粉体表面处理技术

1、HEAT EXCHANGERS石石 岩岩March 21, 2005Content lInstructionlHeat Transfer and Fluid FlowlHeat ExchangerslHeat Exchangers and SystemslSummaryInstructionsHeat transfer is an important function of many industrial processessTransfer heat from one process to anothersTypes of heat exchangers pipe coil exchange

2、rs, double-pipe heat exchanger, shell and tube heat exchanger, reboiler, plate and frame heat exchanger, air cooled heat changer, spiral heat exchangersHeat Transfer and Fluid FlowsConductionsConvectionsRadiant sLaminarsTurbulentConductionsHeat energy is transferred through solid objectssMolecules a

3、bsorb heat energy from a hotter source and then vibrate, so transfer the energy from the hot side to the cooler sidesWhat is more, there are free electrons moving in the metal, so metal have better capability of conductionConvectionsConvection occurs in fluids when warmer molecules move toward coole

4、r moleculessThe movement of the molecules sets up currents in the fluid that redistribute heat energysThe result is that the energy is equally distributedRadiant Heat TransfersHeat is conveyed by electromagnetic wavessRadiant heat transfer is a line-of-sight processsRadiant heat transfer is not used

5、 in a heat exchangerLaminar FlowsLiquid flowing are in parallel fashionsLaminar flow usually exists at low flow ratesL a m i n a r f l o w p r o m o t e s t h e development of static film, which acts as an insulatorTurbulent FlowsTurbulent flow is random movement or mixing of fluidssTurbulent flow a

6、llows molecules of fluid to mix and absorb heat more readily than does laminar flowsTurbulent flow decreases the thickness of static film, increasing the rate of heat transferHeat ExchangerslDouble-pipe heat exchangerlShell and tube heat exchangerlPlate and frame heat exchangerlSpiral heat exchanger

7、lPipe coil exchangerlAir-cooled heat exchangersDouble-Pipe Heat ExchangerlA double-pipe exchanger has a pipe inside a pipeThe outside pipe provides the shell. And the inner pipe provides the tubelThe warm and cool fluids can run parallel flow or countercurrent,because it is more efficientlFlow rates

8、 are very low in a double-pipe heat exchanger Double-Pipe Heat ExchangerlThe chemical processing industry commonly uses hairpin heat exchangerslHairpins are typically rated at 500psig shellside and 500 psig tubesidelFins can be added to the internal tubes external wall to increase heat transferDoubl

9、e-Pipe Heat ExchangersAdvantages:sIts excellent capacity for thermal expansionsIt is easy to install and clean sIts modular design makes it easy to add new sectionssReplacement parts are inexpensive and always in supplysDisadvantagessIt is not as cost effective as most shell and tube exchangers sit

10、requires special gasketsShell and Tube Heat ExchangerssThe shell and tube heat exchanger is the most common style found in industry sAs the tubeside flow enters the exchanger, flow is directed into tubes that run parallel to each other. these tubes run through a shell that has a fluid passing throug

11、h itsHeat energy is transferred through the tube wall into the cooler fluidsHeat transfer occurs primarily through conduction and convectionShell and Tube Heat Exchanger-HeadThe head can be classified as front-end or rear-end types, the front-end head has five primary designs, the rear-end has eight

12、 possible designsShell and Tube Heat Exchanger-ShellsIn most cases, the shell is designed to withstand the greatest temperature and pressure conditionsThe shell is the largest single part of the heat exchangersThe shell can be classified as single-pass, double-pass, split flow, double split flow, di

13、vided flow, cross-flowShell and Tube Heat Exchanger-TubesTube can be plain or dinnedsPlain tubes are commonly used in fabricationsFinned tube are starting to make an impact, fins can be located externally or internallysTube materials include brass, carbon, Shell and Tube Heat Exchanger-Tube SheetsTu

14、be sheets are often described as fixed or floating, single or doublesTube sheets have carefully drilled hole, the ends of the tubes in a heat exchanger are fixed by rolling, welding, or bothsDouble tube sheets are used to prevent tubeside leakage of highly corrosive fluidsShell and Tube Heat Exchang

15、er-Tube SheetlEngineering specifications take into account thermal tube expansionlIf the tube sheet is welded or bolted to the shell, it is called fixedlIf the tube sheet is independently secured to the tub head and is allowed to move freely inside the shell, it is called floating Shell and Tube Hea

16、t Exchanger-BafflelBaffles provide the framework to support and secure the tubes and prevent vibrationlThe baffle layout increases or decreases fluid and directs flow at specific pointslTubeside baffles are built into the heads to direct tubeside flowlIn multipass exchangers, cost goes up with each

17、pass, provide adequate fluid velocities to prevent fouling and to control heat transferShell and Tube Heat Exchanger-BafflelEach segmental baffle supports half of the tubeslBaffles are evenly spaced lSegmental baffles may be horizontal or vertical cutlSystems transferring large quantities of suspend

18、ed solids may use vertical arrangement, which allows liquid and solids to flow around baffleslHorizontal baffles are used in clean service with notches at the bottom to allow liquid drainage on removal from serveShell and Tube Heat Exchanger-BafflelImpingement baffles are used to protect tubing from

19、 direct fluid impactlLongitudinal baffles are used inside the shell to split or divide the flow, increase velocity, and provide superior heat transfer capabilities, longitudinal baffles do not extend the entire length of the exchanger since at some point the fluid must flow around itShell and Tube H

20、eat Exchanger-Tie RodlTie rods and concentric tube spacers keep the baffles in place and evenly spacedlEach hole in the baffle plates is 1/64” larger than the tubes outside diameterShell and Tube Heat Exchanger-Nozzles and Accessory PartlNozzles and accessory partlInlet and outlet nozzles are sized

21、for pressure drop and velocity considerationslThermowells, pressure indicator connections, safety and relief valves, product drains, vents, block valves and control valvesShell and Tube Heat Exchanger-Fixed HeadlIn a fixed head, single pass shell and tube heat exchangers, the tubes are connected to

22、two tube sheets which are firmly attached to the shelll in the multipass Shell and tube heat exchangers, the baffle added to the channel head and the lack of a tube side outlet on the discharge headlTemperature differential is less to 200 Shell and Tube Heat Exchanger-Floating HeadlOne side of the t

23、ube bundle is fixed to the channel head, the other side is unsecuredlFloating head exchangers, with their high cross-sectional areas(fins), are designed for high temperature differentials and high flow rate, produces the highest heat transfer efficiencyShell and Tube Heat Exchanger-U-Tube The tube s

24、heet connects a series of tubes bent in a U shape, the ends of the tubes are secured to the tube sheetlThe total number of tubes is limitslLarge temperature differentials lEach complete U tube has a single fundamental frequencyPlate and Frame Heat ExchangersIt consist of a series of gasketed plates,

25、 sandwiched together by two end plates and compression boltsPlate and Frame Heat ExchangersEasy to disassemble and clean and distribute heat evenly so there are no hot spotssPlates can easily be added or removedsLow fluid resistance time, low fouling, high heat transferPlate and Frame Heat Exchanger

26、sIf gaskets leak, they leak to the outside, and gasket easy to replacesPrevent cross-contamination of productssHigh turbulence and large pressure drop and smallsThis device is best suited for vicious or corrosive fluid slurriessHigh-pressure and high-temperature limitations (2.5MPa and 180) for prot

27、ect internal gasketsGaskets are easily damaged and may not be compatible with process fluidsSpiral Heat ExchangersS p i r a l h e a t exchangers are characterized by a compact c o n c e n t r i c d e s i g n t h a t generates high fluid turbulencesType 1: spiral flow on both sidessType 2: spiral flo

28、w-cross-flowPipe Coil ExchangersP i p e c o i l s a r e submerged in water or sprayed with water to transfer heatsThis type of operation has a low heat transfer coefficient and requires a lot of spaceAir-Cooled Heat ExchangersAir-cooled heat exchangers provide plain or dinned tubes connected to an i

29、nlet and return headersAir is used as the outside medium to transfer heat away from the tubessFans are used in a variety of arrangements to apply forced convection for heat transfer coefficientssProvides a 40 temperature differential between the ambient air and the exiting process fluidAir-Cooled He

30、at Exchangerssimple to construct and cheaper to maintainsCannot fouling or corrosionsLow operating costs and superior high temperature removalsLimited in usesHigh outlet fluid temperature and high initial cost of equipmentsIn cases of loss of containment, they would be fire or explosionHeat Exchange

31、rs and Systems -Parallel and Series Flowsin series flow, the tubeside flow in a multipass heat exchanger is discharged into the tubeside flow of the second exchangersIn parallel flow, the process flow goes through multiple exchangers at the same timeHeat Exchangers and Systems -Cooling TowerssThe system consists of a cooling tower, heat exchanger, pumpsCooling water is pumped into the shellside of a heat exchanger and returned (much hotter) to the top of the co