Water film coefficient

Summer area required (not making any correction for change in water film coefficient or condensing coefficient) ... 

Note An optimum design of this type can be used as a guide for the final selection of an exchanger. However, practical factors of operation must also be considered. In this case, a large number of baffles are required to give the optimum conditions. Because the water-film coefficient is large relative to the air-film coefficient, a reduction in the number of baffles would have little effect on the optimum design. 

The tubes were 0.902 in. ID and 1.000 in. OD, made of admiralty metal, for which /c = 63 Btu/ft-h- F. From these data calculate (a) the steam-film coefficient (based on steam-side area), (6) the water-film coefficient when the water velocity is I ft/s (based on water-side area), (c) value of for the scale in the fouled tube, assuming the clean tube was free of deposit. 

In general, improvements sought were increased water film coefficient, promotion of dropwise steam condensation, and increased boiling heat flux at the same differential temperature and reduced scale deposition. 

F. L. Kallam has studied the film-transfer rate on the outside of tubes that are being cooled with water when the water drips or is sprayed on the tubes. He recommends the following equation for the water film coefficient for the outride of tubes ... 

The coefficient of friction between two unlubricated solids is generally in the range of 0.5-1.0, and it has therefore been a matter of considerable interest that very low values, around 0.03, pertain to objects sliding on ice or snow. The first explanation, proposed by Reynolds in 1901, was that the local pressure caused melting, so that a thin film of water was present. Qualitatively, this explanation is supported by the observation that the coefficient of friction rises rapidly as the remperarure falls, especially below about -10°C, if the sliding speed is small. Moreover, there is little doubt that formation of a water film is actually involved [3,4]. 

Film coefficients for the boiling of liquids other than water have been investigated. Coulson and McNelly [Trans. In.st. Chem. Eng., 34, 247 (1956)] derived the following relation, which also correlated the data of Badger and coworkers [Chem. Metall. Eng., 46, 640 (1939) Chem. Eng., 61(2), 183 (1954) and Trans. Am. Jnst. Chem. Eng., 33, 392 (1937) 35, 17 (1939) 36, 759 (1940)] on water ... 

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... 

The boiling-film coefficient is about 30 percent lower for pure water than it is for salt water or seawater. There is as yet no accepted explanation for the superior performance in salt water. This phenomenon is also seen in evaporation from smooth tubes. 

Tube-side water velocities should be kept within reasonable limits, even though calculations would indicate that improved tube-side film coefficients can be obtained if the water velocity is increased. Table 10-24 suggests guidelines that recognize the possible effects of erosion and corrosion on the system. 

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.

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. 

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... 

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... 

For the film coefficient, tube side, use a mean water temperature of 85°F instead of 95°F. This will lean a little more to the winter operation and be safe for summer, h = 1,400... 

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. 

Tube-side film coefficient. Assume minimum water velocity of 5 ft/sec, using 1-in. X 14 BWG tubes. 

Figure 26.4 illustrates the effect of velocity for water at 16°C on heat transfer coefficients. This graph also plots pressure drop against velocity under the same conditions. The film coefficients are very high and can be obtained for a moderate pressure drop. 

Figure 9.21. Film coefficients of convection for flow of water through a tube at 289 K...

In such a vessel, the thermal resistances to heat transfer arise from the water film on the inside of the coil, the wall of the tube, the film on the outside of the coil, and any scale that may be present on either surface. The overall transfer coefficient may be expressed by ... 

From equations 9.202 and 9.203, the inside film coefficient for the water is given by ... 

The film coefficients for the water jacket were in the range 635-1170 W/nr K for water rates of l. 44—9.23 1/s, respectively. It may be noted that 7.58 1/s corresponds to a vertical velocity of only 0.061 m/s and to a Reynolds number in the annulus of 5350. The thermal resistance of the wall of the pan was important, since with the sulphonator it accounted for 13 per cent of the total resistance at 32.3 K and 31 per cent at 403 K. The change in viscosity with temperature is important when considering these processes, since, for example, the viscosity of the sulphonation liquors ranged from 340 mN s/rn2 al 323 K to 22 mN s/m2 at 40.3 K. 

For purposes of standardisation, 19 mm outer diameter tubes of 1.65 mm wall thickness will be used, and these may be 2.5, 3-6, or 5 m in length. The film coefficient for condensing pentane on the outside of a horizontal tube bundle may be taken as 1.1 kW/m2 K, The condensation is effected by pumping water through the tubes, the initial water temperature being 288 K. 

In a countercurrent-flow heat exchanger, 1.25 kg/s of benzene (specific heat 1.9 kJ/kg K and specific gravity 0.88) is to be cooled front 350 to 300 K with water at 290 K. In the heat exchanger, tubes of 25 mm external and 22 mm internal diameter are employed and the water passes through the tubes. If the film coefficients for the water and benzene are 0.85 and 1.70 kW/nr K respectively and the scale resistance can be neglected, what total length of tube will be required if the minimum quantity of water is to be used and its temperature is not to be allowed to rise above 320 K ... 

Water at 293 K is heated by passing through a 6.1 m coil of 25 mm internal diameter pipe. The thermal conductivity of the pipe wall is 20 W/m K and the wall thickness is 3.2 mm. The coil is heated by condensing steam at 373 K for which the film coefficient is 8 kW/m2 K. When the water velocity in the pipe is I tn/s, ils outlet temperature is 309 K. What will the outlet temperature be if the velocity is increased to 1.3 m/s, if the coefficient of heat transfer to the water in the tube is proportional to the velocity raised to the 0.8 power ... 

It is known that the overall liquid transfer coefficient K a for absorption of S02 in water in a column is 0.003 kmol/sm3 (kmol/m3). Obtain an expression for the overall liquid-film coefficient KLa for absorption of NH3 in water in the same equipment using the same water and gas rates. The diffusivities of S02 and NH3 in air at 273 K are 0.103 and 0.170 cm2/s. S02 dissolves in water, and Henry s constant /7 is equal to 50 (kN/m2)/(kmol/m3). All the data are expressed for the same temperature. 

Figure 12.7. Variation of liquid-film coefficient with liquid flow for the absorption of oxygen in water...

Rathbun RE, Tai DY. 1986. Gas-film coefficients for the volatilization of ethylene dibromide from water. Environ Sci Technol 20 949-952. 

Platford. R.F., Carey, J.H., and Hale, E.J. The environmental significance of surface films Part 1. Octanol-water partition coefficients for DDT and hexachlorobenzene. Environ. PoHut. (SeriesB), 3(2) 125-128, 1982. 

Colloidal liquid aphrons (CLAs), obtained by diluting a polyaphron phase, are postulated to consist of a solvent droplet encapsulated in a thin aqueous film ( soapy-shell ), a structure that is stabilized by the presence of a mixture of nonionic and ionic surfactants [57]. Since Sebba s original reports on biliquid foams [58] and subsequently minute oil droplets encapsulated in a water film [59], these structures have been investigated for use in predispersed solvent extraction (PDSE) processes. Because of a favorable partition coefficient for nonpolar solutes between the oil core of the CLA and a dilute aqueous solution, aphrons have been successfully applied to the extraction of antibiotics [60] and organic pollutants such as dichlorobenzene [61] and 3,4-dichloroaniline [62]. 

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