The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 29
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The typical findensities for flat fins vary from 250 to 800 fins/m (6 to 20 fins/in.), fin thicknesses vary from 0.08 to0.25 mm (0.003 to 0.010 in.), and fin flow lengths from 25 to 250 mm (1 to 10 in.). A tube-fin exchangerhaving flat fins with 400 fins/m (10 fins/in.) has a surface area density of about 720 m2/m3(220 ft2/ft3).These exchangers are extensively used as condensers and evaporators in air-conditioning and refrigerationapplications, as condensers in electric power plants, as oil coolers in propulsive power plants, and asair-cooled exchangers (also referred to as a fin-fan exchanger) in process and power industries.© 1999 by CRC Press LLC4-124Section 4FIGURE 4.5.7 Flat or continuous fins on an array of tubes: On round tubes: (a) wavy fin, (b) multilouver fin, (c)fin with structured surface roughness (dimples), (d) parallel louver fin; (e) louver fin on flat tubes; (f) multilouverfin on elliptical tubes.Regenerators.The regenerator is a storage-type exchanger.
The heat transfer surface or elements are usually referredto as a matrix in the regenerator. In order to have continuous operation, either the matrix must be movedperiodically into and out of the fixed streams of gases, as in a rotary regenerator (Figure 4.5.8a), or thegas flows must be diverted through valves to and from the fixed matrices as in a fixed-matrix regenerator(Figure 4.5.8b). The latter is also sometimes referred to as a periodic-flow regenerator or a reversibleheat accumulator. A third type of regenerator has a fixed matrix (in the disk form) and the fixed streamof gases, but the gases are ducted through rotating hoods (headers) to the matrix as shown in Figure4.5.8c.
This Rothemuhle regenerator is used as an air preheater in some power-generating plants. Thethermodynamically superior counterflow arrangement is usually employed in regenerators.The rotary regenerator is usually a disk type in which the matrix (heat transfer surface) is in a diskform and fluids flow axially. It is rotated by a hub shaft or a peripheral ring gear drive. For a rotaryregenerator, the design of seals to prevent leakage of hot to cold fluids and vice versa becomes a difficulttask, especially if the two fluids are at significantly differing pressures.
Rotating drives also pose achallenging mechanical design problem.Major advantages of rotary regenerators are the following. For a highly compact regenerator, the costof the regenerator surface per unit of heat transfer area is usually substantially lower than that for theequivalent recuperator. A major disadvantage of a regenerator is an unavoidable carryover of a smallfraction of the fluid trapped in the passage to the other fluid stream just after the periodic flow switching.Since fluid contamination (small mixing) is prohibited with liquids, the regenerators are used exclusivelyfor gas-to-gas heat or energy recovery applications. Cross contamination can be minimized significantlyby providing a purge section in the disk and using double-labyrinth seals.Rotary regenerators have been designed for a surface area density of up to about 6600 m2/m3 (20002ft /ft3), and exchanger effectivenesses exceeding 85% for a number of applications.
They can employthinner stock material, resulting in the lowest amount of material for a given effectiveness and pressure© 1999 by CRC Press LLCHeat and Mass TransferFIGURE 4.5.8 Regenerators: (a) rotary, (b) fixed-matrix, and (c) Rothemuhle.© 1999 by CRC Press LLC4-1254-126Section 4drop of any heat exchanger known today. The metal rotary regenerators have been designed for continuousinlet temperatures up to about 790°C (1450°F) and ceramic matrices for higher-temperature applications;these regenerators are designed up to 400 kPa or 60 psi pressure differences between hot and cold gases.Plastic, paper, and wool are used for regenerators operating below 65°C (150°F) inlet temperature ofthe hot gas and 1 atm pressure.
Typical regenerator rotor diameters and rotational speeds are as follows:up to 10 m (33 ft) and 0.5 to 3 rpm for power plant regenerators, 0.25 to 3 m (0.8 to 9.8 ft) and up to10 rpm for air-ventilating regenerators, and up to 0.6 m (24 in.) and up to 18 rpm for vehicularregenerators. Refer to Shah (1994) for the description of fixed-matrix regenerator, also referred to asa periodic-flow, fixed bed, valved, or stationary regenerator.Plate-Type Heat Exchangers.These exchangers are usually built of thin plates (all prime surface). The plates are either smooth orhave some form of corrugations, and they are either flat or wound in an exchanger.
Generally, theseexchangers cannot accommodate very high pressures, temperatures, and pressure and temperature differentials. These exchangers may be further classified as plate, spiral plate, lamella, and plate-coilexchangers as classified in Figure 4.5.1. The plate heat exchanger, being the most important of these, isdescribed next.The plate-and-frame or gasketed PHE consists of a number of thin rectangular corrugated orembossed metal plates sealed around the edges by gaskets and held together in a frame as shown inFigure 4.5.9. The plate pack with fixed and movable end covers is clamped together by long bolts, thuscompressing the gaskets and forming a seal.
Sealing between the two fluids is accomplished by elastomeric molded gaskets (typically 5 mm or 0.2 in. thick) that are fitted in peripheral grooves mentionedearlier. The most conventional flow arrangement is one pass to one pass counterflow with all inlet andoutlet connections on the fixed end cover. By blocking flow through some ports with proper gasketing,either one or both fluids could have more than one pass.
Also more than one exchanger can beaccommodated in a single frame with the use of intermediate connector plates such as up to five“exchangers” or sections to heat, cool, and regenerate heat between raw milk and pasteurized milk in amilk pasteurization application.FIGURE 4.5.9 A plate-and-frame or gasketed PHE.© 1999 by CRC Press LLC4-127Heat and Mass TransferTABLE 4.5.1ExchangersSome Geometric and Operating Condition Characteristics of Plate-and-Frame HeatUnitMaximum surface areaNumber of platesPort sizePlatesThicknessSizeSpacingWidthLength2500 m23–700Up to 400 mm0.5–1.2 mm0.03–3.6 m21.5–5 mm70–1200 mm0.6–5 mOperationPressureTemperatureMaximum port velocityChannel flow ratesMax unit flow ratePerformanceTemperature approachHeat exchanger efficiencyHeat transfer coefficientsfor water-water duties0.1 to 2.5 MPa–40 to 260°C6 m/sec0.05 to 12.5 m3/hr2500 m3/hrAs low as 1°CUp to 93%3000 to 7000 W/m2 KSource: From Shah, R.K., in Encyclopedia of Energy Technology and the Environment, A.
Bision and S.G. Boots, Eds.,John Wiley & Sons, New York, 1994, 1651–1670. With permission.Typical PHE dimensions and performance parameters are given in Table 4.5.1 (Shah, 1994). Anymetal which can be cold-worked is suitable for PHE applications. The most common plate materials arestainless steel (AISI 304 or 316) and titanium. Plates made from Incoloy 825, Inconel 625, HastelloyC-276 are also available. Nickel, cupronickel, and monel are rarely used. Carbon steel is not used becauseof low corrosion resistance for thin plates.
The heat transfer surface area per unit volume for plateexchangers ranges from 120 to 660 m2/m3 (37 to 200 ft2/ft3).In PHEs, the high turbulence due to plates reduces fouling from about 10 to 25% of that of a shelland-tube exchanger. High thermal performance can be achieved in plate exchangers because the highdegree of counterflow in PHEs makes temperature approaches of up to 1°C (2°F) possible. The highthermal effectiveness (up to about 93%) makes low-grade heat recovery economical.
PHEs are mostsuitable for liquid-liquid heat transfer duties.Welded PHEs. One of the limitations of gasketed PHE is the presence of the gaskets which restrictsthe use to compatible fluids and which limits operating temperatures and pressures. In order to overcomethis limitation, a number of welded PHE designs have surfaced with a welded pair of plates for one orboth fluid sides. However, the disadvantage of such design is the loss of disassembling flexibility on thefluid side where the welding is done. Essentially, welding is done around the complete circumferencewhere the gasket is normally placed.
A stacked plate heat exchanger is another welded PHE designfrom Pacinox in which rectangular plates are stacked and welded at the edges. The physical sizelimitations of PHEs (1.2 m wide ´ 4 m long max, 4 ´ 13 ft) are considerably extended to 1.5 m wide´ 20 m long (5 ´ 66 ft) in this exchanger. A maximum surface area of (10,000 m2 or over 100,000 ft2)can be accommodated in one unit. The potential maximum operating temperature is 815°C (1500°F)with an operating pressure of up to 20 MPa (3000 psig) when the stacked plate assembly is placed ina cylindrical pressure vessel.
For operating pressures below 2 MPa (300 psig) and operating temperaturesbelow 200°C (400°F), the plate bundle is not contained in a pressure vessel, but is bolted between twoheavy plates. Some of the applications of this exchanger are catalytic reforming, hydrosulfurization,crude distillation, synthesis converter feed effluent exchanger for methanol, propane condenser, etc.A number of other PHE constructions have been developed to address some of the limitations of theconventional PHEs.
A double-wall PHE is used to avoid mixing of the two fluids. A wide-gap PHE isused for fluids having high fiber content or coarse particles. A graphite PHE is used for highly corrosivefluids. A flow-flex exchanger has plain fins on one side between plates and the other side has conventionalplate channels, and is used to handle asymmetric duties (flow rate ratio of 2 to 1 and higher).A vacuum brazed PHE is a compact PHE for high-temperature and high-pressure duties, and it doesnot have gaskets, tightening bolts, frame, or carrying and guide bars.
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