Finned surfaces

Aoe for effective area of finned surface At for total external area of finned tube Ad for surface area of dirt (scale) deposit ... 

Finned-Surface Appbcation Extended or finned surfaces are often used when one film coefficient is substantially lower than the other, the goal being to make hjAj. A few t ical fin config-... 

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)] ... 

The efficiency depends on the fin geometrical configuration, the fin thermal conductivity, and the heat transfer coefficient at the fin surface. 

A = (fti external finned surface per ft length from Table 10-39 or other specific tube data) (U, net effective tube length) (N number of tubes) (10-5)... 

The heat transfer area, A ft, in an exchanger is usually estahlished as the outside surface of all the plain or hare tubes or the total finned surface on the outside of all the finned tubes in the tube bundle. As will be illustrated later, factors that inherendy are a part of the inside of the tube (such as the inside scale, transfer film coefficient, etc.) are often corrected for convenience to equivalent outside conditions to be consistent. When not stated, transfer area in conventional shell and tube heat exchangers is considered as outside tube area. 

Tubes are copper 1-in. nominal O.D. X 14 BWG (0.083-in. thick at finned section) X 19 fins/in. Wolverine Trufin (standard tube (unfinned) wall thickness = 0.095 in.). Finned surface area/ft length = 0.678 fti/ft. Plain tubes are 0.5463 ftVft. 

Af/Ao ratio of fin surface to total external heated surface. 

The dollars/fT of finned surface or dollars/fT of hare tube surface in a finned unit do not necessarily give the only important factor. 

Determine total dollars per fC of finned surface including standard (or specified) support structure, ladders, etc. 

The lowest dollar value based on complete structure, including the important finned surface. 

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. 

Fouling on the outside of finned surfaces is usually rather small, but must be recognized. Values of 0.0001-0.0015 usually satisfy most fin-side conditions. Finned surfaces should be cleaned periodically to avoid excessive buildup of dust, oil films, bugs, etc. 

This can be converted to finned surface by ratio of fmned/bare surface areas, e. Calculate face area, FAg ... 

Construction materials will be the same as for air-cooled condensers. Aluminium fins on copper tube are the most common for the halocarbons, with stainless steel or aluminium tube for ammonia. Frost or condensed water will form on the fin surface and must be drained away. To permit this, fins will be vertical and the air flow horizontal, with a drain tray provided under. 

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 process is indicated on the chart in Figure 24.9, taking point B as the tube temperature. Since this would be the ultimate dew point temperature of the air for an infinitely sized coil, the point B is termed the apparatus dew point (ADP). In practice, the cooling element will be made of tubes, probably with extended outer surface in the form of fins (see Figure 7.3). Heat transfer from the air to the coolant will vary with the fin height from the tube wall, the materials, and any changes in the coolant temperature which may not be constant. The average coolant temperature will be at some lower point D, and the temperature difference B — D will be a function of the conductivity of the coil. As air at condition A enters the coil, a thin layer will come into contact with the fin surface and will be cooled to B. It will then mix with the remainder of the air between the fins, so that the line AB is a mix line. 

Air coolers Tubes are 0.75-1.OOin. 00, total finned surface 15-20 sqft/sqft bare surface, U = 80-100 Btu/(hr)(sqft bare surface)(°F), fan power input 2-5 PIP/(MBtu/hr), approach 50°F or more. 

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. 

Tong, L. S., A. S. Kitzes, J. Green, and T. D. Stromer, 1967c, Departure from Nucleate Boiling on a Finned Surface Heater Rod, Nuclear Eng. Design 5/386-390. (5)... 

Figure 8.6. Examples of extended surfaces on one or both sides, (a) Radial fins, (b) Serrated radial fins, (c) Studded surface, (d) Joint between tubesheet and low fin tube with three times bare surface, (e) External axial fins, (f) Internal axial fins, (g) Finned surface with internal spiral to promote turbulence, (h) Plate fins on both sides, (i) Tubes and plate fins.

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