Finned tube heat exchangers

Finned Heat Exchangers. Finned tube heat exchangers are used when heat is to be transferred between gases and liquids. The finned tubes provide a large heat transfer area, since gases, which... 

Water-to-water heat exchanger Finned-tube heat exchanger, water in 150-300 850-1700... 

Reboilers and Vaporizers Piate Heat Exchangers Direct-Contact Heat Exchangers Finned Tubes... 

In heat exchangers, fins are used on tubes (externally or internally) to enhance the available surface area for heat transfer. For a plain tube heat exchanger, the overall heat-transfer coefficient is given by ... 

From Figures 5.22 and 5.23, it is seen that there is room to accommodate the thermal expansion of the tubes. In the case of the finned heat exchanger, the tubes can expand in an expandable shell, whereas in the case of the U-tube heat exchanger, expansion room is provided because of the existence of free fixed tubes in a firm shell. 

Figure 9.86a. Heat exchanger showing tubes with longitudinal fins... 

The experience with air-liquid heat exchangers comprising tubes with fins in aluminium shows that dust is a decisive factor in the durability of this equipment, which is often exposed to industrial atmosphere [16]. Dust accumulation at the bottom of fins combined with humidity forms a kind of acidic cataplasm that attacks aluminium. The heat exchange performance as well as the lifetime of this type of equipment are increased by periodical cleaning that eliminates dust accumulated at the bottom of the fins. 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Fig. 8. (a) Heat pipe showing the use of finned tubing for both heating and cooling and (b), heat pipe exchanger ain heater system. ID = induced draft and... 

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. 

Transverse fins upon tubes are used in low-pressure gas sei vices. The primary application is in air-cooled heat exchangers (as discussed under that heading), but shell-and-tube exchangers with these tubes are in sei vice. 

Low-fin tubes (Mfi-in-high fins) provide 2.5 times the surface per lineal foot. Surface required should be divided by 2.5 then use Fig. 11-41 to determine basic cost of the heat exchanger. Actual surface times extra costs (from Table 11-14) should then be added to determine cost of fin-tube exchanger. 

Air-cooled heat exchangers include a tube bundle, which generally has spiral-wound fins upon the tubes, and a fan, which moves air across the tubes and is provided with a driver. Electric motors are the most commonly usea drivers typical drive arrangements require a V belt or a direc t right-angle gear. A plenum and structural supports are basic components. Louvers are often used ... 

HumidiRcation Chambers The air-cooled heat exchanger is provided with humidification chambers in which the air is cooled to a close approach to the wet-bulb temperature before entering the finned-tube bundle of the heat exchanger. 

In order to improve the heat transfer characteristics of air cooled exchangers, the tubes are provided with external fins. These fins can result in a substantial increase in heat transfer surface. Parameters such as bundle length, width and number of tube rows vary with the particular application as well as the particular finned tube design. 

Common to all air cooled heat exchangers is the tube, through which the process fluid flows. To compensate for the poor heat transfer properties of air, which flows across the outside of the tube, and to reduce the overall dimensions of the heat exchanger, external fins are added to the outside of the tube. A wide variety of finned tube types are available for use in air cooled exchangers. These vary in geometry, materials, and methods of construction, which affect both air side thermal performance and air side pressure drop. In addition, particular... 

The L-foot fm covers the tube more or less completely to protect the base tube against corrosive attack, but still leaves a potential corrosive site at the base of the fin adjacent to the preceding fin. The double L-foot is intended to provide complete coverage of the tube, where corrosion would otherwise be a problem. Where corrosion is troublesome, soldered or galvanized tubes may offer a solution. The dimensions of fmned tubes are results of past experience in the design of air cooled heat exchangers. Tube diameters range from about 1.905 cm (0.75 in.) to 5.08 cm (2.0 in,). 

A common type of heat exchanger used in industrial ventilation is the plate fin-and-tube heat exchanger (Fig. 9.7). Liquid or gas flows in the tubes, with a gas or a liquid circulating outside the tubes between the plates. 

Here I /G. is the heat exchanger contact resistance. The reason for rhe contact resistance is that there exists a resistance to heat flow between the outer surface of the pipe and the collar of the plate tins. Normally, the fins are attached to the pipes by mechanical expansion of the tubes out into rhe plate-fin collars. Because of this manufacturing method, the contact will not be ideal. Small gaps between the pipe surface and rhe collar of the tins will occur. 

Webb, R. L. Air-side heat exchanger in finned tube heat exchangers. Heat Transfer Engineering 1 (1980) 3, pp. 3.3-19. 

Kim, N.-H, Yun, f.-H., and Webb, R. L. Heat transfer and friction correlations for wavy plate fin-and-tube heat exchangers. Journal of Heat Transfer 119 (1997) August, pp. 560-567,... 

Equation 2-5 gives a value for U based on the outside surface area of the tube, and therefore the area used in Equation 2-3 must also be the tube outside surface area. Note that Equation 2-5 is based on two fluids exchanging heat energy through a solid divider. If additional heat exchange steps are involved, such as for finned tubes or insulation, then additional terms must be added to the right side of Equation 2-5. Tables 2-1 and 2-2 have basic tube and coil properties for use in Equation 2-5 and Table 2-3 lists the conductivity of different metals. 

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