Film coefficient outside

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

Figure 10-67A. Condensing film coefficients outside horizontal or vertical tubes. (Used by permission Kern, D.Q. Process Heat Transfer, Ed., 1950. McGraw-Hill, Inc. All rights reserved.)...

A, Film coefficient outside bundle, Btu/ft -hr-°F 0 Inside diameter of shell,in. 

In atmospheric air-cooled finned tube exchangers, the air-film coefficient from Eq. (5-64) is sometimes converted to a value based on outside bare surface as follows ... 

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 ... 

Of these special surfaces, only the double-fluted tube has seen extended services. Most of the gain in heat-transfer coefficient is due to the condensing side the flutes tend to collect the condensate and leave the lauds bare [Caruavos, Proc. First Int. Symp. Water Desalination, 2, 205 (1965)]. The coudeusiug-film coefficient (based on the actual outside area, which is 28 percent greater than the nominal area) may be approximated from the equation... 

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. 

In heat transfer applications, this jacket is considered a helical coil if certain factors are used for calculating outside film coefficients. The equivalent heat transfer diameter, D, for a rectangular cross-section is equal to 4 w (w being the width of the annular space). Velocities are calculated from the actual cross-section of the flow area, pw (p being die pitch of die spiral baffle), and die effective mass flowrate W dirough die passage. The effective mass flowrate is approximately 60% of die total mass flowrate of die jacket. 

At > 10,000 the Sieder-Tate equation for straight pipe, 1 -F 3.5(Dg/Dg) can be used to calculate the outside film coefficient. 

Equations for calculating outside film coefficients (ho) of jacketed agitated vessels... 

Consider a single-zone jacket where there is an increase in the jacket flow, and a corresponding increase in the outside film coefficient because hj =f(Njjg, G). Therefore, a two-fold increase in the jacket flow results in an increase in hjj by 2 h . The overall heat transfer coefficient U = l/[FpoL + 1/hj], and a larger outside coefficient subsequently increases the overall heat transfer coefficient. The overall heat flux will increase due to the combined effects of the increased flow and lower jacket outlet temperature. The net result is an increase in the pressure drop. 

Outside Film Coefficient (Shell-and-Tube Exchangers)... 

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 ... 

WTien heat transfer occurs in the presence of suspended solid particles, the chemical reacdon may require mass transfer, etc. However, the mixing part of the operation must be designed to accomplish the process results, which may be those named above (and others) plus the required heat transfer affecting the outside film coefficient, hg, across/around any coils, flat surfaces, or wall jackets. 

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... 

Order of Magnitude of Outside Film Coefficients, h Turbulent Range... 

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. 

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... 

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). 

Film coefficients for turbulent flow that exist on the outside or shell side of the conventional baffled shell and tube exchanger are correlated for hydrocarbons, organic compounds, water, aqueous solutions, and gases by... 

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