Inside film coefficient

Film coefficient outside tube, h = 175 Film coefficient inside tube, hj = 600... 

For turbulent heat transfer in long pipes, the individual film coefficient inside a pipe can be... 

Epq = modulus of elasticity of composite section, psi hi, ho = film coefficients, inside or outside, Btu/ft -hr/°F P = internal pressure, psig Q = heat loss through wall, Btu/ft -hr T, = temperature, outside ambient, °F T(, = temperature, outside ambient during construction, °F... 

Coils For flow inside helical coils, Reynolds number above 10,000, multiply the value of the film coefficient obtained from the apphcable equation for straight tubes by the term (1 + 3..5 D /DJ. 

For the general case, the treatment suggested by Kern (Pmcc.s.s Heat Transfer, McGraw-Hill, New York, 1950, p. 512) is recommended. Because of the wide variations in fin-tube construction, it is convenient to convert all film coefficients to values based on the inside bare surface of the tube. Thus to convert the film coefficient based on outside area (finned side) to a value based on inside area Kern gives the following relationship ... 

The film coefficient h is for the inner wall Dj is the inside diameter of the mixing vessel. The term L N p/ is the Reynolds number for mixing in which L is the diameter and Nr the speed of the agitator. Recommended values of the constants a, b, andm are given in Table 18-2. 

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. 

With tube side condensation, coefficients are generally lower than for comparable shell side condensers. This phenomenon is attributed to (1) lower coolant velocities outside the tubes than are possible with tube side cooling, and (2) increased film thicknesses, namely, film resistance inside the tubes. 

Equations for calculating inside film coefficients (hj) of jacketed agitated vessels... 

The inside film coefficient represents the resistance to heat flow caused by the change in flow regime from turbulent flow in the center of the tube to laminar flow at the tube surface. The inside film coefficient can be calculated from ... 

Although the outside coefficient of a vertical coil is some 13% higher than for a helical coil, the inside coefficient is quite often lower due to the physical arrangement and the lower coefficient if gases are evolved and venting is required. The over-all coefficient may end up about the same as the helical coil. The outside film coefficient for a system varies with in the turbulent region. Thus... 

The heat transfer area, A ft, in an exchanger is usually estahlished as the outside surface of all the plain or hare tubes or the total finned surface on the outside of all the finned tubes in the tube bundle. As will be illustrated later, factors that inherendy are a part of the inside of the tube (such as the inside scale, transfer film coefficient, etc.) are often corrected for convenience to equivalent outside conditions to be consistent. When not stated, transfer area in conventional shell and tube heat exchangers is considered as outside tube area. 

For example, a hot flue gas flows outside a tube and shell exchanger at 900°F (C) while a hot liquid is flowing into the tubes at 325°F (k). The film coefficients have been estimated to be hj = 225°F and h = 16 Btu/(hr) (fti) (°F). Estimate the tube wall temperature using hj as hj corrected to the outside surface for the inside coefficient ... 

Film coefficients defined on an inside tube surface area basis when converted to the larger outside surface area become... 

This value is then used to represent the film coefficient equivalent to the converted inside coefficient, as hj. ... 

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.

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

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

D. For water, the inside film coefficient is represented by Figure 10-50A. Furman presents charts that reduce the expected deviation of the film coefficient from the 20% of Figure 10-50A, 10-50B, 10-50C, and 10-50D. 

Figure 10-50B. Heat transfer film coefficient for water flowing inside 1 in. X 18BWG tubes referred to outside tube surface area for plain tubes. Note the corrections for tubes of wall gauges other than 18 BWG. (Used by permission J. B. Co., Inc., Western Supply Div., Tulsa, Okla.)...

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.)...

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. ... 

Figure 10-103. Kern correlation for natural circulation boiling and sensible film coefficients—outside and inside tubes. (Used by permission Kern, D.Q. Process Heat Transfer, Ed., 1950. McGraw-Hill Book Company. All rights reserved.)...

For organic liquids, evaluate the natural convection film coefficient from Figure 10-103. Equation 10-29 may be used for the inside horizontal tube by multiplying the right side of the equation by 2.25 (1 + 0.010 Gr,i/")/logRe. 

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