Dale Gulley's Heat Exchanger Tips

IntroductionA Workbook Collection of Dale Gulley's Heat Exchanger TipsThis workbook contains a compilation of edited, and formatted valuable and practical "Tips" on HeatExchangers that have been published and offered to the engineering public by Mr. Dale Gulley, an experiencedand recognized authority on Heat Exchanger Design, Fabrication, and Consulting.For some years , I have endeavored to collect as many of Dale's valued advice and "tips" as I possibly could.By doing so, I have gained further insight and knowledge by reading and applying his tips and know-how.Dale is not only an outstanding and recognized heat transfer expert, but he has been a contributing and positivemember of The Tubular Exchanger Manufacturers' Association for many years, advocating the useful andpositive efforts this organzatn has done for the engineerng profession world-wide.In the past 50 years I have arrived at many conclusions and results in dealing with the design, specification,fabrication, and operation of heat exchangers that are identical with Dale's Tips. My experience also coincideswith that of a lot of my past and present engineering colleagues. My field experience has proven Dale'sadvice and Tips to be not only credible - but also valuable in applying heat transfer to process operations.I have put my effort into this compilation in order to make use of this valuable engineering know-how as abasis for Experienced Based Learning when dealing with heat exchangers.Through out this compilation, my personal notes on some of the Tips can be seen off the printed area of theworksheet and to the right-hand side. I have used this method to record my own experience related to the topicand to add empirical support and reinforcement to what Dale describes.Please note that I have used the following spreadsheet and workbook techniques to assist in employing theideas and recommendations expounded by Dale:The bulk of the Tips are organized in the same manner as they are found in Dale's Website. Ihave made use of Exel's Hyperlink feature to facilitate the quick and accurate access to any ofthe topics that are listed and grouped in the Table of Contents. Once you locate a subject ortopic that you want to read or persue in the Table of Contents, all you have to do is click on thesubject and the hyperlink will take you directly to the selected Tip.I have made every effort to convert Dale's original presentation of recommended calculations andequations to a format that allows the reader to immediately employ his/her basic data to make theindicated calculation using Excel's basic spreadsheet feature. The reader can type in the basic datain the YELLOW-filled cells and the resultant calculated answer will be generated in BOLD REDnumbers. This allows the reader to do several "what-if" calculations quickly to get an idea of theperceived effect on the heat exchanger.The various groups of the Tips subject matter are also hyper-linked and a reader can go directly toone of the groups of Tips directly from the Table of Contents.These Tips are compiled and freely distributed with Dale Gulley's permission and approval. I would ask allengineers who are helped and assisted by this contribution to call or email Dale with thanks and gratitude for hiscontribution to heat exchange. Dale is active in heat exchange design, software, and process engineeringout of Tulsa, Oklahoma. Needless to say, his organization can be of great help in a heat exchanger application.Art Montemayor - 05 April 2011

Revision History4/26/2011 - Added Bs factor the second term in the denominator for the equationChris Haslegofor the seal bar calculation on sheet "Calculations".Cheresources.com Admin6/28/2011 - Added three new tips from Gulleyassociates.com with permission.Chris HaslegoBoilingCheresources.com AdminEstimate - critical heat flux for propane chillers.CalculationsEstimate - optimum flow velocity for gas inside tubes.ConstructionLongitudinal baffle heat conduction cures.11/10/2011 - Added five new tips from Gulleyassociates.com with permissionChris HaslegoBoilingCheresources.com AdminKettle Reboilers - Supports or BafflesConstructionDesign Temperatures of Carbon Steel and Low Alloy Tubes and TubesheetsDesign Temperatures of Nonferrous Tubes and TubesheetsMisc.Fouling factors for water(hr-ft2-F/Btu)Fouling Factors for Liquid Hydrocarbons(hr-ft2-F/Btu)4/5/2012 - Added five new tips from Gulleyassociates.com with permissionChris HaslegoBoilingCheresources.com AdminVertical Thermosyphon-Calculate Pressure Drop at The Outlet NozzleVertical Thermosyphon-Design for a Smaller Liquid Preheat ZoneCalculationsEstimate - Hydrocarbon Gas Heat Transfer Coefficient in Shell SideTube Bundle VibrationBest Design Feature to Prevent Bundle VibrationMisc.Viscous Flow - Use More Pressure Drop Than Usual

Estimate - critical heat flux for propane chillers.Estimate - optimum flow velocity for gas inside tubes.Longitudinal baffle heat conduction cures.Kettle Reboilers - Supports or BafflesDesign Temperatures of Carbon Steel and Low Alloy Tubes and TubesheetsDesign Temperatures of Nonferrous Tubes and TubesheetsFouling factors for water(hr-ft2-F/Btu)Fouling Factors for Liquid Hydrocarbons(hr-ft2-F/Btu)Vertical Thermosyphon-Calculate Pressure Drop at The Outlet NozzleVertical Thermosyphon-Design for a Smaller Liquid Preheat ZoneEstimate - Hydrocarbon Gas Heat Transfer Coefficient in Shell SideBest Design Feature to Prevent Bundle VibrationViscous Flow - Use More Pressure Drop Than UsualVertical Thermosyphon-Design for a Smaller Liquid Preheat ZoneVertical Thermosyphon-Calculate Pressure Drop at The Outlet Nozzle

Table of Contents2911 E. 77 Pl., Tulsa, OK 74136 P.O. Box 700295, Tulsa, OK. 74170-0295Phone: (918) 744-0100Air Coolers:1.Air flow accessories - don't overlook these when calculating fan HP2.Box header design - limit of process temperature change3.Connecting bundles of existing coolers for a new service4.Fan drive changes that increase capacity of existing cooler5.Fan drive noise - suggestions on how to reduce6.Maximum motor HP for a fan7.Maximum tube wall temperature for wrap-on fins8.Optimum number of tube rows9.Overall heat transfer rate estimate for hydrocarbons10.When do bare tubes become more efficient than fin tubes?11.When To limit number of tube passes in air coolers12.When to use wind coolersBoiling:1.Avoid mist flow boiling inside tubes2.Kettle reboiler - liquid carryover problem solutions3.Kettle reboiler - shell nozzle arrangement problem4.Kettle reboiler - shell vapor outlet nozzle location5.Kettle reboiler - sizing shell vapor space6.Kettle reboiler - undersized shell effects7.Estimate - pool boiling heat transfer coefficient for hydrocarbons8.Large boiling temperature difference problems9.Lowest limit of boiling temperature difference10.Vertical thermosyphon - choking two phase flow with small outlet nozzle11.Vertical thermosyphon - minimum recirculation rate12.Vertical thermosyphon - check for liquid preheat zone13.Vertical thermosyphon - who sets recirculation rate14.Vertical Thermosyphon-Calculate Pressure Drop at The Outlet Nozzle15.Vertical Thermosyphon-Design for a Smaller Liquid Preheat ZoneCalculations1.What diameter to use to start design of a coil2.Estimate - gas heat transfer coefficient inside tubes3.Estimate - hydrocarbon heat transfer coefficient in tubes4.Estimate - latent heat of hydrocarbons5.Estimate - liquid thermal conductivity of light hydrocarbons6.Estimate - overall heat transfer coefficient in shell & tube7.Estimate - tube length that lowers tube pressure drop8.How to calculate excess surface and overdesign surface9.Use superficial velocities to calculate best heat transfer flow pattern10.L/D equation for heat Transfer coefficient inside tubing11.LMTD correction factor charts for TEMA G and J type shells12.Low LMTD correction factor for divided flow13.What is the lowest LMTD correction to use in shell & tube14.Minimum flow area for shell side inlet nozzle15.How to calculate performance of heat exchangers with plugged tubes16.How to increase heat transfer for low Reynolds numbers17.Calculate when to use seal bars on a bundle to increase heat transfer18.Calculate S & T bundle diameter from number of tubes19.Equation for calculating tube count in shell & tube20.Check for hot tube wall temperature of cooling water21.Sometimes larger tubes are better than small ones22.Weighted MTD23.Estimate - optimum flow velocity for gas inside tubes.24.Estimate - Hydrocarbon Gas Heat Transfer Coefficient in Shell SideCondensing:1.Avoid small baffle cuts in S & T condensers2.Estimate - Condensing heat transfer coefficient for hydrocarbons inside tubing3.Maximum heat transfer rate inside tubes for total condensation4.Quick estimate for reflux condenser LMTD in air cooler5.Reflux (Knockback) condenser comments6.Steam condenser types7.Sulfur condenser - design within tube velocity limits8.Warning about small temperature pinch points in condensers9.When to slope single tube pass tubes in condensing service10.Zone those condensers!11.Estimate - critical heat flux for propane chillers.Construction:1.Benefits of using rotated square pitch in shell & tube2.Caution when using a longitudinal baffle in the shell side3.Using turbulators for tube side laminar flow4.Discussion of types of triple segmental baffles in shell & tube5.Check entrance and exit space for shell nozzles6.Horizontal vs vertical baffle cut in shell & tube7.Is expansion joint required in the shell of a fixed tube sheet?8.Increasing capacity of existing shell & tube exchangers9.Locating vents on the shell side of vertical exchangers10.Optimun gasket location for flanges11.Reinforcing rods as tube inserts to increase heat transfer12.Shell side impingement protection13.Special shell & tube heat exchanger type (NTIW)14.When to consider by-pass strips in shell & tube bundle15.What is too large of temperature change in 2 tube passes ?16.When to rotate square tube pitch in shell & tube exchanger17.Longitudinal baffle heat conduction cures.Heat Recovery:1.Deciding on what fin spacing to use2.Estimate of nozzle size for HRSG3.Face area estimate for HRSG units4.Maximum exhaust gas temperaure for steel fin tubes5.When to use bare tubes in waste heat boilersMaterials:1.Cooling water flowing inside 304SS U-tubesPressure Drop:1.Allowing for fouling in pressure drop calculations2.Allowable pressure drop suggestions3.Allowable shell side pressure drop if a multi-leaf(a.k.a. lamaflex) long baffle is used4.Better baffle window pressure drop equation5.Designing for better use of tube pressure drop6.Effect of 1st tube rows on shell nozzle pressure drop7.Pressure drop on kettle side8.Reducing high shell side pressure drop in fixed tube sheet exchangers9.Use impingement rods instead of plate to lower shell press. drop10.What design pressure drop to use for heavy liquids inside tubes11.Maximum velocity inside tubes12.Calculate shell nozzle pressure drop13.Improve shell side pressure drop calculationsTube Bundle Vibration:1.Features of a new S & T bundle that replaces bundle that vibrated2.Vibration cure when designing shell & tube bundles3.Conditions likely to cause shell & tube bundle vibration4.Cures for vibration in existing bundle5.Best Design Feature to Prevent Bundle VibrationMiscellaneous:1.Allocation of streams in shell & tube2.Articles published by Dale Gulley3.Avoid these fluids when using lowfin tubing4.Best heat transfer flow pattern5.Check liquid thermal conductivity at high reduced temperatures6.Check piping connections when there is under-performance7.Evaluating an exchanger for a new service8.Check heat release curve data for skipping over dewpoints and bubblepoints9.When will exchangers with low-fins be more economical than exchangers with bare tubes?10.Problems with excess heat exchanger surface11.Purchasing warning for shell & tube exchangers12.What is the minimum velocity inside tubing for slurries?13.Suggestions for low-fins and potential S & T bundle vibration14.Choose shell & tube or multi-tube heat exchangers15.Thermal design problem with shell side long baffle16.Trouble shooting article in Hydrocarbon Processing17.Under-surfaced S&T quote18.When to add shell in Series19.When to consider a long baffle in the shell20.Which stream goes inside the tubes of gas/gas exchangers?21.Weighted MTD22.Why did performance decline in a TEMA type F,G or H type shell?23.Zone those condensers24.Viscous Flow - Use More Pressure Drop Than Usual

Air flow accessories - don't overlook these when calculating fan HPBox header design - limit of process temperature changeConnecting bundles of existing coolers for a new serviceFan drive changes that increase capacity of existing coolerFan drive noise - suggestions on how to reduceWhen do bare tubes become more efficient than fin tubes?Overall heat transfer rate estimate for hydrocarbonsOptimum number of tube rowsMaximum tube wall temperature for wrap-on finsMaximum motor HP for a fanWhen To limit number of tube passes in air coolersWhen to use wind coolersAir Coolers:Boiling:CalculationsCondensing:Construction:Heat Recovery:Materials:Pressure Drop:Tube Bundle Vibration:Miscellaneous:Cooling water flowing inside 304SS U-tubesDeciding on what fin spacing to useEstimate of nozzle size for HRSGFace area estimate for HRSG unitsMaximum exhaust gas temperaure for steel fin tubesWhen to use bare tubes in waste heat boilersFeatures of a new S & T bundle that replaces bundle that vibratedVibration cure when designing shell & tube bundlesConditions likely to cause shell & tube bundle vibrationCures for vibration in existing bundleAvoid mist flow boiling inside tubesKettle reboiler - liquid carryover problem solutionsKettle reboiler - shell nozzle arrangement problemKettle reboiler - shell vapor outlet nozzle locationKettle reboiler - sizing shell vapor spaceKettle reboiler - undersized shell effectsEstimate - pool boiling heat transfer coefficient for hydrocarbonsLarge boiling temperature difference problemsLowest limit of boiling temperature differenceVertical thermosyphon - choking two phase flow with small outlet nozzleVertical thermosyphon - minimum recirculation rateVertical thermosyphon - check for liquid preheat zoneVertical thermosyphon - who sets recirculation rateWhat diameter to use to start design of a coilEstimate - gas heat transfer coefficient inside tubesEstimate - hydrocarbon heat transfer coefficient in tubesEstimate - latent heat of hydrocarbonsEstimate - liquid thermal conductivity of light hydrocarbonsEstimate - overall heat transfer coefficient in shell & tubeEstimate - tube length that lowers tube pressure dropHow to calculate excess surface and overdesign surfaceUse superficial velocities to calculate best heat transfer flow patternL/D equation for heat Transfer coefficient inside tubingLMTD correction factor charts for TEMA G and J type shellsLow LMTD correction factor for divided flowWhat is the lowest LMTD correction to use in shell & tubeMinimum flow area for shell side inlet nozzleHow to calculate performance of heat exchangers with plugged tubesHow to increase heat transfer for low Reynolds numbersCalculate when to use seal bars on a bundle to increase heat transferCalculate S & T bundle diameter from number of tubesEquation for calculating tube count in shell & tubeCheck for hot tube wall temperature of cooling waterSometimes larger tubes are better than small onesWeighted MTDAvoid small baffle cuts in S & T condensersEstimate - Condensing heat transfer coefficient for hydrocarbons inside tubingMaximum heat transfer rate inside tubes for total condensationQuick estimate for reflux condenser LMTD in air coolerReflux (Knockback) condenser commentsSteam condenser typesSulfur condenser - design within tube velocity limitsWarning about small temperature pinch points in condensersWhen to slope single tube pass tubes in condensing serviceZone those condensers!Benefits of using rotated square pitch in shell & tubeCaution when using a longitudinal baffle in the shell sideUsing turbulators for tube side laminar flowDiscussion of types of triple segmental baffles in shell & tubeCheck entrance and exit space for shell nozzlesHorizontal vs vertical baffle cut in shell & tubeIs expansion joint required in the shell of a fixed tube sheet?Increasing capacity of existing shell & tube exchangersLocating vents on the shell side of vertical exchangersOptimun gasket location for flangesReinforcing rods as tube inserts to increase heat transferShell side impingement protectionSpecial shell & tube heat exchanger type (NTIW)When to consider by-pass strips in shell & tube bundleWhat is too large of temperature change in 2 tube passes ?When to rotate square tube pitch in shell & tube exchangerAllowing for fouling in pressure drop calculationsAllowable pressure drop suggestionsAllowable shell side pressure drop if a multi-leaf(a.k.a. lamaflex) long baffle is usedBetter baffle window pressure drop equationDesigning for better use of tube pressure dropEffect of 1st tube rows on shell nozzle pressure dropPressure drop on kettle sideReducing high shell side pressure drop in fixed tube sheet exchangersUse impingement rods instead of plate to lower shell press. dropWhat design pressure drop to use for heavy liquids inside tubesMaximum velocity inside tubesCalculate shell nozzle pressure dropImprove shell side pressure drop calculationsWhy did performance decline in a TEMA type F,G or H type shell?Which stream goes inside the tubes of gas/gas exchangers?Allocation of streams in shell & tubeArticles published by Dale GulleyAvoid these fluids when using lowfin tubingBest heat transfer flow patternCheck liquid thermal conductivity at high reduced temperaturesCheck piping connections when there is under-performanceEvaluating an exchanger for a new serviceCheck heat release curve data for skipping over dewpoints and bubblepointsWhen will exchangers with low-fins be more economical than exchangers with bare tubes?Problems with excess heat exchanger surfacePurchasing warning for shell & tube exchangersWhat is the minimum velocity inside tubing for slurries?Suggestions for low-fins and potential S & T bundle vibrationChoose shell & tube or multi-tube heat exchangersThermal design problem with shell side long baffleTrouble shooting article in Hydrocarbon ProcessingUnder-surfaced S&T quoteWhen to add shell in SeriesWhen to consider a long baffle in the shellWeighted MTDZone those condensersLongitudinal baffle heat conduction cures.Estimate - critical heat flux for propane chillers.Estimate - optimum flow velocity for gas inside tubes.Vertical Thermosyphon-Calculate Pressure Drop at The Outlet NozzleVertical Thermosyphon-Design for a Smaller Liquid Preheat ZoneVertical Thermosyphon-Calculate Pressure Drop at The Outlet NozzleVertical Thermosyphon-Design for a Smaller Liquid Preheat ZoneEstimate - Hydrocarbon Gas Heat Transfer Coefficient in Shell SideBest Design Feature to Prevent Bundle VibrationViscous Flow - Use More Pressure Drop Than Usual

Air CoolersNote:Input data into YELLOW cells and receive output in BOLD REDAir flow accessories - don't overlook louvers and screens when calculating fan HPMarch, 2000Air static pressure loss is used to calculate the horsepower required for fans used in process air coolers. Charts andequations in the literature are usually for the tube bundle only. Frequently, air coolers have accessories like louversand fan guards. They may also have hail, bug, or lint screens. Don't overlook the accessory pressure drop becausethey can increase the static pressure as much as 25%.Box header design - limit of process temperature changeMarch, 1998In the design of an Air cooled heat exchanger, avoid imposing too large a temperature change in the box headers.Too much temperature drop between the inlet and outlet tube passes can cause leakage where the tubes meet thetubesheet. If the temperature change of the tube side stream is over approximately 400 oF, then use a split headerdesign. This allows a hot top section to slide past a cooler bottom section.Connecting Bundles of Existing Coolers for a new ServiceApril, 1998When re-using air cooled exchangers in a new service, don't overlook connecting the bundles in a series-parallelarrangement. New air coolers nearly always have the bundle connected in parallel. Arrange the bundles for moreseries type flow to increase the tube side velocity and get higher heat transfer rates. For example, an air cooler withsix bundles could be arranged with four bundles in parallel, connected to two bundles in series. The two seriesbundles would handle the coldest part of the heat load where higher velocity is needed the most.Increase Capacity of Existing Air Cooler with Fan Drive ChangesOctober, 1997If you need to increase the capacity of an air cooler, don't junk it for a new one until you have exhausted thepossibilities on changing the fan and the fan motor. The least expensive change is to increase the fan blade angle if itwill not overload the motor. But check to make sure the blade angle is not already at the maximum. The next bestchange in terms of cost is to increase the fan speed by changing the drive ratio between the fan and the motor. Ifthese changes are not enough you could increase the motor size or change the fan for one with more blades.Suggestions to Reduce Fan Drive NoiseThe most effective solution is to reduce the fan speed by changing the drive ratio between the fan and the motor.Other suggestions are to reduce the fan blade angle or change to a fan with more blades.Maximum Motor HP for a FanAdding more HP to a fan will only work up to a point. The fan efficiency reaches a peak. Then increasing the HPwill produce no more air. An estimate for this HP is:Max HP =17 + 8.4 (Fan Diam - 3.5) =46.4HPFan Diam =7.00feetThis is for fan diameters greater than 3.5 ft.Temperature Limit of Wrap-On Fins for AircoolersJune, 2000Above a certain temperature, it will be too hot for wrap-on fins. Due to thermal expansion, the aluminum fins willlose good contact with the tubing. In this case an integral type fin tube should be used. The summer time air outlettemperature is a very rough approximation. To be more exact, the tube wall temperature needs to be calculated forthe hottest tube row. Then:Twall =Ta + (Th1 - Ta) x Ro x Uc =459oFWhereTwall =temperature of tube wallTa =air outlet temperature =200oFTh1 =temperature inside tube =488oFRo =thermal resistance of air =0.12hr-ft2-oF/BtuUC =clean overall heat transfer coefficient =7.5Btu/hr-ft2-oFExample: Steam is condensing at 488 oF. Assume that the UC is 7.5 and Ro is 0.12.If the air outlet temperature is 200 oF, then:Twall =200 + (488 - 200) x 0.12 x 7.5Twall =459 oFAs you can see, the problem is more severe at high heat transfer rates. Not even the aircooled manufacturers agreeexactly what this maximum tube wall temperature should be. The ASME code for allowable stress of aluminum hasa maximum temperature of 400 oF. I believe this is the upper limit. Then the above example is operating too hot forwrap-on fins.Optimun Number of Tube RowsThe optimum number of tube rows is a function of the maximum acceptable temperature rise of the air side. Thereare three limitations and the smallest air rise of the three should be used. The limitations are:1Limit the LMTD correction factor to a minimum of 0.9 for one tube pass - maximum airoutlet temperature to be the same as the process side outlet temperature.2Minimum temperature difference at the hot end to be 8 to 10 oF.3Maximum air outlet temperature to be 300 oF if tension wound fins are used.Hydrocarbon U Estimate (Air-Coolers)February, 2002In the preliminary design or checking of process air-coolers you need an estimate of the overall heat transfercoefficient (U). An estimate that is based on fin surface can be made from the following:Fluid in Tube sideOverall Heat Transfer CoefficientLiquidsRt = 0.165 x Sqrt (avg. tube viscosity) + 0.145U = 1/RtWhere: viscosity is less than 3 cP.GasesRt = 0.29 x Sqrt (100/OP) + 0.145U = 1/RtWhere OP is the operating pressure in PSIAWhen do Bare Tubes become More Efficient Than Fin Tubes?If the inside heat transfer coefficient beomes too low, fin tubes can become inefficient. This can be the case inheavy oil coolers. If it is expected that the heat transfer coefficient is below approximately 20 Btu/hr-ft2-oF,investigate both bare and fin tubes.When To Limit Tube Passes in an AircoolerNovember, 1999For tube side streams that have a high heat transfer coefficient, it is probably not advantageous to use more thantwo tube passes. This would be for condensing streams like ammonia and steam. This could also be true for highthermal conductivity liquid streams if the LMTD is high. The velocity on these type of streams will have a minoreffect on the overall heat transfer coefficient in the typical aircooler. The major thermal resistance is the air sideheat transfer coefficient.Air Cooler Using WindDecember, 2000Where cooling water is not available and the outlet temperature is not critical, an air cooler can be built thatdepends only on the wind for cooling. It will have the best performance when the tubes have high fins and thetubes are perpendicular to the wind direction. In areas where the wind does not have a prevailing direction,arrange the tubes in a bird cage type pattern. Then there is cooling no matter which way the wind blows. If thereis a prevailing wind direction, use an air cooler bundle that sets on a stand that faces the wind.

BoilingMist Flow Boiling Inside TubesNovember, 2001This is a flow pattern to avoid in heat transfer. The mist flow region is dependent upon velocity, % vapor andstratification effects. In this type of flow the tube wall is mostly dry and the liquid droplets are carried along in a vaporcore. Therefore the heat transfer is much lower because the much higher thermal conductivity of the liquid is in verylittle contact with the tube wall. The higher the % vaporization, the lower the velocity needs to be to avoid mist flow.For example in a vertical tube where the vaporization is 50 % and the vapor density is 1.0 lb/cu ft, the velocity needsto be below approximately 80 ft/sec. If the vaporization is 75 %, the maximum velocity is approximately 30 ft/sec.This comes from the Fair equation. In a horizontal tube where there can be stratification, these maximum velocities aremuch lower. If the mist flow region cannot be avoided, then twisted tape turbulators can be used to increase the heattransfer. They will throw the liquid in the vapor core toward the tube wall.Kettle Reboiler - Location of Vapor Outlet NozzlesArt's Note:When it is necessary to have dry vapor leaving the kettle side, the location of the nozzles is important. The inlet nozzleI agree. I have also found that locating the inlet liquid nozzle directly under the vapor outlet is not good.should not be located directly under the vapor outlet. This probably results in some liquid carryover. When there isIn Amine BKU reboilers I found that locating the inlet rich amine liquid as close to the U-tube bundle tubesheeta single vapor outlet, it is usually centered over the bundle with the inlet nozzle located some distance away. Theregave the best, consistant results in obtaining good solution stripping. This gives the heating mediumhave been cases where someone other than the thermal designer changed the location of this vapor nozzle withoutthe thermal designers OK. In one case the vapor outlet was moved to the back of the kettle resulting in appreciableliquid carryoverKettle Reboiler - Problem Shell Nozzle ArrangementSometimes you see kettle reboilers where the inlet nozzle is directly under the outlet vapor nozzle. This arrangementcreates extra turbulence under the vapor nozzle which affects the amount of liquid entrainment in the outlet vapor. Itis safer to use the conventional nozzle arrangement where the inlet is some lateral distance away unless a demisterpad is used.Another problem with the vertical nozzle arrangement is when the kettle bundle is relatively long and there is asingle pair of nozzles. Then there is no good flow distribution. The boiling zones near the ends of the bundle willhave lower fluid circulation rates and lower heat transfer.Kettle Reboiler - Location of Vapor Outlet NozzlesOctober, 2000When it is necessary to have dry vapor leaving the kettle side, the location of the nozzles is important. The inletnozzle should not be located directly under the vapor outlet. This probably results in some liquid carryover. Whenthere is a single vapor outlet, it is usually centered over the bundle with the inlet nozzle located some distance away.There have been cases where someone other than the thermal designer changed the location of this vapor nozzlewithout the thermal designers OK. In one case the vapor outlet was moved to the back of the kettle resulting inappreciable liquid carryoverSizing the Vapor Space in Kettle ReboilersJune, 1998The size of the kettle is determined by several factors. One factor is to provide enough space to slow the vaporvelocity down enough for nearly all the liquid droplets to fall back down by gravity to the boiling surface. Theamount of entrainment separation to design for depends on the nature of the vapor destination. A distillation towerwith a large disengaging space, low tower efficiency and high reflux rate does not require as much kettle vaporspace as normal. Normally, the vapor outlet is centered over the bundle. Then the vapor comes from two differentdirections as it approaches the outlet nozzle. Only in rare cases are these two vapor streams equal in quantity. Asimplification that has been extensively used is to assume the highest vapor flow is 60% of the total. One casewhere this would cause an undersized vapor space is when there is a much larger temperature difference at one endof the kettle then the other. The minimum height of the vapor space is typically 8 inches. It is higher for high heatflux kettles.Kettle Reboiler - Effect of Undersized Kettle DiameterJuly, 1997What effect will an undersized kettle diameter have? The effect will be a decrease in the boiling coefficient. A boilingcoefficient depends on a nucleate boiling component and a two-phase component that depends on the recirculationrate. An undersized kettle will not have enough space at the sides of the bundle for good recirculation. Anothereffect is high entrainment or even a two-phase mixture going back to the tower.Estimate - Pool Boiling Heat Transfer Coefficient for HydrocarbonsBoil h =22 (t)1.25 =2,925Btu/(hr)(ft2)(oF)Wheret =(tube wall temperature - liquid temperature) =50oFt =temperature, oFLarge Boiling Temperature DifferencesMarch, 1999Large temperature differences in heat exchangers where liquid is vaporized are a warning flag. When the temperaturedifferences reach a certain value, the cooler liquid can no longer reach the heating surface because of a vapor film.This is called film boiling. In this condition, the heat transfer deteriorates because of the lower thermal conductivityof the vapor. If a design analysis shows that the temperature difference is close to causing film boiling, the vaporizershould be started with the boiling side full of relatively cooler liquid. This way, you don't start flashing the liquid.The liquid is slowly heated up to a more stable condition. If the vaporizer is steam heated, the steam pressureshould be reduced which will reduce the temperature difference. With steam heating, take a close look at the designif the LMTD is over 90 oF. This is close to the critical temperature difference where film boiling will start.Lower Limit of Boiling Film Temperature DifferenceFebruary, 1997A reboiler or chiller is best designed so that it doesn't have the lower heat transfer mode of natural convection. Thedividing line between natural convection and boiling depends on the type of tubing used. If steel bare tubes are used,the lower limit of temperature difference between the tube wall and the boiling fluid is approximately 5 oF. We havedesigned hydrocarbon chillers down to the temperature difference of 2 oF using low-finned tubes. Special enhancedtube surfaces can be used for even lower temperature differences than 2 oF.Choking a Vertical ThermosyphonDecember, 1999Choking down on the channel outlet nozzle and piping reduces the circulation rate through a heat exchanger. Sincethe tubeside heat transfer rate depends on velocity, the heat transfer is lower at reduced recirculation rates. A ruleof thumb says that the inside flow area of the channel outlet nozzle and piping should be the same as the flow areainside the tubing. The Shell Oil Company, in an experimental study, showed that a ratio of 0.7 in nozzle flowarea/tube flow area reduced the heat flux by 10%. A ratio of 0.4 cut the heat flux almost in half.An approximate equation for the amount of heat flux reduction is:Reduction = 3.06X -1.63X2 - 0.43 =53.32%Where X = area ratio=0.40Minimum Recirculation Rate in Thermosyphon ReboilersWhen does a recirculation rate become too low (high % vaporization)? When this happens, the tube wall is nolonger wet and the heat transfer diminishes. The guidelines in the literature show the lowest permissible recirculationrates give from 25 to 40% vaporization for hydrocarbons. It has been observed that this threshold is when theoutlet two-phase density (volume basis) is below 1.0 lb/cu-ft. Nearly all thermosyphons have outlet densities abovethis value.Vertical Thermosyphon - Check for Liquid Preheat ZoneFebruary, 2001When designing vertical thermosyphon reboilers with boiling at low operating process fluid pressures, check for thepresence of a liquid preheat zone. Back pressure raises the boiling point at the interface of liquid preheat zone andsubcooled boiling. This boiling point rise creates a liquid zone with relatively low heat transfer and it reduces thetemperature driving force (MTD). If the operating pressure is below approximately 25 PSIA, there should be aliquid preheat zone. The lower the operating pressure, the more likely there is liquid preheat. If there is no liquidpreheat, there may be an input error.Vertical Thermosyphon Recirculation RateDecember, 1997In the design of vertical thermosyphons, the recirculation rate should be set by the process engineer if there will beanything unusual about the connecting piping. The recirculation rate is especially sensitive to the size and configurationof the outlet piping. If the recirculation rate is left for the thermal designer to set, they will have to make pipingassumptions that may be violated later in the actual installation.Estimate - Critical Heat Flux For Propane ChillersA simple equation is presented for a kettle reboiler. It is conservative for very small bundles.The crital heat flux depends on the geometry of the bundle. The following estimate is based on 3/4 inch tubes on 15/16 inch pitch.It is actually good for any tube diameter with a tube pitch/tube diameter ratio of 1.25 and triangular tube pitch.A boiling temperature of -30 F. is assumed for the propane.CHF =32500=12844Btu/h ft2Ds(0.25)CHF = crital heat flux in Btu/(hr)(ft)2Ds = shell bundle diameter in inches =41ExampleWhat is the critical heat flux for a 41 inch diameter bundle?CHF = 32500(41) 0.25CHF = 12,850Kettle Reboilers - Support or Baffles?For kettle reboilers use segmental baffles instead of full supports if shell fouling factor is greater Than 0.002(hr-ft2-F/Btu)Vertical Thermosyphon-Calculate Pressure Drop at The Outlet NozzleA rule of thumb is that the pressure drop at the outlet nozzle should not be greater than 30% of the total static head.There is another tip in this boiling section about choking the flow with a small outlet nozzle. The inside flow area of theoutlet nozzle should be the same or greater than the total flow area inside the tubing. For a channel with a side outletthe pressure drop is composed of a turning loss and a contraction loss The following equations calculate the pressuredrop at the outlet. The pressure drop for expansion into the channel is not included here but is with the tube pressure drop.Ktr = ___1______=0.445575687(used for pressure drop calc)Ds0.3(If Ktr less than 0.40, use 0.40)Kc =0.5 (1 - (No/Ds)2)=0.28077KT = Ktr + Kc=0.726345687Pn = KT = 0.000108 x Vn2 x tp=0.16944Where:Ds = Top channel ID (inches)=14.8Ktr = pressure loss coefficient for turning loss=0.445575687(calculated)Kc = pressure loss coefficient for contraction into nozzle=0.28077KT = total pressure loss coefficient=0.726345687No = Outlet nozzle ID (inches)=9.8Vn = velocity thru nozzle (ft/sec)=120tp = two-phase density (lb/ft3)=0.15Pn = pressure drop thru channel and outlet nozzle (Psi)=0.17Vertical Thermosyphon-Design for a Smaller Liquid Preheat ZoneAt low operating pressures there will be a sensible heat liquid zone with relatively low heat transfer. This is caused by thefact that a small pressure change will cause a large increase in the boiling point. There has been a case where 90% of thetube length was in the sub-cooled phase. What can you change that will decrease the size of the liquid preheat zone andincrease the overall heat transfer?One answer is to evaluate the piping system above the top tubesheet. In order to make an evaluation check the pressure dropat the outlet. There is on this section of the website equations to calculate the pressure drop of a nozzle that is at right angle tothe top channel. Most vertical thermosyphons have the outlet nozzle at right angles to the top channel. There may be a simplechange of enlarging the outlet nozzle that would be the cure. But there needs to be a check to make sure the nozzle andconnecting piping are not so large that there is liquid slip. If enlarging the right angle nozzle and piping is not the answer thenthere are other configerations that will use less outlet pressure drop. Next the pressure drop of using a B type channel witha long radius ell could be tried. If this doesn't do it, try a mitered channel design.Another solution to the problem is to investigate inserts such as swisted tape, wire matrix , or helically coiled.

CalculationsNote:Input data into YELLOW cells and receive output in BOLD REDWhat Coil Diameter to Use to Start DesignOctober, 2002When starting to design a coil or other single continuous tube heat exchanger, the diameter is unknown. An exampleof this is an economizer in a heat recovery system. In this case it is desirable to have a single flow path rather thanusing parallel paths where headers are required. The following gives guidelines for liquids on a diameter selection:SizeUnit Capacity flow rate1 tube3,000-5,000 # / tube / hr1 pipe5,000-10,000 # / tube / hr1 pipe10,000-17,000 # / pipe / hr2 pipe17,000-35,000 # / pipe / hr3 pipe35,000-70,000 # / pipe / hr4 pipe70,000-130,000 # / pipe / hrEstimate Gas Heat Transfer Rate for HydrocarbonsFebruary, 1998If you need to estimate a gas heat transfer rate or see if a program is getting a reasonable gas rate, use the following:h =75 x (Op. pressure/100)1/2 =75Btu/hr-ft2-oFGenerally more accurateOr,h =1.4W0.8 =66Btu/hr-ft2-oFGenerally understatedOperating pressure =100.0Psia.W =123.00lb/tube/hrThis is for inside the tubes. The rate will be lower for the shell side or if there is more than one exchanger.Estimate Hydrocarbon Heat Transfer Coefficient In TubesUse the following equation to estimate the heat transfer coefficient when liquid is flowing inside 3/4 inch tubing:Hio =150 / sqrt(avg. viscosity) =87Btu/Ft2-hr-oFWhere:Viscosity =3.0cP.This is limited to a maximum viscosity of 3 cPEstimate - Latent Heat of HydrocarbonsAn equation from the Bureau of Standards Miscellaneous Publication No. 97 can be used when the Specific Gravityis greater than 0.67 and less than 0.934. It is:Lat heat =(111 - 0.09T)/SG60 =113Btu/lbWhere:Lat heat = The fluid's Latent Heat in Btu/lbT = The fluid temperature in oF =100SG60 = The fluid's Specific gravity @ 60 oF=0.9000(0.67