7027033 efficient-industrial-heat-ex-changers


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7027033 efficient-industrial-heat-ex-changers

  1. 1. 1 INTRODUCTION A heat exchanger is process equipment used for transferring heat fromone fluid to another fluid through a separating wall. Usually heat exchangersare classified according to the functions for which they are employed. The most widely used heat exchanger is the Shell & Tube heatexchanger. It consists of parallel tubes enclosed in a shell. One of the fluidflows through the shell & the other flows through the tubes. The one, whichflows through the shell side, is called as shell side fluid & the one flowingthrough the tubes is called as tube side fluid. " When none of the fluid condenses or evaporates, the unit is called asHeat Exchanger." In this only the sensible heat transfers from the one fluidto another. Degradation is an inevitable process for every heat exchanger, butaffects some to great extent, depending upon the duties they are called uponto perform. Some heat exchangers never achieve their design objective.Their degradation stems from inadequate design or improper execution orpoor workmanship. Others achieve their design objective but thendeteriorates progressively in performance as time wears on. Deterioration may be due to fouling, where there is acceleration ofdeposits that increase the thermal resistance to heat transfer. This diminishesthe heat transfer while simultaneously increasing the compressor and thepump work input because of the partial blockage of fluid conduit. Fouling 1
  2. 2. may be overcome by cleaning, with the potential for the restoration of theheat exchanger to its original performance. Corrosion is another principle source of heat exchanger degradation.Corrosion of heat exchanger structural material arises from variety ofmechanisms and progressively weakens the element to the point where thefailure by the rupture or leakage occur is eminent. The corrosion productswill likely occupy a large volume, partially blocking the flow conduits &increasing the input pump work or inhibiting the mass flow rate of the flow. In heat exchanger the fluid flow do not follow the idealized pathanticipated from the elementary conditions. This departure from ideality canbe very significant indeed. As much as 50% of the fluid can behavedifferently from what is expected. Maldistribution of the flow is the wordoften used to describe unequal flow distribution in several parallel flowpaths found in heat exchanger. The maldistribution of the fluid flow isreduced generally by improving the baffle arrangement & proper designing& placement of the inlet & the outlet nozzle. The measures to combat or repair degradation of performance arediscussed ahead. 2
  3. 3. 2 TYPES OF HEAT EXCHANGER2.1 BASIC CLASSIFICATION (1)2.1.1 Regenerative type These heat exchangers have a single set of flow channels through arelatively solid massive solid matrix. The hot and the cold fluid pass throughthe matrix alternately. When the hot fluid is passing (called the ‘Hot Blow’)heat is transferred form the fluid to heat the matrix. Later when the cold fluidpasses through (called the ‘Cold Blow’), heat is transferred from the matrixto the matrix and the fluid cools. For moderate temperature applications thisheat exchanger is used because they may be made low in cost & the plastichoney comb or any finely divided material as the regenerative matrix.2.1.2 Recuperative type Recuperative Heat Exchanger Plate Heat Exchanger Tubular Heat ExchangerSpiral Plate - Fin Single - Pipe Double pipe Shell & Tube Plate - Coil Plate - Frame Cluster Pipe Fin Tube Fig. 1 It is equipped with separate flow conduits for each fluid. The fluidflows simultaneously through the heat exchanger in separate paths & heat istransferred from hot to the cold fluid across the walls of the flow section. 3
  4. 4. 2.2 CLASSIFICATION BASED ON TYPE OF FLUID FLOW (3)2.2.1 Liquid/Liquid This is by far the most common application of tubular exchangers.Typically, cooling water on one side is used to cool a hot effluent stream.Both the fluids are pumped through the exchanger so that the principal modeof heat transfer is forced convective heat transfer. The relatively high densityof liquid results in very high rates of heat transfer. So there is very littleincentive in conventional situations to use fins or other devices to enhancethe heat transfer.2.2.2 Liquid/Gas It is usually used for air-cooling of hot liquid effluent. The liquid ispumped through the tubes with very high rates of convective heat transfer.The air in cross flow over the tubes may be in forced or free convectivemode. Heat transfer coefficients on the airside are low compared with thoseon the liquid side. Fins are usually added on the outsides (air side) of thetubes to compensate.2.2.3 Gas/Gas This type of heat exchanger is found in the exhaust gas /air preheatingrecuperators of gas turbine systems, steel furnaces & cryogenic gasliquification systems. In many cases one gas is compressed, so the density ishigh, while the other is at the low pressure with a low density. Normally thehigh-density fluid flows inside the tubes. Internal and external fins areprovided to enhance the heat transfer. 4
  5. 5. 2.3 CLASSIFICATION BY FLOW ARRANGEMENTS (3) The flow arrangement helps to determine the overall effectiveness, thecost & the highest achievable temperature in the heated stream. The latteraffect most often dictates the choice of flow arrangement. The fig.2 indicatesthe temperature profile for heating & heated stream, respectively. If thewaste heat stream is to be cooled below the load stream exit, a counter flowheat exchanger must be used. Fig. 2 Thin Cold Fluid Thout Hot Fluid ∆Τ Seperating Surface Cold Fluid Tcout Co - Current Flow Tcin Surface Area A Thin Cold Fluid Tcout Hot Fluid Thout Cold Fluid ∆Τ Counter - Current Flow Tcin Surface Area A Thin Thin Tcout Tcout Tcin Thout Tcin Thout Cross Flow 5
  6. 6. 2.4 TUBULAR HEAT EXCHANGER CLASSIFICATION (1)2.4.1 Clustered pipe heat exchanger L-P Stream It is the development of singlepipe heat exchanger. Two or more H-P Streamtubes are joined by thermallyconducting medium. So that the heat is Soldertransferred between the fluids flowing Fig. 3in the tubes. Sometimes a cluster of tubes is arranged around a central coretube. High – density fluid passes through the core tube. The return stream ofthe low-density fluid passes through the multiple tubes arranged around thecore tube. The construction is favored in small cryogenic counter flow Heatexchanger.2.4.2 Double pipe heat exchanger It consists of central tube contained within a larger tube. It isrelatively cheap, flexible & hence used in smaller units. It is customary tooperate with high pressure and high pressure, high temperature, high densityor corrosive fluid in small inner tube, with less demanding fluid on outertube. Hot Fluid Cold Fluid Fig. 4 6
  7. 7. 2.4.3 Shell & Tube heat exchanger To increase the capacity or reduce the required length, more than oneinternal tube is incorporated within the outer tube enclosure. But the mostcommon form of multi tubular heat exchanger is the one shown in fig. 5.This one is widely used for liquid/liquid heat transfer. The best-knownstandards for the tubular heat exchanger are the TEMA – Standards of theTubular Exchanger Manufacturing Associations, which include the basicnomenclature & classification scheme for Shell & Tube. Heated Fluid Cooled Fluid Cold Fluid Hot Fluid Fig. 5 7
  8. 8. 2.5 PLATE HEAT EXCHANGER CLASSIFICATION (6)2.5.1 Plate & Frame It consists of a series of rectangular, parallel plates held firmlytogether between substantial head frames. The plates have corner ports & aresealed by gaskets around the ports & along the plate edges. Corrugatedplates provide high degree of turbulence even at low flow rates. In thisexchanger, hot fluid passes between alternate pairs of plates, transferringheat to cold fluid in the adjacent spaces. The plates are readily separated forcleaning and heat transfer area can be increased by simply adding moreplates. Plate heat exchangers are relatively effective with viscous fluids withviscosities up to about 30 kg/m.sec (300 poise)2.5.2 Spiral Plate A spiral plate heat exchanger can be considered as plate heatexchanger into which plates are formed into a spiral. The fluids flowbetween the channels formed between the plates. The spiral heat exchangersare compact units. For a given duty the pressure drop over a spiral heat exchanger willusually be lower than that for the equivalent shell and tube heat exchanger.Spiral heat exchanger give true counter current flow and can be used wherethe temperature correction factor for a shell and tube heat exchanger wouldbe too low. Because they are easily cleaned and turbulence in channels ishigh, spiral heat exchanger can be used for very dirty process fluids andslurries. 8
  9. 9. 2.6 SPECIAL PURPOSE HEAT EXCHANGER (6)2.6.1 Scraped surface heat exchanger Spring Clip Inner pipe "J" Spring Scraper Blade Fig. 6 Shell and tube heat exchanger is basically a double pipe heatexchanger with fairly large central tube, 100 to 300 mm in diameter,jacketed with steam or cooling liquid. The scrapping mechanically rotatingshaft provided with one or more longitudinal scrapping blades isincorporated in inner pipe to scrape the inside surface. The process fluid(viscous liquid) flows at low velocity through the inside pipe and cooling orheating medium flows through the annular space created between twoconcentric pipes. The rotating scrapper continuously scrapes the surface thuspreventing localized heating and facilitating rapid heat transfer. Liquid-solid suspensions, viscous aqueous and organic solution andfood products, such as organic juice concentration are often heated or cooledin such type of exchanger. It is widely used in paraffin wax plants. 9
  10. 10. 2.6.2 Finned tube Heat exchanger When the heat transfer coefficient of one of the process fluids is very lowas compared to the other, the overall HTC becomes approximately equal tothe lower coefficient. This reduces the capacity per unit area of heat transfersurface, making it necessary to provide very large heat transfer area. Suchsituations often arise in, 1. heating of viscous liquids. 2. heating of air or gas stream by condensing steam. Air or gas side HTC is very low in comparison of film coefficient on thecondensing side. In such cases it is possible to increase the heat transfer byincreasing / extending the surface area on the side with limiting coefficient(air, gas or viscous liquid side) with the help of fins. The heat transfer area is substantially increased by attaching the metalpieces. "The metal pieces employed to extend or increase the heat transfersurface are known as fins". The fins are most commonly employed onoutside of the tubes. According to the flow of the gas, longitudinal andtransverse fins are used.2.6.3 Graphite Block heat exchanger Generally heat exchangers are made from various metals and alloys,suitable to process streams. But corrosive liquids like H2SO4, HCl etc.require the use of exotic metals as titanium, tantalum, zirconium and others. In such cases, graphite heat exchangers are well suited for handlingcorrosive fluids. Graphite is inert towards most corrosive fluids and has veryhigh thermal conductivity. Graphite being very soft, these exchangers aremade in cubic or cylindrical blocks. 10
  11. 11. 2.6.4 Jackets and cooling coils in vessels In chemical industries a number of reactions are carried out in agitatedvessel. In such cases, addition or removal of heat is conveniently done byheat transfer surface, surface that can be in the form of jacket fitted outsidethe vessel or the helical coil fitted to inside. Jackets as well as helical coils are used for heating or cooling purposedepending upon the situation. Helical Vessel Coil Jacket Baffle Agitator Fig. 7 11
  12. 12. 3 IMPORTANCE OF HEAT EXCHANGER3.1 INTRODUCTION Heat recuperators or heat exchangers as they are called so, are piecesof equipment, which can abstract sensible heat from one stream of flowingfluid and supply it to another stream. They are essential features of allproduction process in chemical industry. Because of importance ofimproving heat recovery, consequent on the very rise in prime energy costs.Heat exchangers are becoming increasingly important in the heating &ventilating field as well.3.2 MAIN USES OF HEAT RECUPERATORS (4) 1. To extract useful heat from the waste hot liquid & gases. The heat is transferred to secondary fluids, which can then be used for either space heating or for the supply of preheated water to the boiler. 2. For normal heat transfer from the stream heaters or flues to circulating air, in order to raise this air to the required working temperature. 3. For normal operating of air-conditioning equipment, in which, the heat is being abstracted from room air by refrigeration fluid or by chilled air. 4. For heat recovery from exhaust air, flue gases & other sensible heat source. 12
  13. 13. 3.3 UNIT OPERATIONS (3)3.3.1 Exhaust – Gas stream Recuperation is the most promising candidate for heat recovery fromhigh temperature exhaust gas streams. As shown in fig.8 the hot gases willbe cooled by the incoming combustion air which will be supplied to thesame furnace. Because of the temperature of the gases leaving the furnace,the heat exchanger being selected is the radiation recuperator. This is theconcentric tube heat exchanger, which replaces the present stack. The incoming combustion air is needed to cool the base of therecuperator & thus the parallel flow occurs. In figure, temperature profilesketch is drawn. It is seen that, in parallel flow heat exchanger, heat recoveryceases when the two streams approach a common exit temperature. Cooled Exhaust Gas Heated Furnace Tc Th Air Height of Chimney as st G Coo au ling Exh Gas Cool Furnace Air Th Temperature Tc Hot Exhaust Gas Fig. 8 13
  14. 14. 3.3.2 Boiler economizer An economizer is constructed as a bundle of finned tubes, installed inboiler breaching. Boiler feed water flows through the tube to be heated bythe exhaust gases. The extent of the heat recovery in the economizer may belimited by the lowest allowable exhaust gas temperature in the exhaust stack. The exhaust gases may contain water vapor both from the combustionair & from the combustions of hydrogen that is contained in the fuel. If theexhaust gases are cooled below the dew point of the water vapor,condensation will occur & may cause damage to the structural material. 300°F Flue Exhaust 220°F Feed water Finned tube from deaerator Economiser Boiler Exhaust 500°F Water Tube Boler Fig. 9 14
  15. 15. 4 CORROSION IN HEAT EXCHANGER (1)4.1 INTRODUCTION Corrosion is defined as "The degradation of a material because ofreaction with environment". It is the part of the cycle of growth and decay that is natural order ofthings. Corrosion is principal cause of failure for engineering systems. Theannual cost of corrosion runs grater than costs of floods, and earthquakes.4.2 UNIFORM OR GENERAL CORROSION Uniform or general corrosion is the most common form of corrosion.It is characterized by chemical or electrochemical reactions that proceeduniformly over the entire exposed surface or a substantial portion of thatsurface. The metal becomes progressively thinner and eventually failsbecause of the stress produced on it. This type of corrosion is easy to handle. The rate of decompositioncan be can be determined by comparatively simple immersion test of thespecimen in the fluid. The life of the equipment can therefore be predictedand extended to the degree required by the addition of corrosion allowanceto the metal wall thickness to sustain the pressure or the other stress loadingapplied. 15
  16. 16. Prevention of Uniform general corrosion Uniform corrosion can be prevented or reduced by the selection ofappropriate materials (including internal coatings), the addition of corrosioninhibitors to the fluid, treatment of fluids to remove corrosive elements andthe use of the sacrificial cathodic protection or impressed electricalpotentials. Other forms of corrosion are difficult to predict. They tend to belocalized and concentrated with the consequent premature or unexpectedfailure.4.3 GALVANIC OR TWO METAL Copper ZincCORROSION When two dissimilar metals are Electrolyteimmersed in a corrosive or electricallyconductive solution, a voltage willbecome established between them. Itthe metals are then connected byelectrically conducting path, a smallcurrent will pass continuously Fig. 10between them. The principle is shownin fig.10. Corrosion of less corrosion resistant metal is accelerated and thatof the more resistant metal is decreased, as compared with their behaviorwhen they are not coupled electrically. The less resistant metal is describedas Anodic and the more resistant metal as ‘Cathodic’. Usually corrosion ofthe cathode is virtually eliminated. The combination of dissimilar metals and a corrosive or electricallyconductive medium constitutes a galvanic cell. The various metals and 16
  17. 17. alloys, along with other materials of interest can be arranged in order ofdecreasing corrosion resistance as shown in table. The noble metals leading the list are cathodic and the least subject tocorrosion. Those at the bottom are anodic and most subjected to attack. Thecombination of metal from the upper half of the table with any other furtherdown the table will establish a galvanic cell with the potential to acceleratethe rate of corrosion of the anode, lower in the table, while decreasing thecorrosion rate of the cathode. The effect increase for the metals that isfurther apart in table. Magnesium will rapidly corrode in seawater inconjunction with a titanium cathode, but less rapidly in combination withaluminum or zinc.Prevention of Galvanic Corrosion Use a single material or a combination of materials that are close in thegalvanic series. 1. Avoid the use of small ratio of anode area to the cathode area. Use equal areas or large ratio of anode to cathode area. 2. Electrically insulate dissimilar metals where possible. This recommendation is shown in fig.11 Insulating Sleeve Insulating Washer Nut Bolt Pipe Valve Fig. 11 17
  18. 18. 3. Local failure of the protective coating, particularly at the anode can result in small anode to cathode area, marked by accelerated galvanic corrosion. Maintain all coatings in good condition, especially at the anode. 4. Avoid the use of riveted or threaded joints in favor of welded or brazed joints. 5. Install a sacrificial anode lower in the galvanic series than both the materials involved in the process equipment.4.4 CREVICE CORROSION It is charachterised by the intense localcorrosion in the crevices and other shieldedareas on the metal surfaces exposed to stagnant Crevicecorrosive liquids. It can occur where anyundistributed liquid film exists, such as at asmall hole, gasket - flange interface, lap joints,surface deposits, and the crevice under bolt and Crevice Fig. 12rivet heads. Relative to heat exchangers, it isimportant to note that nonmetallic deposits (fouling) of sand, or crystallinesolids may act as a shield and create the necessary stagnant condition theessence of crevice corrosion. The mechanism of crevice corrosion is associated with the depletionof the oxygen in the stagnant liquid pool, which results in the corrosion ofthe metal walls adjacent to the crevice. This type of corrosion occurs withmany fluids but is particularly intense with those containing chlorides. Thenature of electrochemical process is such that the corrosion attack is 18
  19. 19. localized within the stagnant or shielded area while the surrounding surfacesover which the fluid moves suffer little or no damage. Some time is required between the initial establishment to theconditions for the crevice corrosion and the occurrence of the visibledamage, which is called the incubation period.Prevention of Crevice Corrosion 1. Use welded butt joints instead of bolted or riveted joints. Good welds with deep penetration are required to avoid porosity and crevices on the inside if the joint is welded on one side only. 2. Eliminate crevices by continuous welding by solder or brazing filling and by caulking. 3. Design to eliminate the sharp corners, crevices and the stagnant areas and complete drainage. 4. Clean at regular intervals. 5. Eliminate the solids suspended in the fluids, if possible. 6. Weld tubes to the tube sheet, instead of rolling.4.5 PITTING CORROSION Pitting corrosion is the phenomenonwhereby an extremely localized attack resultsin the formation of the holes in the metalsurface that eventually perforates the walls. Itis shown in the fig.13. The holes or pits are ofvarious sizes and may be isolated or grouped Fig. 13very closely. 19
  20. 20. The mechanism of pitting is very close to crevice corrosion. Pitsusually grow in the direction of gravitational action i.e. downward formhorizontal surfaces. They sometimes develop on vertical surfaces, but onlyin very exceptional cases do pits grow upward form the bottoms ofhorizontal surfaces. As with crevice corrosion an incubation period is required beforepitting corrosion starts; thereafter, it continues at an accelerated rate. Furthermore once below the surface, the pits tend to spread out, undermining thesurface as shown in figure. This particularly is unfortunate for the smallsurface pits can easily become obscured by the corrosion products or othersediments and the deposits. Failure as leak resulting from the completeperforation of the metal wall therefore occurs suddenly and unexpectedly. Most pitting corrosion arises from the action of the chloride orchlorine containing ions. The process of establishing a pit site is unstableand is interrupted by any movement of the fluid over the surface. Thus,pitting corrosion is rarely found in metal surface over which fluids moveconstantly. Even in these few cases it can be reduced if the fluid velocity isincreased. Often a heat exchanger pump or a tube carrying a corrosive fluidshows no sign of pitting corrosion when in service but rapidly deteriorates ifthe plant is shutdown and the fluid not drained from the system. Stainless steel alloys are particularly susceptible to pitting corrosionattack. Carbon steel is more resistant to pitting than stainless steel.Prevention of pitting corrosion The principal measure is to use material that is known to be resistantto pitting. These include: 20
  21. 21. Titanium, Hastelloy C or Chloriment 20, Type 316 stainless steel, Type 304stainless steel (Pits badly in chloride solution).4.6 EROSION CORROSION Erosion corrosion is the Water Flowterm used to describe corrosion Corrosion Corrosion Film Original metal pits surfacethat is accelerated as a result ofincrease in the relative motionbetween the corrosive fluid and Fig. 14the metal wall. The process isusually a combination of chemical or electrochemical decomposition andmechanical wear action. Erosion corrosion therefore differs from most otherforms of corrosion, where the rate of attack is highest under stagnant or low-velocity conditions. Erosion corrosion can be recognized by the appearance of thegrooves, gullies, and waves in the directional pattern, similar to sandformations on the shorelines. Fig.14 is a sketch of the erosion corrosioncorrosion pattern on a condenser tube wall. Failure by erosion corrosion canoccur in a relative short time (a matter of weeks or months). It often comesas a surprise, following satisfactorily tests for the corrosion susceptibility ofthe specimen submerged in the corrosive fluid under static condition. Metals that depend for their corrosion resistance on the formation of aprotective surface film are particularly susceptible to attack by the erosioncorrosion. Aluminum and stainless steel are in this category. The protectivefilm is eroded by mechanical scrubbing, exposing the soft core to chemicalor electrochemical attack in addition to the continued mechanical wear. 21
  22. 22. Many fluids that are not normally considered aggressive corrosionagents can promote erosion corrosion. High velocity gases and vapors athigh temperature may oxidize a metal and then physically strip off theotherwise protective scale. Many erosion corrosion failures in heat exchanger, occurs in the tubeside, particularly at the tube inlet; the process is frequently called inlet-tubecorrosion. It arises essentially from the highly turbulent flow ensuing as aconsequence of the sudden change in the section as the fluid leaves the inletbonnet and enters the reduced flow section of the tubes. An increase in therate of erosion corrosion as the velocity increases. For many materials thereappears to be a critical value, above which the rate of attack increases.Prevention of Erosion corrosion 1. Use materials with superior resistance to erosion corrosion. 2. Design for minimal erosion corrosion. 3. Change the environment. 4. Use protective coating. 5. Provide cathodic protection.4.7 STRESS CORROSION Stress corrosion is the name given to the process whereby the cracksappear in the metals subject simultaneously to a tensile stress and specificcorrosive media. The metal is generally not subjected to appreciable uniformcorrosion attack but is penetrated by fine cracks that progress by expandingover more of the surface and proceeding further into the wall. The cracksmay or may not be branched. They may proceed along the grain boundaries 22
  23. 23. only or may be transgranular and advance with no preference to follow thegrain boundaries. Stress corrosion cracks develop in specific metal-fluid combinationwhen the stress level is above a minimum level that depends on thetemperature, alloy structure, and environment. In some materials minimumstress levels for crack formation are as low as 10% of the yield stress. Inother cases the critical value may be as high as 70%. For stress corrosion cracks to initiate, the stress must be tensile incharacter and exceed the critical level referred to above. They are inducedfrom any source, including residual welding stress. Stress corrosion oftenoccurs in lightly loaded structures that are not stress relived after fabrication. Not all metal fluids are susceptible to cracking. Stainless steels crackwith fluids containing chloride but not with ammoniacal fluids, whereasbrasses crack in ammonia but not in chlorides. It is likely that stress corrosion cracks are initiated at a corrosion pit orother surface regularity. The base of the pit acts as a stress raiser so the localstress concentration is very high. Once a crack is started, the stress at the tipof the crack is very high and the fosters continuing development of thecrack. As the crack penetrates further into the metal, the remaining wallsection assumes the whole load. The general stress level is therefore raisedand is further magnified at the tip of the crack, so the rate of propagation isaccelerated. Eventually the metal fails suddenly and catastrophically whenthe stress in the remaining metal exceeds the ultimate.Prevention of Stress Corrosion 1. Lower the stress level below the critical threshold level by reducing the fluid pressure or increasing he wall thickness. 23
  24. 24. 2. Relieve the stress by annealing. 3. Change the metal alloy to one that is less subjected to stress corrosion cracking in the given environment. 4. Modify the corrosion fluid by process treatment or by adding corrosion inhibitors, such as phosphates.4.8 HYDROGEN DAMAGE Hydrogen damage is a term applied to the variety of consequencesfollowed by exposure of metal to hydrogen. Hydrogen may exist in themono atomic form (H) or the diatomic form (H2). Atomic hydrogen candiffuse through many metals. Molecular hydrogen cannot do this, nor canany other chemical species. There are various source of atomic hydrogen,including high temperature atmospheres, corrosion and electrochemicalprocess. Corrosion and cathodic protection, electroplating, and electrolysis,all produce hydrogen ions, which reduce to atomic hydrogen molecules.Some substances (sulfide ions, phosphorus and arsenic compounds) inhibitthe reduction of hydrogen ions, leading to a concentration of atomichydrogen on the metal surfaces. The hydrogen damages are of four distincttypes.4.8.1 Hydrogen Blistering The production of hydrogen ions will, in some way, result in theaggregation of hydrogen ions, atomic hydrogen and molecular hydrogen onthe metal surface of a heat exchanger. Some of the atomic hydrogen willdiffuse into and through the metal before reducing to molecular hydrogen onthe outer surface. 24
  25. 25. The atomic hydrogen diffusing through the metal will enter any voidsin the metal. Some will then reduce to molecular hydrogen, which cannotpermeate the metal wall. The equilibrium pressure for atomic pressure forthe atomic and the molecular hydrogen is several hundred thousandatmospheres so the one way accumulative process continues, giving rise tovery high pressures - far exceeding the yield stress of the material. Thegrowth appears as "Blisters" on the wall of the heat exchanger.4.8.2 Hydrogen Embrittlement It arises from the source as blistering - the penetration of apparentlysolid metal by atomic hydrogen. In some metals the hydrogen reacts to formbrittle hydride compounds. In others the mechanism of embrittlement is notknown. Alloys are most susceptible to cracking from hydrogenembrittlement at their highest strength levels. The tendency to embrittlementincreases with the hydrogen concentration in the metal.4.8.3 Decarbonisation and Hydrogen attack It is associated with metals exposed to high temperature gas streamscontaining hydrogen and variety of other gases. Decarbonisation is theremoval of carbon from a steel alloy on exposure to hydrogen at hightemperatures. It results in reduction of tensile strength and increase inductility and creep rate. Hydrogen attack is the interaction of metals or analloy constituent with hydrogen at high temperature. 25
  26. 26. Prevention of Hydrogen Damage 1. Use of void free steels. 2. Use of metallic, inorganic and organic coatings and the liners in steel vessels. The liner must be impervious to hydrogen penetration and resistant to other media in the vessel. Carbon steel clad with nickel is sometimes used. Rubber, plastic and brick liners are also used. 3. Addition of inhibitors to reduce corrosion and the rate of hydrogen - ion production. These are economically feasible in closed circulating systems. 4. Fluid treatment to remove hydrogen – generating compounds such as sulphides, cyanides and phosphorous containing ions. 5. Use of low hydrogen welding rods and the maintenance of dry conditions during welding operations. Water and water vapor sources are major sources of hydrogen. 26
  27. 27. 5 MALDISTRIBUTION OF FLUID FLOW5.1 INTRODUCTION The fluid flows do not follow the idealized paths anticipated from theelementary considerations. These departures form ideality can be verysignificant indeed. As much as 50% of the fluid can behave differently fromwhat is expected, based on the simplistic model. The maldistribution of flowis a term often used to describe unequal flow distribution in the severalparallel flow paths found in most heat exchangers.5.2 THE TINKER DIAGRAM (1) Flow on the shell side of the shell and tube heat exchanger, wasclassified by Tinker, into a number of separate streams, as representeddiagrammatically in fig.15, 16. The A stream represents flows that occur inthe clearance between the baffles tube holes and the tubes. Flow is due topressure drop between the upstream and the downstream sides of the baffle.The B stream is the true cross flow stream, passing through the tube bundlesand performing the real function of the shell-side fluid. The C stream bypasses the tube bundle and flows in the annulusbetween the shell and the tube bundle. This is highly ineffective use of thefluid. If the tube bundle shell clearance is greater than the tube pitch, it isadvisable to include a sealing device to inhibit bypass flow. The sealingdevices can be stripes, rods or dummy tubes, as shown in fig.16. 27
  28. 28. The F stream includes other bypass streams that arise when the tubepartitions of the multipass tube bundles are arranged parallel to the directionof the main cross flow stream. The D stream is leak flow that occurs in theclearance space between the edge of the baffle and the shell. This representsdirect loss of fluid, for it serves no useful heat-transfer function. D A C A B B A Fig. 15 Tinker diagram Bypass stealing strips Dummy rods or tubes Fig. 16 Seal for by – pass flowNote : For more information on “Maldistribution of Fluid Flow” refer TEMA (Tubular Exchanger Manufacturers Asso.) 28
  29. 29. 5.3 PARALLEL - PATH FLOW (1) Flow paths in the tube side of shell and tube heat exchanger cannot bemade absolutely identical and fluid flows are incredibly sensitive toapparently trivial differences between one path and another. When thenumber of parallel paths is limited to two or three and the paths are highlyrestricted, the difference in channel mass flow rates may be as high as 90percent. The flow is then function of some power of the principal flowresistance parameter e.g. the third power of the width of a slit or the squareof the cross-section area of a flow aperture. Tube distortion in bending or the squashing resulting from improperhandling fabrications, can contribute appreciably to flow maldistribution asshown in fig.17. A difference in the mass rate of flow through the tubecarries the implication that the flow velocity is significantly different. Theheat transfer rate depends on the fluid velocity and the tube wall and thefluid temperatures depend on the heat transfer. Low mass flow and fluidvelocity in some tubes may give rise to high fluid and wall temperatureswith accelerated corrosion and fouling deposition rates. The fouling depositsand products of corrosion exacerbate the difference in flow resistancebetween one tube and the other and further diminish the mass flow in tubesalready starved of fluid. The process is a cancer feeding on itself. Alternative solutions to heat transfer problem are also explored.Special heat exchangers are shown in the fig.18. The flow channel are ofvariable geometry designs to incorporate a compensatory feedbackmechanism, acting to adjust the duct geometry to ensure uniform distributionof flow in various channels. The miniature high performance heat exchangerwas designed to achieve huge NTU of 200 (The NTU of most of industrialexchanger is less than 5). Even with great attention to manufacturing detail, 29
  30. 30. the early high performance heat exchangers were unable to exceed an NTUof 33. With the compensation feed back geometry, values of 167 wereachieved. Tube deformation increases flow resistance. Tube subject to erosion corrosion at the site of deformation Fig. 17 Cold Flow Hot Flow Fig. 18 30
  31. 31. 5.4 STAGNANT AREAS (1) Disappointing heat exchanger thermal performance often arises fromthe creation of stagnant areas in the fluid – flow circuits. In stagnant or semistagnant areas the fluid velocities are, by definitions, zero or negligibly low.The consequences are often very serious. The obvious effect is that with lowfluid velocity area for heat transfer is not effectively utilized. Less obviousbut of greater importance is the fact that corrosion and fouling processes arehighly accelerated under stagnant conditions. Sediments in slurries aggregatein the low velocity areas. Surface temperature in the low velocity areas maybe appreciably higher than the mean design condition, which furtheraccelerates the chemical reactions exacerbating the corrosion and foulingprocesses. A common location of semi stagnant fluid zones in shell and tube heatexchanger is the region on the shell side between the tube sheet and the inletand outlet nozzles (fig.19). It is necessary to establish the centerlines of theinlet and outlet nozzles some distance from the tube sheets so as toaccommodate the nozzle flanges and to provide sufficient shell strength inthe high stress areas near the tube sheets. The existence of some low velocityregions on the shell side near the ends of the tubes is then virtuallyinescapable but is frequently overlooked by inexperienced thermaldesigners. They fail to add extension to the calculated tube length tocompensate for the “dead area”. Baffle design and placement are the principal means by which toensure adequate fluid velocities on the shell side and a well – regulated,dispersed flow. Even good designs can be hopelessly compromised if theyare improperly or inadequately executed. Excess clearance of the baffles inthe shell will certainly facilitate loading the tube bundle in the shell during 31
  32. 32. the construction. However, that clearance will lead to substantial bypassingof the fluid at the periphery of the baffle, so that little of the fluid actuallytraverses the tube bundle. Excessive clearance of the tube holes will greatlyfacilitate construction, but again will result in a proportion of the fluid notpassing through the tube bundle as intended. In figure upper diagram shows the tube bundle correctly installed. Inlower diagram the bundle has been reversed. It is immediately clear that thecompartments between the tube sheets and the first and last baffles arecompletely stagnant and virtually useless for heat transfer. The effectivenessof the tube bundle is reduced by as much as 40 percent. (a) Stagnant areas (b) fig. 19 (a) correct (b) incorrect placement of the tube bundle in shell and tube heat exchanger 32
  33. 33. 6 FOULING6.1 INTRODUCTION (2) Most process application involve fluids that form some type ofadhering film or scale on to the surface onto the inside or outside of the tubewall separating the two systems. These deposits may vary in nature (brittle,gummy), texture thickness, thermal conductivity, ease of removal etc.Although there are deposits on the clean tube or the bundle, the designpractice is to attempt to compensate for the reduction in heat transferthrough these deposits by considering them as resistance to heat transfer.These resistances or fouling factors have not been accurately determined formany fluids and metal combinations. Yet the general practice is to “throwin” a fouling factor. This can be disastrous to an otherwise good technicalevaluation of the expected performance of the unit. Actually considerableattention has to be given to such value as the temperature range, whichaffects the deposits, the metal surface (steel copper, nickel) as it affects theadherence of the deposit and the fluid velocity as it flows over the deposit orelse moves the material at such a velocity to reduce the scaling or fouling. The percentage effect of the fouling factor on the effective overallheat transfer coefficient is considerable more on units with the normally highvalue of the clean unfouled coefficient than for one of low value. Forexample an unit with clean overall HTC of 400 when corrected for 0.003 thetotal ends up with effective coefficient of 180, but a unit with clean 33
  34. 34. coefficient of 60, when corrected for 0.003 fouling allowance, shows aneffective coefficient of 50.5 as shown in the graph (Fig.20). Fig. 20 Effect of fouling resistance on transfer rates (2) 34
  35. 35. F.F. U 0.286 3.5 0.25 4.5 0.182 5.5 0.125 8 After 16 Months 0.0825 12 After 6 Months Clean 0.04 25 0.02 0.01 Gas outside tubes 500 100 50 30 20 17 15 Gas inside tubes Flow Rate Fig. 21 Graph for prediction of fouling and HTC as a function of velocity over a period of time (2) The above (fig.21) working chart presents a plot of actual operatingUa values to allow projection back to infinity and to establish the basefouling factor after the operating elapsed time. The flow rate inside oroutside the tubes is plotted against the overall heat transfer coefficient, U. As the value of B or the fouling factor increases with time, theengineer can determine when the condition will approach that time whencleaning of exchanger will be required. Gas flows are used because usuallygas film controls in a gas – liquid exchanger. Fouling factors are suggested by TEMA in table below. These valuesare predominantly for the petroleum operations, although portions of thetable are applicable to general use and to petrochemical process. 35
  36. 36. GUIDE TO FOULING RESISTANCES (2) Fouling resistance for Industrial fluidsOils:Fuel oil 0.005Quench oil 0.004Gases and vapors:Steam (non oil – bearing) 0.005Compressed air 0.001Ammonia vapor 0.001Chlorine vapor 0.002Coal flue gas 0.010Liquids:Refrigerant liquids 0.001Ammonia liquid (oil – bearing) 0.003Co2 liquid 0.001Chlorine liquid 0.002 Fouling resistances for chemical processing streamsGases and vapors:Acid gases 0.002Solvent vapors 0.001Liquids:MEA and DEA solutions 0.002Caustics solutions 0.002Vegetable oils 0.003 Fouling resistance for natural gasoline processing streamGases and vapors:Natural gas 0.001Overhead products 0.002Liquids:Rich oil 0.002Natural gasoline 0.001Crude and vacuum liquids:Gasoline 0.002Kerosene 0.003Light gas oil 0.003Heavy gas oil 0.005 36
  37. 37. 6.2 GENERAL CONSIDERATIONS (2) Fig.22 shows data on some fluids showing the effects of velocity andtemperature. Also see fig.23. The fouling factors are applied as a part of the overall HTC to both theinside and the outside of the heat transfer surface using the factors that applyto the appropriate material or fluid. As a rule the fouling factors are appliedwithout correcting for the inside diameter to outside diameter, because thesedifferences are not known, to any degree of accuracy. To fouling resistanceof significant magnitude, a correction is made to convert all values to theoutside surface of the tube. Sometimes only one factor is selected torepresent both sides of the transfer fouling film or scales. In the tables the representative or typical fouling resistances arereferenced to the surface of the exchanger on which the fouling occurs - thatis, the inside or the outside tubes. Unless the specific plant/equipment datarepresents fouling in question, the estimates listed in table are the reasonablestarting point. It is not wise to keep changing the estimated fouling toachieve the specific overall HTC, U. Fouling can be generally kept tominimum provided the proper and general cleaning of the surface takesplace. Unless a fabricator is guaranteeing the performance of the exchangerin a specific process service they cannot and most likely will not accept theresponsibility for the fouling effects on the heat transfer surface. Therefore,the owner must expect to specify a value to use in the thermal design of theequipment. This value must be determined with considerable examinationsof the fouling range, both inside and the outside of the tubes and bydetermining the effects of these have on the surface area requirements. Just alarge unit may not be the proper answer. 37
  38. 38. Fig.. 22 Fouling factors as a function of time & temperature 0.03 . .P M to Oil black F 2° gFouling Resistance - ro or ri -3 atin °C Lamp ax 0.02 - 86 ric t W hal Asp Lub in d ra f Roa Pa 0.01 Scale - Boiler CaSO4 Co ke Cracking Coil 0.02 0.04 0.06 0.08 0.10 Thickness of layer - Inches Fig. 23 Fouling resistance offered by various substances 38
  39. 39. 6.3 OVERALL HEAT TRANSFER COEFFICIENT ‘U’ (2) In a heat exchanger the process of heat transfer from hot fluid to coldfluid involves various conductive and convective process. This can beindividually represented in terms of thermal resistances. The summation ofindividual resistances is the total thermal resistance and its inverse is theoverall HTC, U. That is, 1 = 1 + Ao 1 + Rfo + Ao Rfi + Rw U ho Ai hi AiWhere,U = overall heat transfer coefficient based on outside area of tube wallA = area of tube wallh = convective heat transfer coefficientRf = thermal resistance due to foulingRw = thermal resistance due to wall conduction and suffixes ‘i’ and ‘o’ refer to the inner and outer tubes, respectively. It is customary in design work for the heat transfer coefficient ho andhi to be determined from complicated relations involving the Nusselt,Prandtl, Reynolds and Grashof numbers. Similarly, the thermal resistance isdetermined from calculations involving properties and dimensions of thematerial of the tube walls. Such detailed process is not involved indetermining the fouling resistance, the so called fouling factors Rf and Rfo.The uncertainty is such that one simply includes arbitrary values of thefouling factor selected from the sources based on the experience. The lessexperience on has, the less confidence one will have in the eventual result. 39
  40. 40. 6.4 FOULING AS A FUNCTION OF TIME (1) The assumption of constant Avalues for the internal and the external B Dfouling factors implies that, when put Ein service, the new heat exchanger Cinstantaneously deteriorates to thefouled condition. Of course it does notdo this, but instead deteriorates Timeprogressively. Considerable time, Fig. 24years, perhaps may elapse before it arrives at the condition where it can nolonger perform adequately and must be cleaned. The build up of fouling resistance as a function of time may followvarious forms as indicated in fig.24. Curve A describes a process startingwith clean surfaces having zero fouling resistance, which then develops atconstant rate with time. Curve B describes a process where the foulingresistance develops at a progressively diminishing rate. The family of curvesC, D and E all share a lengthy incubation or induction period in which thereis little or no build up of fouling resistance, followed by a rapidly increasingbuild up. There is therefore a substantial time lapse before the heat exchangerfouling resistance approaches the design value arbitrarily selected from someexperience based source. When first put into service, the heat exchanger willoperate with a reduced thermal resistance and therefore with surplus of heattransfer area. In many cases involving boiling, the fouling resistance is theprincipal resistance. Thus, when the heat exchanger is new, the availabletemperature difference may be so great as to carry the process into the film 40
  41. 41. boiling region, with the possibility of enhanced surface corrosion andconsequent accelerated development of fouling resistance. In other cases the new heat exchanger with zero fouling resistancemay be so effective as to overcool the process stream. To compensate thecooling water flow may be reduced, with the result that the water velocity isdecreased and the water temperature increased. Both these factors are highlyconducive to fouling on the water – side. The provision of excess allowancefor fouling or an excess heat transfer area “just to be on the safe side” doesnot automatically increase the interval before cleaning is necessary; quitelikely it has the reverse effect. The excess area has the reduced flowvelocities and elevated temperatures, so the exchanger deteriorates inperformance at drastic rates.6.5 MECHANISMS OF FOULING (1) Various mechanisms of fouling have been recognized and can becategorized as follows: 1. Precipitation or scaling fouling : Precipitation on hot surfaces or due to inverse solubility. 2. Particulate or scaling fouling : Suspended particles settle on heat transfer surface. 3. Chemical reaction fouling : Deposits formed by chemical reaction in the fluid systems. 4. Corrosion fouling : corrosion products produced by a reaction between fluid and the heat transfer surface and tube surface becomes fouled. 5. Solidification fouling : Liquid and/or components in liquid solution solidify on tube surface. 41
  42. 42. 6. Biological fouling : Biological organisms attach to heat transfer surface and build a surface to prevent good fluid contact with the tube surface. Fouling occurs to some extent in all systems where liquids, gases andvapors are being heated or cooled. The process may involve boiling,condensing or heat transfer without phase change. The greatest source offouling, principally inverse solubility crystallization and chemical reactionsoccurs on hot surfaces in heating process without phase change. Coolingprocesses without phase change also results in appreciable fouling as a resultof particulate deposition, sedimentation and chemical reaction.6.6 EFFECTS OF SURFACE MATERIAL AND STRUCTURE (1) By the time the fouling deposit hascovered most of the surface, the materialand the finish of the wall has becomeirrelevant; the primary effect is duringthe incubation or the induction period.Different materials have differentcatalytic actions with various fluids and Timemay promote or inhibit the reactive Fig. 25process responsible for initial fouling. The figure shows typical foulingresistance development histories during the induction period for carbon-steel, stainless – steel and brass surfaces exposed to brackish water streamsunder constant flow conditions. Polished surfaces resist the growth of fouling deposits but are highlysusceptible to corrosive action that roughens the surface and increase the 42
  43. 43. potential crystallization sites. Improperly cleaned heat exchangers withresidual fouling deposits on the surface will degrade by fouling more readilythan those restored to the “as new” clean condition.6.7 EFFECT OF FLUID VELOCITY There is much evidencesuggesting fluid velocity as the mostimportant parameter affecting fouling.In most cases, an increase in velocitydecreases both the rate of foulingdeposit formation and the ultimate level Timeattained, as shown by the typical Fig. 26development histories given by fig. Improvement tends to be atprogressively diminishing rate. Doubling the fluid velocity from a low valuemay halve the fouling resistance. Doubling it again may halve the remainingresistance. However, the second doubling requires an increase to four timesthe original velocity and gains only a reduction of one quarter the originalthermal resistance. In addition to decreasing the fouling, the higher velocity increases theheat-transfer coefficients so that a double – barraled reduction in the size andcost of the heat exchanger might be anticipated. With reduced fouling therewill also be a decrease in the maintenance requirements and cost. However itmust be recalled that the pressure drop is a function of the square of the fluidvelocity. Doubling the fluid velocity increases the pressure drop by fourtimes, increasing both the capital cost and operating cost of the pumping. 43
  44. 44. 6.8 EFFECT OF TEMPERATURE Temperature has a pronouncedeffect on fouling that can begeneralized as shown in fig. The rateof development of fouling resistanceand the ultimate stable level bothincrease as the temperature increases. TimeTemperature refers to either or both of Fig. 27the surface temperature and the fluid bulk temperature. The rates ofchemical and inverse crystallization including catalytic effects, are stronglydependent on temperature, which explains the increase in fouling rate. Therate of removal of fouling deposits is less a function of temperature thanfluid velocity. Therefore an increase in the rate of deposition with noincrease in removal will result in a higher ultimate stable level.6.9 EFFECT OF BAFFLE & TUBE PATTERN (1) The relative propensity to fouling and the ease with which cleaningcan be accomplished are important factors in selecting the type of exchangerfor a given application. On the shell side, baffle designs and tubearrangements are influenced by fouling and cleaning considerations.Because high velocity is important to minimize fouling, it is clear that thebaffle arrangement shown in fig.28(a). would lead to many stagnant areas inthe shell - side flow, with consequent high fouling. The baffle arrangementshown in fig.28(b) has fewer stagnant areas and a longer mean flow path. Ifthe shell side mass flow were the same in both exchangers, the velocity infig. (b) would be much greater than that in fig.28(a). Of course the pressuredrop and cost of pumping increases as the square of the fluid velocity. 44
  45. 45. Tubes are generally arranged in the triangular in the triangular orsquare pattern shown in fig. Triangular arrangements allow for inclusion ofthe greatest number of tubes in a given shell diameter and for the strongesttube-sheet ligaments. However they are much difficult to clean withmechanical scrapers and brushes than square tube arrangements. Exchangerslikely to require periodic cleaning on the shell side should therefore havesquare tube arrangements. Of course their may be other compelling reasonsto override this general rule, so as to increase the tube count or takeadvantage of the stronger tube - sheet ligaments of triangular arrangements. (a) (b) Fig. 28 Baffle designs affecting fluid velocity at the creation of stagnant areas Square Triangular Fig. 29 Triangular and square pitch pattern 45
  46. 46. 6.10 PRACTICAL FOULING FACTORS (2) It is customary for the purchaser to specify the fouling resistance usedin the thermal design of the exchanger. The exposition will do little toincrease users confidence in the value of the fouling resistance marked onthe exchanger specifications sheets; however they should have a clearerunderstanding of the uncertainties prevailing in the specifications. Manyusers have their own private collection of fouling factors, based on pastexperience with similar equipment under equivalent conditions. These arethe most reliable data. However, the indiscriminate application of thesefactors to equipment larger in size and the operating under more arduousconditions is of questionable validity. The uncertainty increases the moreone departs from past experience. 46
  47. 47. 7 ENERGY CONSERVATION TECHNIQUES IN HEAT EXCHANGER7.1 INTRODUCTION Fouling factor plays a major role in overall HTC of heat exchanger. Itdecides the area required for heat transfer. The higher the value of ‘U’, lesserwill be the area required for heat transfer. This area required is directlyproportional to the energy required for pumping of the fluid and pressuredrop. A = Q / (U . ∆Tm )Where,Q = Total heat transferU = overall heat transfer coefficient (HTC)∆Tm = Log mean temperature differenceA = Area of heat transfer7.2 MODE OF OPERATION (4) It is always feasible with counter current heat exchangers to have aheat donating fluid entering the heat exchanger, at say, 150oC and leavingthe exchanger at 80oC, while the heat receiving fluid is heated up from 40oCto 120oC or more. This is impossible to achieve with co – current operation. Since in counter current mode of operation the hottest inflow faces thewarmest out flow, the vale of ∆T i.e. (th – tc) throughout the heat exchangeris constant. By and large the efficiency of such heat exchanger is directlyproportional to their length and the surface area of calendria. 47
  48. 48. Co – current operation is used, 1. When it is necessary to transfer as much as heat possible from heat donating fluid to the heat receiving fluid. 2. When the difference in the temperature between the fluid is less. 3. When the temperature of the heat donating fluid leaving the heat exchanger is lower than the temperature of the heat receiving fluid leaving the heat exchanger.7.3 FLUID FLOW CHARACTERISTICS (4) In a stream line flow, liquid molecules flow along in a parallel fashion& in consequence, heat transfer from the center of the fluid to the walls ofheat exchanger tubes proceed by conduction only. As table below shows,thermal conductivness of fluids are remarkably poor compared with those ofmetals. Thermal Conductivity of Metals and Fluids (4) Thermal Thermal Material Conductivity Material Conductivity W/moK at 20oC W/moK at 20oC Aluminum 237 Water 1.967 Copper 166 Toluene 0.44 Iron 147 Petrol 0.47 Magnesium 159 Oil 0.75 Silver 427 Glycerol 0.97 Zinc 115 Air 0.025 It is therefore necessary to ensure that the fluids in heat exchangersmove turbulently i.e. in such a fashion that constant mixing occurs. 48
  49. 49. When turbulent motion occurs, one can accept that the entire body ofthe fluid has the same temperature because of the turbulence. The onlyconduction heat transfer needed is across the boundary layer. Turbulence canbe inducted in a fluid if the Reynolds number exceeds about 2000. NRe = Dvρ µWhere,D = Diameter of pipe containing fluid (m)v = velocity of fluid (m/s)ρ = Density of the fluid (Kg/m3)µ = Viscosity of fluid (Kg/m.s)7.4 PRESSURE DROP AND PUMPING POWER (7) Apart from heat transfer requirements an important consideration inheat exchange design, is the pressure drop or pumping cost. The size of theheat exchanger can be reduced, by forcing the fluids through it at highervelocities thereby increasing the overall heat transfer coefficient. But highervelocities will result in larger pressure drops and corresponding largerpumping costs. The selection of optimum pipe size also has a bearing on thepumping cost. For a given flow rate, the smaller diameter pipe may involveless initial (capital) cost but definitely higher pumping cost for the life ofheat exchanger. It is known that the pressure drop of an incompressible fluids flowingthrough pipes and fittings is ∆p ∝ m2Where m is the mass flow rate. 49
  50. 50. The power requirement to pump fluid in steady state is given by, Power = v dp = (m/ρ) ∆p ~ m3 So the power requirement is proportional to the cube of the mass flowrate of the fluid and it may be further increased by dividing it by pump (fanor compressor) efficiency. Since the pumping cost increases tremendouslywith the higher velocities, a compromise between the larger overall HTCand corresponding velocities will have to be made. A – overall HTC B – Pumping Power Above graph (fig.30) explains C – Pressure drop D – Fouling factorthat at higher fluid velocity fouling D C B,will be reduced but will require A, Annual Costhigher pumping power and higherpressure drops with increasedoverall HTC. At lower fluid Optimisationvelocities, pumping power willreduce and reduce pressure drop, but Fluid Velocity Fig. 30 Optimization for fluid velocitywith less overall HTC and higherfouling factor. Hence optimization is done where a velocity of fluid is decided whichwill give economical pressure drops and heat transfer, since higher annualcost is directly related to higher energy requirements. Hence optimizationhelps in cutting the annual cost and conserving energy. 50
  51. 51. 7.5 RUBBER BALL CLEANING (5) Fig. 31 (a) The basic principle of cleaning with sponge rubber balls is tofrequently wipe clean the inside of the tube while the unit is in operation.Since the balls are slightly larger in diameter than the tube, they arecompressed as they travel the length. This constant rubbing action keeps thewalls clean and virtually free from deposits. Thus suspended solids are keptmoving and not allowed to settle, while bacterial fouling is wiped quicklyaway. Pits do not form as deposits are prevented. The balls are selected inaccordance with the installation, their specific gravity being nearly equal tothat of cooling media. Therefore, they distribute themselves in ahomogeneous fashion. They travel the length of the tube forced by thepressure differential between the inlet and the outlet. The balls surfaceallows a certain amount of water to follow through the area of contact withthe wall, flushing away accumulated deposits ahead of the ball. They areavailable in various degrees of resiliency, depending on requirement. 51
  52. 52. An abrasive coated ball is alsoavailable for situations where thecooling water tubes have already beenheavily fouled. Here the effect isgentle souring that removes the scaleslowly but steadily, until the tube isready to be maintained by the normalsponge-rubber ball. Heat - transferefficiency climbs steadily throughoutthis treatment. Fig. 31 (b) The balls are circulated in closed loop, including the heat exchangeras shown in fig. At the discharge end they are caught in a screen installeddirectly in the line. They are then rerouted through the collector back to thecondenser ball - injection nozzles to ensure that the balls are uniformlydistributed. At the collector unit, the balls can be counted or checked for size. Thenumber required for a particular service is a function of the number ofcooling tubes. Naturally, some wear occurs so that the balls must beeventually replaced. These cleaning systems can be retrofitted into most existing heatexchangers, although some modifications of piping or unit design may berequired. The slight increase in pumping resistance due to pressure dropacross the screening device is more than offsets by the reduction in foulingresistance in the heat exchanger tubes. The most effective way to takeadvantage of these systems is for its installation at the design stage. A filterprevents solid debris from entering the water box of the heat exchanger. 52
  53. 53. Located in the cooling water inlet, it is flushed as need without shuttingdown or bypassing the filter.Examples of continuous tube cleaning Fig. 32 Before and after use of rubber ball cleaning A typical case is shown in the "Before and "After" graphs (Fig.32).An instance involved stainless steel tubing, where the rubber systemmaintained a cleanliness factor and a backpressure of 1.49 in. Hg. After1,800 hr of operations, the tube cleaning system was taken out of service fortesting purposes. During a month of operations without cleaning, the heatexchanger back – pressure climbed to 1.65 in. Hg and the cleanliness factordropped from 98 to 81%. When the cleaning was restarted, the originalbackpressure and the cleanliness was recovered in 10 days. After extensive testing, it was proved that the continuous system washighly economical and produced superior performance over manualcleaning. Continuous cleaning gives 17% better performance than manualcleaning. Continuous cleaning and filtering systems maintain a high level ofheat exchanger efficiency. The ball cleaning scheme results in fuel saving,fewer outages and reduction or elimination of cleaning chemicals. 53
  54. 54. 7.6 PLATE OVER TUBULAR HEAT EXCHANGER (5)7.6.1 Introduction The continuous search forgreater economy and efficiencyhas led to the development ofmany different types of heat Fig. 33exchanger, other than the popularshell and tube. Some of these have been highly successful in particular fieldsof application. Briefly, a plate heat exchanger consists of number of corrugated metalsheets provided with gaskets and corner portals (to achieve the desired flowarrangement, each fluid passes through alternate channels). Plates are spacedclose together, with nominal gaps ranging from 2 to 5 mm. The plates arecorrugated so that the very high degree of turbulence is achieved. One of themost widely used plates, are of the following relationship: NNu = (0.374) NRe0.668 NPr0.333 ( µ / µw) Pumping cost In the fig. it can be seen that for a given energy loss (HP / unit area),the plate heat exchanger produces higher film coefficient than does a tubularunit (considering the flow inside the tube). When accessing various heat exchanger types, the question ofpumping should be considered, since these will probably represent by far thegreatest of the operating costs. Plate heat exchangers are by far the best inthis respect. 54
  55. 55. Fig. 34 Advantages of PHE over Fig. 35 Performance of plate heat tubular heat exchanger exchanger7.6.3 Fouling factors in plate heat exchangers Fouling factors required in plate heat exchangers are small comparedto those commonly used in shell and tube designs for six reasons: 1. High degree of turbulence, maintain solids in suspension. 2. Heat transfer surfaces are smooth. For some types, a mirror finish may be available. 3. No dead spaces where fluid can stagnate, as in case of shell and tube. 4. Since the plate is necessarily of a material not subject to massive corrosion (being relatively thin), deposits of corrosion products to which fouling can adhere are absent. 5. High film coefficients tend to lead to lower surface temperature for the cold fluid (the cold fluid is the culprit as far as fouling is concerned). 6. Extreme simplicity of cleaning. The small hold up volume and very large turbulence in plate heat exchanger (plus the absence of dead spaces) mean that the chemical cleaning methods are rapid and effective. 55
  56. 56. 7.7 ADVANCES IN HEAT EXCHANGER TECHNOLOGY7.7.1 Spiral tube heat exchanger (9) Fig. 36 Heliflow Heat Exchanger The Graham Heliflow is a unique type of shell and tube heatexchanger. The tubes in the Heliflow are arranged in parallel, starting withan inlet manifold on one end, and terminating at an outlet manifold on theopposite end. The tube bundle is wound into a helical pattern. This coiledconstruction creates a spiral flow path for the fluid inside the coil. Each tube is in close contact with the tube above and below it. Thecoiled tube bundle is fit into a two – piece casing. When the casing istightened, it is designed to slightly compress the tubes. Because of the tightfit, the shell side fluid is forced to circulate in a spiral pattern, which iscreated by the open spaces between the coils. The unique arrangement of the Heliflow Heat Exchanger createsspiral flow paths for both tubeside and shellside fluids, providing 100% truecountercurrent-flow design. The spiral pattern also promotes turbulence,leading to increased heat transfer rates. In addition, there are no baffles ordead spaces that lead to inefficiencies commonly found in other types of 56
  57. 57. shell and tube exchangers. The net result is a Heliflow Heat Exchanger thatis up to 40% more efficient than a standard shell and tube. Originally built for use in boiler sample cooling over 60 years ago,there are thousands of Graham Heliflow heat exchangers being used today inhundreds of services. Many units have been in operation for well over 40years. The service life of a Heliflow varies with the application, but its manyfeatures add to its reliability when compared to a shell and tube exchanger. No gaskets are required for the tube side of the Heliflow. Aggressivefluids are often placed tube side for this reason. No gaskets on the tube sidewill minimize the chance of leakage. The spring-like coil of the Heliflowreduces stresses caused by thermal expansion of the tube material. Heliflow can do the job for you in a fraction of the space required bytypical straight shell and tube exchangers. With higher heat transferefficiencies, the surface area required is normally less than a straight shelland tube. Smaller surface requirements, and the coiled tube design result in avery compact unit. Access space required for maintenance or inspection isvery small compared to straight shell and tube exchangers. The only spacerequired for a Heliflow is to remove the casing, which allows inspection ofboth the entire tube bundle and shellside of the exchanger. You can mount aHeliflow on columns, nozzles, walls, ceilings, or in-line; certain sizesrequire no support. A Heliflow is easy to maintain. The casing of the unit can be removedwithout disturbing any of the piping connections. Once the casing isremoved, the entire tube bundle is exposed for inspection. With the casingremoved, the shellside of the unit can easily be cleaned in place. 57
  58. 58. 7.7.2 Fluidized bed heat exchanger (10) Fig. 37(A) Self cleaning heat exchanger with Fig. 37(B) Self cleaning heat exchanger with Cyclone widened outlet channel Self-cleaning heat exchange technology applying a fluidized bed ofparticles through the tubes of a vertical shell and tube exchanger wasdeveloped in the early 1970s for sea – water desalination service. Since thattime, several generations of technological advancements have made themodern self-cleaning heat exchanger the best solution for most severelyfouling liquids. In the 90s, a chemical plant in the United States compared for theirseverely fouling application a conventional solution versus the installation ofself – cleaning heat exchangers. The result of this comparison is also shownin table 1. 58
  59. 59. Table: Comparison of self cleaning heat exchanger v/s conventional heat exchanger (10) SELF – CLEANING CONVENTIONAL HEAT HEAT EXCHANGER EXCHANGER Heat transfer surface 4,600 m2 24,000 m2 Pumping power 840 kW 2,100 kW Number of cleanings per year 0 12 As could be expected, but also convinced by a test, plant managementdecided in favor of the self-cleaning configuration. During operation, theexpectations for the self-cleaning heat exchangers were fully met and evenbetter than that: After 26 months of continuous operation, the self-cleaningheat exchangers still have not been cleaned. This striking example of the self-cleaning heat exchange technologyand a large number of improvements and new developments havesubstantially increased the potential applications, which can benefit fromthis unique self-cleaning heat exchange technology. These improvementsand developments leading to new and very interesting applications will bediscussed in the next paragraphs.Principles of Operation The principle of operation with respect to the original configuration ofthe self-cleaning heat exchanger employing an external down comer isshown in figure 1. The fouling liquid is fed upward through a vertical shelland tube exchanger that has specially designed inlet and outlet channels.Solid particles are also fed at the inlet where an internal flow distributionsystem provides a uniform distribution of the liquid and suspended particles 59
  60. 60. throughout the internal surface of the bundle. The particles are carriedthrough the tubes by the upward flow of liquid where they impart a mildscraping effect on the wall of the heat exchange tubes, thereby removing anydeposit at an early stage of formation. These particles can be cut metal wire,glass or ceramic balls with diameters varying from 1 to 4 mm. At the top,within the separator, connected to the outlet channel, the particles disengagefrom the liquid and are returned to the inlet channel through a downcomerand the cycle is repeated. Figure 2 shows an improved configuration. Now,the particles disengage from the liquid in a widened outlet channel and, then,are again returned to the inlet channel through an external downcomer andare recirculated continuously. For both configurations, the process liquid fedto the exchanger is divided into a main flow and a control flow that sweepsthe cleaning particles into the exchanger. By varying the control flow, it isnow possible to control the amount of particles in the tubes. This provides acontrol of aggressiveness of the cleaning mechanism. It allows the particlecirculation to be either continuous or intermittent.7.7.3 Helixchanger heat exchanger (11) Heat exchanger fouling has been very costly for the industry both interms of capital costs of heat exchanger banks as well as operation andmaintenance costs associated with it. The HELIXCHANGER heatexchanger, when applied in typically fouling services, has proven to be veryeffective in reducing the fouling rates significantly. Three to four timeslonger run-lengths are achieved between bundle cleaning operations. Properattention is required in designing the heat exchangers placed at the hot endof crude oil pre-heat operations where temperatures and velocity thresholdsare highly dependent on heat exchanger geometry. The helical baffle design 60
  61. 61. offers great flexibility in selecting the optimum helix angles to maintain thedesired flow velocities and temperature profiles to keep the conditions belowthe “fouling threshold”. In a Helixchanger heat exchanger, the quadrant shaped baffle platesare arranged at an angle to the tube axis in a sequential pattern, creating ahelical flow path through the tube bundle. Baffle plates act as guide vanesrather than forming a flow channel as in conventionally baffled heatexchangers. Uniformly higher flow velocities achieved in a Helixchangerheat exchanger offer enhanced convective heat transfer coefficients. Helicalbaffles address the thermodynamics of shell – side flow by reducing the flowdispersion primarily responsible for reducing heat exchanger effectiveness.Least dispersion (high Peclet numbers) achieved with the helical bafflearrangements approach that of a plug flow condition resulting in highthermal effectiveness of the heat exchanger. In a Helixchanger heat exchanger, the conventional segmental baffleplates are replaced by quadrant shaped baffles positioned at an angle to thetube axis creating a uniform velocity helical flow through the tube bundle.Near plug flow conditions are achieved in a Helixchanger heat exchangerwith little back-flow and eddies. Exchanger run lengths are increased by twoto three times those achieved using the conventionally baffled shell and tubeheat exchangers. Heat exchanger performance is maintained at a higher levelfor longer periods of time with consequent savings in total life cycle costs(TLCC) of owning and operating Helixchanger heat exchanger banks.Feedback on operating units, are presented to illustrate the improvedperformance and economics achieved by employing the Helixchanger heatexchangers. 61
  62. 62. Helixchanger heat exchangers have demonstrated significantimprovements in the fouling behavior of heat exchangers in operation. In aHelixchanger heat exchanger, the quadrant shaped shellside baffle plates arearranged at an angle to the tube axis creating a helical flow pattern on theshellside. Uniform velocities and near plug flow conditions achieved in aHelixchanger heat exchanger, provide low fouling characteristics, orderinglonger heat exchanger run-lengths between scheduled cleaning of tubebundles. Fig. 38 Fig. 39 HTC using helical baffles of various angles 62
  63. 63. Fig. 40 Performance of segmental bundles Fig. 41 Performance of Helix bundles Although it may be observed from the graphs that the HELIX bundlesshow marginal improvement in the drop in overall heat transfer coefficientwith time in the initial stages, it has since achieved and sustained anasymptotic level of performance much higher than the performance levelachieved in the earlier segmental bundles. The HELIX bundles arereportedly expected to achieve more than three years of continuousoperation, thus increasing the run-length by three times. 63
  64. 64. Earlier segmental bundles required two to three times cleaning in thistime period. The HELIX bundles have achieved significantly enhanced heattransfer performance and have sustained this performance for a long periodof time. Three to four times longer run-length has already been achievedwith these bundles. 64
  65. 65. 8 CONCLUSION In this seminar various heat exchanger types, along with theirapplications have been given. Various types of trouble – shooting and non –ideal behavior of heat exchanger, along with its causes and prevention havebeen discussed in this seminar. It is generally seen that even though shell and tube heat exchangergives less heat transfer for a particular pressure drop than in plate or spiraltube heat exchanger, but still is widely used in Chemical Process Industries,due to its rugged construction and various design and trouble - shooting dataavailable to the designers, which is not the case for other type of heatexchangers, even if they are having better efficiency. From energy aspect, proper cleaning of heat exchangers and regularmaintenance to reduce fouling and if possible to avoid corrosion, is needed.Lesser the fouling, which is the main cause for lower heat transfer in the heatexchanger, lesser will be the wastage of energy, and higher will be theefficiency of heat exchanger. Upcoming technologies like the fluidized bed heat exchanger, spiraltube heat exchanger and helical shaped baffles, although not heavily used inindustry but in near future, where energy resources will become scares andneed of highly efficient heat exchangers will be the need of hour, moreadvanced, complex and compact heat exchangers like mentioned above willbe in demand, which helps in reducing the fouling or in some caseseliminates fouling. 65