Specifying Shell-and-Tube Heat ExchangersUnderstand what heat exchanger design specialists needto know — and remember, you know your process bestAsif RazaShell-and-tube heat exchangers are one of the most important and commonly usedprocess equipment items in the chemical process industries (CPI). If you are working ona project during either the basic or the detailed engineering phase, there is a good chancethat you will need to specify one or more shell-and-tube exchangers — and perhaps manyof them.While the actual design will likely be done by a specialist at an equipment vendor orwithin your own company, you still need to fill out a process datasheet for each heatexchanger and in due course, review the vendor’s detailed proposal. You know yourprocess best, and it is a bad idea to rely on the vendor always to make the right decisions.This article shows you the basics of specifying and selecting shell-and-tube heatexchangers: the process information and preliminary design decisions needed to fill outthe datasheet, and how to check any corresponding assumptions made by the vendor.Although it does not go into detail on the design procedure, the article is also a goodstarting point if you intend to design the heat exchanger yourself.Figure 1. Which fluid goes on the shellside and which on the tubeside? There is nostraightforward answer, but the guidelines presented here will help you decideFigure 1. Which fluid goes on the shellside andwhich on the tubeside? There is no straightforwardanswer, but the guidelines presented here willhelp you decideDatasheet informationThough every company is likely to have its own heat exchanger datasheet, most of themlook much like the sample shown in Figure 2 (p. 49). To complete the datasheet youwill need to know:
1. The composition and normal flowrate of the process fluid(s), and the temperaturechange required. Refer to heat and material balances.2. Process fluid properties — density, viscosity and thermal conductivity — at theoperating temperature and pressure.
Figure 2. A typical datasheet for a shell-and-tube heat exchanger lists all the informationrequired for a detailed design Source: TEMA (Tubular Exchanger ManufacturersAssociation, Inc.; Tarrytown, N.Y.; www.tema.org).Which fluid on which side?Next comes your first design decision: Which fluid goes on the shellside and which onthe tubeside (Figure 1)? There is no straightforward answer, but some considerations andrules of thumb outlined in an online reference (smartprocessdesign.com) andincorporating the author’s experience are summarized here: Corrosive fluids are best kept to the tubeside. Since the tubeside has less metal than theshellside, this will minimize the use of expensive metals that may be needed to withstandthe fluids’ corrosive properties. Fluids at extreme pressures and temperatures are preferably kept to the tubeside,because they are likely to require a greater metal thickness, or more expensive materialsof construction. The tubes, being smaller in diameter than the shell, withstand higherpressures. Fluids that need to be kept at a high velocity, such as water or propylene glycol forcooling, should be kept on the tubeside. Dirty fluids, or streams that are otherwise likely to cause fouling, should go on thetubeside. This is because the tubes are easier to clean than the shell. For instance, it isoften possible to clean the tubes by water jetting, having simply opened the head of theexchanger, without needing to remove the tube bundle. The shell and the outside of thetube bundle, on the other hand, are harder to clean mechanically, and chemical cleaningis often the only option. The shellside offers a larger cross-section for vapor flow, and hence lower pressuredrops. Process vapors to be condensed are therefore normally placed on the shellside,though the tubeside is generally used for condensing steam. The baffles on the shellside help to ensure good mixing, which reduces the effects oflaminar flow and therefore tends to increase heat-transfer coefficients. Hence you will getbetter heat transfer if viscous fluids are kept on the shellside — I confirmed this recentlyon a project involving a very viscous polymer. Twisted tubes, static mixers or tube inserts increase turbulence and thus heat-transfercoefficients on the tubeside by reducing the effects of laminar flow. Because these areusually proprietary technologies, however, your ability to check the vendor’sperformance claims may be limited. If you think you would benefit from one of thesetechnologies, work closely with the vendor and be sure to evaluate all the options.
In heat exchanger designs that feature gaskets or floating heads, the shellside typicallyis not a suitable location for fluids that are hazardous, corrosive or especially valuable,because the risk of leaks is too high. Such fluids should therefore normally go on thetubeside. Exchangers featuring all-welded construction can safely carry hazardous fluidson the shellside, though you should remember the difficulty of cleaning the shellside. Thermal expansion may be an issue if one of the fluids undergoes a temperature changeof more than 150–200 C (300–400 F). In this case you would normally put the hightemperature-change fluid on the shellside, which is better able to handle largetemperature changes in certain exchanger designs. In summary, the fluids preferred on the tubeside are the following:– Cooling water– The more-fouling, erosive or corrosive fluid– The less-viscous fluid– The fluid at higher pressure– The hotter fluid– The smaller volumetric flowrate.Remember, however, that none of the suggestions above is definitive. Use them as astarting point, but if they indicate a different fluid arrangement from what has been usedin the past in your plant or industry, you may find that there is a good reason. If twosuggestions conflict, or the performance of your initial configuration looks unsatisfactory— because the predicted pressure drop or heat-transfer performance does not meet yourrequirements — do not be afraid to reverse the arrangement of the two fluids and seewhether that improves matters.More key decisionsAllowable pressure drop. You will have to understand the process thoroughly before youcan attempt to specify the pressure drop on each side of the heat exchanger. As a rule ofthumb, start with 10 psi on both the shellside and the tubeside. If there is a pumpupstream of the heat exchanger, there probably will be no concern about pressure drop aslong as the pump can handle this. For gases, if there is a compressor upstream, checkwith your equipment-design engineer that it can provide the necessary pressure drop. Forcooling water, check for constraints on the allowable return pressure at the battery limitof the unit.
Sometimes the need to optimize the heat exchanger means that you will have to take ahigher pressure drop than originally specified. A higher pressure drop means highervelocity, which in turns gives a higher Reynolds number and a higher heat-transfercoefficient. Give the heat exchanger vendor an allowable pressure drop as high asrealistically possible to allow flexibility in optimizing the design. Once the designer hasconfirmed the calculated pressure drop, pass this value on to your rotary equipmentengineer, who will need it for sizing pumps and compressors.Fouling factors. These are very important in sizing the heat exchanger. Do not expect thevendor to provide you with fouling factors. A higher fouling factor translates to a lowerdesign heat-transfer coefficient ( U d) and a larger required surface area. Fouling factorscan often be taken from existing plant data. If these are not available, you will have toassume a value taken from company guidelines or published sources (Table 1). Makesure that your customer — whether internal or external — is in agreement with yourassumed fouling factor. Designing with a too-high fouling factor will result in anoversized heat exchanger that will cost you more and probably will not work as intended.Table 1. typical fouling factorsFluidTypical fouling factor (ft2· F·h/Btu)Fuel oil0.005Steam (clean)0.0005Exhaust steam (oil bearing)0.001Refrigerant vapors (oil bearing)0.002Compressed air0.002Industrial organic heat-transfer media0.001Refrigerant liquids0.001Hydraulic fluid0.001Molten heat-transfer salts0.0005Acid gas0.001Solvent vapors0.001MEA and DEA solutions0.002DEG and TEG solutions0.002Caustic solutions0.002Vegetable oils0.003Lean oil0.002Cooling water0.001Natural gas0.001Atmospheric tower overhead vapors0.001
Vacuum overhead vapors0.002Specifying appropriate fouling factors is important but not always easy. In the absence ofoperating experience, pick figures from reliable published sources.Source: TEMAExcess area. The difference between the design heat-transfer coefficient and the serviceheat-transfer coefficient provides a safety factor, often known as “excess area” because itis equivalent to specifying a larger heat-transfer area than necessary. The excess area isusually a minimum of 10%, but can be up to 30%. Choose a value from your plant’s orunit’s design basis, or ask your customer.Heating and cooling curve. If the heat exchanger will be used to condense or vaporizeprocess fluids, the vendor will require a corresponding heating or cooling curve showinghow the vapor fraction varies with temperature, and the corresponding thermal propertiesof the liquid and vapor fractions. A heating or cooling curve with 8–10 points can easilybe generated using simulation software.Design temperature and pressure. Calculate the design temperature and pressure on boththe shellside and the tubeside by adding an appropriate safety margin to the maximumvalues expected in service. Consider the following guidelines:1. To arrive at the design temperature, add a margin of 30 C (50 F) to the maximumallowable operating temperature of the exchanger.2. Similarly, the design pressure can be calculated by adding an appropriate margin to themaximum allowable operating pressure.3. If the process hazard analysis has identified tube rupture as a hazard, to avoid the needto design a pressure relief valve for the tube rupture case, the shellside design pressuremust be at least 77% of the tubeside design pressure (the “10/13” rule). For instance, ifthe tubeside design pressure is 500 psig, the minimum shellside design pressure shouldbe 500 10/13 385 psig. The logic of this is that ASME codes require the shell to behydraulically tested at 1.3 times its design pressure, so tube rupture — which is generallyconsidered an unlikely event — would not pressurize the shell beyond its test pressure.4. When deciding the design temperature, consider routine operations such as steaming ofthe heat exchanger during maintenance.Design codes. Under ASME rules, if the operating pressure is higher than 15 psig, thenthe heat exchanger is considered a pressure vessel, and the pressure-vessel design codeASME section VIII, Div. 1 or 2 applies. Similar logic applies to different pressure vesselcodes used outside the U.S. and Canada; make sure you use the code appropriate to thecountry in which the equipment will be used.
The Tubular Exchanger Manufacturers Assn., Inc. (TEMA; Tarrytown, N.Y.;www.tema.org) issues its own design and manufacturing codes. There are threecategories: TEMA C, B and R. In simple terms, TEMA C applies mostly to water, oil andair at low or moderate pressures and temperatures, and is the most cost-effective standardin cases where it is applicable. TEMA B is for chemicals and petrochemicals at highertemperatures and pressures. TEMA R, for severe service involving high pressures andtemperatures, is widely used in petroleum refineries, and is the most expensive option.Inappropriate TEMA ratings will significantly increase the cost of a heat exchanger, sochoose carefully based on existing plant data or suitable guidelines.Keep in mind that you do not necessarily have to design your heat exchanger to TEMAstandards. In particular, TEMA B and R standards enforce a minimum tube diameterwhich could lead to too-low velocities if the tubeside flowrate is small. The resulting lowheat-transfer coefficient may require a large and expensive heat exchanger. In suchsituations, it may be best not to design your heat exchanger to TEMA standards.Heat exchanger type . It is very important to specify the correct type of heat exchangerfor the application (Figure 3, p.50), and in this case there are no right or wrong answers.Here is a list of criteria that will help you in making a decision:
Figure 3. TEMA exchanger-type codes provide a shorthand for different basic designsand construction methods1. If the fluids are relatively clean and the difference in temperature between the shellsideand the tubeside is not very high (around 100 C / 200 F), then consider a BEM (fixedtubesheet) design. Typical applications are condensers; liquid-liquid, gas-gas, and gasliquid heating and cooling; and vertical thermosyphons.2. If the heat exchanger must accommodate a significant amount of thermal expansionbetween shell and tubes (more than 100 C / 200 F), consider type BEU, in which thetubes are free to expand. Keep in mind that BEU exchanger tubes can only be cleanedchemically, not mechanically, so these exchangers are best suited to clean service on boththe shellside and the tubeside.
3. For a chiller with refrigerant evaporating on the shellside and cooling a process fluidon the tubeside, consider a heat exchanger of type BKU.4. Similarly to Point 2 above, if the difference in operating temperature between shellsideand tubeside is more than 100 C (200 F), consider a design with hairpin tubes, a floatinghead or a floating tubesheet (types P–W). These types are best suited to dirty fluids, andmay be either horizontal or vertical.5. If you encounter a temperature cross — that is, if the outlet temperature of the hot fluidis below the outlet temperature of the cold fluid — then you cannot use a single BEM orBEU type heat exchanger. Consider a BFS type with a two-pass shell and a longitudinalbaffle, or two shells in series. Other types of heat exchanger, such as spiral and platetypes, are fully countercurrent and so better suited to handling temperature crosses.Material of construction. Do not trust the vendor to pick the right material ofconstruction for your service. That is your job. That said, do not take responsibility forthe material of construction unless you have agreed it with the user or verified it with anappropriate expert.Tube-to-tubesheet joints. These determine the integrity of your shell-and-tube heatexchanger. The basic guidelines are the following:1. For a design pressure of less than 300 psig and a design temperature below 180 C(350 F), use rolled and expanded tube-to-tubesheet joints. These are used primarily forwater, air and oil service.2. For higher design pressures or temperatures, use grooved, rolled and expanded tubejoints.3. When dealing with light hydrocarbons or other flammable fluids, even at low pressureand temperature, consider seal welding.4. For hydrocarbons or flammable vapors at high pressures and temperatures, consideradditional welding for strength.Special instructions. This category covers specifications including the tube pitch, baffletype, minimum tube diameter, tube length and orientation of the heat exchanger. Usecustomer specifications or guidance where available, and ask vendors whether these willhave any implications. If no specifications are available, use your judgment. For instance,if your shellside fluid is very fouling, use a square tube pitch to aid cleanability. Decreasebaffle spacing to increase turbulence, and thus heat-transfer coefficient, on the shellside.If you have a height limitation, ask the vendor to limit the tube length.Reviewing vendor quotes
After you have received your quotes it is time to review them and select a vendor. Hereare the most important points to look for: Basic process requirements: For both fluids (shellside and tubeside), the vendor’sspecification should match your specified flowrate, operating temperature and pressure,and properties such as density, viscosity, and thermal conductivity. Materials of construction, design pressure and design temperature as per yourinstructions. Fluid velocity: Should generally be in the range of 3–8 ft/s on both the tubeside and theshellside. Lower velocities will mean lower heat-transfer coefficients and larger requiredsurface areas. Compare the calculated clean heat-transfer coefficient ( U c) and the design heat-transfercoefficient ( U d) with typical values from your company sources or published literature(Table 2, p. 51). Do not expect close matches — each application is different, and heattransfer coefficients depend on many factors. If the U values proposed by the vendor arevery different from what you would expect, however, then the design may be at fault. Insuch a situation, review the design with the vendor.Table 2. typical Design heat-transfer coefficientsHot fluidCold fluidUd (Btu/h· F·ft2)WaterWater250–500Aqueous solutionAqueous solution250–500Light organicsLight organics40–75Medium organicsWater50–125Heavy organicsHeavy organics10–40Heavy organicsLight organics30–60Light organicsHeavy organics10–40If a vendor’s calculated heat-transfer coefficients are reasonably close to reliablepublished values, the thermal design is probably correct. Do not expect an exact match.Light organics are fluids with viscosities less than 0.5 cP. Medium organics are 0.5–1 cP,and heavy organics are above 1 cP.Source: “Process Heat Transfer”, Donald Q. Kern, McGraw-Hill Companies, 1950. Check the heat-transfer area. Different vendors will propose different values based onvarying exchanger geometry and calculated heat-transfer coefficients. Pick a geometrythat meets your requirements best. Check the heat duty and make sure it matches your specified value.
Check the code requirements. Check that the vendor has complied with any special instructions including tubediameter, tube pitch, tube length, baffle type, baffle pitch, and excess area. Check the price and delivery schedule for the heat exchanger. Weigh all the options and select a vendor.Close coordination with the heat exchanger vendor and a solid understanding of theprocess requirements are essential to heat exchanger design and selection. Byunderstanding different kinds of heat exchangers and developing a solid understanding ofheat-transfer coefficients, fouling factors and so on, you will be on the right track todesign and select the most appropriate heat exchanger for your process.Edited by Charles ButcherAuthorAsif Raza (Mississauga, Ont., Canada; Phone: 905–607–1335; Email:asifraza [email protected]) is an equipment design engineer at Praxair Canada. His workinvolves the design and specification of major equipment, such as cryogenic centrifugalpumps, shell-and-tube heat exchangers, vessels and vaporizers. He has more than 15years of experience in process design. His interests include sizing and specifying majorequipment, P&ID development, process simulation and selection of control logic. Beforejoining Praxair he was lead process engineer at Zeton Inc., where his work involveddesign and fabrication of pilot plants for research and development. Previously heworked with companies including Bantrel and SNC Lavalin. He holds a B.Tech degree inchemical engineering from Amravati University, India. Raza is a registered professionalengineer in the province of Ontario and is also a member of Ontario Society ofProfessional Engineers.
www.tema.org) issues its own design and manufacturing codes. There are three categories: TEMA C, B and R. In simple terms, TEMA C applies mostly to water, oil and air at low or moderate pressures and temperatures, and is the most cost-effective standard in cases where it is applicable. TEMA