ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 48-2012 HEAT EXCHANGERS.pdf

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1、48.1CHAPTER 48HEAT EXCHANGERSFundamentals 48.1Types of Heat Exchangers. 48.1Components. 48.4Application 48.4Selection Criteria 48.5Installation 48.6EAT EXCHANGERS transfer heat from one fluid to anotherH without the fluids coming in direct contact with each other.Heat transfer occurs in a heat excha

2、nger when a fluid changes froma liquid to a vapor (evaporator), a vapor to a liquid (condenser), orwhen two fluids transfer heat without a phase change. The transferof energy is caused by a temperature difference.In most HVAC transferring heatto a liquid by condensing steam is a common example.This

3、chapter describes some of the fundamentals, types, compo-nents, applications, selection criteria, and installation of heatexchangers. Chapter 4 of the 2009 ASHRAE HandbookFunda-mentals covers the subject of heat transfer. Specific applications ofheat exchangers are detailed in other chapters of this

4、 and other vol-umes of the Handbook series.FUNDAMENTALSWhen heat is exchanged between two fluids flowing through aheat exchanger, the rate of heat transferred may be calculated usingQ = UAtm(1)whereU = overall coefficient of heat transfer from fluid to fluidA = heat transfer area of the heat exchang

5、er associated with Utm= log mean temperature difference (LMTD)For a heat exchanger with a constant U, the tmis calculated astm= Cf(2)where the temperature distribution is as shown in Figure 1 and Cfisa correction factor (less than 1.0) that is applied to heat exchangerconfigurations that do not foll

6、ow a true counterflow design.Figure 1 illustrates a temperature cross, where the outlet tem-perature of the heating fluid is less than the outlet temperature of thefluid being heated (T2 t2). A temperature cross can only beobtained with a heat exchanger that has a 100% true counterflowarrangement.Th

7、e overall coefficient U is affected by the physical arrangementof the surface area A. For a given load, not all heat exchangers withequal surface areas perform equally. For this reason, load conditionsmust be defined when selecting a heat exchanger for a specificapplication.The load for each fluid s

8、tream can be calculated asQ = mcp(tin tout)(3)The value of tmis an important factor in heat exchanger selec-tion. If the value tmis high, a relatively small heat exchange sur-face area is required for a given load. The economic effect is that theheat exchanger must be designed to accommodate the for

9、ces andmovements associated with large temperature differences. Whenthe approach temperature (the difference between T2and t1) issmall, tmis also small and a relatively large A is required.Chapter 4 of the 2009 ASHRAE HandbookFundamentalsdescribes an alternative method of evaluating heat exchanger p

10、er-formance that involves the exchanger heat transfer effectiveness and number of exchanger transfer units (NTU). This method isbased on the same assumptions as the logarithmic mean temperaturedifference method described previously.TYPES OF HEAT EXCHANGERSMost heat exchangers for HVAC brass and stai

11、nless steel arealso used. The inlet and outlet nozzles can be made with standardflange openings in various orientations to suit piping needs. Thenozzles are sized to avoid excessive fluid velocity and impinge-ment on the tubes opposite a shell inlet connection.Baffles, tube supports, tie rods, and s

12、pacers are usually madeof steel; brass and stainless steel are also available. The numberand spacing of baffles controls the velocity and, therefore, a sig-nificant portion of the shell-side heat transfer coefficient and pres-sure drop.Tubes are usually made of copper; special grades of brass andsta

13、inless steel can be specified. The tube diameter, gage, andmaterial affect the heat transfer coefficient and performance.Tubesheets are available in the same materials as baffles,although the materials do not have to be the same in a given heatexchanger. Tubesheets are drilled for a specific tube la

14、yout calledpitch. The holes are sometimes serrated to improve the tube-to-tubesheet joint.Heads are usually cast iron or fabricated steel. Cast brass and caststainless steel are available in limited sizes. Heads can be customfabricated in most metals. The inlet and outlet nozzles can bemade with sta

15、ndard flange openings. Figures 3, 4, and 5 illustratethree different head configurations that offer different levels ofserviceability and ease of installation.Plate ComponentsFigure 14 illustrates the various components of a gasketed plateand frame heat exchanger. The materials of construction and p

16、ur-pose of the components are as follows:Fixed frame plates are usually made of carbon steel. Single-passunits have inlet and outlet connections for both fluids located onthe fixed frame plate. Connections are usually NPT or stud portdesign to accommodate ANSI flanges. NPT connections are car-bon st

17、eel or stainless steel. Stud port connections can be lined withmetallic or rubber-type materials to protect against corrosion.Movable pressure plates can be moved along the length of thecarrying bar to allow removal, replacement, or addition of plates.They are made of carbon steel. Multiple-pass uni

18、ts have someconnections located on the movable pressure plate.Plate packs are made up of multiple heat transfer (channel) platesand gaskets. Plates are made of pressable metals, such as 316 or304 stainless steel or titanium. They are formed with corruga-tions, typically in a herringbone or chevron p

19、attern. The angle ofthese patterns affects the thermal performance and pressure dropof a given flow channel.Compression bolts compress the plate back between the move-able pressure and fixed frame plates. The dimension between thetwo is critical and is specified by the unit manufacturer for a givenp

20、late pack configuration.Carrying and guide bars support and align the channel plates.The upper bar is called a carrying bar, the lower a guide bar. Theyare made of stainless steel, aluminum, or carbon steel with zincchromate finish.Support columns support the carrying and guide bars on largerplate h

21、eat exchangers.Splashguards are required in the United States by OSHA toenclose exterior channel plate and gasket surfaces. They are usu-ally formed from aluminum.Drip pans made of stainless steel are often installed under plateheat exchangers to contain leakage on start-up or shut down, gas-ket fai

22、lure, or condensation.APPLICATIONHeat exchangers are used when the primary energy source isavailable for multiple purposes, uses a different medium, or its tem-perature or pressure is not in the design limits. Most of the followingFig. 11 Double-Wall Plate Heat ExchangerFig. 12 Double-Wall Plate Hea

23、t ExchangerFig. 12 Exploded View of Straight-Tube Heat ExchangerFig. 13 Exploded View of Straight-Tube Heat ExchangerFig. 13 Components of a Gasketed Plate Heat ExchangerFig. 14 Components of a Gasketed Plate Heat ExchangerHeat Exchangers 48.5examples are discussed in other chapters and volumes of t

24、heASHRAE Handbook. Heat exchangers are usedTo condense steam from a boiler to produce hot water for centralwater systemsFor service water for potable and nonpotable applications, whichis often heated by a converter and hot-water or steam boilers, withor without a storage tankTo meet special temperat

25、ure requirements of parts of a system orto protect against freezing in isolated terminal units (coils) andcooling tower basinsTo isolate two systems operating at different pressures whiletransferring thermal energy between themIn energy-saving applications such as condensate cooling, ventcondensing,

26、 boiler blowdown, thermal storage, and chiller bypass(free cooling)In many refrigeration applications as evaporators, condensers,and liquid coolersSELECTION CRITERIAA heat exchanger is often selected by a computer program thatoptimizes the selection for the given design. A manufacturer shouldprovide

27、 detailed selection guidance for both a shell-and-tube andplate exchanger for a given set of conditions.Thermal/Mechanical DesignShell-and-tube heat exchangers are designed first to be pressurevessels and second to transfer heat. Plate heat exchangers aredesigned to transfer heat efficiently within

28、certain temperature andpressure limits.Thermal Performance. The thermal performance of a heatexchanger is a function of the size and geometry of the heat transfersurface area. Heat transfer surface materials also affect perfor-mance; for instance, copper has a higher coefficient of heat transferthan

29、 stainless steel.Flow rates (velocity), viscosity, and thermal conductivity of thefluids are significant factors in determining the overall heat transfercoefficient U. In addition, the fluid to be heated should be on thetube side because the overall U of a shell-and-tube unit is oftenreduced if the

30、fluid to be heated is on the shell side. Properly selected shell-and-tube heat exchangers use tube passoptions and shell-side baffle spacing to maximize velocity (turbu-lence) without causing tube erosion. The ability to maximizevelocity on each side of a heat exchanger is particularly importantwhen

31、 the two fluids flow rates are dissimilar. However, fluidvelocity in the shell-and-tube heat exchanger is limited to avoidtube erosion. U-tube exchangers have lower tube-side velocitylimits than straight-tube units due to the thinner tube wall in the Ubends.Shell-and-tube heat exchangers can be cons

32、tructed for split-shellflow design (see Figure 6) to accommodate unusual conditions.These units have one shell inlet connection and two outlet connec-tions.Plate heat exchangers typically have U-factors three to fivetimes higher than shell-and-tube heat exchangers. The high turbu-lence created by th

33、e corrugated plate design increases convectionand increases the U-factor. The plate design achieves a large tem-perature cross at a 1 K approach because of the counterflow fluidpath and high U-factor.Thermal Stress. Heat exchangers must accommodate the ther-mal stresses associated with large tempera

34、ture differences. U-tubeunits offer superior economic performance over straight-tubeunits with removable tube bundles under extreme conditions.Units with fixed tubesheets do not handle large temperature differ-ences well.Gasketed plate units have a differential pressure/temperaturelimitation (DPTL),

35、 which is the maximum difference in operatingpressure of the two fluids at a specific temperature. A unit rated for2 MPa at 125C might have a DPTL of 1.5 MPa at 95C.Pressure Drop. Fluid velocity and normal limitations on tubelength tend to result in relatively low pressure drops in shell-and-tube he

36、at exchangers. Plate units tend to have larger pressure dropsunless the velocity is limited. Often a pressure drop limitation ratherthan a thermal performance requirement determines the surface areain a plate unit.Fouling. Often, excess surface area is specified to allow for scaleaccumulation on hea

37、t transfer surfaces without a significant reduc-tion of performance. This fouling factor or allowance is appliedwhen sizing the unit. Fouling allowance is better specified as a per-centage of excess area rather than as a resistance to heat transfer.Shell-and-tube exchangers with properly sized tubes

38、 can handlesuspended solids better than plate units with narrow flow channels.The high fluid velocity and turbulence in plate exchangers makethem less susceptible to fouling.The addition of surface area (tube length) to a shell-and-tubeexchanger does not affect fluid velocity, and, therefore, has li

39、ttleeffect on thermal performance. This characteristic makes a foulingallowance practical. This is not the case in plate units, for which thenumber of parallel flow channels determines velocity. This meansthat as plate pairs are added to meet a load (heat transfer surfacearea) requirement, the numbe

40、r of channels increases and results indecreased fluid velocity. This lower velocity reduces performanceand requires additional plate pairs, which further reduces perfor-mance.CostOn applications with temperature crosses and close approaches,plate heat exchangers usually have the lowest initial cost.

41、 Widetemperature approaches often favor shell-and-tube units. If theapplication requires stainless steel, the plate unit may be moreeconomical.ServiceabilityShell-and-tube heat exchangers have different degrees of ser-viceability. The type of header used facilitates access to the inside ofthe tubes.

42、 The heads illustrated in Figures 3, 6, and 7 can be easilyremoved without special pipe arrangements. The tube bundles in allof the shell-and-tube units illustrated, except the fixed-tubesheetunit (Figure 6), can be replaced after the head is removed if they arepiped with proper clearance.The diamet

43、er and configuration of the tubes are significant indetermining whether the inside of tubes of straight-tube units can bemechanically cleaned. Figure 7 shows a type of head that allowscleaning or inspection inside tubes after the channel cover isremoved.Plate heat exchangers can be serviced by slidi

44、ng the movablepressure plate back along the carrying bars. Individual plates can beremoved for cleaning, regasketing, or replacement. Plate pairs canbe added for additional capacity. Complete replacement plate packscan be installed.Space RequirementsCost-effective and efficient shell-and-tube heat e

45、xchangershave small-diameter, long tubes. This configuration often chal-lenges the designer when allocating space required for service andmaintenance. For this reason, many shell-and-tube selections havelarge diameters and short lengths. Although this selection per-forms well, it often costs more th

46、an a smaller-diameter unit withequal surface area. Be careful to provide adequate maintenanceclearance around heat exchangers. For shell-and-tube units, spaceshould be left clear so the tube bundle can be removed.Plate heat exchangers tend to provide the most compact design interms of surface area f

47、or a given space.48.6 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)SteamMost HVAC applications using steam are designed with shell-and-tube units. Plate heat exchangers are used in specialized indus-trial and food processes with steam. INSTALLATIONControl. Heat exchangers are usually controlle

48、d by a valve witha temperature sensor. The sensor is placed in the flow stream of thefluid to be heated or cooled. The valve regulates flow on the otherside of the heat exchanger to achieve the sensor set-point tempera-ture. Chapter 46 discusses control valves.Piping. Heat exchangers should be piped

49、 such that air is easilyvented. Pipes must be able to be drained and accessible for service. Pressure Relief. Safety pressure relief valves should be installedon both sides between the heat exchanger and shutoff valves toguard against damage from thermal expansion when the unit is notin service, as well as to protect against overpressurization.Flow Path. The intended flow path of each fluid on both sides ofa heat exchanger design should be followed. Failure to connect tothe correct inlet and outlet connections may reduce performance.Condensate Removal. Heat exchangers that co