Fluid-Film Coefficients

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...

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. ... 

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 ... 

Good heat transfer on the outside of the reactor tube is essential but not sufficient because the heat transfer is limited at low flow rates at the inside film coefficient in the reacting stream. The same holds between catalyst particles and the streaming fluid, as in the case between the fluid and inside tube wall. This is why these reactors frequently exhibit ignition-extinction phenomena and non-reproducibility of results. Laboratory research workers untrained in the field of reactor thermal stability usually observe that the rate is not a continuous function of the temperature, as the Arrhenius relationship predicts, but that a definite minimum temperature is required to start the reaction. This is not a property of the reaction but a characteristic of the given system consisting of a reaction and a particular reactor. 

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. 

For sheU-aiKl-l-ube heat exchangers with shell-side balile. die shell-side fluid flow is perpendicular to the tubes. In this arrangement, the outside film coefficient can be calculated from the following equation ... 

Effects of viscosity on Process Fluid Heat Transfer Film Coefficient... 

Ug = mean mixer side film coefficient of tank temperature k = thermal conductivity of fluid, Btu/hr/sq ft/°F/ft d = d, = tube OD, ft T = tank diameter, ft... 

Najjar, Bell, and Maddox studied the influence of physical property data on calculated heat transfer film coefficients and concluded that accurate fluid property data is extremely important when calculating heat transfer coefficients using the relationships offered by Dittus-Boelter, Sieder-Tate, and Petukhov. Therefore, the designer must strive to arrive at good consistent physical/thermal property data for these calculations. 

Sometimes one of the fluid-side scale resistances can be neglected or assumed to be so small as to be of little value, in which case only the significant resistances and/or film coefficients need to be used in arriving at the overall coefficient, U. Note that Aq, Aj, and can be substituted by d , dit, and d g respectively. Theoretically, d g and should be the logarithmic average, but for most practical cases, the use of the arithmetic average is completely satisfactory. 

Film Coefficients with Fluid Inside Tubes, Forced Convection... 

Figure 10-50C. Tube-side (inside tubes) liquid film heat transfer coefficient for Dowtherm . A fluid inside pipes/tubes, turbulent flow only. Note h= average film coefficient, Btu/hr-ft -°F d = inside tube diameter, in. G = mass velocity, Ib/sec/ft v = fluid velocity, ft/sec k = thermal conductivity, Btu/hr (ft )(°F/ft) n, = viscosity, lb/(hr)(ft) Cp = specific heat, Btu/(lb)(°F). (Used by permission Engineering Manual for Dowtherm Heat Transfer Fluids, 1991. The Dow Chemical Co.)...

Figure 10-51. Convection inside film coefficient for gases and low viscosity fluids inside tubes—heating and cooling. (Used by permission McAdams, W. H. Heat Transmission, 2"= Ed., 1942. McGraw-Hill, Inc. All rights reserved.)...

Calculate the film coefficient for fluid on side of tube opposite from the one associated with the boiling or vaporizing operation. 

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. 

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