The deposition of material on heat transfer surfaces — commonly known as fouling — significantly affects the thermal and mechanical performance of heat exchangers. Fouling increases the overall thermal resistance and lowers the overall heat transfer coefficient of heat exchangers. In addition, it impedes fluid flow, accelerates corrosion and increases pressure drop across the heat exchanger. Cleaning fouled heat exchangers presents a significant challenge to maintenance and operation personnel in the process industries.
Common fouling mechanisms include:
• Particulate Fouling. This results from sediment of dust, rust, fine solids and other entrained solids.
• Crystallization Fouling. Calcium carbonate is the predominant component of the hard, tenacious scale deposit from water and is particularly apparent in processes involving heat transfer. A concentration of dissolved solids by repeated partial evaporation of the water is the main factor that causes calcium carbonate scale. Even soft water eventually will become scale forming when concentrated multiple (two, three, four or more) times.
• Biological Fouling. This occurs when biological organisms grow on heat exchanger surfaces. Problems arise from algae and other microbes such as barnacles and zebra mussels. At certain times of the year when microbes typically bloom, colonies several inches thick may grow across the heat exchanger, affecting thermal performance.
• Chemical Reaction Fouling. This type of fouling occurs when the depositions are formed as a result of chemical reaction.
• Corrosion Fouling. This results from a chemical reaction that involves the heat exchanger surface material.
The costs related to fouling are myriad. Costs for extra fuel occur when fouling leads to extra fuel burning in boilers and furnaces or when other energy such as electricity or process steam is needed to overcome the effects of fouling. It is calculated that additional fuel and energy costs equate to billions of dollars each year.
Additional costs are attributed to maintenance charges for the removal of fouling deposits. Manufacturers of heat exchangers say that 15 percent of all factory maintenance costs are credited to heat exchangers, and of that, 50 percent is due to fouling. Add to these the cost of a plant shutdown due to a fouled exchanger, and the effect of fouling is only too plain.
Cleaning Methods for Fouled Exchangers
Four popular methods of removing fouling include:
• Mechanical cleaning using brushes and scraping.
• Chemical cleaning using solvent or chemicals.
• High velocity cleaning using water jets.
• Electronic water treatment.
This article will focus on electronic scale removal as a means of handling heat exchanger fouling. Electronic water treatment is effective in removing fouling in heat exchangers, including rust, scale and biological fouling such as zebra mussels.
In the process industries, it is common for chemicals to be added either because the water is being used for “scrubbing” or cleaning, or to achieve a chemical mineral effect as part of the production process and thereby increasing the scaling tendency. Scaling deposits are common in flow lines subject to changes of pressure or temperature. Regardless of how hard water effects are achieved, the outcome is the same. Scale formation results in reduced-diameter or blocked pipes, reduced heat transfer efficiency, seized pumps, inoperable valves, misleading meter readings and defective heating elements.
Electronic Water Conditioning for Handling Fouled Exchangers
Electronic water treatment is a non-invasive system utilizing a solenoid coil or coils wrapped around the pipework to be treated. A signal generator — of which the frequency is continuously changed — supplies current to the coils. The pulse-shaped current creates an induced electric field concentric around the axis inside the pipe. With this arrangement, any charged particle or ion moving within the field experiences a so-called Lorenz force generated by the interaction between charged particles and magnetic and electric fields. Research at a major university in Philadelphia confirmed that the Lorenz Force is unchanged, irrespective of flow rate. Generated magnetic fields have been measured and have been found to be below 1 Gauss, lower than the earth magnetic field strength.
This technology overcomes one disadvantage of permanent-magnet devices: that they work well only within a certain flow rate window, and that at higher and lower flows, the performance drops to zero.
Electronic water treatment products affect the formation of scale by increasing the homogeneous precipitation rate of calcium carbonate and certain other minerals. The ability to adjust power, frequency and coil configurations on-site allows performance to be optimized.
Electronic water treatment also can be used for biological fouling. The impact of zebra mussels extends from the Great Lakes watershed to the mouth of the Mississippi River. The mussels affect industry by clogging pipes and intake structure. Crustaceans and zebra mussels are similar in the way they use calcium — a key component of pipe scaling. They both convert calcium in a free-ion form to calcium carbonate to construct their shell or exoskeleton.
A study undertaken by Aquatic Sciences Inc., an international underwater inspection service, found that electronic water treatment technology was successful in removing calcium carbonate and thereby reducing the level of zebra mussel infestation.
The fouling of heat exchangers in the processing industries is a chronic operating problem. Costs due to additional fuel consumption and maintenance, loss of production and downtime have been estimated as 0.25 percent of the gross national product of industrialized countries. Electronic water treatment offers companies a method of reducing fouling on heat exchangers.
Web Exclusive: What Causes Scale