Extended Surface Heat Exchangers

For extended surfaces, which include fins mounted perpendicularly to the tubes or spiral-wound fins, pin fins, plate fins, and so on, friction data for the specific surface involved should be used. For details, see Kays and London (Compact Heat Exchangers, 2d ed., McGraw-HiU, New York, 1964). If specific data are unavailable, the correlation by Gunter and Shaw (Trans. ASME, 67, 643-660 [1945]) may be used as an approximation. 

Above this size, the flow of air over the condenser surface will be by forced convection, i.e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 1 and 10 1. 

Extended surfaces created from integrally formed low transverse fins can be used in conventional shell-and-tube heat exchangers to enhance the outside film transfer coefficient. Low transverse fins can increase the surface area by a factor of around 2.5 relative to plain tubes. 

Extended surface heat exchanger tubing to improve heat transfer... 

Other significant uses of PCBs included heat exchangers and hydraulic fluids. Prior to controls PCBs were also used in adhesives, coatings, plasticizers and inks for microencapsulating dyes for carbonless duplicating paper as extenders in pesticide formulations and catalyst carriers in olefin polymerizations to impart hydrophobicity to materials and surfaces in bactericide formulations (combined with insecticides), and in immersion oil for microscopes. Mixed with chloronaphthalenes, PCBs were also used in wire and cable insulation in the mine and shipbuilding industries (ref. 80, p. 455). 

Double-pipe heat exchangers are widely used for smaller flow rates. When the heat-transfer coefficient outside is too low, a solution consists of using longitudinal finned tubes as an extended surface. 

The ambient air moved in a cyclone path through the heat exchanger as indicated in Fig. 3. Spiral fins in the stack gas chamber and inner ambient air chamber improve the heat transfer process by increasing the exposed surface area and by extending the dwell time of stack gas and ambient air. 

We shall defer part of our analysis of conduction-convection systems to Chap. 10 on heat exchangers. For the present we wish to examine some simple extended-surface problems. Consider the one-dimensional fin exposed to a surrounding fluid at a temperature T as shown in Fig. 2-9. The temperature of the base of the fin is T0. We approach the problem by making an energy balance on an element of the fin of thickness dx as shown in the figure. Thus... 

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