Film outside tubes

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. 

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

Film Coeffidents with Fluids Outside Tubes Forced Convection... 

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.)...

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... 

In horizontal condensers (outside tubes), for N tubes in a vertical row, with the condensate flowing uniformly from one tube to the one below without extensive splashing, the mean condensing coefficient, h j, for the entire row of N tubes (per Knudsen in reference 94A) is related to a film coefficient for the top, h, single tube by ... 

The area concerned with the subcooling only can be evaluated using a film coefficient calculated from Figure 10-54 for liquids outside tubes. This assumes that the liquid being cooled is held in the area around the tube by a level control or pipe seal, allowing drainage at the rate it builds up and covering a portion of the tubes. 

Calculate the shell-side dry-gas film coefficient, hg or h, for outside tube conditions. Assume a baffle spacing or about equal to one shell diameter. Use the shell-side method described in Equation 10-48 and Figure 10-54. This is necessary for inlet conditions and then must be checked and recalculated if sufficient change occurs in the mass flow rate, G, to yield a change in hg. 

Nucleate boiling is boiling at the tube surfece at a temperature difference between outside tube surface temperature and the fluid body, less than the critical temperature difference. At and beyond the critical temperature difference, metastable and film boiling take place. These produce lower transfer coefficients as the temperature difference increases. 

Film coefficients of mass transfer inside or outside tubes are important in membrane processes using tube-type or the so-called hollow fiber membranes. In the case where flow inside the tubes is turbulent, the dimensionless Equation 6.25a, b (analogous to Equation 5.8a, b for heat transfer) provide the film coefficients of mass transfer k [7]... 

In the case where the fluid flow outside tubes is normal or oblique to a tube bundle, approximate values of the film coefficient of mass transfer can be estimated by using Equation 6.27a [7], which is analogous to Equation 5.12a ... 

Kirkbride (K17), 1934 Flow of water and 4 oils outside tubes, JVro = 0.04-2000. Film thicknesses (maximum wave heights) measured by micrometer. Wavy flow is described, and corrections to Nusselt theory derived for heat transfer in laminar wavy film flow. 

Brauer (B14), 1956 Extensive experimental study of film flow outside tube 4.3X130 cm. films of water, water + surfactant, aqueous diethylene glycol solutions, kinematic viscosity 0.9-12.7 cs. Nr = 20-1800. Data on film thicknesses, waves, maximum and minimum thicknesses, characteristic Reynolds numbers of flow, onset of rippling and turbulence, wall shear stress, etc. 

Belkin et al. (B4), 1959 Experimental studies of water films flowing outside tubes observations of film thicknesses, onset of rippling. Nr, = 50-7500 (mostly turbulent zone). 

Extensive survey of flow and heat transfer in liquid films flowing outside tubes. Measurements of temperature and velocity profiles in films of various liquids are reported, and a heat transfer mechanism is proposed. 

The most outstanding of these equations that I have used and carried around with me are Eqs. (5.2) and (5.3). These equations actually determine the tube wall heat-transfer film resistance. This is the resistance of significance to any heat-transfer event in a tube. If there were no resistance to heat flow, all materials in contact would immediately reach an equilibrium temperature. We all know this is not the case for instance, when you pick up (without a rag or gloves) a hot object—such as a hot skillet cooking a porterhouse steak—the small amount of moisture on your skin serves to keep your fingers from being burned severely before you throw the pan down. This holds true for the subject at hand also, as there is a distinct film on both the inside and outside of the tube wall that resists the flow of heat transfer. This film reaches a steady state of resistance to heat flow. Several factors play a role in this film resistance, called hi for the inside tube film and h0 for the outside tube wall film. 

Now consider the outside of the tube, where the flowing fluid is on the shell side of the exchanger. For the outside tube wall, the film offering heat-flow resistance is denoted h0, which has the same units as h, (Btu/h ft2 °F). The following equation is also from Kern [1],... 

Equations (5.2) and (5.3) derive ht and h0 tube wall film heat flow resistances. Next a common tube wall film resistance is established. The convention here is to let the outside tube wall be the basis. Therefore the inside tube wall ht is to be converted to the outside tube wall basis and designated hio. This is a simple multiplication ... 

If the recommended factor of 0.7 on the refractory area is used, the effective area of the tubes is [22.0-1- (0.7)(3.18)]/22.0= 1.10 mVm of actual area. The exact evaluation of the outside tube temperature from the known oil temperature would involve a knowledge of the oil-film coefficient, tube-wall resistance, and rate of heat flow into the tube, the evaluation usually involving trial and error. However, for the present purpose the temperature drop through the tube wall and oil film will be assumed to be 41.7°C ( 5°F), making the tube surface temperatures 357°C (675°F) and 468°C (875°F) the average is 412°C (775°F). The radiating gas temperature is... 

Recently, Hedrick [12] developed a new correlation for the heat transfer coefficient in the transition region between laminar and turbulent flow. The equations for determining the inside film coefficient and based on the outside tube diameter, hj , are ... 

Estimate the overall heat transfer coefficient U, based on the inside tube surface area, of a shell-and-tube-type vapor condenser, in which cooling water at 25 °C flows through stainless steel tubes, 25 mm i.d. and 30 mm o.d., at a velocity of 1.2 m s 1. It can be assumed that the film coefficient of condensing vapor at the outside tube surface is 2000kcalh m 2 C, and that the fouling factor of the water side is 5000 kcal h-1m-2oC 1. 

Individual heat-transfer coefficient, W/m - C or Btu/ft -h-°F h, for outside of coil h for inside of tube hj, for inner wall of jacket h , for outside of tube h of gas film near tube wall of packed... 

The relationships developed for these two cases are different and will be discussed below. Equipment using falling film heat transfer can be classified into vertical and horizontal systems. The vertical systems can include falling films on the inside or outside of tubes, or alternatively (in plate-type evaporators) on vertical flat plates. Generally, the liquid films are sufficiently thin to be treated as equivalent to the flat plate case for all of these configurations. Another important case is that of falling films on tube banks, as illustrated in Fig. 15.141 the... 

Film-type tube apparatus) Multitube apparatus with an absorbent falling-film on the tube insides. Cocurrent and countercurrent flow of gas phase and falling-film (both phases are coherent). Simple design with the possibility of heat removal by a cooling agent outside the tube shell. Low gas pressure drop, small interfacial area, small liquid mass transfer coefficient. 

The thermal conductivity of scale such as CaCOs is approximately 0.03 W cm K and that of CaS04 is around 0.003 W cm K . Producticai of steam at 600 psi (40 atm) in a 4-cm OD tube of S A 210 carbon steel 3.4 mm thick (K = 0.41 W/cm K) that requires a heat flux of 40 W/m has a temperature gradient across the steel tube (32 K) and a water boundary film of about 2 mm (40 K). Water temperature is approximately 250°C, and outside-tube temperature is around 325°C, both well below the safe limit of 525°C. 

An incompressible fluid flows upward through a small circular tube and then downward on the outside of the tube in a falling film. The tube radius is R. Determine ... 

In cases of combined heat transfer for a heat exchanger, there are two values for h. There is the convective heat transfer coefficient h for the fluid film inside the tubes and a convective heat transfer coefficient hg for the fluid film outside the tubes. The thermal conductivity, k, and thickness. Ax, of the tube wall must also be accounted for. An additional term Uo, called the overall heat transfer coefficient, must be used instead. It is common practice to relate the total rate of heat transfer, Q to the cross-sectional area for heat transfer and the overall heat transfer coefficient Uq. The relationship of the overall heat transfer coefficient to the individual conduction and convection terms is shown in Figure 6.5. 

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