The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 28
Текст из файла (страница 28)
The portion of the surface which alsoseparates the fluids is referred to as a primary or direct surface. To increase heat transfer area, appendagesknown as fins may be intimately connected to the primary surface to provide an extended, secondary,or indirect surface. Thus, the addition of fins reduces the thermal resistance on that side and therebyincreases the net heat transfer from the surface for the same temperature difference.Heat exchangers may be classified according to transfer process, construction, flow arrangement,surface compactness, number of fluids, and heat transfer mechanisms as shown in Figure 4.5.1.FIGURE 4.5.1 Classification of heat exchangers.© 1999 by CRC Press LLC4-120Section 4A gas-to-fluid heat exchanger is referred to as a compact heat exchanger if it incorporates a heattransfer surface having a surface area density above about 700 m2/m3 (213 ft2/ft3) on at least one of thefluid sides which usually has gas flow.
It is referred to as a laminar flow heat exchanger if the surfacearea density is above about 3000 m2/m3 (914 ft2/ft3), and as a micro heat exchanger if the surface areadensity is above about 10,000 m2/m3 (3050 ft2/ft3). A liquid/two-phase heat exchanger is referred to asa compact heat exchanger is the surface area density on any one fluid side is above about 400 m2/m3(122 ft2/ft3). A typical process industry shell-and-tube exchanger has a surface area density of less than100 m2/m3 on one fluid side with plain tubes, and two to three times that with the high-fin-density lowfinned tubing. Plate-fin, tube-fin, and rotary regenerators are examples of compact heat exchangers forgas flows on one or both fluid sides, and gasketed and welded plate heat exchangers are examples ofcompact heat exchangers for liquid flows.Types and DescriptionGas-to-Fluid Exchangers.The important design and operating considerations for compact extended surface exchangers are (1)usually at least one of the fluids is a gas or specific liquid that has low h; (2) fluids must be clean andrelatively noncorrosive because of small hydraulic diameter (Dh) flow passages and no easy techniquesfor mechanically cleaning them; (3) the fluid pumping power (i.e., pressure drop) design constraint isoften equally as important as the heat transfer rate; (4) operating pressures and temperatures are somewhatlimited compared with shell-and-tube exchangers as a result of the joining of the fins to plates or tubessuch as brazing, mechanical expansion, etc.; (5) with the use of highly compact surfaces, the resultantshape of a gas-to-fluid exchanger is one having a large frontal area and a short flow length (the headerdesign of a compact heat exchanger is thus important for a uniform flow distribution among the verylarge number of small flow passages); (6) the market potential must be large enough to warrant thesizable manufacturing research and tooling costs for new forms to be developed.Some advantages of plate-fin exchangers over conventional shell-and-tube exchangers are as follows.Compact heat exchangers, generally fabricated from thin metallic plates, yield large heat transfer surfacearea per unit volume (b), typically up to ten times greater than the 50 to 100 m2/m3 provided by a shelland-tube exchanger for general process application and from 1000 to 6000 m2/m3 for highly compactgas side surfaces.
Compact liquid or two-phase side surfaces have a b ratio ranging from 500 to 600m2/m3. A compact exchanger provides a tighter temperature control; thus it is useful for heat-sensitivematerials, improves the product (e.g., refining fats from edible oil) and its quality (such as a catalystbed). Also, a compact exchanger could provide rapid heating or cooling of a process stream, thusimproving the product quality. The plate-fin exchangers can accommodate multiple (up to 12 or more)fluid streams in one exchanger unit with proper manifolding, thus allowing process integration and costeffective compact solutions.Fouling is one of the potential major problems in compact heat exchangers (except for plate-andframe heat exchangers), particularly those having a variety of fin geometries or very fine circular ornoncircular flow passages that cannot be cleaned mechanically.
Chemical cleaning may be possible;thermal baking and subsequent rinsing is possible for small-size units. Hence, extended surface compactheat exchangers may not be used in heavy fouling applications.Liquid-to-Liquid Exchangers.Liquid-to-liquid and phase-change exchangers are plate-and-frame and welded plate heat exchangers(PHE), spiral plate, and printed circuit exchangers; some of them are described next in some detail alongwith other compact heat exchangers and their applications.Plate-Fin Heat Exchangers.This type of exchanger has “corrugated” fins or spacers sandwiched between parallel plates (referred toas plates or parting sheets) as shown in Figure 4.5.2. Sometimes fins are incorporated in a flat tube withrounded corners (referred to as a formed tube), thus eliminating a need for the side bars.
If liquid or© 1999 by CRC Press LLCHeat and Mass Transfer4-121FIGURE 4.5.2 Typical components of a plate-fin exchanger.phase-change fluid flows on the other side, the parting sheet is usually replaced by a flat tube with orwithout inserts/webs. Other plate-fin constructions include drawn-cup (see Figure 4.5.3) or tube-and-FIGURE 4.5.3 U-channel ribbed plates and multilouver fin automotive evaporator. (Courtesy of Delphi HarrisonThermal Systems, Lockport, NY.)center configurations. Fins are die- or roll-formed and are attached to the plates by brazing, soldering,adhesive bonding, welding, mechanical fit, or extrusion.
Fins may be used on both sides in gas-to-gasheat exchangers. In gas-to-liquid applications, fins are usually used only on the gas side; if employedon the liquid side, they are used primarily for structural strength and flow-mixing purposes. Fins arealso sometimes used for pressure containment and rigidity.© 1999 by CRC Press LLC4-122Section 4FIGURE 4.5.4 Fin geometries for plate-fin heat exchangers: (a) plain triangular fin, (b) plain rectangular fin, (c)wavy fin, (d) offset strip fin, (e) multilouver fin, and (f) perforated fin.Plate fins are categorized as (1) plain (i.e., uncut) and straight fins, such as plain triangular andrectangular fins; (2) plain but wavy fins (wavy in the main fluid flow direction); and (3) interrupted finssuch as offset strip, louver, and perforated. Examples of commonly used fins are shown in Figure 4.5.4.Plate-fin exchangers have been built with a surface area density of up to about 5900 m2/m3 (18002ft /ft3).
There is a total freedom of selecting fin surface area on each fluid side, as required by the design,by varying fin height and fin density. Although typical fin densities are 120 to 700 fins/m (3 to 18 fins/in.),applications exist for as many as 2100 fins/m (53 fins/in.). Common fin thicknesses range from 0.05 to0.25 mm (0.002 to 0.010 in.). Fin heights range from 2 to 25 mm (0.08 to 1.0 in.). A plate-fin exchangerwith 600 fins/m (15.2 fins/in.) provides about 1300 m2 (400 ft2/ft3) of heat transfer surface area per cubicmeter volume occupied by the fins. Plate-fin exchangers are manufactured in virtually all shapes andsizes, and made from a variety of materials.Tube-Fin Heat Exchangers.In this type of exchanger, round and rectangular tubes are the most common, although elliptical tubesare also used.
Fins are generally used on the outside, but they may be used on the inside of the tubesin some applications. They are attached to the tubes by a tight mechanical fit, tension winding, adhesivebonding, soldering, brazing, welding, or extrusion. Fins on the outside of the tubes may be categorizedas follows: (1) normal fins on individual tubes, referred to as individually finned tubes or simply asfinned tubes, as shown in Figures 4.5.6 and 4.5.5a; (2) flat or continuous (plain, wavy, or interrupted)external fins on an array of tubes, as shown in Figures 4.5.7 and 4.5.5b; (3) longitudinal fins on individualtubes. The exchanger having flat (continuous) fins on tubes has also been referred to as a plate-fin andtube exchanger in the literature.
In order to avoid confusion with plate-fin surfaces, we will refer to itas a tube-fin exchanger having flat (plain, wavy, or interrupted) fins. Individually finned tubes are probablymore rugged and practical in large tube-fin exchangers. Shell-and-tube exchangers sometimes employlow-finned tubes to increase the surface area on the shell side when the shell-side heat transfer coefficientis low compared with the tube side coefficient. The exchanger with flat fins is usually less expensive ona unit heat transfer surface area basis because of its simple and mass-production-type construction© 1999 by CRC Press LLCHeat and Mass Transfer4-123FIGURE 4.5.5 (a) Individually finned tubes, (b) flat or continuous fins on an array of tubes.FIGURE 4.5.6 Individually finned tubes: (a) helical, (b) annular disk, (c) segmented, (d) studded, (e) wire loop,and (f) slotted helical.features.
Longitudinal fins are generally used in condensing applications and for viscous fluids in doublepipe heat exchangers.Tube-fin exchangers can withstand high pressures on the tube side. The highest temperature is againlimited by the type of bonding, the materials employed, and the material thickness. Tube-fin exchangerswith an area density of about 3300 m2/m3 (1000 ft2/ft2) are commercially available. On the fin side, thedesired surface area can be employed by using the proper fin density and fin geometry.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
