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1、精选优质文档-倾情为你奉上Heat ExchangersN. P. Cheremisinoff, Heat Transfer Equipment. The McGraw-Hill Company, 2000Classification of Heat ExchangersHeat exchangers are devices that provide the flow of thermal energy between two or more fluids at different temperatures. Heat exchangers are used in a wide variety

2、 of applications. These include power production; process, chemical, and food industries; electronics; environmental engineering; waste heat recovery; manufacturing industry; and air-conditioning, refrigeration, and space applications. Heat exchangers may be classified according to the following mai

3、n criteria:1. Recuperates and regenerators2. Transfer processes: direct contact and indirect contact3. Heat transfer mechanisms: single phase and two phase4. Flow arrangements: parallel, counter, and cross flowsThe preceding four main criteria are illustrated in Figure 1.Recuperation and Regeneratio

4、nThe conventional heat exchangers shown diagrammatically in Figure la with heat transfer between two fluids is called a recuperate, because the hot stream A recovers (recuperates) some of the heat from stream B. The heat transfer is through a separating wall or through the interface between the stre

5、ams as in the case of direct contact type of heat exchangers (Figure lc).In regenerators or in storage-type heat exchangers, the same flow passage (matrix) is alternately occupied by one of the two fluids. The hot fluid stores the thermal energy in the matrix; during the cold-fluid flow through the

6、same passage later, energy stored will be extracted from the matrix. Therefore, thermal energy is not transferred through the wall as in a direct transfer type of heat exchanger. This cyclic principle is illustrated in Figure lb. While the solid is in the cold stream A it loses heat; while it is in

7、the hot stream B it gains heat (i. e., it is regenerated). Some examples of storage-type heat exchangers are rotary regenerator for preheating the air in a large coal-fired steam power plant, gas turbine rotary regenerator, and fixed-matrix air presenters for blast furnace stoves, steel furnaces, op

8、en-hearth steel melting furnaces, and glass furnaces.Criteria used in the classification of heat exchangers, Regenerators can be classified as follows:1. Rotary regenerator2. Fixed-matrix regeneratorRotary regenerators can be further subclassified as:1. Disk type2. Drum typeIn a disk-type regenerato

9、r, the heat transfer surface is in a disk form and fluids flow axially. In a drum type, the matrix is in a hollow drum form and fluids flow radially.These regenerators are periodic flow heat exchangers. In rotary regenerators, the operation is continuous. To have this, the matrix moves periodically

10、in and out of the fixed stream of gases. A rotary regenerator can be used for air heating. There are two kinds of regenerative air presenters used in convectional power plants: the rotating-plate type and the stationary-plate type. The rotor of the rotating-plate air heater is mounted within box hou

11、sing and is installed with the heating surface in the form of plates. As the rotor rotates slowly, the heating surface is exposed alternately to flue gases and to the entering air. When the heating surface is placed in the flue gas stream, the heating surface is heated; and then when it is rotated b

12、y mechanical devices into the air stream, the stored heat is released to the air flow. Thus, the air stream is heated. In the stationary-plate air heater, the heating plates are stationary, while cold-air hoods-both top and bottom-are rotated across the heating plates; the heat transfer principles a

13、re the same as those of the rotating-plate regenerative air heater. In a fixed-matrix regenerator, the gas flows must be diverted to and from the fixed matrices. Regenerators are compact heat exchangers and they are designed for surface area density of up to approximately 6 600 m2 /m3.Transfer Proce

14、ssesAccording to transfer processes, heat exchangers are classified as direct contact type and indirect contact type.In direct contact type heat exchangers, heat is transferred between the cold and hot fluids through a direct contact between these fluids. There is no wall between hot and cold stream

15、s, and the heat transfer occurs through the interface between two streams as illustrated in Figure lc. In direct contact-type heat exchangers the streams are two immiscible liquids, a gas-liquid pair, or a solid particle-fluid combination. Spray and tray condensers and cooling towers are good exampl

16、es of such heat exchangers.In an indirect contact type heat exchanger, the heat energy is exchanged between hot and cold fluids through a heat transfer surface (i.e. a wall separating the fluids). The cold and hot fluids flow simultaneously while heat energy is transferred through a separating wall

17、as illustrated in Figure 16. Id. The fluids are not mixed.Indirect contact- and direct transfer-type heat exchangers are also called recuperates cooling towers; and tray condensers are examples of recuperates.Heat Transfer MechanismsHeat exchanger equipment can also be classified according to the he

18、at transfer mechanisms as:1. Single-phase convection on both sides2. Single-phase convection on one side, rwo-phase convection on other side3. Two-phase convection on both sidesIn heat exchangers like economizers and air heaters in boilers, compressor intercoolers, automotive radiators, regenerators

19、, oil coolers, space heaters, etc., single-phase convection occurs on both sides.Condensers, boilers and steam generators used in pressurized water reactors, power plants, evaporators, and radiators used in air-conditioning and space heating include the mechanisms of condensation, boiling, and radia

20、tion on one of the surfaces of the heat exchanger. Two-phase heat transfer could also occur on each side of the heat exchanger such as condensing on one side and boiling on the other side of the heat transfer surface. However, without phase change, we may also have a two-phase flow heat transfer mod

21、e as in the case of fluidized beds where a mixture of gas and solid particles transports heat to or from a heat transfer surface.Flow ArrangementsHeat exchangers may be classified according to the fluid-flow path through the heat exchanger. The three basic configurations are1. Parallel flow2. Counte

22、r flow3. Cross flowIn parallel flow (concurrent) heat exchangers, the two fluid streams enter together at one end, flow through in the same direction, and leave together at the other end. In counterflow (countercurrent) heat exchangers, two fluid streams flow in opposite directions. In single-crossf

23、low heat exchangers, one fluid flows through the heat transfer surface at right angles to the flow path of the other fluid. Multipass crossflow configurations can also be arranged by having the basic arrangements in series. For example, in a U-baffled tube single-pass shell-and-tube heat exchanger,

24、one fluid flows through the U-tube while the other fluid flows first downward and then upward, crossing the flow path of the other fluid stream, which is also referred to as crosscounter, cross-parallel flow arrangements.The multipass flow arrangements are frequently used in heat exchanger designs,

25、especially in shell-and-tube heat exchangers with baffles. The main difference between the flow arrangements lies in the temperature distribution along the length of the heat exchanger, and the relative amounts of heat transfer under given temperature specifications for specified heat exchanger surf

26、aces (i.e., for given flow and specified temperatures, a counterflow heat exchanger requires a minimum area, a parallel flow heat exchanger requires a maximum area, while a crossflow heat exchanger requires an area in between).In the crossflow arrangement, the flow may be called mixed or unmixed, de

27、pending on the design. If both hot and cold fluids flow through individual flow channels with no fluid mixing between adjacent flow channels, each fluid stream is said to be unmixed. If one fluid flows inside the tubes, thus is not free to move in the transverse direction, and therefore is considere

28、d unmixed; on the other hand if another fluid is free to move in the transverse direction and mix itself, and therefore is called unmixed-mixed crossflow heat exchanger.General Design TerminoJogyThe rate of heat transfer from one fluid to another through a metal wall is proportional to the overall h

29、eat transfer coefficient, to the area of the metal wall, and to the temperature difference between the hot and cold fluid: Q = Uo A MTDeWhen specifying a heat exchanger, the designer nearly always knows or can readily calculate the Q and MTDe terms for the process conditions. It is necessary only to

30、 evaluate the coefficient Uo in order to arrive at a proper value of the necessary heat transfer area. Unfortunately, Uo is a function of the actual exchanger design as well as of the fluid properties and fouling rates. For this reason, the design of a heat exchanger requires a trial-and-error calcu

31、lation.The general procedure used in heat-exchanger design is as follows:1. Establish Q from process considerations.2. Establish MTDe from process considerations. Exchanger type and tube arrange ment will have some effect on MTD? as explained subsequently.3. Assume Uo is the overall duty coefficient

32、.4. Calculate an assumed A from the assumed Uo.5. Determine the physical dimensions of the applicable type heat exchanger from the calculated A.6. Calculate the fluid pressure drops through the exchanger and modify internals if required to obtain a reasonable balance between pressure drop and exchan

33、ger size.7. Calculate Uo from physical properties of the fluids, fouling factors, and the exchanger layout.8. Calculate A based on Q and the calculated valued of Uo and MTDe.9. Compare A calculated with A assumed and repeat the calculations until they are equal. (For almost any value of U0 there is

34、an exchanger design that satisfies the criterion that A calculated equals A assumed. However, only a few of these designs are reasonable. )When heat flows from a fluid on one side of a tube to a fluid on the other side of a tube, it must overcome the following resistances:1. Ri0 is the resistance of

35、 the fluid laminar film on the inside of the tube,2. rio, is the resistance (fouling factor) of foreign material deposited on the inside of the tube,3. rm is the resistance of the metal wall,4. ro is the resistance (fouling factor) of foreign material deposited on the outside of the tube,5. Ro is th

36、e resistance of the fluid laminar film on the outside of the tube.The sum of these five resistances is R, the total resistance; andUo=l/Rt,The fouling factors rio and ro are estimated based on experience or taken from typical values listed in steam tables. The term rw is calculated from the thicknes

37、s and thermal conductivity of the metal wall. Rio and Ro are functions of mass velocity and physical properties of the fluid and are evaluated from the applicable correlations in temrs of hio and h where 1 /Ro = ho, and 1 /Rio = ho. The h terms are known as the film coefficients.The resistance terms

38、 contain an area dimension, m2, which usually refers to the square meters of surface area at which the resistance occurs. Since the resistance terms must be added to obtain the overall resistance, the area dimensions of each term must refer to the same surface area rather than to its own area. This

39、rationalizes the terms and makes them additive. Standard practice is to use the outside tube area as the basis for calculation and specification of exchangers. As shown earlier, the usual nomenclature indicates this by the subscript io. For example, hio is the inside coefficient based on the outside

40、 tube area. For a tube or pipe, hio = hi ( di/ do), where hi is the inside coefficient based on the inside tube area. This factor is already included in the correlations presented.The commercially clean coefficient is the overall coefficient which can be expected when a new exchanger is first placed

41、 into service and before process fouling of the tubes occurs. It is calculated according to the following: l/Uc = Rc = Rio + Ro + rw+0. 00025The 0.00025 term is an estimated resistance to heat transfer to allow for fouling in a new exchanger due to tube roller lubricants, mild corrosion from hydrost

42、atic testing water, and so on. It is assumed that this allowance is split evenly between the shell-side and the tube-side surfaces.The operating temperatures of the exchanger are usually set by process conditions. However, in certain cases, the exchanger designer will establish the operating tempera

43、tures.Temperature of Streams to StorageThe maximum temperature of a stream going to atmospheric storage generally is set by safety, economics, or special process considerations.A stream going to an atmospheric tank at sea level should not exceed that temperature at which its true vapor pressure is 8

44、9.6 kPa abs. This value is reduced 11.3 kPa for each 1, 000 m elevation. For heavy streams whose true vapor pressure is difficult to determine, the maximum temperature to tankage should be the lower value of either 28t below the ASTM initial boiling point or 8t below the minimum flash point.Streams

45、should not be sent to tankage at temperatures above 90 to 120. Operation in or above this temperature range could cause the tank water heel to flash to steam, resulting in a boll over.The selection of the optimum temperature of a stream going to cone-roof tankage is generally based on an economic ba

46、lance between the cost of incremental cooler surface and cooling water and the savings due to reduced vapor losses.Opportunity for optimizing the temperature of a stream going to storage is greatest for intermediate products. However, special considerations are required for the following cases;1. St

47、reams that are stored prior to a process requiring refrigeration of the feed.2. Steams whose properties are permanently degraded by high storage temperature.3. Streams those are stored prior to blending. The storage temperatures for such streams should be chosen after considering the properties and

48、temperature of the blend, assuming no heat loss in intermediate tankage.The maximum allowable cooling-water outlet temperatures, set by fouling considerations, for coolers other than box coolers are: Salt water, 48 Brackish water, 51 Fresh water, 54The maximum temperature used for a project should b

49、e checked with the affiliate, since this has an important bearing on the economics of surface selection.An equally, if not more, important criteria is the maximum allowable cooling-water temperature. This is the average film temperature at the water outlet. These limits are: Salt water, 60 Fresh water, 65For box coolers, the maximum cooling-water outlet temperature is 65 for both salt and fres