Industrial Heat Exchanger: Operation and Maintenance to Minimize Fouling and Corrosion

Heat exchanger is equipment used to transfer heat from one fluid to another. It has extensive domestic and industrial applications. Extensive technical literature is available on heat exchanger design, operation and maintenance, but it is widely scattered throughout the industrial bulletins, industrial design codes and standard, technical journals, etc. The purpose of this book chapter is to consolidate into basic background and concepts design of heat exchangers, operation, cleaning and green technology maintenance on heat exchanger closely related to the industrial practices.


Introduction
Heat exchanger plays an important role in industrial application. It is implemented for the purposes of heating and cooling of large-scale industrial process fluids [1]. Heat exchanger is a dynamic design which can be customized to suit any industrial process depending on the temperature, pressure, type of fluid, phase flow, density, chemical composition, viscosity and many other thermodynamic properties [2,3]. Due to global energy crisis, an efficient heat recovery or dissipation of heat has become a vital challenge for Scientists and Engineers [4].
Heat exchangers are designed to optimize the surface area of the wall between two fluids to maximize the efficiency, while minimizing resistance to fluid flow through the exchangers within constrain of material cost. The performance of heat exchanging surfaces could be enhanced by the addition of corrugations or fins in heat exchanger, which increase surface area and may channel fluid flow or induce turbulence [5]. Efficiency of industrial heat exchangers could be online monitored by tracking the overall heat transfer coefficient based on its temperatures which tends to decline over time due to fouling [6].
Potential damage towards equipment caused by formation of scale can be very costly if processed water is not treated correctly. Chemicals are commonly used to treat the water in the industry. A total of 7.3 billion dollar worth chemicals per year in the U.S. is released into the air, dumped in streams and buried in landfills every year. Forty percent of these chemicals is purchased by industry for control of scale in cooling tower, boiler and other heat transfer equipment. This percentage also represents more than 2 billion dollar of toxic waste which contribute to trillion of gallon contaminated water disposed annually into the earth which belongs to all of us.
Maintenance of fouled tubular heat exchangers can be performed by several methods such as acid cleaning, sandblasting, high-pressure water jet, bullet cleaning or drill rods. In largescale cooling water systems for heat exchangers, water treatment such as purification, addition of chemicals, catalytic approach, etc., are used to minimize fouling of the heat exchanging equipment [7]. Other water treatment processes are also used in steam systems for power plants to minimize fouling and corrosion of the heat exchanger and other equipment. Most of the chemical and additives used for fouling and corrosion mitigation are hazardous to the environment [8]. So, the days have come to apply chemicals of approaches benign to the environment [9-11].

About industrial heat exchanger
An industrial heat exchanger is heat transfer equipment that utilizes a thermal energy exchange process between two or more medium available at different temperature. Industrial heat exchangers are applied in various industrial applications such as power plant generation, petroleum oil and gas industry, chemical processing plant, transportation, alternate fuels, cryogenic, air conditioning and refrigeration, heat recovery and other industries. In addition, heat exchangers are the equipment always closely related to our daily life, for example, evaporators, air preheaters, automobile radiators, condensers and oil coolers. In most heat exchangers, a heat transfer surface separates the fluid which incorporates a wide range of different flow configuration to achieve the desired performance in different applications. Heat exchangers could be classified in many different ways. Generally, industrial heat exchangers have been classified according to construction, transfer processes, degrees of surface compactness, flow arrangements, pass arrangements, phase of the process fluids and heat transfer mechanisms as seen in Figure 1.

Basic design concepts for heat exchanger
The design concepts of heat exchanger must meet normal process requirements specified through service conditions for combinations of un-corroded and corroded conditions and the clean and fouled conditions. One of the critical criteria of heat exchanger design is the exchanger must be designed for ease of maintenance, which usually means cleaning or replacement of parts, tubing, fittings, etc. damaged by ageing, vibration, corrosion or erosion throughout the service period.
Hence, a heat exchanger design should be as simple as possible particularly if heavy fouling is expected. By minimize temperature in conjunction with the choice of fluid velocity and reducing the concentration of foulant precursors, will reduce the incidence of potential fouling. Moreover, highest flowing velocity should be allowed under the constraints of pressure drop and erosion from the flow. In addition, material selection within constrained cost retards the build-up of deposits and allows shorter residence time. It should also be compatible in terms of pH, corrosion and not only just heat exchanger, but also in terms of heat equipment and transfer lines of the heat exchanger.

Fouling
Fouling is always defined as the formation and accumulation of unwanted materials deposit onto the processing equipment surfaces. These normally very low thermal conductivity materials form an insulation on the surface which can extremely deteriorate the performance of the surface to transfer heat under the temperature difference for which it was designed [13]. On top of this, fouling increases the resistance to fluid flow, resulting in higher pressure drop across the heat exchanger. Many types of fouling can occur on the heat transfer surfaces, for examples, crystallization fouling, particulate fouling, corrosion fouling, chemical reaction fouling, biological fouling and solidification fouling [14]. Fouling can have a very costly effect in the industries which eventually increases fuel usage, interrupts operation, production losses and enhances maintenance costs [15].
The fouling is formed in five stages which can be summarized as initiation of fouling, transport to the surface, attachment to the surface, removal from the surface and ageing at the surface [16]. There are a few parameters influencing the fouling factors, such as pH [9], velocity [17], bulk temperature of fluid [18], temperature of the heat transfer surface, surface structure [19] and roughness [20,21].
The overall fouling process is usually considered to be the net result of two simultaneous sub processes: a deposition process and a removal process as shown in Figure 2. As illustrated in

Corrosion
Environment features such as soil, atmosphere, water or aqueous solutions commonly attack general metal and alloys. The deterioration of these metals is known as corrosion. It is agreeable that corrosion happens due to electrochemical mechanism. Premature failures in various equipment are caused by corrosion in most commercial processes and engineering operations, leading to unwanted issues. This includes pricey breakdown, un-schedule shutdown and increases in maintenance cost.
This downtime worsens in fields such as chemical industries, oil refining, sea and land electric power plant, paper manufacture, air conditioning, refrigerator, food and liquor manufacturing. Hence, general info and mechanism of corrosion will bring great interest to public and  industry [24]. The corrosion process is affected by various parameters as show in Figure 5. Hence, these criteria should take consideration into the design basics of heat exchangers.

Cost imposed due to fouling
Apart from the high cost of heat exchanger fouling, very few work have been reported to accurate determine economic penalties causes by fouling. Therefore, these attribute cost to difference aspect of heat exchanger design and operation. However, reliable knowledge of fouling economics is desirable in order to evaluate the cost efficiency of various mitigation strategies [26,27]. The total fouling-related costs involve the following:

Capital expenditure
Excessive surface area required to overcome the heavy fouling conditions, costs for stronger foundation, provision for extra spaces and increased transportation and installation costs.

Energy costs
Costs for extra fuel required if fouling leads to extra fuel burning in heat exchanging equipment to overcome the effect of fouling.

Maintenance costs
Costs for removal of fouling deposits, costs for chemicals or other operating costs for antifouling devices.

Cost of production loss
Planned or unplanned plant shutdowns due to fouling in heat exchangers can cause large production losses. These losses are often considered to be the main cost of fouling and very difficult to estimate.

Extra environmental management cost
Cost for disposing large amount of chemical/additives used for fouling mitigation.

Current efforts to solve fouling deposition and corrosion problems
A lot of works have been done to reduce fouling formation and control of corrosion. In recent years, numerous methods have been developed to control fouling and corrosion [29]. These methods can be classified as chemical means (inhibitor), mechanical means, changing the phases of the solution, electromagnetic fields, electrostatic fields, acoustic fields, ultraviolet light, radiation or catalytic treatment, surface treatment, green additives, fibre as a suspension, etc. In the past, chromate was a successful chemical agent for corrosion protection and crystal growth control until it was banned. In the past, most of the methods used, chemical/additives for fouling and corrosion mitigation are hazardous to the environment. So, the days have come to apply green technology method and chemicals approaches benign to the environment [9][10][11].

Fouling mitigation by green technology (catalytic mitigation and green additive)
Physical water treatment (PWT) is a good alternative for safe and efficient nonchemical fouling mitigation method. Examples of PWT include permanent magnets [32], solenoid coil devices [33], green additive [34] and catalytic materials and alloys [35].
To mitigate scaling on heat transfer surfaces, chemical additives are often used, but chemicals are expensive and pose hazard threat to the environment and health. Mitigation of calcium sulphate dehydrates scale formation on heat exchanger surfaces by using natural wood pulp fibre was conducted by Kazi [36] and others in University of Malaya. Experimental work was designed and fabricated to study the use of natural wood pulp fibre as a means of fouling mitigation as seen in Table 2 and Figure 6. Test specimen materials SS316, brass, copper, aluminium Table 2. Experiments set-up for fouling mitigation by incorporating green additives [36,37].

Cleaning of heat exchanger
In order to maintain or restore efficiency of the heat exchanger, it is often necessary to clean the heat exchangers. Methods of cleaning may be classified into two groups: online and offline cleaning [38]. In some applications, the cleaning can be done online to maintain acceptable performance without interruption of operation. In other cases, offline cleaning must be used.

Online cleaning
Online cleaning generally utilizes a mechanical method designed for only tube side and requires no disassembly. The advantages of online cleaning are the continuity of service of the heat exchanger with the hope that no cleaning-mandated downtime will occur. However, it adds extra cost of a new heat exchanger installation or the large cost of retrofits and there is no assurance that all the tubes would be cleaned sufficiently.

I. Circulation of sponge rubber balls [39]
The technique is capable of preventing the accumulation of particulate matter, biofilm formation and scale and corrosion product deposition. It is only applicable to flow through the inside of tubes.

II. Two phases of the ferrous sulphate treatment
The first phase involves the initial laying down of the protective film. The second phase involves the maintenance of the film, which would be otherwise destroyed by the shear effects of flow.

III.
Chlorination used for combat bio-fouling [40] IV. Scale inhibitors [10,41,42] V. Magnetic devices [10,43,44] VI. Sonic technology [45] High and low frequency sound emitters (horns) are used to relief fouling problems on heat exchangers. The use of sound is much less effective in sticky and tenacious deposits that are generally associated with slagging.
VII. Online chemical cleaning [46] Injection of chemical solutions into the process streams for the cleaning purposes.
VIII. The use of radiation [47] Radiation sterilization of microbial-laden water, the use of ultraviolet light and Gamma rays have been considered for a long time.

Offline cleaning
An alternative to online cleaning is to stop operation and clean the heat exchanger. Offline cleaning can be classified into offline chemically cleaning or by mechanical means. The cleaning method preferred without the need to dismantle the heat exchangers, but usually it is necessary to have access to the inside surfaces. It would be prudent to consider the installation of a "standby" heat exchanger, thereby providing the opportunity to clean the fouled heat exchanger while at the same time maintain the production.

Offline mechanical cleaning
a. Tube drilling and rodding [28] Devices may be applied to the rotating shaft including drills, cutting and buffing tools and brushes that may be made from different materials, for example, steels or nylon, brasses depending on the tube material and the nature of the deposit.

b. Cleaning with explosives
Used of controlled explosions, where the energy to remove the deposits, is transmitted by a shock wave in the air adjacent to the surface to be cleaned or by the general vibration of tubes brought about the explosion. It is a relatively new innovation introduced in boiler plant cleaning. It is possible to begin the cleaning process, while the structure is still hot.
c. Thermal shock [48] Changes in temperature particularly rapid changes, cause cracking of foulant layer with the possibility of flaking. This technique is similar to steam soaking. The water flushing carries away the dislodged material and it is repeated until clean surfaces are obtained.
Inhibitor hydrofluoric acid is by far the most effective agent but cannot be used if deposits contain more that 1% w/v calcium.
b. Chlorinated or aromatic solvents followed by washing are suitable for heavy organic deposits for example, tars and polymers (foulant) [50].

Conclusion
Fouling and corrosion are the major unresolved crisis in heat exchanger operation. Though the fouling deposition problems and the impact to the economy are a serious concern, still there is lack of awareness in concerned authorities. In addition, the penalties of corrosion are numerous and varied and the effects of these on the efficient, reliable and safe operation of equipment or structures are often more serious than the simple loss of a mass of metal. Therefore, the present paper will promote concerned organization in different countries, seriousness of this problem and application of possible mitigation approach.
For an industry, the proper cleaning method and control play important role to reduce the production costs. Production cost significantly increases due to chemical usage, maintenance work and downtime loss and water wastage. Consequently, the related authorities need to comprehend the importance of corrosion control, fouling cleaning and enforce a specific standard of cleaning procedure in the industries.