Finned surface high fins

High Fins To calculate heat-transfer coefficients for cross-flow to a transversely finned surface, it is best to use a correlation based on experimental data for that surface. Such data are not often available, and a more general correlation must be used, making allowance for the possible error. Probably the best general correlation for bundles of finned tubes is given by Schmidt [Knltetechnik, 15, 98-102, 370-378 (1963)] ... 

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. 

Base Bare-tuhe external surface 1 in. o.d. hy 12 B.W.G. hy 24 ft. 0 in. steel tube with 8 aluminum fins per inch V -in. high. Steel headers. 150 lh./sq. in. design pressure. V-helt drive and explosion-proof motor. Bare-tuhe surface 0.262 sq. ft./ft. Fin-tuhe surface/hare-tuhe surface ratio is 16.9. 

Coefficients are based on outside bare tube surface for 1-in. O.D. lubes with 8 extruded Al fins/in., /ain. high, 16.9 surface ratio. 

Ambient air entering tbe oral cavity during oral breathing confronts a variety of surface structures. Inspired air initially passes between highly vascular lips and across the teeth, w hich can be viewed as a series of heat transfer fins. The tongue and buccal surfaces (both rough, highly vascular... 

In a dusty industrial environment, finned heaters are more liable than plain tube heaters to become blocked or coated with dust. If this dust is greasy, it will bake on the high-temperature surfaces, reducing the rate of heat transfer and in-... 

When the fluid in the tubes yields a low film coefficient, the amount of tinned surface area is adjusted, as suggested, to provide an economical and compatible area. A high ratio of outside tinned surface to bare tube surface is of tittle value when the outside air and inside fluid coefficients are about the same. The tubes are usually on 2-in. or V2 -in. triangular (60°) spacing. Fin thickness usually varies from... 

These two may not be the same. In some instances, high-finned surface area but low bare tube surfece means that a lot of tall (sometimes less efficient) fins are crowded onto the tube. In this case, horsepower might be expected to be higher. 

Transfer rate, Btu/(hr) (ft ) (°F), based on outside fin tube surface for 1-in. O.D. tubes with /g in. high aluminum fins spaced 11 per in. 

Despite the ability to adapt spray painting techniques to cope with a multitude of shapes and sizes, all surfaces must be accessible for painting (and indeed for preparation). It is worth remembering that high output spray equipment is infinitely more cumbersome than the smaller set-ups used in laboratories. Thus, fins and ribs must be wellspaced with their edges made round and smooth to retain paint more readily. 

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. 

Unsealed products will be affected by low humidity of the air in the cooled space and may suffer dehydration. Conversely, some food products such as fresh meat will deteriorate in high humidities. Since the dewpoint of the air approaches the fm surface temperature of the evaporator (see also Chapter 24), the inside humidity is a function of the coil AT. That is to say, the colder the fin surface, the... 

The fin surface area will not be as effective as the bare tube surface, as the heat has to be conducted along the fin. This is allowed for in design by the use of a fin effectiveness, or fin efficiency, factor. The basic equations describing heat transfer from a fin are derived in Volume 1, Chapter 9 see also Kern (1950). The fin effectiveness is a function of the fin dimensions and the thermal conductivity of the fin material. Fins are therefore usually made from metals with a high thermal conductivity for copper and aluminium the effectiveness will typically be between 0.9 to 0.95. 

Early bubbling FBC units were designed to burn coal, and the heat released was removed by heat transfer to in-bed tubes and/or to the water-wall tubes used to enclose the furnace. These surfaces experienced high rates of metal loss through the combined effects of erosion and abrasion. Protective measures such as plasma-sprayed coatings and metal fins to disrupt the solids flow pattern were used. These were effective for only short periods before requiring replacement, and so maintenance requirements were high. 

Banks of radial high-fin tubes e = (bare tube surface)/(total surface of finned tube)... 

Tubes are 0.75-1.00 in. OD, with 7-11 fins/in. and 0.5-0.625 in. high, with a total surface 15-20 times bare surface of the tube. Fans are 4-12ft/dia, develop pressures of 0.5-1.5in. water, and require power inputs of 2-5HP/MBtu/hr or about 7.5HP/ 100 sqft of exchanger cross section. Spacings of fans along the length of the equipment do not exceed 1.8 times the width of the cooler. Face velocities are about 10 ft/sec at a depth of three rows and 8 ft/sec at a depth of six rows. 

Air cooler data are based on 50 mm tubes with aluminum fins 16-18 mm high spaced 2.5-3 mm apart coefficients based on bare tube surface. Excerpted from HEDH, 1983. 

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