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利用TASC进行管壳式换热器设计

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利用TASC进行管壳式换热器设计 管壳式换热器 管壳式换热器进行 利用TASC
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Shell and Tube Geometry 1Shell and Tube Geometry Contents • Standards • Shell and head types • Tubes: arrangement, type, selection • Baffles: type, cut, orientation • Clearances • Fluid allocation 2Shell and Tube Geometry Standards • The mechanical design and construction of S&T exchangers is usually based on TEMA 7th Edition 1988. • TEMA supplements the ASME Pressure Vessel code. • TEMA provides 3 letter designation Shell Front end stationary head type Rear end head type 3Shell and Tube Geometry E F One-pass shell Two-pass shell with longitudinal baffle Shell types • E-type shell should be used if possible but • F shell gives pure counter-current flow with two tube passes (avoids very long exchangers) Note: longitudinal baffles are difficult to seal with the shell especially when reinserting the shell after maintenance 4Shell and Tube Geometry More shell types • Often only used for horizontal thermosyphon reboilers (with the shell-side flow upwards) • Longitudinal baffle then more a distribution plate - may be perforated G H Split flowDouble-split flow 5Shell and Tube Geometry JX K Still more shell types • J & X give low shell side Dp • Dividing J called I by HTFS Divided flow Cross flow Kettle-type reboiler 6Shell and Tube Geometry Summary of shell types • E-type shells are standard • G and H shells are normally only used for horizontal thermosyphon reboilers • J and X shells are used if allowable pressure drop can not be achieved in an E shell • For services that need multiple shells and removable bundles, F shells should be considered as alternative • K type is only used as a reboiler • Note that TASC also handles “double-pipe” exchangers 7Shell and Tube Geometry Front and rear heads Rear Head ShellTube bundle Front Head BES 8Shell and Tube Geometry Front head types AB Channel and removable cover 平盖管箱 Bonnet (integral cover) 封头管箱 • A-type is standard for dirty tube side • B-type for clean tube side duties. Use if possible since cheap and simple. 9Shell and Tube Geometry More front head types C N • C-type with removable shell for hazardous tube-side fluids, heavy bundles or services that need frequent shell-side cleaning 可拆卸管束与管板为一体的管箱 • N-type for fixed for hazardous fluids on shell side Channel integral with tube sheet 10Shell and Tube Geometry Summary of front head selection • B type standard for clean tube side fluids • A type standard for dirty tube side fluids • Consider C type for •Hazardous tubeside fluids •Heavy tube bundles •Frequent shellside cleaning • Consider N type for fixed tubesheet exchangers with hazardous tubeside fluids • Consider D type (or bonnet welded to tubesheet) for high pressure • Consider conical for single tube pass (not actually a TEMA type) 11Shell and Tube Geometry Rear head types These fall into three general types • fixed tube sheet (L, M, N) similar to (A, B, C) • U-tube • floating head (P, S, T, W) (填料函式浮头,钩圈式浮头,可抽式浮头,套环填料函式浮 头 Use fixed tube sheet if T low*, otherwise use other types to allow for differential thermal expansion You can use bellows in shell to allow for expansion but these are special items which have pressure limitations (max. 35 bar) *For preliminary scoping studies, take below 500C as being low 12Shell and Tube Geometry Fixed head types Give small shell-to-bundle clearance 13Shell and Tube Geometry Floating head types T has simpler construction than S but gives large shell with large shell-to- bundle clearance Split backing ring 14Shell and Tube Geometry P and W floating heads • Rarely used except on small units • Note that you cannot have a pass partition plate in a W type thus limiting it to 2 (or 1) pass 15Shell and Tube Geometry Rear head selection • fixed tube sheet (L, M, N) - low shell-baffle clearance • U-tube - simple design but difficult to clean - low shell- baffle clearance • floating head (P, S, T, W) - S most common •S gives higher shell-to-baffle clearance •T gives highest shell-to-baffle clearance •W limited to 2 (or 1) passes U-tube 16Shell and Tube Geometry Shell-to-baffle clearance 0.51.01.52.02.50 Shell diameter, m Clearance, mm 0 150 100 50 Fixed and U-tube P and S T 17Shell and Tube Geometry Rear Head Selection • Fixed tubesheets (L, M, N) provided that •no overstressing due to differential expansion •shellside does not need mechanical cleaning • Fixed tubesheet with bellows provided that •shellside fluid is not hazardous •shellside pressure is below 35 bar (500 psi) •shellside does not need mechanical cleaning • U-Tube provided that •tubeside will not need mechanical cleaning •countercurrent flow is not required (unless F shell ok) • Split backing Ring Floating Head (S type) • P,T,W floating head 18Shell and Tube Geometry TEMA designation - example BES 19Shell and Tube Geometry Shell diameter • For sizes up to 610 mm (24 in) normally use standard pipe sizes • British / US standard sizes are 152, 203, 254, 305, 356, 406, 508 and 610 mm (6, 8, 10, 12, 14, 16, 18 and 24 in) nominal bore • For sizes 356 mm (14 in) and above actual inside diameter is 19 mm (0.75 in) less • 152 mm (6 in) minimum size for most process applications 20Shell and Tube Geometry Shell diameter (cont.) • Larger shells made from rolled plate • Size limited by drilling and machining capacity of manufacturer but can be up to 3000 mm (118 in) or more • For rolled shells any diameter possible but most designers prefer to work in steps of 50 mm (2 in) • Note that mechanical design requires that the shell thickness is roughly proportional to the shell diameter so big shells can be expensive 21Shell and Tube Geometry Tube Selection • Outside diameter •TEMA lists 9 standard sizes, 6.35 to 50.8 mm (0.25 to 2 in) •Only two in common use •19.05 mm (0.75 in) standard •25.4 mm (1 in) for low tubeside pressure drop • Wall thickness •TEMA give recommended values (adequate for normal temperatures and pressures) •For high internal pressure refer to TEMA •For high external pressures refer to pressure vessel codes •Note that codes refer to minimum (not average) thickness •For U tubes watch for thinning at bends 22Shell and Tube Geometry Tube length and no. passes • Length •TEMA standard lengths are 2.44, 3.05, 3.66, 4.27, 4.88 and 6.1 m (8, 10, 12, 14, 16 and 20 ft) •6.1 m sensible maximum length for petroleum refineries and chemical plants where space restricted •For gas plants and special applications much longer lengths are possible - 20 m (60 ft) or more •Watch for transportation problems • Number of tubeside passes •Generally the more the better •higher coefficient but higher pressure drop •Except single pass E shells or 2 pass F shells •true counter current flow and therefore better temperature difference 23Shell and Tube Geometry Tube layout • Use 900 (or 450 ) if cleaning required • Use 300 (or 600 ) otherwise - higher packing density • Little difference between 300 & 600 and between 900 & 450 • 300 & 900 normally used Triangular - 30o Rotated Triangular - 60o Square - 90o Rotated Square - 45o 24Shell and Tube Geometry Tube pitch • If shell side mechanical cleaning required clearance between tubes must be 6.35 mm (0.25 in) or larger •e.g. 19.05 mm tubes on 25.4 mm pitch (0.75 inch on 1 inch pitch) • If tube welding is required clearance may have to be increased (refer to manufacturer) • Otherwise use TEMA minimum (1.25 times tube outside diameter) •e.g. 19.05 mm tubes on 23.81 mm pitch (0.75 inch on 15/16 inch) 25Shell and Tube Geometry Low fin tubes Sometimes called integral fin tubes • Traditionally used for condensation of vapours with low surface tension, e.g. Refrigerants • Useful to enhance single phase where benefit is from extra surface area • Databank in TASC • Common fin pitches: 12, 19, 26, 35 fins per inch Root dia. Fin dia. Fin pitch Fin thickness Fin height 26Shell and Tube Geometry Tube inserts • TASC includes methods for twisted tape inserts • Can be useful for retrofit • Other patent inserts are available 27Shell and Tube Geometry Nozzles/impingement protection • Impingement plate sometimes required • Vapour belts excellent but expensive Vapour belt 28Shell and Tube Geometry TEMA requirement Impingement plate required by TEMA for • Corrosive, abrasive fluids • Saturated vapours or two phase fluids • Non corrosive, non abrasive, single phase fluids with ru2 greater than 2230 kg/m s2 (1500 lb/ft s2) • Corrosive, abrasive liquids with ru2 greater than 740 kg/m s2 (500 lb/ft s2) or liquids at boiling point • Whether plate fitted or not shell entrance and exit area must be such that ru2 does not exceed 5950 kg/m s2 (4000 lb/ft s2) 29Shell and Tube Geometry Baffles To give cross flow and support the tubes 30Shell and Tube Geometry Baffles Types Single segmental Double segmental Ds h Baffle Cut (%) = h/Ds x 100 Ds Window Region h 31Shell and Tube Geometry Choice of baffles • Single segmental baffles standard • Use double segmental baffles if lower Dp required • Use ‘no tubes in window’ if vibrations problems likely (with intermediate tube supports) • Use unbaffled only if really necessary (coefficient low) • Other types used less often •Rod baffles - really a grid support not a baffle •Triple segmental •Disc & doughnut •Helical / spiral flow inducing 32Shell and Tube Geometry Rod baffles and NTIW Tubes Windows with no tubes Intermediate baffles Rod baffles 33Shell and Tube Geometry Baffle Cut • Maximum to ensure full tube support is 45 % • Minimum to ensure good shellside flow distribution is 15 % (or 10 % for ‘no tubes in window’) • Rule of thumb to equalize the cross flow and window flow areas • Orientation •Vertical cut (side/side flow) for shell side condensers •Horizontal cut (up/over flow) otherwise (to avoid large bypass under nozzles) 34Shell and Tube Geometry Baffle spacing • Maximum baffle spacing usually half maximum unsupported length (given in TEMA) • No problem with ‘no tubes in window’ as intermediate supports can be used • Minimum spacing recommended by TEMA is 50.8 mm (2 in) • Small baffle spacings (457.2 mm, diameteral clearance = 0.3969 mm • Shell/baffle - Defaults •From 2.54 to 7.62 mm (diameteral) depending on shell diameter • Bundle/Shell - Defaults •Type L,M,N,P,U,W: 12.7 mm •Type S: From 34.9 to 54.0 mm (diameteral) depending on shell diameter •Type T: Type S clearance + 76.2 mm 36Shell and Tube Geometry Leakage and bypass • Leakage and bypass reduce the cross flow and hence lower the coefficient • Also cause axial mixing which may reduce the MTD with close temperature approach • Sealing strips often used to reduce bypass Tube-to-baffle leakage Bypass Use of sealing strips Shell-to-baffle leakage 37Shell and Tube Geometry Allocation of fluids • Put dirty stream on the tube side - easier to clean inside the tubes • Put high pressure stream in the tubes to avoid thick, expensive shell • When special materials required for one stream, put that one in the tubes to avoid expensive shell • Cross flow gives higher coefficients than in plane tubes, hence put fluid with lowest coefficient on the shell side • If no obvious benefit, try streams both ways and see which gives best design (TASC makes this easy) 38Shell and Tube Geometry Example 1 Debutaniser overhead condenser Hot sideCold side FluidLight hydrocarbonCooling water CorrosiveNoNo Pressure(bar)4.95.0 Temp. In/Out (oC) 46 / 4220 / 30 Vap. fract. In/Out 1 / 00 / 0 Fouling res. (m2K/W)0.000090.00018 39Shell and Tube Geometry Example 2 Crude tank outlet heater Hot sideCold side FluidCrude oilSteam CorrosiveNoNo Pressure(bar)2.010 Temp. In/Out (oC)10 / 75180 / 180 Vap. fract. In/Out0 / 01 / 0 Fouling res. (m2K/W)0.00050.0001 40Shell and Tube Geometry
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