Extended Surfaces Fins, Pins

Let us now search for means of increasing heat transfer from a surface. For this purpose, consider a surface at temperature Tw transferring heat by convection to an ambient at temperature Too (Fig. 2.24). As we learned in Chapter 1, the rate of heat transfer from this surface may be evaluated in terms of Newton s cooling law [Eq. (1.53)], 

Clearly, Qc may be increased by increasing the temperature difference AT, by increasing the heat transfer coefficient, or by increasing the heat transfer area. The temperature difference is usually dictated by the nature of practical problems and cannot be altered the control of the heat transfer coefficient by using different fluids and/or increased flow of these fluids is the subject of the convection heat transfer the increase of the heat transfer area is the concern of this section. 

Having learned the purpose, construction, types, and application of extended surfaces, we proceed now to the main objective of this section, the study of heat transfer from extended surfaces. Because of transversal convection, the temperature in extended surfaces varies longitudinally, and Eq. (2.104) cannot be used directly. We must first evaluate the temperature distribution, and then the heat transfer in terms of this temperature distribution. 

Consider an extended surface with variable cross section [Fig. 2.28(a)). Assume 

Consider an infinitely long fin with a specified base temperature To (Fig. 2.29). We wish to find the temperature distribution in and the heat transfer from the fin. 

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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. 

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... 

Heat transfer to the tubes on the furnace walls is predominantly by radiation. In modern designs this radiant section is surmounted by a smaller section in which the combustion gases flow over banks of tubes and transfer heat by convection. Extended surface tubes, with fins or pins, are used in the convection section to improve the heat transfer from the combustion gases. Plain tubes known as shock tubes are used in the bottom rows of the convection section to act as a heat shield from the hot gases in the radiant section. Heat transfer in the shield section will be by both radiation and convection. The tube sizes used will normally be between 75 and 150 mm diameter. The tube size and number of passes used depend on the application and the process-fluid flow rate. Typical tube velocities will be from 1 to 2 m/s for heaters, with lower rates used for reactors. Carbon steel is used for low temperature duties stainless steel and special alloy steels, for elevated temperatures. For high temperatures, a material that resists creep must be used. 

Fig. 1.14 Examples of extended surface, a straight fins, b pins, c circular fins...

This differential equation covers all forms of extended surfaces, as long as the aforementioned assumptions are met. The different fin or pin shapes are expressed by the terms Aq = Aq(x) for the cross sectional area and Af = Af(x) for the fin surface area over which the heat is released. So for a straight fin of width b perpendicular to the drawing plane in Fig. 2.11, with a profile function y = y(x) we obtain the following for the two areas... 

TYPES OF EXTENDED SURFACE. Two common types of extended surfaces are available, examples of which are shown in Fig. 15.14. Longitudinal fins are used when the direction of flow of the fluid is parallel to the axis of the tube transverse fins are used when the direction of flow of the fluid is across the tubes. Spikes, pins, studs, or spines are also used to extend surfaces, and tubes carrying these can be used for either direction of flow. In all types, it is important that the fins be in tight contact with the tube, both for structural reasons and to ensure good thermal contact between the base of the fin and the wall. 

SURFACE FIEATING - The exterior surface of a heating unit. Extended heating surface (or extended surface), consisting of fins, pins, or ribs which receive heat by conduction from the prime surface. Prime surface heating surface having the heating medium on one side and air (or extended surface) on the other. 

Increased heat transfer of surface area in contact with the coolant is accomplished by the use of extended surfaces, plates or pin fins, giving the heat transfer rate Qy by a fin or fin structure as... 

The use of fins or extended surfaces in plate-fin or similar exchangers greatly increases the heat transfer area. However, the effectiveness of the entire surface area is usually less than 100% due to the temperature gradient along the fin. Jakob has shown that the effectiveness of the fin area, rjj-, for plate fins and straight pin fins is given by... 

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