cooling of fluids, without phase change, in turbulent flow through circular tubes, the following dimensionless equation [2] is well established. In the following equations all fluid properties are evaluated at the arithmetic-mean bulk temperature. 

The film coefficient h for turbulent flow ofwater through a tube can be estimated by the following dimensional equation [1]  

Flow through tubes is sometimes laminar, when fluid viscosity is very high or the conduit diameter is very small, as in the case of hollow fibers. The values of h for laminar flow through tubes can be predicted by the following dimensionless equation [1, 3]  

Equations 5.8a and 5.9a give h for straight tubes. It is known that the values of h for fluids flowing through coils increase somewhat with decreasing radius of helix. However, in practice the values of h for straight tubes can be used in this situation. 

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For film coefficients many situations exist. Table 1 and Figure 1 give ballpark estimates of film resistance at reasonable design velocities. 

Figure 12.43 can be used to estimate condensate film coefficients in the absence of appreciable vapour shear. Horizontal and downward vertical vapour flow will increase the rate of heat transfer, and the use of Figure 12.43 will give conservative values for most practical condenser designs. 

Table 4 presents equations for use in the estimation of heat-transfer film coefficients for condensation, boiling, and natural convection. Values showing the general range of film coefficients for various situations are indicated in... 

The heater, cooler, and heat exchanger models allow the design engineer to enter estimates of film transfer coefficients, and hence calculate the exchanger area. As with distillation columns, the designer must remember to add a design factor to the sizes predicted by the model. Design factors are discussed in Section 1.7. 

Examples 13-1 and 13-2 demonstrate that for the same set of film coefficients (w L, riG) and compositions xjU yj+ approximately the same values of yn are obtained by use of vaporization plate efficiencies (based on the modified Murphree model) as are obtained by use of Murphree plate efficiencies for binary mixtures. This result suggests that the existing correlations for riL and riG given in the Bubble-Tray Design Manual4 may be used to estimate the Ej-s for the modified Murphree model for multicomponent mixtures. This statement arises from the fact that the formulas for the vaporization plate efficiencies which were used in the computations for binary mixtures are precisely the same ones that... 

The estimations of film thickess and mass transfer coefficient are demonstrated in Example 8.1. 

The value of ap, the mass transfer area per volume, is usually estimated as 6/dp. A number of correlations have been developed to estimate the film coefficient. The Wakoa and Funazkri (1978) correlation appears to be quite accurate... 

Tables 10-16, 10-17, 10-18, and 10-18A give general estimating overall coefficients, and Table 10-19 gives the range of a few common film coefficients. Table 10-20 illustrates the effect of tube-wall resistance for some special construction materials. Table 10-20A lists estimating coefficients for glass-lined vessels. Also see Reference 215. See Table 10-24 for suggested water rates inside tubes.

Figure 10-118 a from Equation 10-184. d. Nucleate boiling coefficient, h, from Table 10-30 or other source. (An estimate of the film temperature drop is required.)... 

There are several correlations for estimating the film mass transfer coefficient, kf, in a batch system. In this work, we estimated kf from the initial concentration decay curve when the diffusion resistance does not prevail [3]. The value of kf obtained firom the initial concentration decay curve is given in Table 2. In this study, the pore diffusion coefficient. Dp, and surface diffusion coefficient, are estimated by pore diffusion model (PDM) and surface diffusion model (SDM) [4], The estimated values of kf. Dp, and A for the phenoxyacetic acids are listed in Table 2. 

Below a Reynolds number of about 2000 the flow in pipes will be laminar. Providing the natural convection effects are small, which will normally be so in forced convection, the following equation can be used to estimate the film heat-transfer coefficient ... 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

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