Fouling may be defined as the accumulation and formation of unwanted materials on the surfaces of processing equipment, which can seriously deteriorate the capacity of the surface to transfer heat under the temperature difference conditions for which it was designed (Mostafa M. Awad). Heat exchangers are process equipment in which heat is continuously or semi-continuously transferred from a hot to a cold fluid directly or indirectly through a heat transfer surface that separates the two fluids. Heat exchangers consist primarily of bundles of pipes, tubes or plate coils (Hassan Al-Haj Ibrahim).
The process of fouling could be represented by the equation below (S. N. Kazi):
dmdt=md – mrWhere:
dmdt = Net deposition rates
md = Deposition rate
mr = Removal rate
Figure 1: Fouling of Heat Exchanger
Fouling can occur on any fluid-solid surface and have other adverse effects besides reduction of heat transfer. It can affects the operation of equipment in two ways (Mostafa M. Awad):
The fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of heat exchangers.
When deposition occurs, the cross sectional area is reduced, which causes an increase in pressure drop across the apparatus.
Many types of fouling can occur on the heat transfer surfaces. The generally favoured scheme for the classification of the heat transfer fouling is based on the different physical and chemical processes involved. Most fouling situations are virtually unique. Fouling can be classified into the following categories (Bott, T. Reg):
Crystallization or Precipitation Fouling
Chemical Reaction Fouling
Fouling can occur as a result of the fluids being handled and their constituents in combination with the operating conditions such as temperature and velocity. Almost any solid or semi-solid material can become a heat exchanger foulant, but some materials that are commonly encountered in industrial operations as foulants include (Mostafa M. Awad):
Airborne dusts and grit
Waterborne mud and silts
Calcium and magnesium salts
Biological substances such as bacteria, fungi and algae
Oils, waxes and greases
Heavy organic deposits such as polymers, tars
EFFECTS OF FOULING
Fouling on process equipment surfaces can have a significant, negative impact on the operational efficiency of the unit. On most industries today, a major economic drain may be caused by fouling (Hassan Al-Haj Ibrahim). These are effects on heat transport, pressure drop and heat exchanger design.
Figure 2: Thermal Resistances for clean and fouled tubes (Mostafa M. Awad)
Precipitation of mineral salts as a scale on the surface of the conduit and cause obstruction of fluid flow, impedance of heat transfer, wear of metal parts, localized corrosion attack and unscheduled equipment shutdown. The deposit layer provides an additional resistance to heat transfer (S. N. Kazi). The performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer and causes the rate of heat transfer in a heat exchanger to decrease. The net effect of these accumulations on heat transfer is represented by a fouling factor, which is a measure of the thermal resistance introduced by fouling.
The overall heat transfer coefficient for a smooth tubular heat exchanger under deposited conditions, Uf can be obtained by adding the inside and outside thermal resistances:
Uf = 1AOAihi+AoRfiAi +AOlndOdi2?kL+RfO+1hO Where Rfi and RfOrepresent resistances for the outer and inner surfaces of the tubes.
The thermal resistance due to fouling is evaluated generally based on experiments as difference in the overall specific resistances of the fouled and clean wall:
Rf=1Uf-1UdWhere, the overall heat transfer coefficient Uf can also be evaluated by using the rate equation:
Uf=QA x ?Tf The heat flow rate, Q and temperature difference ?Tf (the temperature difference between heated surface and the bulk liquid) are experimentally obtained. A is the exposed area of the heat exchanging surface to the liquid.
Pressure loss is considered more critical than loss in heat transfer due to fouling in heat exchanger. Fouling results in a finite layer. Thus in a tubular heat exchanger, the deposited layer roughens the surface, diminishes the inner and raises the outer dimension of the tubes. The inside diameter of the tube decreases and roughness of the tube increases due to fouling which, causes an increase in pressure drop. Pressure drop inside a tube of a heat exchanger under fouled and clean state can be correlated as follows (S. N. Kazi):
?Pf ?Pc = ff fc dc df umf umc 2Considering that the mass flow rates under clean and fouled conditions are the same, the mass flow rate can be represented as:
m= ?um AcrThus, the equation become:
?Pf ?Pc = ff fc dc df 5The magnitude of df of scaled tube can be obtained from equation below:
df = dcexp-2kc Rf dC The thickness, tf of deposit layer can be obtained from:
tf = 0.5dc1-exp-2kc Rf dC For a known total fouling resistance, the tube diameter under fouled conditions can be evaluated on knowing the thermal conductivity of the deposits. Non-uniform thermal conductivity may result from the multi layers of fouling deposits. Depending on situations the fouling layer is considered composed solely of one material. In some occasions to ease calculations ff is considered equal to fc (S. N. Kazi).
Heat Exchanger Design
Fouling rate is a dominating factor in designing a particular heat exchanger (S. N. Kazi). For the fouling allowance, there are different approaches are used to provide an allowance for fouling resistance. They all result into an excess heat transfer surface area. A few methods include such as specifying the fouling resistances, the cleanliness factor, or the percentage over surface.
A fouling resistance is prescribed on each side of the surface where fouling is anticipated. A lower overall heat transfer coefficient is resulted. To achieve the specified heat transfer, excess surface area is provided. Until the specified value of the fouling resistance is reached, the performance of the heat exchanger will be satisfactory (S. N. Kazi).
According to Tubular Exchanger Manufacturers Association (TEMA) is referenced source of fouling factors used in the design of heat exchangers. Plant data, proprietary research data, personal and company experience are other sources of fouling resistance data that could be used in design. These are minimize fouling by considering design features, design features to facilitate fouling control, fouling and operation of heat exchangers, fouling control strategies and heat exchanger with green additives.
Minimize Fouling by considering Design Features
Fouling prone fluid stream should be placed on the tube side as cleaning is easier. Generally higher fluid velocity and lower tube wall temperature retard fouling accumulation. Velocity of 1.8 m/s is a widely accepted figure for tube side flow of a heat exchanger. Fouling deposits are always found heavy in the region of low velocity at the vicinity of baffles in the shell side of the shell and tube heat exchangers (S. N. Kazi).
Design features to facilitate fouling control
Heat exchangers require cleaning at certain intervals. Online cleaning can be employed to control fouling by extending cleaning cycle. At construction and installation phase of a plant on-line cleaning system could be installed at ease. A heat exchanger with removable head and straight tube would be easy to clean and maintain. On site cleaning facilities are to be provided with options of keeping isolation valves and connection provisions for cleaning hoses which could lead to chemical cleaning (S. N. Kazi).
Fouling and operation of heat exchangers
Provision of excess surface area in heat exchangers for curbing fouling may lead to operation problem and fouling build. Generally high heat transfer area enhances total heat transfer which raises the out let temperature. By changing process parameters such as flow, surface temperature leads to higher fouling (S. N. Kazi).
Fouling control strategies
A number of strategies are applied for fouling control. In operating condition additives are added. On-line or off-line surface cleaning techniques are other options. To control fouling under different consequences are consolidated by some researchers as stated in Table 1:
On-line techniques Off-line techniques
Use and control of appropriate additives: Inhibitors, Antiscalants, Dispursants, Acids,
Air jet Disassembly and manual cleaning:
Liquid jet, Steam, Air jet.
Sponge balls, brushes, sonic horns, soot blowers, chains and scrappers, thermal shock, air bumping Chemical cleaning
Table 1: Various techniques adapted to control fouling (S. N. Kazi)
Heat Exchanger with green additives
Many additives were developed for retardation of fouling but many of them found carcinogenic in nature. Now researchers are heading towards green additives. Chemistry and analysis are underway. Lab analysis and performances will be subsequently achieved. In near future users are looking for a breakthrough in this field (S. N. Kazi).
Specifying the fouling resistances or oversizing result in added heat transfer surface, the excess surface area can result in problems during start up and bring about conditions that can, in fact, encourage excess fouling due to low velocity (Mostafa M. Awad). In order to prevent or mitigate the impact of fouling problems, these methods such as chemical methods, mechanical methods (S. N. Kazi), electronic anti-fouling technology (Tangient LLC) and plant design and construction (Hassan Al-Haj Ibrahim) can be applied.
Chemical additives developed by many companies have been extensively used to mitigate fouling in the industrial sector. Various additives can be used to prevent scaling. Some of the common water additives are EDTA (sequestering agent), polyphosphates and polyphosphonates (threshold agents) and polycarboxylic acid and its derivatives (sequestering and threshold treatment). Sequestering agents such as EDTA complex strongly with the scaling cations such as Ca++, Mg++, and Cu++ in exchange with Na+, thus preventing scaling as well as removing any scale formed previously.
Polyphosphates and polyphosphonates as threshold agents are also used to reduce scaling in boilers and cooling water systems. Crystal modifying agents (e.g. Polycarboxylic acid) distort the crystal habit and inhibit the formation of large crystals. The distorted crystals do not settle on the heat transfer surface, they remain suspended in the bulk solution (S. N. Kazi).
Mechanical methods can be divided into two categories according to their ways of action. These are Brute force methods such as high-pressure jets, lances, drills and Mild methods such as brushes and sponge balls. Muller-Steinhagen has reported that several mechanical methods have been developed in recent years. The following mechanisms predict the modern methods:
Breakage of deposits during brief overheating due to differential thermal expansions of heat transfer surface and deposits,
Mechanical vibration of the heat transfer surfaces,
Increased shear stress at the fluid/deposit interface, and
Reduced stickiness of the heat transfer surface.
The deposits which are not strongly adhere to the surface can be removed by increasing the flow velocity. Muller-Steinhagen and Midis reported that alumina deposits were removed completely when the flow velocity was increased for a short period of time after a fouling run (S. N. Kazi).
Figure 3: Fouling Resistance as function of flow velocity
Besides that, another methods in mechanical is hydro-blasting method (Tangient LLC). This method is the most common method. However, it requires to shut down the heat exchanger. Hydro-blasting method will sends high pressure water through the tubes to remove the solid build ups. Drills with drill bits or brushes are also alternatives used to clean plugged tubes. Shooting hydro pressure/air, metal scrapers or rubber plugs through the tubes is a faster method for mechanical cleaning. There are a few disadvantages when using this method, these are include having to shut down the heat exchanger, increase in labour cost, time and corrosion (Tangient LLC).
Electronic Anti-Fouling Technology
It is a physical process that can be used in order to reduce fouling in heat exchangers. In this technology, CaCO3 will be used to exhibit inverses solubility characteristics. The temperature at the walls of a heat exchanger is greater than the temperature at the pipes feeding the heat exchanger. Therefore, CaCO3 will precipitated onto the exchanger walls if the walls act as nucleation sites. This technique causes the CaCO3 to precipitate in the bulk solution before the water enters the heat exchangers (Tangient LLC). The principle of this technology is explained in Figure 4.
Figure 4: Principle behind how the EAF works (Tangient LLC)
The EAF device consists of a solenoid coil wrapped around the feed tube. A time varying magnetic field is produced which in turn produces an oscillating electrical field. The oscillating electrical field produced causes molecular agitation in the bulk solution, this causes the following reaction to occur leading to the precipitation of Calcium Carbonate in the bulk solution.
Figure 5: Schematic diagram of EAF unit (Tangient LLC)
This precipitated CaCO3 enters and exits the exchanger with the bulk fluid. By precipitating the CaCO3 in the feeding tube the opportunity for precipitation on the exchanger walls is greatly reduced (Tangient LLC).
Plant design and construction
Fouling mitigation and control require scientific considerations in design and construction. In general, high turbulence, absence of stagnant areas, uniform fluid flow and smooth surfaces reduce fouling and the need for frequent cleaning (Hassan Al-Haj Ibrahim). The factors that need to be considered in the designs include the extra surface required. The purposes are to ensure that the heat exchangers will meet process specifications up to shut down for cleaning, the additional pressure drop expected due to fouling, and the choice of appropriate construction materials. The designers must also consider the mechanical arrangements that may be necessary for fouling inspection or fouling removal and cleaning.
However, fouling resistance in the shell and tube heat exchangers are usually much greater than in other types of heat exchangers (Hassan Al-Haj Ibrahim). This is because in the shell side in particular lower fluid flow velocities and low-velocity or stagnant regions, encourage the accumulation of foulants. Table 2 represents fouling risk and effects for different types of heat exchangers:
Type Shell and Tube Plate Spiral Plate Air Cooled Lamella Plate Fin Coiled Tube Double Pipe Graphite Scraped Surface
risk Very poor Very Good Good Poor Fair Poor Fair Fair Fair Very Good
effect Poor Good Good Very Poor Poor Very Poor Poor Fair Poor Good
Table 2: Fouling Risk and effects for different types of heat exchangers (Hassan Al-Haj Ibrahim)
If fouling is expected on the tube side, some engineers recommend using larger diameter tubes (a minimum of 25 mm OD) (Mukherjee R.) The use of corrugated tubes has been shown to be beneficial in minimising the effects of at least two of the common types of fouling mechanisms, deposition fouling because of an enhanced level of turbulence generated at lower velocities, and chemical fouling because the enhanced heat transfer coefficients produced by the corrugated tube result in tube wall temperatures closer to the bulk fluid temperature of the working fluids.