A heat exchanger is a device for transferring heat from one medium to another. During operation, there is an accumulation of undesired substances on a surface (fouling), which retards the heat exchanging capability of the equipment, enhances pressure loss and augments requisite pumping power.
Periodic cleaning of heat exchangers is necessary to prevent a decrease in performance below an acceptable level, even if the equipment is well-designed and the fluid treatment is effective. Conditions in the heat exchanger may deviate from the original design criteria due to changes in flow rates and temperatures, equipment malfunctions, the entrance of air and bacteria and changes in the fluid composition or up-stream corrosion, all of which may promote fouling. Corrosion processes may take place under the deposit. Fouling rates may increase due to the deposit surface roughness, and irregular behavior of the exchanger may occur due to build-up and removal of deposits.
The design of a heat exchanger is significantly influenced by the materials used, process parameters, continuous service in the system and the potential for fouling. Even with those parameters considered, the development of fouling may deteriorate heat exchanger performance to the extent that removal from service is required for cleaning or replacement.
Therefore, it is recommended to remove non-protective deposits soon after their formation. Preventive measures of fouling will prolong heat exchanger service time and reduce the economic impact on operating costs.
Categories of Fouling
Particulate fouling: Deposition of small suspended solid particles, such as clay, silt or iron oxide on heat transfer surfaces of any orientation. Heavy particles settle on a horizontal surface due to gravity, and fine particles settle onto heat transfer surfaces at any orientation. Unburned fuels or ashes deposition on boiler tubes and dust deposition on air-cooled condensers are examples.
Precipitation fouling: Called sedimentation or crystallization fouling, dissolved inorganic salts, normally present in heat exchanger fluids, become supersaturated at the heat transfer surface. The following processes may cause supersaturation:
- Evaporation of solvent
- Cooling below solubility limit for a solution with normal solubility, e.g. increasing solubility with increasing temperature
- Heating above the solubility limit for solutions with inverse solubility, such as
CaCO3, CaSo4, Ca3(PO4), CaSiO3, Ca(OH)2, Mg(OH)2, MgSiO3, Na2SO4, Li2SO4 and Li2CO3 in water
- Mixing of streams with different composition
- Variation of pH, which affects the solubility of CO2 in water
Crystallization usually starts at especially active points or nucleation sites, such as scratches and pits. After an induction period, this type of adherent fouling spreads to cover the entire surface and requires vigorous mechanical or chemical treatment to be removed.
High temperatures and an increase in salt concentration will augment the fouling rate. This often occurs in heat exchangers of process industries, boilers and evaporators.
Chemical reaction fouling: This includes deposit formation on the heat transfer surface by a chemical reaction in which the surface material itself does not participate. This type of fouling is often incredibly tenacious and needs extraordinary measures to clean.
Corrosion fouling: This is a type of fouling caused by a chemical reaction, occurring when the surface reacts with the fluid and becomes corroded. For example, the presence of sulfur in fuel can cause corrosion in gas and oil fired boilers.
Corrosion is often predisposed to the liquid side of the heat exchanger. In some cases, the product of corrosion may be swept away downstream and cause deposition on surfaces there.
Biological fouling: This includes the development and deposition of organic films, consisting of micro-organisms, growth of macro-organisms and their products, such as bacteria and the attachment of macro-organisms, such as mussels, algae, etc. on the heat transfer surfaces. Microbial fouling always precedes fouling by macro-organisms, with the micro-organisms acting as the nutrient source for the macro-organisms.
Many types of bacteria will produce slime on the heat transfer surfaces and attract other types of foulants. Larger growth, such as seaweed and other organic fibers, restricts the fluid flow and often causes pitting of the metal.
Mixed Fouling: Several fouling mechanisms usually occur at the same time. This is nearly always mutually reinforcing.
Heat Exchanger Cleaning Methods
Heat exchangers may be cleaned by mechanical or chemical methods or by a combination of both. In some situations, on-line cleaning can be done without interruption of operation. At other times, off-line cleaning is required.
Off-Line Cleaning Methods
Off-line chemical cleaning is a technique that is used very frequently to clean heat exchangers. In general, this type of cleaning is designed to dissolve the deposit by means of a chemical reaction with the cleaning fluid. Chemical cleaning has the advantage over mechanical cleaning by its ability to clean difficult-to-reach areas.
There is no mechanical damage to the bundle from chemical cleaning, although some risk of corrosion damage, due to a reaction of the tube material with the cleaning fluid, exists. The problem may be overcome through proper flushing of the unit. Drawbacks to off-line chemical cleaning include potential corrosion damage, handling of hazardous chemicals and the use of a complex procedure.
Chemical cleaning methods have several advantages over mechanical methods:
- Relatively quick
- No risk of mechanical damage to surfaces
- Chemical solutions that reach normally inaccessible areas
- Less labor intensive than mechanical cleaning
- Can be performed onsite
Most chemical cleans include five distinct procedures, each being monitored for results before proceeding to the next.
- Alkaline Clean: The purpose of the alkaline clean is to remove the organic portion of the deposit (oil and fat) and render the inorganic surface hydrophilic. It is essential to make the following acid cleaning stage effective.
- Rinses: Before and after each chemical step, high-pressure water flushes are used to physically remove loose and softened material.
- Acid Cleaning: Once the surface is hydrophilic, the deposit is softened and/or dissolved by application of the appropriate acid blend. The blend usually contains an inhibitor, which prevents corrosion of the base metal. The analysis of the spent acid strength and the concentration of dissolved scale species indicates the acid clean effectiveness.
- Rinses: After the acid stage, water rinsing is required to remove loose debris, sludge and residual acid. Water rinsing may be accompanied by inert gas purging and sequestrate addition, depending on the cleaning technique and the plant configuration.
- Passivation: After the acid and rinse stages, the base metal is exposed and in a very active state. If left exposed to the atmosphere, the surface would rapidly re-oxidize. A passivation process forms a tightly adherent, protective oxide film on the base metal.
Some applications may require modifications of the above sequence. The selection of the cleaning agent and the cleaning procedure strongly depend on the type of deposit and the material and configuration of the installation, as well as economic and environmental considerations.
Off-line Mechanical Cleaning Methods
Mechanical cleaning is a labor-intensive process that requires heat exchangers be taken off-line and dismantled. Steam-blasting and hydro-blasting are probably the most common mechanical cleaning methods.
For very tenacious deposits, sand can be added to the pressurized water to increase the cleaning efficiency. However, blasting may not eliminate all deposits, and some significant roughness can remain.
While blasting is an effective method for the shell side of the tube bundle, several different methods can be used for the inside of straight tubes:
- The continuous cleaning sponge ball system can also be used as a transportable, off-line cleaning system, particularly if used with corundum-coated sponge balls.
- Very dirty and plugged tubes can also be cleaned with drills equipped with drill bits, brushes or bit-brush combinations.
- Using air or hydro pressure, rubber plugs or metal scrapers can be shot through the tubes. Rubber plugs are ineffective for hard deposits, while shooting metal scrapers through the tubes at a water pressure of 35 bar and a scraper velocity of 3 m/s to 6 m/s results in the removal of most deposits.
In general, water pressure systems are safer than air pressure systems, due to the compressibility and subsequent rapid expansion of gasses.
Most mechanical cleaning methods remove not only the deposit, but also the protective oxide layer. Under certain circumstances, this may create a corrosion problem. On the other hand, regular cleaning removes deposit and avoids severe fouling problems, so a combination of chemical and mechanical cleaning may be the solution.
Recent Developments in Heat Exchanger Cleaning
Although significant progress has been made in recent years towards the mitigation of heat exchanger fouling, the challenge to reduce its impact on heat exchanger performance remains. Many mitigation and cleaning techniques are currently part of regular plant operation, and were developed by an empirical trial and error approach.
These antifouling techniques have little or no relationship to academic research findings, since industry and academic research institutions have traditionally approached the problem of fouling from different perspectives. A closer collaboration between the two communities is required to optimize the effectiveness of mitigation methods and to develop new strategies for fouling mitigation.
Since 1995, conferences on heat exchanger fouling and cleaning have been held at bi-yearly intervals to promote innovative thinking and to explore new theoretical and practical approaches. Experts from industry, academia and government research centers from around the world present their latest research and technological developments in the areas of fouling mitigation and cleaning technologies. The participants have included from the academic community, Hans Müller-Steinhagen, M. Reza Malayeri and A. Paul Watkinson.
The next conference, Heat Exchanger Fouling and Cleaning XII – 2017, organized by Heat Transfer Research Inc. (HTRI), is scheduled for June 11-16, 2017, in Madrid.
An article published by G.E. Saxon, Jr. at the Conference on Heat Exchanger Fouling and Cleaning – 2015, addresses high-pressure vs. low-pressure cleaning. A brief description of his methods follows.
Hydro-Blasting Versus Mechanical Tube Cleaning
Hydro-blasting using up to 40,000 or more PSI of water pressure has been the preferred method for cleaning industrial heat exchangers. It requires multiple water trucks, cleaning apparatus/pumps onsite and numerous technicians to operate the equipment and ensure safety. The cleaning time is lengthy, and the environmental impact of using thousands of gallons of water to clean one heat exchanger is significant.
Newer cleaning systems utilizing mechanical tube cleaners operate at far lower water pressures, typically under 700 PSI. The cleaning components of low-pressure mechanical systems are smaller, more specialized and require fewer technicians.
Fewer technicians mean less unit congestion, a lower safety risk and less cost. The environmental impact of mechanical cleaning with low-pressure water is much lower than with high-pressure water methods, with an average 90% less contaminate wastewater requiring treatment.
The three most common low-pressure water mechanical tube cleaning systems in use are tube shooting, brushing and drilling. One system or a combination may be more effective than hydro-blasting at removing the most tenacious deposits including particulate and biological fouling, baked-on hard deposits, calcium-carbonate, acrylic, asphalt high-density polyethylene, iron oxide and others.
Tube shooting methods utilize a mechanical tube cleaner (pig) propelled through a tube using low-pressure water at under 700 PSI. They are available in a variety of sizes, materials and configurations. Typical configurations include mechanical tube cleaners with stainless-steel wire brushes or spring-tension metal blades.
Special application mechanical tube cleaners are also available to score and break apart calcium-carbonate deposits or U-Tube cleaners. They are designed to thoroughly clean the bends of u-tube units. Low-pressure water is used to propel the mechanical tube cleaner down the length of the tube, and it also flushes out the deposits as they are loosened.
Brushing or drilling may become necessary when dealing with thick, hard, baked-on deposits. The Excaliber™ system uses a brush mounted to the tip of a flexible shaft, rotating at up to 2,500 RPM, and low-pressure water to remove deposits.
It is ideal for cleaning heat exchangers in tight locations. The output water flow of only 3 to 7 GPM generates far less wastewater than hydro-blasting operations.
A ridged shaft system HydroDrill™ is recommended when the tube is completely blocked or the deposit is too tenacious for a flexible shaft. This system uses a drill bit mounted to one end of a rigid shaft.
As the shaft rotates, the unit pumps water at 250 PSI through the shaft, pushing out deposits as they are loosened. The HydroDrill has a very low output water flow at only 2 to 3 GPM, keeping wastewater to an absolute minimum.
Numerous heat exchanger cleaning methods, both off-line and on-line, are available. The method selected, the cleaning agent and the cleaning procedure will depend on the type of deposit and the material and configuration of the installation, along with economic and environmental considerations. Therefore, the selection of a cleaning and maintenance service company, one with the technical knowledge, experience and the appropriate equipment, is critical.