Enhanced condenser tubes

FIGURE 11.20 Performance of enhanced condenser tubes compared to a smooth tube [28],... 

Admiralty Brass and Naval Brass are 30 and 40% zinc alloys, respectively, to which a 1% tin addition has been added. Resistance to dezincification of Cu—Zn alloys is increased by tin additions. Therefore, these alloys are important for thein corrosion resistance in condenser tube appHcations. In these, as weU as the other higher zinc compositions, it is common to use other alloying additives to enhance corrosion resistance. In particular, a small amount (0.02—0.10 wt %) of arsenic (C443), antimony (C444), or phosphoms (C445) is added to control dezincification. When any of these elements are used, the alloy is referred as being "inhibited." For good stress corrosion resistance, it is recommended that these alloys be used in the fiiUy annealed condition or in the cold worked plus stress reHef annealed condition. 

Iron is added in small (usually 0.5—1.0 wt %) amounts to increase strength. More importantly, iron additions also enhance corrosion resistance, especially when precautions are taken to retain the iron in solution. Precipitation of the iron—nickel-rich phase does not result in strengthening and can cause degradation of corrosion resistance (47). A small (up to 1.0 wt %) amount of manganese is usually added to both react with sulfur and deoxidi2e the melt. These copper alloys are most commonly applied where corrosion resistance is paramount, as in condenser tube or heat exchangers. 

Nozu, S., Honda, H. and Nishida, S., Condensation ofZeotropic CFC114-CFC113 Refrigerant Mixture in the Annulus of a Double-Tube Coil with an Enhanced Inner Tube, Experimental Thermal and Fluid Science, Vol. 11, pp364-371, 1995. 

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

Carnavos [155] reported typical overall improvements that can be realized with a variety of commercially available enhanced horizontal condenser tubes. The heat flux for single 130-mm-long tubes, in most cases with outside diameters of 19 mm, is plotted in Fig. 11.22 against AT/m for 12 tubes qualitatively described in the accompanying table. The overall heat transfer performance gain of the enhanced tubes over the smooth tube is as high as 175 percent. Internal enhancement is a substantial contributor to the overall performance, since the more effective external enhancements produce a large decrease in the shell-side thermal resistance. 

D. Eissenberg, Tests of an Enhanced Horizontal Tube Condenser Under Conditions of Horizontal Steam Cross Flow, in Heat Transfer 1970, vol. 1, paper HE2.1, Elsevier, Amsterdam, 1970. 

P. J. Marto, R. J. Reilly, and J. H. Fenner, An Experimental Comparison of Enhanced Heat Transfer Condenser Tubing, in Advances in Enhanced Heat Transfer, J. M. Chenoweth, J. Kaellis, J. W. Michel, and S. Shenkman eds., pp. 1-9, ASME, New York, 1979. 

Discussion on materials for steam-water drcuit is in progress. Titanium has been selected for condenser tubing because of its enhanced resistance against seawater corrosion and extensive world wide experience. 

Where an under-slung condenser has been specified, the provision of a basement to the engine room offers the attraction of compactness at the expense of enhanced civil works, while alternatively, the specification of pannier condensers can obviate the need for a basement and will simplify the foundation design, but will considerably increase the floor area requirements. The condensing plant itself consists essentially of banks of tubes through which cooling water flows and around which exhausted steam from the turbine is condensed to form a vacuum. Such tubes have traditionally been made of brass, but where severe corrosion conditions exist, cupro-nickel is sometimes used. 

Experiments have shown that Eq. (11.63) tends to underestimate the heat transfer rate from a column of tubes, and is thus conservative for design purposes. The actual heat transfer rates are enhanced by several factors not accounted for in the theoretical model discussed above. These factors include such effects as splashing of the film when it impinges on a lower tube, additional condensation on the subcooled film as it falls between tubes, and uneven run-off because of bowed or slightly inclined tubes. 

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