OPTIMIZING PROCESS VACUUM CONDENSERS Graham Corporatio - PDF document

OPTIMIZING PROCESSVACUUM CONDENSERS Graham CorporationP.O. Box 719,20 Florence AvenueBatavia, N.Y. 14021-0719Phone: 716-343-2216Fax: 716-343-l 097Email: equipment@graham-mfg.comWebsite: http://www.graham-mfg.comINSIDE; REPRINTED FROM CHEMICAL ENGINEERING Vacuum condensers play a critical role insupporting vacuum processing operations.Although they may appear similar toatmospheric units, vacuum condensers havetheir own special designs, considerations andinstallation needs. By adding vacuumcondensers, precondensers andintercondensers (Figure l), system costefficiency can be optimized. Vacuumcondensing systems permit reclamation ofhigh value product by use of a precondenser,or reduce operating costs withintercondensers.A precondenser placed between the vacuumvessel and ejector system will recovervaluable process vapors and reduce vaporload to an ejector system minimizing thesystem’s capital and operating costs.Similarly, an intercondenser positionedbetween ejector stages can condense motivesteam and process vapors and reduce vaporload to downstream ejectors as well as lowercapital and operating costs.Vacuum condensers cannot be designed orconsidered as typical process heat exchangers.Doing so will result in less than optimalperformance with increased utility andcondensate treatment costs. For instance,internal geometry may not be modeled wellby standard heat transfer software becausecondenser design is proprietary and variesfrom one manufacturer to another. Also, tube-field layout and baffling are oftenunconventional and not suited for standardsoftware. It is also vital to incorporate ejectoroperation into vacuum condenser design.A number of primary CPI processes (rangingfrom glycerin manufacture to urea prilling)use vacuum condensers each requiring aspecial design that depends on the type ofvacuum condenser needed. For example, inurea plants, the main vacuum condensers areoutfitted with spray nozzles above the tubefield for removal of solidified productbuildup.Vacuum condenser systemsThe prevalent type of vacuum condensers areshell-and-tube. These look similar externallyto conventional shell-and-tube heatexchangers; however, their internal geometryis notably different. The major componentsof a vacuum condenser (Figure 2) include:lTubesheet( s)lSupport plateslChannels or bonnetsThe design and optimum operation of avacuum condenser is application specific, anddetermined by its tube-field layout and flowbaffling. These geometries strongly affectcondensation efficiency and pressure dropminimization. Under sub-atmosphericconditions, the need to minimize pressuredrop is the key design consideration. Pressuredrop across a vacuum condenser reducescondensation efficiency or product recoveryand, therefore, increases the operating cost ofa vacuum system.Vessel geometry affects both vapordistribution and flow pattern, whichultimately impacts condenser performanceand pressure drop. Poor flow distributionmay result in localized “dead spots” in acondenser that essentially reduce effectiveheat transfer surface area. Furthermore,improper baffling may result innoncondensable binding and, consequently, aloss in the system’s efficiency and vacuum.At higher vacuum levels, the design ofvacuum condensers becomes more critical andthe units are characterized by uniquegeometries or features. For instance, inglycerin plant condensers, which operatebelow 10 mm Hg, spacing between tubesvaries. Initially, the top tube row has spacingincreased to 1.62 times tube diameter. Thisallows high specific volume vapors todistribute above the tube field, and flow intothe bundle at velocities suitable for lowpressure drop. Tube spacing is then OPTIMIZING PROCESS VACUUM CONDENSERS James R. Lines and David W. Tice, Graham CorporationDesigning theseunits properlyinvolves morethan just usingstandard heat-transfer software reduced to a normal 1.25 times tube diameternear the final tube row, which ensures thatvelocities are sufficiently high to maintainproper heat transfer.Types of vacuum condensersThe geometries of surface condensersgenerally follow three basic designs thatcomply with standard nomenclatureestablished by the Tubular ExchangerManufacturers Assn. (TEMA; Tarrytown,N.Y.):1. Shellside-condensing design fixed tubesheettype, designated as: AXL, BXM, AEL orBEM. Figure 3 provides a clearer descriptionof the various “mix and match” geometriesand their designations2. Shellside-condensing design removablebundle type: AXS, AXU, AES or AEU3. Tubeside-condensing design fixed tubesheettype: AEL or BEMShellside condensingKey features of vacuum condensers withshellside condensation include:lVapor inlet connectionlVapor distribution space above the tubefieldMain condensing zonelNoncondensable-gas cooling and finalcondensing zonelNoncondensable-gas outlet connection (orvapor outlet)lCondensate outlet connectionCondensers with shell diameters greater than26 in. often have a longitudinal baffle thatruns virtually the entire tube length. Thistype of condenser is denoted as a TEMAcrossflow “X” shell. A majority of thecondensation occurs in the tube field prior tothe longitudinal baffle.Noncondensable gases and associated vaporsof saturation are drawn underneath thelongitudinal baffle by a low-pressure regioncreated by a downstream ejector, which isdesigned for that purpose. Asnoncondensables and vapors are drawnunderneath the longitudinal baffle, that changein direction separates condensate from thevapors. Condensate drops down via gravityto the bottom of the shell and is subsequentlydrained from the unit. Meanwhilenoncondensables and associated vapors aredrawn through tubes beneath the longitudinalbaffle for additional cooling and condensation.This separation of condensate fromnoncondensables and remaining vaporspermits final cooling ofnoncondensables to atemperature below thebulk condensatetemperature.Furthermore, tubesbeneath a longitudinalbaffle contain the coldestcooling water. Thisenables a system designwhereby finalnoncondensable gas andthe saturated vaporoutlet temperature isbelow the cooling wateroutlet temperature.Units with smallerdiameter shells (less than26 in.), denoted asTEMA “E” shells, arecharacterized by “up andover” baffles in the finalnoncondensable coolingsection. Here again, themajority of condensationtakes place in the tubefield area before the “up and over” bafflesection. Internal geometry is such that there isseparation of the condensate fromnoncondensables and vapors of saturation.Only noncondensables and associated vaporsof saturation are drawn into the “up andover” baffle section to ensure that heattransfer is maximized. Once again, it ispossible to cool noncondensables to atemperature below the cooling water outlettemperature or below the average condensatetemperature.In either case of shellside condensing, thedominant design factor is to coolnoncondensables to the coldest temperaturepossible, while at the same time maintainingminimum pressure loss. Ensuring thatnoncondensables are cooled to the lowesttemperature possible minimizes the amountof condensable vapors that saturate thosenoncondensable gases. Effective condenseroptimization requires cooling noncondensables to within 10-15°F of theinlet cooling-water temperature. This servesto minimize the amount of vapors thatsaturate the noncondensables and must behandled by a downstream ejector.Tubeside condensingAlthough shellside condensation is moreprevalent, tubeside condensing may also beused. In this case, cooling water is on theshellside, while noncondensables and vaporsare directed through the tubes. In thisconfiguration, vapors and condensate remainin intimate contact throughout the heattransfer area and exit this area together at thesame location. The shellside is baffled (as inany typical heat exchanger) because theshellside fluid is simply water.One special feature of tubeside condensers isin the bottom head, where the condensatedrops to an outlet drain and noncondensablegases are extracted through a connection onthe side of the head.Noncondensable gasesDue to the sub-atmospheric condition ofvacuum systems, air inleakage is always apotential problem. In addition, a particularprocess may already have variousnoncondensable gases in the process load.With noncondensables being present,condensation occurs along the cooling curve,and vapors of saturation exit the condenseralong with the noncondensables.The tube-field layout is designed to separatecondensate from noncondensables and theirvapors of saturation. It is common to havenoncondensables, along with their vapors ofsaturation, exit a condenser at one locationwhile condensate exits another.Flow distribution above the tube field isimportant so as to ensure that vapors andnoncondensables enter the bundle uniformlyand that there is full utilization of availableheat transfer area. Also, pressure drop isminimized by proper flow distribution, thusreducing utility and capital costs.Figure 4 shows heat release curves for theextreme cases of low noncondensable and highnoncondensable flow. Note the shape of therespective curves and the effect thatnoncondensable load has on logarithmic meantemperature difference (LMTD), heat transferrate and required surface area.Noncondensable gases serve to lower LMTDand heat transfer rate, while consequentlyincreasing required surface area of thecondenser.Precondenser pressure dropPressure drop in a precondenser has acompounded impact. Depending on theprocess, precondensers are positioned torecover valued overhead vapors as condensateprior to their introduction to an ejectorsystem. As pressure drop increases, morecondensable vapors exit the precondenserwith noncondensable gas. Not only does thisreduce the amount of condensable vaporrecovered, it increases the gas load to theejector system and its compressionrequirements. As load and compression rangeincreases, so do utility requirements andwastewater treatment costs. Pressure dropacross the intercondenser similarly increasesutility requirements for an ejector system.Table 1, p. 102, highlights the impact ofpressure drop across a precondenser.System interdependencyWithin a vacuum system, there is aninterdependency between an ejector andintercondenser. This relationship must beunderstood for optimum design and to ensurereliable operation. An intercondenser isdesigned to handle discharge load from apreceding ejector at a pressure equal to, orbelow, that which is achievable by thatejector. Furthermore, the intercondenser must condense the condensable vapors and coolnoncondensables in a manner that satisfies the capability ofthe next following ejector.Should an intercondenser not satisfy the dischargecapabilities of its preceding ejector or the suction capacityof the ejector that follows it, a discontinuity occurs. Theresult is that the preceding ejector ceases proper operation,resulting in a sharp rise in the operating pressure of thevacuum vessel, which ultimately affects product quality. Itis for this reason that ejector-condenser interdependencymust be understood and taken into account.Equipment installationProper installation of vacuum condensers is important forsmooth operation. Typical plant layouts allow vacuumcondenser condensate to drain by gravity to a condensatereceiver. The leg height of the condensate drain must besufficient to ensure that condensate is not lifted into theintercondenser because of the vacuum operation.A straight vertical drain leg is preferred. This may notalways be possible, however. Should a layout require anoffset, horizontal runs of pipe should not be used.Horizontal piping runs allow the formation of air pockets,which offer additional resistance to drainage, and maycause the flooding of a condenser.The suggested practice is to lay out a drain leg with no lessthen a 45 deg angle, measuring from the horizontal axis,and ensuring at least a 5ft straight length prior to theangled run of piping. Remember to always take intoaccount the operating pressure of the condensate receiver.As the condensate receiver’s operating pressure increases,so does required drain leg height. Figure 5, above, showsacceptable drain design.Equipment layoutPressure drop due to piping between components is just asimportant as pressure drop across a condenser. Keepingpipe diameter equivalent to connection size on thecondenser is one key to minimizing piping loss. Also, oneshould maintain interconnecting piping as short as possible.Furthermore, always try to position a precondenser or first-stage ejector as close to a vacuum vessel as possible. If atall possible, directly connect the two items; sometimes it ispossible to mount a precondenser directly atop a vacuumvessel. First stage ejectors may be coupled directly to thevacuum vessel, as well.Remember the importance and negative impact of even asmall pressure drop loss in a high vacuum processingsystem. A 2 mmHg pressure loss due to piping has agreater impact on equipment size, utility and cost when thatpressure drop is taken at 15 mm Hg absolute rather than at80 mm Hg absolute pressure.Edited by David J. Deutsch