Bare exchange

As already mentioned by Sham and Schliiter, from eq.(2.22) v in LDA can easily be obtained. It is also straightforward to obtain the local in the HF approximation to this only the bare exchange E vg (in this case g g ) is kept and g replaced by g in eq (2.22 . This local exchange potential is equivalegg to an e fective local potential derived by Talman and Shadwick from a minimization procedure. For atoms, such as carbon, neon and aluminium they demonstrated, that this local exchange potential results in total energies, which exceed the HF total energies by less than 0.005%. 

Here, the first and second terms on the r.h.s. are again the bare exchange and correlation contribution to. The Si and C num-... 

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 ... 

TABLE 11-5 Overall Coefficients for Air-Cooled Exchangers on Bare-Tube Basis... 

Typical Transfer Coefficients For Air-Cooied Exchangers Based on Outside Bare Tube Surface... 

The price of air-cooled exchangers should be obtained from vendors if possible. If not, then by coirelating in-house historical data on a basis of /ft of bare surface vs. total bare surface. Correction factors for materials of construction. pressure, numbers of tube rows, and tube length must be used. Literature data on air coolers is available (Reference 15). but it should be the last resort. In any event, at least one air-cooled heat exchanger in each project should be priced by a vendor to calibrate the historical data to reflect the supply and demand situation at the expected time of procurement. 

The procedure for calculating the number of tubes required for an aerial cooler is similar to that for a shell- and-tube exchanger. Table 3-6 shows approximate overall heat transfer coefficients. Ub should be used when the outside surface area of the bare tube (neglecting fins) is used in the heat... 

The two basic types of tubes are (a) plain or bare and (b) finned—external or internal, see Figures 10-4A-E, 10-10, and 10-11. The plain tube is used in the usual heat exchange application. However, the advantages of the more common externally finned tube are becoming better identified. These tubes are performing exceptionally well in applications in which their best features can be used. 

Figure 10-9C. Coil Assembly for bare tube Heliflow exchanger. Tube sizes range from V4 -V4 in. O.D. Tube-side manifold connections are shown for inlet and outlet fluid. (Used by permission Graham Manufacturing Company, Inc., Bui. HHE-30 1992.)...

The usual applications for finned tubes are in heat transfer involving gases on the outside of the tube. Other applications also exist, such as condensers, and in fouling service where the finned tube has been shown to be beneficial. The total gross external surface in a finned exchanger is many times that of the same number of plain or bare tubes. 

Because finned tubes of the low-fm design are standard tubes, the inside heat exchange and pressure drop performance will be the same as determined for plain or bare tubes. Use the appropriate information from earlier design sections. 

Air-cooled exchangers use atmospheric air on the outside of high-fmned tubes (except bare tubes are used in a few applications) to cool or condense fluids flowing through the inside of the tubes. 

A = total exchanger bare tube heat transfer surface, fF Cp = specific heat, Btu/ (lb) (°F) t = air temperature, °F T = hot fluid temperature, °F U = overall heat transfer coefficient (rate),... 

A = total exchanger bare tube heat transfer, ft or, net external surface area of tubes exposed to fluid heat transfer, ft or, area available for heat transfer, ft (for conduction heat transfer, A is a cross-sectional area, taken normally in the direction of heat flow, ft2). 

Gentry, G. G., R. K. Young, and W. M. Small, RODbaffle Heat Exchanger Thermal-Hydraulic Predictive Methods for Bare and Low-Finned Tubes, National Heat Transfer Gonference, Niagara Falls, NY, Aug. 5-8, (1984). 

Little work has been done on bare lithium metal that is well defined and free of surface film [15-24], Odziemkowski and Irish [15] showed that for carefully purified LiAsF6 tetrahydrofuran (THF) and 2-methyltetrahydrofuran 2Me-THF electrolytes the exchange-current density and corrosion potential on the lithium surface immediately after cutting in situ, are primarily determined by two reactions anodic dissolution of lithium, and cathodic reduc-... 

Figure 6.3a, b. Shell and tube heat exchangers. Time base mid-2004 Purchased cost = (bare cost from figure) x Type factor x Pressure factor... 

Fins are used to increase the effective surface area of heat-exchanger tubing. Many different types of fin have been developed, but the plain transverse fin shown in Figure 12.66 is the most commonly used type for process heat exchangers. Typical fin dimensions are pitch 2.0 to 4.0 mm, height 12 to 16 mm ratio of fin area to bare tube area 15 1 to 20 1. 

F.O.B. equipment cost of a carbon steel exchanger rated at 150 psi is 23,000. See Appendix B, Fig. B-3. The bare module factor is 3.39 (only one exchanger it would be 3.29 if there were two exchangers. See Table B-l). 

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