A heat exchanger (HE) is a system intended to effectively pass heat from a place to the next, whether the substances are isolated by a strong barrier not to combine, or if the substances are touched directly. The theory of strong turbulence and counter-flows provides for effective heat transfer. HE is commonly used for ventilation, air cooling, space heating (SH), power generation domestic hot water (DHW), and chemical processing. For commercial applications, the heat exchanger plays a significant function. This is used for the warming and refrigeration of a large amount of manufacturing process fluids. The heat exchanger is a complex system that can be modified based on the temperature, heat, flow-form, phase movement, thickness, chemical properties, and several other thermodynamic properties to match any industrial operation. An effective power regeneration or power dissipation has been a critical task for researchers and engineers because of the worldwide power crisis.
Fig 1. Straight tube heat exchanger
Prospective damage to infrastructure resulting from scale forming may be very costly if the liquid water is not handled properly. Chemicals are widely used in industry to purify water. A huge amount of contaminants per year is emitted into the environment, poured into waterways, and deposited each year in landfill sites. Industry consumes forty percent of contaminants for size monitoring in the storage tank, boiler, and other thermal energy facilities. A large percentage of contaminants reflects and over 2 billion in hazardous waste that leads to a trillion in gallons of polluted water dumped onto the Planet per year. The insulation of the pipes and sheets is being damaged by rust, localized overheating, presumably owing to the impaction of regional heat flux, induced by heat water vaporing (Ian Travers, 2018).
Repairs of clogged tubular heat exchangers may be carried out using many techniques such as acid washing, sandblasting, water stream high speed, projectile washing, or drill sticks. Treated water, such as purification, the introduction of additives, catalytic solution, etc., are used in large-scale cooling water sources for heat exchangers to reduce heat exchange machinery fouling. Many water management methods are often used in power station steam systems to eliminate the heat exchanger as well as other machinery getting fouled and corroded. Some of the compounds and substances used to foul and reduce the corrosion are potentially dangerous. Therefore, the days have arrived to add healthy solution substances to the atmosphere.
There is a lack of certainty about the main reasons causing the failure of exchangers. Reduced HE performance attributable to fouling, reflects a rise in fuel usage with effects not just on expense but also on heat resource management. Independent of the tube material, the other most reliable means of ensuring the tubes reach their maximum life span and heat exchange capacity is to hold them clean any moment the tube particles, subsidence and micro-fouling are removed, the layers are restored down to the bare metal, allowing the most successful temperature difference and the pipe itself in a fresh lifespan (Peter Diakow, 2016).
Tiredness, corrosion exhaustion, stress corrosion-cracking (SCC), and tensile fracturing are the commonly observed failure modes. Corrosion reflects the mechanical degradation of HE surface building products under the violent control of moving fluids and the touching atmosphere. Other applied mechanical processes are essential for HE layout and activity as well as corrosion, such as corrosion arises in contacting surfaces between vibrated and slipped metals during load. Fouling and corrosion reflect HE activity-induced results that should be regarded both for the design of a new HE and for the function of a current exchanger (Schwartz, 1982).
Fig 2. Corrosion mechanism
Types of Heat Exchangers Failures
Such failures may take 7 specific forms: metal corrosion, handle for air or water, friction, thermal exhaustion, freezing, thermal distribution, and lack of energy for the cooling.
Fluid overload speed either on the heat-exchanger shells or the tube side may cause harmful corrosion when metal sheds from the piping. Some erosion that has been present is exacerbated when corrosion destroys the secured and protected layer of the tube and exposes the fresh surface to further invasion. Many issues with metal corrosion arise within the tubing. Recommended optimum velocity in the pipes and inlet nozzle is a feature of several factors including tube size, treated fluid, and heat. Products like titanium, stainless, and aluminum-nickel can handle higher tube speeds than copper. The U-curve of U-type heat-exchangers and the duct openings are by far the most erosion sensitive regions. Copper is typically restricted to 7.5 fps, 10 to 11 fps for the other components. As water passes into copper pipes, the velocity will be less than 7.5 fps whether it comprises or is diluted with suspended particles.
Fig 3. Metal erosion
The excessive friction from devices such as air -compressors or cooling machines may contribute to tube collapse in the shape of a strain stress crack or tube degradation at the contact point with the baffle plates. Heat exchangers should be separated from movements of this kind. Shell-side liquid velocities above 4 fps will cause harmful tubular vibrations. Causing a slashing motion with baffles on help points. Vibrations caused by pace may often trigger fatigue failures when acting to harden the piping at baffling multiple touchpoints or in U-bend places before a fatigue fracture develops.
Fig 4. Noise and vibration in a fluid-pipe
Fig 5. Vibration-inducing split
Tubing may fail due to fatigue induced by cumulative stresses of repetitive heat treatment, especially in the U- bend region. This question is significantly compounded as the variation in temperature throughout the U-bend conduit decreases. The change in temperature induces tube bending and creates a force that works optimally before the material’s compressive properties are surpassed and cracks (Addepalli, 2015).
Fig 6. Thermal fatigue-As failure
Typically the crack travels radially across the pipeline, resulting in multiple complete breakages. In other instances, the fracture just happens halfway through the pipeline, and then continues through it lengthwise.
Fig 7. Heat exchanger showing thermal fatigue
Chemically Induced Corrosion
External characteristics such as soil, climate, liquids, or aqueous solutions typically impact iron and alloys in particular. Degradation of these materials is referred to as corrosion. For most industries, premature faults of different machinery are triggered by corrosion which leads to unwanted problems e.g expensive failure, untimely termination, and repair expenses. Wet vapor pressure drop is one of the main causes of uniform corrosion as well as erosion-corrosion. This contributes to the decomposition of unalloyed metals. Regardless of the fall of water, the streaming medium has a corrosive impact on the first baffle. The exposure of chemicals to the protective coating shows that the blank steel has little tolerance to corrosion to prevent corrosion from water vapor.
Fig 8. Cross-section of tube damage by pitting corrosion
Fig 9. Uniform corrosion
This slow increase in corrosion increasing in places such as pharmaceutical factories, oil mining, electric power plants at sea and ground, paper processing, air conditioning, refrigeration, food, and liquor outputs. Basic knowledge of the corrosion process could be helpful for better understanding and improvements. The general rate of corrosion is calculated by the substance condition. Several criteria that reduce the risk of corrosion are extreme heat, low pH, high residual chlorine, high total dissolved solids (TDS) material, high durability, low water vapor density, shifts inflow system, and strong ion concentrations such as sulfate, nitrate, chlorine, oxygen, and titanium (Faes, 2019).
Fig 10. Intergranular corrosion of the tube
Fig 11. Crevice corrosion under the gasket of a plate heat exchanger
Fouling is often characterized as the creation and aggregation on surfaces of machine tools of undesirable materials accumulated on them. Such usually very relatively low thermal materials form a sheet layer that will significantly worsen the layer’s ability to carry power at the thermal gradient it was engineered for. On account of this, fouling raises the fluid movement resistance, leading to a greater pressure decrease around the heat exchanger. Fouling could have a very expensive impact on the enterprises which subsequently enhances fuel consumption, inhibits service, losses in output, and increases operational costs. The fouling is produced in five steps that can be described as fouling activation, surface transfer, surface addition, surface extraction, and surface maturity. Many variables influence the fouling process such as pH, density, temperature, surface composition, and friction.
Fig 12. Fouling process
Fig 13. Surface deposits
Solution for heat exchanger failures
Fouling and corrosion
A huge amount of work is undertaken to reduce fouling and prevent corrosion with numerous strategies being established in recent times. In the past, before it was prohibited chromate was an effective chemical product for corrosion protection and conservation of crystal production. The corrosion inhibitor polyphosphate has been added to supplement chromate-based additives (Ibrahim, 2012).
Fig 14. Solution for fouling
Fig 15. Fouling
Physical water-treatment (PWT) is a strong choice for a safe and effective process of prevention of nonchemical fouling. Interpretations of PWT include electromagnets, solenoid tubes, natural chemicals, and catalytic and metallic materials. Therefore, more particulate matter is present in a solution at relatively higher heats, so it is possible that more contaminants will, therefore, be found in the deposited layer and would render the deposit layer thinner. A reduced deposit layer promotes the extraction cycle which at relatively higher temperatures is interpreted as an exponential fouling curve. Improved bulk crystallization can often decrease liquid ionic strength, thereby reducing surface crystal growth due to decreased supersaturation (Teng, 2017).
Fig 16. Cleaning of the heat exchanger
Cleaning of the heat exchanger
Cleaning the heat exchangers is also required to preserve or improve its performance. Cleaning strategies can be divided into two groups: in service and offline cleaning. The cleaning may be performed in service in certain applications to preserve reasonable efficiency without interrupting service. Offline washing may be done in many situations.
Fig 17. Cleaning of the heat exchanger
To gain the ideal process requirements of the modern systems, there will be many advancements in the heat exchangers. Potential advantages of certain requirements can be assessed that will ultimately upgrade and improve the exchanger system.
Such cathodic protection can be achieved by growing the propensity for degradation, which is performed through two different methods in action. A “sacrificial anode” should be attached to the covered device. The design uses handheld anodes to shield the tubes from heat exchangers. Even before the heat exchanger, multiple cylinders with a size significantly smaller than the tube diameter are inserted into the fluid and then removed from the fluid after the tool. Such balls are constructed of either metal or silicone that have metallic components on the rim (A.N. Campbell, 2016). This metal has to be less honorable than the covered tubes, and as they fall in touch with the pipes they can behave like an anode.
Fig 18. Cathodic protection of the heat exchanger tubes by placing the anodes 82 and 90 in the end chamber (Deivasigamani & Preiser, 2013)
Fig 19. Cathodic protection to avoid biofouling of the tube sheet and corrosion of the tubes (Inagaki et al., 2012)
One electrochemical approach to avoid corrosion is a defense against anodes. An external energy source is needed as in the case with Impressed Current Cathodic Protection (ICCP). Nevertheless, it does help to move the covered metal ‘s ability in the anodic direction. The benefit of anodic defense is that only a short circuit is needed after the passivation of the metal. The present often gives a direct example of the level of corrosion. Nevertheless, in comparison to cathodic protection, it only decreases the occurrence of rust but never prevents it altogether (Allahkaram, 2011).
Fig 20. Cathodic protection of heat exchanger tubes with mobile sacrificial anodes (SA) (Ivusic, 2004)
Cathodic and anodic protection techniques may be categorized as chemical means (inhibitors), some mechanical means, alteration in solvent steps, electromagnetic forces, electrostatic forces, electromagnetic fields, UV rays, surface coating, organic-additions, suspension fiber, etc. Different types of polymers were used to inhibit the deposition of calcium phosphate crystals that include maleic anhydride (MA), hydroxy propyl-acrylate (HPA), sulfonated styrene (SS), and sulfonic acid. These polymers are considered very effective in minimizing the deposition and decreasing the corrosion of calcium phosphate.
Fig 21. Surface deposition
Advancement can be accomplished by developments in computer technologies. There is a validated vessel testing technique that provides profiling of all tubing inside the vessel to avoid destroying the tubing: eddy current testing. A decaying tube can also be impossible to identify by traditional testing procedures (visual, stress measurements, etc.) before a major collapse happens and thousands of dollars are spent. The probability of such a loss may be controlled by utilizing eddy current assessment.
The correct cleaning process and monitoring play a significant role in decreasing the cost. Development costs escalate dramatically due to solvent usage, repair activities, and lack of maintenance as well as wastage of energy. The relevant authorities also ought to consider the value of corrosion prevention, fouling cleanup, and implementing a particular industrial cleaning process.
A.N. Campbell, L. D. (2016). Investigating the Impact of Internal and External Convection on Thermal. HAZARDS 26 ,SYMPOSIUM SERIES NO 161 , 1-10.
Addepalli, S. E. (2015). Degradation study of heat exchangers. . Procedia Cirp, 137-142.
Allahkaram, S. R. (2011). Failure analysis of heat exchanger tubes of four gas coolers. . Engineering Failure Analysis,, 1108-1114.
Faes, W. L. (2019). Corrosion and corrosion prevention in heat exchangers. Corrosion reviews,, 131-155.
Ian Travers, P. C. (2018). Thinking in Outcomes – Maintaining Focus on What Process Safety. HAZARDS 28, SYMPOSIUM SERIES NO 163, 1-5.
Ibrahim, H. A. (2012). Fouling in heat exchangers. . MATLAB-A fundamental tool for scientific computing and engineering applications, 57-96.
Peter Diakow, K. T. (2016). Large-Scale Vented Deflagration Tests . HAZARDS 26 , SYMPOSIUM SERIES NO 161 , 1-9.
Schwartz, M. P. (1982). Four types of heat exchanger failures. ITT Bell & Gosset.
Teng, K. H.-S. (2017). Industrial Heat Exchanger: Operation and Maintenance to Minimize Fouling and Corrosion.