Heat Exchangers

HEAT EXCHANGERS

Heat treatment refers to the procedure of heating metals with theobjective of altering their outward or chemical properties. Theheating does not have an effect on the shape and size. The objectiveof heat-treating involves creating a heat-treated part, which has thestrength of performing specific functions. The treatment differs inrelation to the kind of metal being employed as well as theanticipated power for the treated section. Steps to achieving heattreatment include heat-treating to a specified temperature, ensuringthe temperature remains constant for some period and metal cooling toattain room temperature. The paper discusses heat exchangers asequipment used in heat treatment.

Equipment

Heat exchangers refer to equipments, which transfer heat, mainlyfluid, from one substance to a different one via conduction(Servicing America’s Energy,2014). The fluids can be in gas form or liquid that depends onthe kind of heat exchanger being employed. They are preserved in anumber of pipes to permit them circulate. Heat exchangers function inline with the rules of thermal equilibrium that dictates the movementof heat amid objects. They can be located in motor vehicles, homesand industries. In homes, they are inbuilt in the storage heaters andrefrigerators. In industries, heat exchangers are for the basic roleof cooling to ensure volatile substances do not overheat.

Figure1: industrial heat exchanger

There are three basic categorizations of heat exchangers in relationto the arrangement of flow. These are:

Parallel-flow – both fluids pass through the heat exchangersimultaneously, and move parallel to the opposite side (ServicingAmerica’s Energy,2014).

Figure2: heat exchanger parallel-flow

Counter-flow – the fluids pass through the exchanger via differentdirections, from opposite sides (ServicingAmerica’s Energy,2014). The flow is the most effective because it makes itpossible to transfer more heat per every mass. This is because thestandard temperature disparity along the unit length is more.

Cross-flow – The fluids move almost vertical to each other via theexchanger. For effectiveness, the design of heat exchangers is suchthat it maximizes the surface area for the wall amid both fluids,whereas reducing fluid flow resistance via the exchanger (ServicingAmerica’s Energy,2014). It is possible for the performance of the exchanger tobe influenced by the inclusion of corrugations from any of thedirections, as they enhance surface area and might direct fluid flowin the process inducing turbulence.

The diverse kinds of heat exchangers work in dissimilar manners, byemploying varied flow arrangements, apparatus as well as designproperties. However, all heat exchangers work with the aim ofdirectly or indirectly exposing a warm medium to one that is coolthus the name heat exchange. This is achieved via the utilization ofa group of tubes housed in some kind of casing. Heat exchanger belts,fans, coolers, condensers, extra tubes or lines, in line withdifferent parts and equipment function to enhance heating and coolingeffectiveness or advance flow. Heat exchangers are fundamentally twochambers that have been made separate using a wall, where everychamber comprises one kind of fluid matter. Heat moves from a hotobject like the furnace to a first fluid that heat the fluid whilecooling the object. The circulating fluid moves its heat to a secondfluid via the wall dividing the chambers, in the process making thesecond fluid warm and eliminating high temperature from the first.The main intention is cooling the primary hot substances throughphases.

Current State-of-the-art

In the past 30 years, important developments have been achieved inthe technology of heat exchangers (Lara-Curzioet al, 2007). One such technology is the compact heatexchanger, which minimizes the length scale through which heat aswell as mass transfer happens. It has made it possible to create higheffective heat exchangers. For metal alloys, the compact heatexchangers are employed in car radiators, HVAC as well as theprocessing of petrochemicals. The inclusion of super-alloys incompact heat exchangers has created the present state of the art forheat exchangers working under high temperatures (Lara-Curzioet al, 2007). Compact heat exchangers that have beenmanufactured using super-alloys, though, lack ample material ormechanical properties to enable the sulfuric acid breakdownprocedure. This has resulted in the advancement to using ceramic heatexchangers, which work as decomposers (Sommers et al, 2010).

In energy recovery uses as well as air conditioning, heat exchangersare useful to the general effectiveness, expense and system size.Current heat exchangers depend largely in plate designs, frequentlymade with copper and aluminum. Advancements in material science,specifically developments in ceramics, as well as ceramic matrixcompounds, create advent opportunities towards the use of improvedheat exchanger designs (Sommers et al, 2010). The two major benefitsof employing ceramic materials in the creation of heat exchangersover other previous metallic materials, is their ability to resisttemperature and oxidization.

Ceramics

Density

Moderate to high

Thermal conductivity

Low to high

Temperature

High (&lt1650 degrees Celsius)

Chemical resistance

Excellent

Mechanical properties

Good

Shape and join

Difficult

Cost

Moderate to high

Main weaknesses

Inherent brittleness

Figure 3: concise propertyfeatures for ceramic substances

Ceramic materials have the ability to endure operating temperatures(1400 degrees Celsius), which surpass those of traditional metalalloys (Sommers et al, 2010). For instance, the mass materialtemperature in a heat exchanger, which has been made from carbonsteel is not to surpass 425 degrees Celsius. Likewise, the bulksubstance temperature for a heat exchanger made of stainless steelmust not be more than 650 degrees Celsius (Sommers et al, 2010). Thismeans that the heat exchanger needs to be safeguarded in severaluses. It is possible to achieve thermal protection through anenvironmental obstruction coating, which covers the metal having theimpact of including a thermal resistance to heat transfer, hence,minimizing the general per unit performance. In different instances,the unit works via the parallel flow mode in place of thecounter-flow means to ensure a reduced general material temperature.The operation mode has the impact of enhancing the heat exchanger’slifetime in turn reducing the general thermal unit effectiveness. Adifferent widely used method is dilution of air where ambient air isincorporated in the exhaust gases, which are upstream. The method hasthe impact of reducing the general heat exchanger effectiveness.

The second main benefit of ceramic-based heat exchangers derives fromtheir ability at resisting corrosion as well as chemical erosion(Sommers et al, 2010). Corrosion that happens under regularsituations is intensified through high operating temperatures. Inaddition, corrosion can happen in numerous diverse forms via a drainout gas stream. An illustration is an exhaust flow having oxygen, andable to attack a metal surface. In the illustration, oxygen diffusionto the material results in scaling. Though the scaling at firstcreates a protective surface, the discontinuous utilization of theheat exchanger and ensuing thermal cycling may result in flaking off,of the scale. This exposes the basic material to more attack.Different probable gaseous elements involve sulfur and carbon thatare capable of diffusing within the grain boundaries. The movement ofsulfur to grain boundaries creates eutectics, which melt at lowtemperatures contrary to the melting temperature of material. Whencarbon diffuses to the metallic, surface it leads to carbidescreation that can result in residual stress.

Safety Issues

Common safety issues are rusting, cracking, clogging andoverheating. Rusting arises from compression, seeping humidifiers, aswell as leaking air conditioners. The same applies to heat exchangersused in traditional furnaces. The mere disparity is that for heatexchangers from highly effective furnaces, it is anticipated thatcondensing will happen. The ability not to rust following exposure tocondensation depends on the manufacture of heat exchangers andmaterials employed. Tertiary in addition to secondary heat exchangershas a higher probability of rusting. Cracks are more likely to format the primary heat exchanger. This is where temperatures are highwhile flare impingement could be a safety issue within the heatexchanger. Clogging arises from condensation as well as collection ofsoot. The drainage shaft passages for greatly effective heatingsystem exchangers are likely to be constricted to almost half an inchresulting in clogging. Overheating is the outcome of excessivetemperature increase normally arising from insufficient flow of airthrough the heat exchanger. It is notable with retrofit furnaces thathave restricted pipe size. It might also become an issue for adventinstallations when care fails to be taken in the proper setting offan speeds. Fan speeds need to be properly set to guarantee freemovement of air through heat exchangers. The availability of airconditioner coils reduces the flow of air and unwarranted increase intemperature, which is more probable in air conditioner systems.

In instances where it is possible to assess the heat exchangerburner, it is necessary to look for proof of rust, flaking oraluminum sheets peeling from heat exchangers made of steel, or sanddeposits for those that have a coating. In other cases, it isprobable to examine the second or third exchangers in the fansection. Some furnaces are created in manners that make it simple todraw the fan from its compartment and check for any issues. Any proofof moisture from the heat exchangers exterior implies that condensateis seeping out via a crack. It could also imply the presence of ahole, which resonates to a safety issue in the heat exchangers. Inthe presence of a crack, the switch differentiating heat becomesdissatisfied and requires shutting down. It is difficult to discoverclogging. Nonetheless, when the visible plastic condensate tubing hasdirt or soot within the tube wall, it is possible to presume thepresence of soot in heat exchangers as well. An operationalexamination endorsed for high effectiveness furnaces involvesverifying the rise of temperature transversely of heat exchangers,during when a steady condition is apparent for the unit. It isnecessary to ensure an increase in temperature does not surpass theone expected as indicated in the data plate, a situation that couldnegate the service contract of the manufacturer. It could alsosignificantly minimize the heat exchangers life expectancy. Suchissues arise from undersize ducts, slow pace, dirty filters, blowermotor and ducts.

Advantages and Disadvantages

Heat exchangers are employed in nuclear plants, as well as differingmajor large-scale industry uses. When using nuclear reactors,materials like uranium create massive heat levels that need to beeliminated to ensure the reactor does not overheat (Wang,Sunden &amp Manglik, 2002). A basic fluid that could be wateror metal that has been heated to form a liquid like sodium picks thereactor’s high temperature. Pumps direct the basic fluid to theheat exchanger, transferring heat to water, and turning it to steam.As a result, the steam propels a turbine that propels an electricgenerator. The steam dissolves to water that moves back through theheat exchanger, making it a repetitive procedure. Heat exchangers areemployed in all kinds of industrial procedures. The kind as well assize for every procedure may be customized, because they all have thesame guidelines. In place of employing unnecessary coolant, mostindustries opt to pipe in chemicals, which require heating. Thissaves heat energy, which could contrary become waste.

Advantages

In general, the advantages of heat exchangers include adaptability,affordable and easy to maintain and used in industries. Adaptability– most current heat exchangers are pliable, hence can be employedfor different functions. They are now compact making them functionalin lesser and lesser items (Sam,2012). Though former heat exchangers were frequently as big asa refrigerator, from 2011 they have become portable. In addition,they are flexible. Flexible since the fluid employed in mediating theheat exchange may altered to design requirements. Affordable and easyto maintain – the fabrication expenses for heat exchangers havedropped with the advancement in knowledge. The heat exchangers areless costly compared to traditional models. Most are now easy toclean making maintenance simpler. For instance, plate heat exchangersmake it possible to take away and put back the plates instead ofdismantling the entire system (Sam,2012). Used in industries – being a tool, it comprises ofnumerous industrial uses. The coolant from radiator coils from theinterior combustion engine is an illustration of the heat exchanger.Additionally, the equipments are employed at chemical plants, fortreating sewages, nuclear power centers and in petroleum refinery.

Plate heat exchangers – they provide efficient heat transfer dueto the huge surface areas as well as ridged plates, which make itpossible for effective heat transfer to happen. Every plate becomespressed via a chevron-shaped method resulting in the formation ofsoaring turbulent flow, outstanding fluid delivery, and an enhancedsurface area (Sam, 2012).The fluid movement amid plates is probable to be wide, having arelevant reduction in foul rates. Simple to maintain – this is dueto the ability to dissemble and clean the plates. The plates arehighly corrosive and have a reduced fouling speed meaning theyrequire less maintenance compared to different heat exchangers.Compact structure – they are small and effective because on thecompact structure that makes it possible to preserve space within theheat exchanger surrounding, in addition to material expense (Wang,Sunden &amp Manglik, 2002). Cost effective – the exchangersare small and employ minimal material in their production, hence arefrequently the most efficient economic alternative in case of a heattransfer function. Plate exchangers are easy to expand to accommodateadvent uses or enhance flow rate, frequently opposing the requirementto buy another heat exchanger. Materials – they can be found indifferent materials, formulated to fit particular uses. The platescan be in steel, tantalum, titanium or nickel among other materials(Wang, Sunden &amp Manglik, 2002).

Shell and tube heat exchanger – it comprises of tubes within ashell and is a heavy duty. This makes it a better option for use inprocessing industries, as it is better at handling temperatures thathave a high of 900 degrees Celsius and above (Sam,2012). Fluid temperature is maximum at wall contrary toflowing stream making it necessary to have even heat redistribution.There are various pros deriving from the makeup of these heatexchangers. They include the capability to handle high pressures andtemperatures, handling fluids in all states, simple to take apartwhen doing cleaning or repairing, and the ability to change designwith the aim of meeting operating stipulations.

Disadvantages

The disadvantages of heat exchangers derive from leakage in additionto declines in pressure. Leakage may happen during the transfer ofheat, and is hard to avoid or amend. It is a major safety issue,which is expensive to repair and if not detected in time causes moredamage to the equipment. In actuality, most heat exchangers requireto be dismantled completely for it to become possible to repair aleakage. In the similar manner, when pressure declines, for instancein the plate type, it mandates the examination of each plate torectify the fault.

Plate heat exchangers – do not apply in all uses. In instances ofintense temperature disparity amid fluids, it is advisable to employthe shell and tube. This because with plates, high pressure loss mayoccur because of the intense level of instability arising from narrowflowing channels (Sam, 2012).In applications where low pressure loss in needed, it is necessary touse a different heat exchanger. Plates that are gasketed have alimitation for temperature fluid, which is high. Shell and tube –despite handling high temperatures, it is only probable to employ asingle unit per duty. High heat loss happens, which makes insulationnecessary. In terms of spending, it is costly compared to plate heatexchangers due to the large space needed. After some time, vibrationscause harm to the heat exchanger. When the heat exchanger is harmed,optimization of baffle position becomes necessary to minimizevibrations, as well as ease vibrations and ensure more durability forthe heat exchanger (Sam, 2012).Falling film heat exchangers – fluid moves in from the film topflowing down because of gravity, which makes flooding unavoidable attimes. Drops in pressure from film breaking, which makes the heatexchangers require ongoing heat removal limit the flow movement.Evaporation may happen deteriorating components, while the liquidmust be equivalently supplied (Sam,2012).

Figure 4: falling filmheat exchanger example

Future Trends

Heat exchangers market is segmented depending on kind and uses. Themain kinds are plate, shell and tube. Uses are largely in thechemical, gas and oil industries, power generation, food andbeverage, as well as HVAC (MarketReports Online, 2015).There has been remarkable developments in heat exchangers, and it isanticipated that because of global energy infrastructure growth,enhancing demand within the nuclear power sector, and a developinginsistence on the reduction of heat energy expenses by improvingenergy effectiveness, heat exchangers are more likely to becomeaffordable and used in more areas (MarketReports Online, 2015).Developments in the manufacture of heat exchangers will imply a needfor changing old models. Industries will adopt the use of high-endheat exchangers that save energy.

References

Lara-Curzio, E., Salem, J. A., Zhu,D., &amp American Ceramic Society. (2007).&nbspMechanical propertiesand performance of engineering ceramics and composites III: Acollection of paperspresented at the 31st International Conference on Advanced Ceramicsand Composites,January 21-26, 2007, Daytona Beach, Florida.Hoboken, N.J: Wiley- Interscience.

Market Reports Online.(2015). Global heat exchanger market – trends and opportunities. Retrieved from: http://www.marketreportsonline.com/390048.html

Sam, K. K. (2012). Comparisonbetween four types of heat exchangers. InclusiveScience and Engineering,1-1. Retrieved from:http://www.inclusive-science- engineering.com/comparison-between-four-types-of-heat-exchangers/

Servicing America’s Energy.(2014). Industrial heat exchangers: What they are, how they work andwhy they are needed. Retrieved from:http://setxind.com/downstream/industrial-heat- exchangers-what-they-are-how-they-work-and-why-they-are-needed/

Sommers, A., Wang, Q., Han, X., TJoen, Y., Park, Y &amp Jacobi, A.(2010). Ceramics and ceramic matrix composites for heat exchangersin advanced thermal systems – a review. Applied ThermalEngineering, 30, 1-15.

Wang, L., Sunden, B., &ampManglik, R. M. (2002).&nbspPlateheat exchangers: Design, applications andperformance.Southampton: WIT.

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