June 2025

Heat Exchange/Management

Application of systems engineering principles in a reliability assessment of air-cooled heat exchangers

This article proposes a holistic remaining life assessment and reliability engineering framework developed based on systems engineering principles to cover both mechanical integrity (header boxes and fin tubes) and dynamic reliability aspects (fin blades, electrical motors, gearboxes and propellers).

IP: 10.3.106.231

Air-cooled heat exchangers are some of the most important assets in process plant facilities for ensuring continued operational excellence and production rates. Numerous remaining life assessment (RLA) and reliability engineering (RE) methodologies are available to perform a condition assessment of air-cooled heat exchangers. Most methodologies concentrate on the mechanical integrity aspects of air-cooled heat exchangers, such as the header box and fin tubes life assessment using recognized and generally accepted good engineering practices (RAGAGEP) to primarily address loss of containment (process fluid) scenarios.  

A significant systems engineering challenge exists due to the lack of a comprehensive methodology that holistically addresses both the static and dynamic components of air-cooled heat exchangers. Traditional methodologies often overlook the dynamic components—such as fin blades, electrical motors, gearboxes and propellers (fans)—that are integral to the overall performance and reliability of the air-cooled heat exchangers. The absence of an integrated framework that considers the interdependencies and interactions between mechanical and dynamic components complicates the task of accurately assessing the remaining life and ensuring the reliability of the entire system. 

This article proposes a holistic RLA and reliability engineering framework developed based on systems engineering principles to cover both mechanical integrity (header boxes and fin tubes) and dynamic reliability aspects (fin blades, electrical motors, gearboxes and propellers). By employing a systems engineering approach, the framework ensures a comprehensive assessment of the interdependencies among all sub-systems, thereby enhancing the overall reliability and maintainability of air-cooled heat exchangers. The systems engineering-based assessment framework enables asset integrity, reliability and maintenance engineering professionals to produce a technically robust, coherent and auditable system reliability report, addressing both static and dynamic subsystems of air-cooled heat exchangers and ultimately contributing to improved asset performance, regulatory compliance and longer useful life. 

An air-cooled heat exchanger—or fin cooler—is the largest set of packaged equipment in a refinery or similar process plant that has both stationary and rotating components. The annual maintenance spend on air-cooled heat exchangers can be $2 MM–$5 MM, depending on the size, process complexity, and distillation or processing capacity of a plant. Air-cooled heat exchangers are used to cool the process fluid and achieve the required condensation in a particular process unit.  

An air-cooled heat exchanger operates on the principle that hot process fluid always enters on the tube side (flows inside finned tubes and header box), while the air at the ambient conditions flows over the finned surfaces of the tubes. Heat is rejected to the atmosphere (air) from the process fluid through convection that cools the process fluid to the desired temperature. The operating principle of an air-cooled heat exchanger is depicted in FIG. 1. 

FIG. 1. Basic air-cooled heat exchangers operating principle.^1,2  

Air-cooled heat exchangers can be classified and configured into several categories or types, as detailed in TABLE 1. Several arrangements and configurations for air-cooled heat exchangers are depicted in FIGS. 2–4. 

FIG. 2. Air-cooled heat exchanger fin types and dynamic (mechanical) components.² 

FIG. 3. Air-cooled heat exchanger configurations [induced (top) and forced (bottom) drafts].² 

Key challenges in conducting a reliability assessment of air-cooled heat exchangers.

The maintenance and reliability of complex assets like an air-cooled heat exchanger require a framework or methodology^1,3 that caters to both stationary and rotating components and is compliant with applicable in-service inspection codes or industry recommended practices for all components. Each air-cooled heat exchanger can be broken down into three types of components: 

• Process fluid containment (static) components of the tube bundle 
• Header box  
• Finned tubes  
• Structural (static) components 
• Fan ring (housing) and plenum assembly  
• Column structure supporting the air-cooled heat exchanger 
• Dynamic or rotating components 
• Electrical motor (including bearing) 
• Fan, fan shaft and blade assembly  
• Gearbox or belt. 

It is clear from the above list that no single in-service inspection code or industry best practice is sufficient to conduct a comprehensive reliability engineering study for air-cooled heat exchangers. It becomes imperative to break down air-cooled heat exchanger equipment into components and then assess each individual component per its unique in-service inspection code or available industry best practice. For example, the header box and finned tubes relate to mechanical integrity aspects of air-cooled heat exchanger equipment reliability since the primary mode of failure for these two would be loss of containment (process fluid).  

The air-cooled heat exchanger header box and tube are usually designed in accordance with ASME Section VIII Div.1.4 The industry recommended practices for header box and finned tube are in accordance with API RP 661 and internal standard ISO 13706 that have design, fabrication, inspection, testing and shipping requirements for air-cooled heat exchangers in the petroleum and natural gas industries.5,6 

Similarly, the fan ring assembly and structural components can be evaluated using non-destructive techniques, such as the ultrasonic spot or grid methods, to assess the thickness loss due to metallic corrosion. Structural components may also suffer minor cracking damage due to the vibration of mounted dynamic components such as electrical motors or fans (propellers). 

Dynamic components like an electrical motor, fan and gearbox can be assessed using reliability centered maintenance methods or condition-based (predictive) maintenance by calculating failure rate(s) and further adjusting with key specific factors to compute the reliability levels for dynamic components.7 The failure frequencies for dynamic components can be taken from a reliability failure mode library developed either in-house or commercially available in the market. 

Subsequently, reliability engineers can then perform holistic reliability or remaining life assessments and recommend repairs, replacements and the continued operation of some components or the whole air-cooled heat exchanger. The holistic analysis of air-cooled heat exchangers will require a comprehensive framework to unite the individual reliability improvement methodologies for mechanical integrity, structural and dynamic components. 

Systems engineering framework to assess equipment reliability.

A systems engineering framework1 to perform a condition and remaining life assessment on air-cooled heat exchangers in the process plant industry is depicted in FIG. 5.  

FIG. 5. Systems engineering framework for air-cooled heat exchangers.¹ 

The systems engineering framework in FIG. 5 was developed based on a poster that was originally presented at the 2024 AiCHE Spring Meeting and 20th Global Congress on Process Safety in New Orleans, Louisiana (U.S.). This systems engineering framework (holistic methodology) can be adopted and implemented by reliability engineers in the steps detailed in the following sections. 

Step 1: Criticality analysis or initial prioritization. Perform a failure mode effects and criticality assessment (FMECA) on all air-cooled heat exchangers in the facility or plant to define or assign a criticality ranking.8,9 A second-level FMECA may also be done to assess and break down each air-cooled heat exchanger into major components with relative criticality.  

The individual assessments applicable to different component groups are depicted in the systems engineering framework in FIG. 5, are listed below and will be discussed at length in the subsequent steps: 

• The mechanical integrity assessment applies to the header box and finned tube bundle. 

• The structural integrity assessment applies to column (beam) structures, fan rings and plenum chamber assemblies. 

• Reliability centered maintenance applies to dynamic components like electrical motors (including bearings), fan shafts, blade assemblies, and gearboxes or belt drives. 

This step describes a typical FMECA methodology to conduct the initial prioritization or criticality assessment on all the air-cooled heat exchangers in a facility. 

FMECA is a risk-based prioritization technique (methodology) used to identify, analyze and report the following for an equipment, group of equipment or a process.^8–11 All parameters, categories and classifications relevant to the FMECA process are defined below: 

• Anticipated failure mode(s) of equipment (system) and its components (subsystems) 

• Likelihood of all anticipated failure mode(s) for equipment (system) and components (subsystems): classified as occurrence (O) 

• Consequence of all anticipated failure mode(s) for equipment (system) and components (subsystems): categorized as severity (S) 

• Detection or mitigation capabilities for all anticipated failure mode(s) for equipment (system) and components (subsystems): categorized as detection (D) 

• Risk ranking or criticality analysis of equipment (system) and components (subsystems): calculated using equation risk priority number (RPN) = S*O*D 

• Prioritization or optimization of resources for improving efficiency of all lifecycle steps (design, construction, operation, retirement) of an equipment, process, or system. 

The severity, occurrence and detection scales for performing the FMECA method on air-cooled heat exchangers in the process plant industry are described in the poster technical paper referenced in literature (FIG. 1).¹ 

A fully populated sample FMECA worksheet is shown in TABLE 2 for a major air-cooled heat exchanger function. A similar assessment can be adopted for all air-cooled heat exchanger functions within the FMECA study scope. The output from the FMECA process will be a list of high, medium and low criticality air-cooled heat exchangers on a plant or facility level.  

Step 2: Mechanical integrity assessment. A mechanical integrity assessment that is applicable to critical (ACHE) header-box and finned tube bundle(s) is conducted in Step 2. Note that a preliminary criticality (FMECA) analysis must have already been completed per Step 1. The major assessments required in this step are detailed in the following sections. 

Non-destructive evaluation (NDE). Conduct a visual inspection of all header-box and finned tube bundles,5,6 both onstream and during shutdown, as needed, per API RP 661 and ISO 13706. Industry best practices for NDEs on air-cooled heat exchangers are to check the following: 

• Damage to fins due to vibration, debris, or sand and dust clogging. This affects the heat dissipation capacity of the fins and reduces the thermal efficiency of the air-cooled heat exchanger. This may also lead to local hot spots on the finned tube bundle(s), affecting the mechanical integrity of the air-cooled heat exchanger and potentially leading to a loss of containment scenario. 

• Mechanical fatigue damage on tube ends connecting to the header-box due to vibration. 

• Check for minor gas leaks using a gas detector if the visual inspection is done onstream during operation. Inaccessible areas can be checked using a binocular device. 

• Local hot spots due to clogs, dust and debris on fins can lead to reduced thermal efficiency or—in a worst-case example—a loss of containment scenario and must be examined using thermography technique. 

An NDE can identify thinning and cracking damage on the header box and finned tubes (bundles) per API RP 661 and ASME Section V Code. RAGAGEP for major NDEs on finned tube bundle(s) are listed below:^5,6,12 

• Eddy current testing (ECT) is good for external and internal defects (pits). Limited flaw depth and defects parallel to probe winding and scan direction are not detectable. This is suitable for non-ferromagnetic materials. 

• Internal rotary inspection system (IRIS) is an ultrasonic immersion pulse echo technique. It requires a clean tube to enable precise maneuvering of the probe, and detects internal and external defects. It requires ferromagnetic (carbon and duplex stainless steel) tube materials and has a heavy water usage. This technique is low speed but accurate (excellent sizing of defects). 

The air-cooled heat exchanger header box can be inspected for cracking using a combination of below surface and volumetric NDE techniques.^4,1 

• Surface NDEs such as ECT and a wet fluorescent magnetic test (WFMT). An alternative field current measurement (ACFM) coupled with a WFMT can also be used to detect and verify surface bearing cracks. 

• Volumetric NDEs are angle beam ultrasonic techniques (UT) such as time-of-flight diffraction (TOFD), phased array ultrasonic testing (PAUT) or shear wave ultrasonic testing (SWUT).  

An air-cooled heat exchanger header box can be inspected for thinning using an ultrasonic technique (UT) spot for general thinning, and an ultrasonic scan, grid or automated UT for local thinning. Radiography technique (RT) can also be used for smaller geometries or air-cooled heat exchanger nozzle connections.  

Fitness for service (FFS) evaluation. FFS engineering evaluations are data-intensive, multi-disciplinary and quantitative assessments used to compute and verify the structural integrity of equipment with a particular type of flaw, defect or damage.¹³  

API RP 579’s Part 4, which focuses on general metal loss and Level-1 assessment on air-cooled heat exchangers, primarily entails refining minimum thickness (Tmin) or adjusting the maximum allowable working pressure (MAWP) as part of a re-rate analysis.¹³ The cross-sectional shape of the air-cooled heat exchanger header-box is rectangular; therefore, the regular pressure vessel calculations applicable to cylinder, elliptical heads, etc., will not apply. The ASME section VIII Div. 1’s mandatory appendix 13 titled design of vessels with non-circular cross section must be utilized to refine the Tmin for the header-box or to re-rate the air-cooled heat exchanger to a lower MAWP to enable operating with an identified thinning or cracking flaw.^4,13 

Similarly, API RP 579’s Part 5 can be utilized for local thinning damage evaluation along with the applicable ASME Section VIII. Div. 1 construction code equations for the non-circular vessel design. API RP 579’s Part 9 contains the FFS evaluation methods to perform detailed calculations for crack-like flaws and can be used in similar fashion to assess the remaining life.^4,13 

Bundle repair, replacement and upgrade strategy (inspection planning). This section will discuss quantitative and semi-quantitative risk-based inspection planning techniques—qualitative techniques for mechanical integrity-related inspection planning are not deemed fit by industry experts and are rarely used.^10,11 

An industry best practice for performing quantitative risk-based inspection study on heat exchangers or air-cooled heat exchangers is based on the recommended practice API RP 581 titled, “Risk-based inspection methodology.” API RP 581’s Part 5 describes a quantitative methodology for heat exchanger or air-cooled heat exchanger tube bundles. API RP 581 describes three different options (methods) for performing fully quantitative probability of failure calculations for tube bundles.¹¹ These methods are described in TABLE 3 with the perceived adoption and implementation challenges.¹⁴ Note that a consequence assessment is not discussed in this section since it is possible to benchmark a common method of consequence analysis and use different probability of failure (POF) or damage rate calculation methods for risk calculations. The parameter β is defined as a Weibull shape factor and denotes how failure rate develops over time, and η is the Weibull characteristic life parameter that denotes the time at which 63% of the units have failed. The relationship is such that when β is equal to 1, the mean-time to failure (MTTF)—the sum of operational lifespans for all the air-cooled heat exchangers; the number of air-cooled heat exchangers—is equal to the parameter η. 

MTTF is specified for the air-cooled heat exchanger tube bundle to denote or assess the replacement frequency since the reliability metric MTTF is used for non-repairable components or systems. Since air-cooled heat exchangers can keep running with plugged or retubing, the MTTF is a suitable metric that informs the full replacement of the tube bundle due to not being able to be repaired any further.^7,9 

Step 3: Structural integrity assessment. A structural integrity assessment applicable to critical (air-cooled heat exchanger) column structure and fan ring housing should be conducted in Step 3 as described below. The air-cooled heat exchanger structure is composed of columns, support beams and connected bracings. The height of the air cooler fan assembly is typically kept high enough to allow the necessary volume of air flow through the finned tube bundle.  

NDE should be performed per each facility’s established inspection guidelines for the process unit structure. The following lists industry best practice suggestions for NDEs on both the structure and fan ring, plenum assembly. 

• Check for vibration-induced damage or deformation on the bolted joints and welds between various components such as columns, bracings and support beams. Particularly, deformation at the location of anchor bolts is often noted if there is excessive vibration in air-cooled heat exchanger operation.  

• Perform ultrasonic thickness (UT) spot measurements to check for thinning damage if there are signs of material yielding, spalling of coatings due to unexpected high concentration of stresses, or force distribution arising out of both vibration and corrosion. RT can also be performed if the UT is infeasible due to geometrical limitations of the air-cooled heat exchanger structural component(s). Similarly, surface cracking NDEs such as dye penetration testing (DPT) can be performed to verify the presence of surface cracks on structural components. Cracking is rare on the plenum and fan ring components. 

The FFS or finite element analysis (FEA) evaluations^6,12,13 of the damaged structural components are rare since the rate and extent of damage on structural components are less than on process containment components such as header-box and finned tube(s).  

Step 4: Reliability centered maintenance. The reliability centered maintenance (RCM) methodology is based on applying a combination of professional intuition and a rigorous statistical approach to drive the facility maintenance program in a logical and optimized manner. The fundamental principles of RCM analysis, the RCM work process and the resulting maintenance strategies will be described in this section for air-cooled heat exchanger reliability and maintenance assessments.^7,9 

The evergreen RCM work process and its five major steps are depicted in FIG. 6. Note that the RCM process applies to only high and medium criticality air-cooled heat exchangers as classified from the FMECA study in Step 1. 

FIG. 6. The RCM process for air-cooled heat exchangers.^1,7,9 

TABLE 4 highlights an example of anticipated functional failures, resulting in failure modes on a system and component level for a particular air-cooled heat exchanger system function. It is essential to quantify the potential consequences in terms of safety and economics using an industry established consequence model. 

The following four major maintenance strategies are the outcome of an RCM analysis: 

• Run to malfunction (failure)—This strategy applies to systems (equipment) or components where the consequence of failure and the mitigation benefit is less than the cost of the required maintenance tasks. These equipment are assessed to have low criticality.  

• Preventive or calendar-based maintenance—This strategy applies to systems (equipment) or components where the real-time condition monitoring (maintenance) is not cost effective and cannot be justified due to medium and low criticality of the function(s) per an FMECA study. The greasing preventive maintenance (PM) program17,18 for the fan and motor bearings of an air-cooled heat exchanger and the mechanical PM program for dynamic parts such as belt and pulley, fan blades and bearings are explained in literature.¹ 

• Predictive or condition-based maintenance—This strategy applies to systems (equipment) or components where the real-time condition monitoring and testing efforts and related costs are justified due to medium and high criticality of the equipment function(s) per an FMECA study. Vibration monitoring (PdM) and thermography programs7,9 to identify local temperature differences or hot spots on air-cooled heat exchanger bundles and electrical motors during operation using infrared waves are explained in literature.¹ 

• Proactive maintenance—This strategy revolves around using the lessons learned from the past inspection, maintenance, testing and root cause analysis reports and bringing about significant, permanent and cost-effective organizational changes in the future equipment design, technical specifications, inspection techniques and maintenance procedures. 

Step 5: Performance control, cost benefit analysis and cross-functional team workshop. This operational aspect (post design and construction) of air-cooled heat exchanger maintenance is of critical importance due to its dynamic nature. The operation of air-cooled heat exchangers involves many variables, such as the variability in process fluid composition, process fluid rate, inlet temperature of the fluid, and changes in ambient conditions of the air both daily and seasonally. Both undercooling (less heat duty) and overcooling (more heat duty) can be detrimental to air-cooled heat exchanger mechanical integrity and dynamic component reliability. 

The process fluid containment (mechanical integrity) components—such as the header box and finned tube bundle—require performance control in the form of corrosion control, monitoring and mitigation to prevent loss of containment scenarios that may potentially lead to production loss, process safety incidents or similar operational inefficiencies. Corrosion control and monitoring can be achieved through the development and implementation of integrity operating windows (IOWs) for equipment in a process unit.^16,19,20 IOWs are also referred to as reliability or integrity operating limits in the industry and entail listing all process fluid and operating parameter ranges for equipment and as well as the material of construction to prevent, control or mitigate specific damage mechanism(s) and susceptibilities. IOWs should be developed for all air-cooled heat exchangers within a process plant; an exceedance must be accounted for in risk-based inspection planning or other mechanical integrity assessments such as fitness for service, repair and replacement initiatives. 

For example, there are recommended industry practices on corrosion control, inspection and mitigation for the reactor effluent air coolers in the hydrotreating process unit of a refinery. An example of integrity or reliability operating window parameters for reactor effluent air coolers is described in TABLE 5. Note that the establishment of reliable limits (ranges) for these IOW parameters depends on the specific needs of the process system and credible damage mechanism(s). 

Similarly, IOW parameters for other air-cooled heat exchangers in a process plant or system can be developed to establish their ranges in line with the required thermal efficiency and production rates for that process unit.  

Performance control for dynamic component reliability. Performance control or optimization of cooling in air-cooled heat exchanger operation beyond the management of IOWs is accomplished through air flow control. Air flow control depends on the dynamic components of the air-cooled heat exchanger and ambient air conditions, including seasonal variations.^2,21 The air flow rate directly impacts the overall heat duty of the air-cooled heat exchanger for a given ambient condition. Additionally, noise control can also be a performance control parameter for air-cooled heat exchangers due to regulatory or ergonomic requirements. 

The goal of the performance control step is to strategically install upgrades of air-cooled heat exchanger dynamic components that are causing thermal efficiency issues due to undesirable air flow control and unoptimized heat duty during air-cooled heat exchanger operation. The upgrades in the form of adjustment, repair and the replacement of dynamic components of air-cooled heat exchanger are listed below.^2,21 

The list is not exhaustive, but rather only a sample of performance control measures to achieve the desired air flow control during air-cooled heat exchanger operation. 

• Adjustment of fan blade pitch angle, regulating fan speed with variable frequency drives for air flow control  

• Radial clearance of the fan blade for effective control of air flow and noise 

• Composite material of construction for the fan blades to improve air flow significantly and permanently (e.g., fiberglass aluminum composite rather than aluminum). 

Cost benefit analysis. A cost benefit analysis can be conducted individually for all three sets of air-cooled heat exchanger components described in Steps 2, 3 and 4. The best practice is to assess each major repair, replacement or predictive maintenance strategy—developed as part of Steps 2, 3 and 4—in terms of benefit-to-cost ratio analysis. This enables the reliability engineering team to monitor and control the lifecycle cost of an air-cooled heat exchanger with the maximum benefits in terms of improved availability and reliability.  

Takeaways. This article has outlined a thorough reliability engineering or RLA framework (methodology) developed based on systems engineering principles covering both the mechanical integrity (header box and fin tubes) and reliability (fin blades, electrical motor, gear box and propeller) aspects of air-cooled heat exchanger equipment. The goal is to enable the asset integrity (reliability and maintenance) engineering community to conduct and generate a technical, coherent and auditable reliability assessment report for air-cooled heat exchangers.  

It is recommended that a comprehensive air-cooled heat exchanger asset integrity and reliability manual (AIRM) be prepared if the air-cooled heat exchangers are in large numbers across several facilities of a large process plant conglomerate. The air-cooled heat exchanger AIRM should include execution templates (or formats) for all steps of this holistic methodology (framework). The AIRM templates will help evaluate all three component types of air-cooled heat exchanger consistently and present the reliability assessment results methodically to plant and corporate management. 

LITERATURE CITED

1 Narang, D., “Engineering framework to assess equipment reliability of air fin coolers in process plant industries,” 2024 AiCHE Spring Meeting and 20th Global Congress on Process Safety, 25 March 2024, online: https://www.aiche.org/academy/conferences/aiche-spring-meeting-and-global-congress-on-process-safety/2024/proceeding/paper/55cm-engineering-framework-assess-equipment-reliability-air-fin-coolers-process-plant-industries 

2 Hudson Products Corp., “The basics of air-cooled heat exchangers,” Chart Industries, 2007, online: https://files.chartindustries.com/hudson/BasicsofACHEBrochure-Web.pdf 

3 Narang, D., “Remaining life assessment of upstream well stimulation service units,” Inspectioneering, July 12, 2021, online: https://inspectioneering.com/blog/2021-07-12/9743/remaining-life-assessment-of-upstream-well-stimulation-service-units 

4 American Society of Mechanical Engineers (ASME), “BPVC.VIII.1: Rules for construction of pressure vessels—Division 1,” 2025. 

5 International Organization for Standardization (ISO) 13706, “Petroleum, petrochemical and natural gas industries—Air-cooled heat exchangers,” 2011. 

6 American Petroleum Institute (API) Standard 661, “Petroleum, petrochemical and natural gas industries—Air-cooled heat exchangers,” July 2013. 

7 Chalifoux, A. and J. Baird, Reliability centered maintenance (RCM) guide: Operating a more effective maintenance program, Defense Technical Information Center, U.S. Army Corps of Engineers, Construction Engineering Research Laboratory, Champaign, Illinois (U.S.), 1999. 

8 Narang, D., “Application of FMECA methodology in reliability assessment of complex assets in process plant industries,” Reliability, Maintenance & Managing Risk Conference, American Society of Quality (ASQ), July 25–26, 2024, online: https://asqrrd.org/rmmr-2024-presentations/ 

9 U.S. Department of Defense (DOD) Military Standard MIL-STD-1629A, “Procedures for performing a failure mode, effects and criticality analysis,” November 24, 1980, online: https://www.dsiintl.com/wp-content/uploads/2017/04/mil_std_1629a.pdf 

10 American Petroleum Institute (API) Recommended Practice 580, “Risk-based inspection,” 2016. 

11 American Petroleum Institute (API) Recommended Practice 581, “Risk-based inspection technology,” 2016. 

12 American Society of Mechanical Engineers (ASME), “BPVC.V: Non-destructive examination, 2025. 

13 American Petroleum Institute (API) 579-1/ American Society of Mechanical Engineers (ASME) FFS-1, “Fitness for service,” 2021. 

14 Narang, D., “Selection and integration of risk-based inspection tool in hydrocarbon facilities,” American Petroleum Institute (API) Inspection and Mechanical Integrity Summit, Galveston, Texas (U.S.), February 2017. 

15 Narang, D., “Secrets to becoming a well-informed risk-based inspection professional,” Inspectioneering Journal, May/June 2018. 

16 American Petroleum Institute (API) Recommended Practice API 932A, “A study of corrosion in hydroprocessing reactor effluent air cooler systems,” September 2002, online: https://tajhizkala.ir/doc/API/API_932A_2002_,_A_Study_of_Corrosion.pdf 

17 SKF, “Principles of bearing-selection-process/lubrication/suitable-grease,” lubrication (grease) selection and application by SKF, January 9, 2024, online:  https://www.skf.com/us/products/super-precision-bearings/principles/bearing-selection-process/lubrication/suitable-grease 

18 SKF, “SKF engineering tools,” January 9, 2024, online: https://www.skf.com/us/services 

19 American Petroleum Institute (API) Recommended Practice API 584, “Integrity operating windows,” May 2014. 

20 American Petroleum Institute (API) Recommended Practice API 932-B, “Design, materials, fabrication, operation, and inspection guidelines for corrosion control in hydroprocessing reactor effluent air cooler (REAC) systems,” June 2019. 

21 Giammaruti, R., “Performance improvement to existing air-cooled heat exchangers,” Cooling Technology Institute, 2004. 

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