Tube wall thickness

Effect of Uncertainties in Thermal Design Parameters. The parameters that are used ia the basic siting calculations of a heat exchanger iaclude heat-transfer coefficients tube dimensions, eg, tube diameter and wall thickness and physical properties, eg, thermal conductivity, density, viscosity, and specific heat. Nominal or mean values of these parameters are used ia the basic siting calculations. In reaUty, there are uncertainties ia these nominal values. For example, heat-transfer correlations from which one computes convective heat-transfer coefficients have data spreads around the mean values. Because heat-transfer tubes caimot be produced ia precise dimensions, tube wall thickness varies over a range of the mean value. In addition, the thermal conductivity of tube wall material cannot be measured exactiy, a dding to the uncertainty ia the design and performance calculations. 

The values of CJs are experimentally determined for all uncertain parameters. The larger the value of O, the larger the data spread, and the greater the level of uncertainty. This effect of data spread must be incorporated into the design of a heat exchanger. For example, consider the convective heat-transfer coefficient, where the probabiUty of the tme value of h falling below the mean value h is of concern. Or consider the effect of tube wall thickness, /, where a value of /greater than the mean value /is of concern. 

Fabrication Expanding the tube into the tube sheet reduces the tube wall thickness and work-hardens the metal. The induced stresses can lead to stress corrosion. Differential expansion between tubes and shell in fixed-tube-sheet exchangers can develop stresses, which lead to stress corrosion. 

Figure 13.5 Plug-type dezincification on the internal surface of a brass condenser tube. Note the extreme porosity of the copper plugs. Tube wall thickness was 0.040 in. (0.10 cm). Compare to Fig. 13.13. (Courtesy of National Association of Corrosion Engineers, Corrosion 89 Paper No. 197 by H. M. Herro.)...

Microstructural examinations revealed V-shaped openings along the tube seam, some extending into as much as 50% of the tube wall thickness. The incompletely closed seam provided a crevice in which differential concentration cells developed (see Chap. 2, Crevice Corrosion ). The resulting localized corrosion caused the observed pits. 

Such defects result from abnormal manufacturing operations such as insufficient lubrication between the metal and the mandrel during the tube-forming process. The lubricant may have been contaminated. Measurement indicated that some of these defects penetrated 8% of the tube wall thickness. Defects of this type can act as corrosion-initiation sites in a sufficiently aggressive environment. 

Fig. 16. Effect of tube-wall thickness on the film flow rate [from Hewitt el al. (H6)]. Based on tests with water in uniformly heated stainless steel tubes, d — 0.366 in., P = 55 psia, G = 0.219 x 106 lb/hr-ft2.

If these are packed in a square configuration with a negligible tube wall thickness, each tube occupies an area 1 cm in the tube bundle. The cross-sectional area of the tube bundle is 590 cm so the diameter of the column is... 

Fig. 157. Minimum charge diameter for stable detonation in iron tubes (wall thickness 3 mm). Initiation by No. 8 detonator and 27 g compressed TNT 5% water f — detonation, —miss 0% water H------------------------detonation, o—miss [71].

The specific heat for water is 4.2 kJ kg-1 K 1 and that of the process fluid is 3.4 kJ kg-1 K 1. The process fluid side film heat transfer coefficient is 2500 W m 2 K 1 and the cooling water side heat transfer coefficient is 1200 W raT2 K 1. The tube wall thickness is 3 mm and the thermal conductivity is 220 W m 1 K 1. 

Tube-wall thickness is usually specified by the Birmingham wire gauge, and variations from the nominal thickness may be + 10 percent for average-wall tubes and + 22 percent for minimum-wall tubes. Pressure, temperature, corrosion, and allowances for expanding the individual tubes into the tube sheets must be taken into consideration when the thickness is determined. 

A horizontal heat exchanger has seven steel tubes enclosed in a shell having an ID of 5.0 in. The OD of the tubes is 1.0 in., and the tube wall thickness is 0.10 in. Pure ethyl alcohol flows through the l.O-in.-OD tubes. The ethyl alcohol enters the unit at 150°F and leaves at 100°F. Water at 70°F enters the shell side of the unit and flows countercurrent to the ethyl alcohol. It is necessary to cool 50,000 lb of ethyl alcohol... 

The second term in the denominator of Equation 4.11 represents the convective resistance to heat transfer caused by the inside fluid film on the scale surface. The third term is the conductive resistance caused by the tube wall, which is usually small, because the thermal conductivity of many metals is large. We will neglect the conductive resistance to heat transfer, unless the thermal conductivity is very small and tube wall thickness large. The fourth term is the convective resistance to heat transfer of the outside fluid film on the scale surface. After substituting Equations 4.13 and 4.14 into Equation 4.11,... 

This team of men can produce tubing and rod of any desired dimensions and can maintain the tube wall-thickness within remarkably small tolerances. It is interesting to note that 3 mm diameter tube is about 250 ft (75 m) long when fully drawn out. (Fig. 1.1.)... 

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