Film coefficients, determination

Example Buckingham Pi Method—Heat-Transfer Film Coefficient It is desired to determine a complete set of dimensionless groups with which to correlate experimental data on the film coefficient of heat transfer between the walls of a straight conduit with circular cross section and a fluid flowing in that conduit. The variables and the dimensional constant believed to be involved and their dimensions in the engineering system are given below ... 

The physical properties of the liquid, rather than those of the vapor, are used For determining the film coefficient for condensation. Nus-selt [2. Ver. Dt.sch. Ing., 60, 541, 569 (1916)] derived theoretical relationships for predicting the film coefficient of heat transfer for condensation of a pure saturated vapor. A number of simplifying assumptions were used in the derivation. 

FIG. 5-9 Chart for determining film coefficient for film-type condensation of pure vapor, based on Eqs. 5-88 and 5-93. For vertical tubes multiply by 1.2. If 4F/ J exceeds 2100, use Fig. 5-10. is in U.S. customary units to convert feet to... 

The proportionality constant, h, is influenced by the nature of the fluid and the nature of the agitation and is determined experimentally. If agila-tion does not exist, h is only influenced by the nature of the fluid and is called the film coefficient. 

Determine from experimental data the Reynolds number exponent for the s) stem, or use information in Figure 5-40 if the systems of Table 5-8 can be considered similar, use proper coefficients and solve for outside film coefficient, hg. 

Often, a reasonable and convenient way to understand the heat transfer process in a heat exchanger unit is to break down the types of heat transfer that must occur such as, vapor subcooling to dew point, condensation, and liquid subcooling. Each of these demands heat transfer of a different type, using different AT values, film coefficients, and fouling factors. This is illustrated in Figure 10-36. It is possible to properly determine a weighted overall temperature... 

An exchanger has heen examined, and the following individual coefficients and resistances determined. What is the overall coefficient of heat transfer referenced to outside coefficients (Methods for determining these film coefficients are given later). 

Figure 10-45. Chart for determining U-clean from tube-side and shell-side fluid film coefficients no fouling included. Note s = shell side, t = tube-side. (Used by permission (q...

Determine tube-side film coefficient, hj, based on inside tube surface (Figure 10-48). 

Determine the tube-side film coefficient for water, using Figure 10-50A or 10-50B. For other liquids and gases, use Figure 10-46. Correct hj to the outside tube surface by... 

Determine the shell-side film coefficient for an assumed bafHe spacing. 

Determine the inside film coefficient by methods previously oudined for convection. 

This unit consists of two pipes or tubes, the smaller centered inside the larger as shown in Figure 10-92. One fluid flows in the annulus between the tubes the other flows inside the smaller tube. The heat transfer surface is considered as the outside surface of the inner pipe. The fluid film coefficient for the fluid inside the inner tube is determined the same as for any straight tube using Figures 10-46-10-52 or by the applicable relations correcting to the O.D. of the inner tube. For the fluid in the annulus, the same relations apply (Equation 10-47), except that the diameter, D, must be the equivalent diameter, D,.. The value of h obtained is applicable directly to the point desired — that is, the outer surface of the inner tube. ... 

To calculate the outside film coefficient, you need to know the difference in temperature of the condensing vapor (T, ) and the pipe wall temperature (L). The pipe wall temperature is determined hy trial-and-error calculations using the following equation/ ... 

Determine the tube-side film coefficient for convection or condensation as required, by methods previously described. 

Due to the development of the data, the method requires the use of a single tube boiling film coefficient. Using this to reach the overall bundle transfer The overall U value is determined for a theoretical boiling coefficient of an unfouled tube (single) (this is an iterative procedure). See reference 90 also. 

Determine outside film coefficient for spray or drip cooling using the equation of McAdams as presented hy Kem. ... 

Determine the inside film coefficient using Equation 10-41 and Figure 10-46 for tube-side heat transfer. If two or more coils are in parallel, be certain that the flow rate per pipe is used in determining hj. Correct hj to outside of tube, giving hjo. Note that Figure 10-46 also applies to cast iron cooling sections. 

Calculate the tube-side film coefficient for finned tube, hj. If water, use Figure 10-50A or 10-50B if other fluid, use Equation 10-44 or 10-47. Use an assumed or process determined tube-side velocity or other film fixing characteristic. 

Figure 10-153A. For determination of h , shell-side (finned side) film coefficient h K for longitudinal fins, flow laminar. h must be...

After determining the h from the preceding figures, the film coefficient must he corrected for fm efficiency using Figure 10-154. 

The mass transfer equations, Equations (ll.l)-(ll.lO), remain valid when A, replaces A,. Equations (11.27) and (11.28) contain one independent variable, 2, and two dependent variables, ai and Ug. There are also two auxiliary variables, the interfacial compositions a and a. They can be determined using Equations (11.5) and (11.6) (with A, replacing A). The general case regards K/f in Equation (11.4) as a function of composition. When Henry s law applies throughout the composition range, overall coefficients can be used instead of the individual film coefficients. This allows immediate elimination of the interface compositions ... 

To determine the condensing film coefficient using Equation 15.83 requires the number of tubes to be known. 

For non-Newtonian liquids and suspensions, an apparent viscosity is determined using correlations which include power input and the Reynolds number. Scale-up comparisons based on heat generation data only were determined by comparison of results from RC1 experiments and from a 675-liter reactor [208]. In the experiments, a Bingham plastic fluid was used to determine the film heat transfer coefficient. This presents a worst case because of the low thermal conductivity of the Bingham plastic. Calculated inside film heat transfer coefficients determined in the RC1 tests were about 60% lower than the values determined in the pilot plant reactor, even though substantial effort was made to obtain both geometric and kinematic similarity in the pilot reactor. 

The gas film coefficient is dependent on turbulence in the boundary layer over the water body. Table 4.1 provides Schmidt and Prandtl numbers for air and water. In water, Schmidt and Prandtl numbers on the order of 1,000 and 10, respectively, results in the entire concentration boundary layer being inside of the laminar sublayer of the momentum boundary layer. In air, both the Schmidt and Prandtl numbers are on the order of 1. This means that the analogy between momentum, heat, and mass transport is more precise for air than for water, and the techniques apphed to determine momentum transport away from an interface may be more applicable to heat and mass transport in air than they are to the liquid side of the interface. 

We also need to develop the theories for hquid film coefficient to use in the aforementioned equations. For drops that are close to spherical, without separation, Levich (1962) assumed that the concentration boundary layer developed as the bubble interface moved from the top to the bottom of a spherical bubble. Then, it is possible to use the concepts applied in Section 8.C and some relations for the streamlines around a bubble to determine Kl. ... 

The influence of wind is predominant in determining the liquid film coefficient for lakes, reservoirs, oceans, and many estuaries. Wind creates a shear on the water surface and generates turbulence below and on the water surface. Thus, this section deals with the measurement and prediction of the wind influence on liquid film coefficient. 

Dual Tracer Technique. The dual tracer measurement technique utilizes two gas tracers with diffusion coefficients that are substantially different, such as He and SFe. This technique can also be utilized with one volatile (gas) tracer and one nonvolatile tracer. We will derive the relevant equations to determine hquid film coefficient from the diffusion equation for both cases, beginning with the two gas tracers. 

The evaporation of water is generally used to determine the gas film coefficient. A loss of heat in the water body can also be related to the gas film coefficient because the process of evaporation requires a significant amount of heat, and heat transfer across the water surface is analogous to evaporation if other sources and sinks of heat are taken into account. Although the techniques of Section 8.D can be used to determine the gas film coefficient over water bodies, they are still iterative, location specific, and dependent on fetch or wind duration. For that reason, investigators have developed empirical relationships to characterize gas film coefficient from field measurements of evaporation or temperature. Then, the air-water transfer of a nonvolatile compound is given as... 

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